6.E: Acid-Base Equilibria (Exercises)
Polyprotic Acids
1. Which of the following concentrations would be practically equal in a calculation of the equilibrium concentrations in a 0.134- M solution of H 2 CO 3 , a diprotic acid:
- \(\ce{[H3O+]}\),
- \([OH^−]\)
- \([H_2CO_3]\)
- \(\ce{[HCO3- ]}\)
- \(\ce{[CO3^2- ]}\)
No calculations are needed to answer this question.
- Answer
- [H 3 O + ] and \(\ce{[HCO3- ]}\) are equal, H 3 O + and \(\ce{HCO3-}\) are practically equal
2. Calculate the concentration of each species present in a 0.050- M solution of H 2 S.
3. Calculate the concentration of each species present in a 0.010- M solution of phthalic acid, C 6 H 4 (CO 2 H) 2 .
- Answer
- \(\ce{C6H4(CO2H)2}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{C6H4(CO2H)(CO2)-}(aq) \hspace{20px} K_\ce{a}=1.1×10^{−3}\)
- \(\ce{C6H4(CO2H)(CO2)}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{C6H4(CO2)2^2-}(aq) \hspace{20px} K_\ce{a}=3.9×10^{−6}\)
- [C 6 H 4 (CO 2 H) 2 ] 7.2 × 10 −3 M , [C 6 H 4 (CO 2 H)(CO 2 ) − ] = [H 3 O + ] 2.8 × 10 −3 M , \(\ce{[C6H4(CO2)2^2- ]}\)3.9 × 10 −6 M , [OH − ] 3.6 × 10 −12 M
4. Salicylic acid, HOC 6 H 4 CO 2 H, and its derivatives have been used as pain relievers for a long time. Salicylic acid occurs in small amounts in the leaves, bark, and roots of some vegetation (most notably historically in the bark of the willow tree). Extracts of these plants have been used as medications for centuries. The acid was first isolated in the laboratory in 1838.
a. Both functional groups of salicylic acid ionize in water, with K a = 1.0 × 10 −3 for the—CO 2 H group and 4.2 × 10 −13 for the −OH group. What is the pH of a saturated solution of the acid (solubility = 1.8 g/L)?
b. Aspirin was discovered as a result of efforts to produce a derivative of salicylic acid that would not be irritating to the stomach lining. Aspirin is acetylsalicylic acid, CH 3 CO 2 C 6 H 4 CO 2 H. The −CO 2 H functional group is still present, but its acidity is reduced, K a = 3.0 × 10 −4 . What is the pH of a solution of aspirin with the same concentration as a saturated solution of salicylic acid (See Part a)?
c. Under some conditions, aspirin reacts with water and forms a solution of salicylic acid and acetic acid:
\[\ce{CH3CO2C6H4CO2H}(aq)+\ce{H2O}(l)⟶\ce{HOC6H4CO2H}(aq)+\ce{CH3CO2H}(aq)\]
d. Which of the acids salicylic acid or acetic acid produces more hydronium ions in solution such a solution?
e. What are the concentrations of molecules and ions in a solution produced by the hydrolysis of 0.50 g of aspirin dissolved in enough water to give 75 mL of solution?
5. The ion HTe − is an amphiprotic species; it can act as either an acid or a base.
- Wh at is K a for the acid reaction of HTe − with H 2 O?
- What is K b for the reaction in which HTe − functions as a base in water?
- Demonstrate whether or not the second ionization of H 2 Te can be neglected in the calculation of [HTe − ] in a 0.10 M solution of H 2 Te.
- Answer
- a. (K_{\ce a2}=1×10^{−5};\)
- b. \(K_\ce{b}=4.3×10^{−12};\)
- c. \(\ce{\dfrac{[Te^2- ][H3O+]}{[HTe- ]}}=\dfrac{(x)(0.0141+x)}{(0.0141−x)}≈\dfrac{(x)(0.0141)}{0.0141}=1×10^{−5}\). Solving for x gives 1 × 10 −5 M . Therefore, compared with 0.014 M , this value is negligible (0.071%).
Buffers
6. Explain why a buffer can be prepared from a mixture of NH 4 Cl and NaOH but not from NH 3 and NaOH.
7. Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the acid H 3 PO 4 and a salt of its conjugate base NaH 2 PO 4 .
- Answer
- Excess H 3 O + is removed primarily by the reaction:
- \(\ce{H3O+}(aq)+\ce{H2PO4-}(aq)⟶\ce{H3PO4}(aq)+\ce{H2O}(l)\) Excess base is removed by the reaction: \(\ce{OH-}(aq)+\ce{H3PO4}(aq)⟶\ce{H2PO4-}(aq)+\ce{H2O}(l)\)
8. Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the base NH 3 and a salt of its conjugate acid NH 4 Cl.
9. What is [H 3 O + ] in a solution of 0.25 M CH 3 CO 2 H and 0.030 M NaCH 3 CO 2 ?
\(\ce{CH3CO2H}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{CH3CO2-}(aq) \hspace{20px} K_\ce{a}=1.8×10^{−5}\)
- Answer
- [H 3 O + ] = 1.5 × 10 −4 M
10. What is [H 3 O + ] in a solution of 0.075 M HNO 2 and 0.030 M NaNO 2 ?
- Answer
- \(\ce{HNO2}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{NO2-}(aq) \hspace{20px} K_\ce{a}=4.5×10^{−5}\)
11. What is [OH − ] in a solution of 0.125 M CH 3 NH 2 and 0.130 M CH 3 NH 3 Cl?
- Answer
- \(\ce{CH3NH2}(aq)+\ce{H2O}(l)⇌\ce{CH3NH3+}(aq)+\ce{OH-}(aq) \hspace{20px} K_\ce{b}=4.4×10^{−4}\)
- [OH − ] = 4.2 × 10 −4 M
12. What is [OH − ] in a solution of 1.25 M NH 3 and 0.78 M NH 4 NO 3 ?
- Answer
- \(\ce{NH3}(aq)+\ce{H2O}(l)⇌\ce{NH4+}(aq)+\ce{OH-}(aq) \hspace{20px} K_\ce{b}=1.8×10^{−5}\)
13. What concentration of NH 4 NO 3 is required to make [OH − ] = 1.0 × 10 −5 in a 0.200- M solution of NH 3 ?
- Answer
- [NH 4 NO 3 ] = 0.36 M
14. What concentration of NaF is required to make [H 3 O + ] = 2.3 × 10 −4 in a 0.300- M solution of HF?
15. What is the effect on the concentration of acetic acid, hydronium ion, and acetate ion when the following are added to an acidic buffer solution of equal concentrations of acetic acid and sodium acetate:
- HCl
- KCH 3 CO 2
- NaCl
- KOH
- CH 3 CO 2 H
- Answer
- a. The added HCl will increase the concentration of H 3 O + slightly, which will react with \(\ce{CH3CO2-}\) and produce CH 3 CO 2 H in the process. Thus, \(\ce{[CH3CO2- ]}\) decreases and [CH 3 CO 2 H] increases.
- b. The added KCH 3 CO 2 will increase the concentration of \(\ce{[CH3CO2- ]}\) which will react with H 3 O + and produce CH 3 CO 2 H in the process. Thus, [H 3 O + ] decreases slightly and [CH 3 CO 2 H] increases.
- c. The added NaCl will have no effect on the concentration of the ions.
- d. The added KOH will produce OH − ions, which will react with the H 3 O + , thus reducing [H 3 O + ]. Some additional CH 3 CO 2 H will dissociate, producing \(\ce{[CH3CO2- ]}\) ions in the process. Thus, [CH 3 CO 2 H] decreases slightly and \(\ce{[CH3CO2- ]}\) increases.
- e. The added CH 3 CO 2 H will increase its concentration, causing more of it to dissociate and producing more \(\ce{[CH3CO2- ]}\) and H 3 O + in the process. Thus, [H 3 O + ] increases slightly and \(\ce{[CH3CO2- ]}\) increases.
16. What is the effect on the concentration of ammonia, hydroxide ion, and ammonium ion when the following are added to a basic buffer solution of equal concentrations of ammonia and ammonium nitrate:
- KI
- NH 3
- HI
- NaOH
- NH 4 Cl
17. What will be the pH of a buffer solution prepared from 0.20 mol NH 3 , 0.40 mol NH 4 NO 3 , and just enough water to give 1.00 L of solution?
- Answer
- pH = 8.95
18. Calculate the pH of a buffer solution prepared from 0.155 mol of phosphoric acid, 0.250 mole of KH 2 PO 4 , and enough water to make 0.500 L of solution.
19. How much solid NaCH 3 CO 2 •3H 2 O must be added to 0.300 L of a 0.50- M acetic acid solution to give a buffer with a pH of 5.00? (Hint: Assume a negligible change in volume as the solid is added.)
- Answer
- 37 g (0.27 mol)
20. What mass of NH 4 Cl must be added to 0.750 L of a 0.100- M solution of NH 3 to give a buffer solution with a pH of 9.26? (Hint: Assume a negligible change in volume as the solid is added.)
21. A buffer solution is prepared from equal volumes of 0.200 M acetic acid and 0.600 M sodium acetate. Use 1.80 × 10 −5 as K a for acetic acid.
a. What is the pH of the solution?
b. Is the solution acidic or basic?
22. What is the pH of a solution that results when 3.00 mL of 0.034 M HCl is added to 0.200 L of the original buffer?
- pH = 5.222;
- The solution is acidic. (c) pH = 5.221
23. A 5.36–g sample of NH 4 Cl was added to 25.0 mL of 1.00 M NaOH and the resulting solution diluted to 0.100 L.
- What is the pH of this buffer solution?
- Is the solution acidic or basic?
- What is the pH of a solution that results when 3.00 mL of 0.034 M HCl is added to the solution?
24. Which acid in [link] is most appropriate for preparation of a buffer solution with a pH of 3.1? Explain your choice.
- Answer
- To prepare the best buffer for a weak acid HA and its salt, the ratio \(\dfrac{\ce{[H3O+]}}{K_\ce{a}}\) should be as close to 1 as possible for effective buffer action. The [H 3 O + ] concentration in a buffer of pH 3.1 is [H 3 O + ] = 10 −3.1 = 7.94 × 10 −4 M
- We can now solve for K a of the best acid as follows:
- \(\dfrac{\ce{[H3O+]}}{K_\ce{a}}=1\)
- \(K_\ce{a}=\dfrac{\ce{[H3O+]}}{1}=7.94×10^{−4}\)
- In [link] , the acid with the closest K a to 7.94 × 10 −4 is HF, with a K a of 7.2 × 10 −4 .
25. Which acid in [link] is most appropriate for preparation of a buffer solution with a pH of 3.7? Explain your choice.
26. Which base in [link] is most appropriate for preparation of a buffer solution with a pH of 10.65? Explain your choice.
- Answer
- For buffers with pHs > 7, you should use a weak base and its salt. The most effective buffer will have a ratio \(\dfrac{\ce{[OH- ]}}{K_\ce{b}}\) that is as close to 1 as possible. The pOH of the buffer is 14.00 − 10.65 = 3.35. Therefore, [OH − ] is [OH − ] = 10 −pOH = 10 −3.35 = 4.467 × 10 −4 M .
- We can now solve for K b of the best base as follows: \(\dfrac{\ce{[OH- ]}}{K_\ce{b}}=1\) K b = [OH − ] = 4.47 × 10 −4 In [link] , the base with the closest K b to 4.47 × 10 −4 is CH 3 NH 2 , with a K b = 4.4 × 10 −4 .
27. Which base in [link] is most appropriate for preparation of a buffer solution with a pH of 9.20? Explain your choice.
28. Saccharin, C 7 H 4 NSO 3 H, is a weak acid ( K a = 2.1 × 10 −2 ). If 0.250 L of diet cola with a buffered pH of 5.48 was prepared from 2.00 × 10 −3 g of sodium saccharide, Na(C 7 H 4 NSO 3 ), what are the final concentrations of saccharine and sodium saccharide in the solution?
- Answer
- The molar mass of sodium saccharide is 205.169 g/mol. Using the abbreviations HA for saccharin and NaA for sodium saccharide the number of moles of NaA in the solution is:
- 9.75 × 10 −6 mol. This ionizes initially to form saccharin ions, A − , with: [A − ] = 3.9 × 10 −5 M
29. What is the pH of 1.000 L of a solution of 100.0 g of glutamic acid (C 5 H 9 NO 4 , a diprotic acid; K 1 = 8.5 × 10 −5 , K 2 = 3.39 × 10 −10 ) to which has been added 20.0 g of NaOH during the preparation of monosodium glutamate, the flavoring agent? What is the pH when exactly 1 mol of NaOH per mole of acid has been added?
Acid-Base Titrations
30. Explain how to choose the appropriate acid-base indicator for the titration of a weak base with a strong acid.
- Answer
- At the equivalence point in the titration of a weak base with a strong acid, the resulting solution is slightly acidic due to the presence of the conjugate acid. Thus, pick an indicator that changes color in the acidic range and brackets the pH at the equivalence point. Methyl orange is a good example.
31. Explain why an acid-base indicator changes color over a range of pH values rather than at a specific pH.
32. Why can we ignore the contribution of water to the concentrations of H 3 O + in the solutions of following acids:
- 0.0092 M HClO, a weak acid
- 0.0810 M HCN, a weak acid
- 0.120 M \(\ce{Fe(H2O)6^2+}\) a weak acid, K a = 1.6 × 10 −7
but not the contribution of water to the concentration of OH − ?
- Answer
- In an acid solution, the only source of OH − ions is water. We use K w to calculate the concentration. If the contribution from water was neglected, the concentration of OH − would be zero.
33. Why can we ignore the contribution of water to the concentration of OH − in a solution of the following bases:
0.0784 M C 6 H 5 NH 2 , a weak base
0.11 M (CH 3 ) 3 N, a weak base
but not the contribution of water to the concentration of H 3 O + ?
34. Draw a curve for a series of solutions of HF. Plot [H 3 O + ] total on the vertical axis and the total concentration of HF (the sum of the concentrations of both the ionized and nonionized HF molecules) on the horizontal axis. Let the total concentration of HF vary from 1 × 10 −10 M to 1 × 10 −2 M .
- Answer
35. Draw a curve similar to that shown in Figure for a series of solutions of NH 3 . Plot [OH − ] on the vertical axis and the total concentration of NH 3 (both ionized and nonionized NH 3 molecules) on the horizontal axis. Let the total concentration of NH 3 vary from 1 × 10 −10 M to 1 × 10 −2 M .
36. Calculate the pH at the following points in a titration of 40 mL (0.040 L) of 0.100 M barbituric acid ( K a = 9.8 × 10 −5 ) with 0.100 M KOH.
a. no KOH added
b. 20 mL of KOH solution added
c. 39 mL of KOH solution added
d. 40 mL of KOH solution added
e. 41 mL of KOH solution added
- Answer
- a. pH = 2.50; b. pH = 4.01; c. pH = 5.60; d. pH = 8.35; e. pH = 11.08
37. The indicator dinitrophenol is an acid with a K a of 1.1 × 10 −4 . In a 1.0 × 10 −4 - M solution, it is colorless in acid and yellow in base. Calculate the pH range over which it goes from 10% ionized (colorless) to 90% ionized (yellow).


2.2A: Covalent Bonds and Other Bonds and Interactions
Covalent bonds result from a sharing of electrons between two atoms and hold most biomolecules together.
- Compare the relative strength of different types of bonding interactions
Key Points
- A polar covalent bond arises when two atoms of different electronegativity share two electrons unequally.
- A non-polar covalent bond is one in which the electrons are shared equally between two atoms.
- Hydrogen bonds and Van Der Waals are responsible for the folding of proteins, the binding of ligands to proteins, and many other processes between molecules.
Key Terms
- hydrogen bond : A weak bond in which a hydrogen atom in one molecule is attracted to an electronegative atom (usually nitrogen or oxygen) in the same or different molecule.
- covalent bond : A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
- dipole : Any object (such as a magnet, polar molecule or antenna), that is oppositely charged at two points (or poles).
EXAMPLES
Adenosine triphosphate, or ATP, is the most-commonly used cofactor in all of biology. Its biosynthesis involves breaking the triple bond of molecular nitrogen, or N 2 , followed by the formation of several carbon-nitrogen single and double bonds.
The octet rule can be satisfied by the sharing of electrons between atoms to form covalent bonds. These bonds are stronger and much more common than are ionic bonds in the molecules of living organisms. Covalent bonds are commonly found in carbon-based organic molecules, such as DNA and proteins. Covalent bonds are also found in inorganic molecules such as H 2 O, CO 2 , and O 2 . One, two, or three pairs of electrons may be shared between two atoms, making single, double, and triple bonds, respectively. The more covalent bonds between two atoms, the stronger their connection. Thus, triple bonds are the strongest.
The strength of different levels of covalent bonding is one of the main reasons living organisms have a difficult time in acquiring nitrogen for use in constructing nitrogenous molecules, even though molecular nitrogen, N 2 , is the most abundant gas in the atmosphere. Molecular nitrogen consists of two nitrogen atoms triple bonded to each other. The resulting strong triple bond makes it difficult for living systems to break apart this nitrogen in order to use it as constituents of biomolecules, such as proteins, DNA, and RNA.
The formation of water molecules is an example of covalent bonding. The hydrogen and oxygen atoms that combine to form water molecules are bound together by covalent bonds. The electron from the hydrogen splits its time between the incomplete outer shell of the hydrogen atom and the incomplete outer shell of the oxygen atom. In return, the oxygen atom shares one of its electrons with the hydrogen atom, creating a two-electron single covalent bond. To completely fill the outer shell of oxygen, which has six electrons in its outer shell, two electrons (one from each hydrogen atom) are needed. Each hydrogen atom needs only a single electron to fill its outer shell, hence the well-known formula H 2 O. The electrons that are shared between the two elements fill the outer shell of each, making both elements more stable.
Polar Covalent Bonds
There are two types of covalent bonds: polar and nonpolar. In a polar covalent bond, the electrons are unequally shared by the atoms because they are more attracted to one nucleus than the other. The relative attraction of an atom to an electron is known as its electronegativity: atoms that are more attracted to an electron are considered to be more electronegative. Because of the unequal distribution of electrons between the atoms of different elements, a slightly positive (δ+) or slightly negative (δ-) charge develops. This partial charge is known as a dipole; this is an important property of water and accounts for many of its characteristics. The dipole in water occurs because oxygen has a higher electronegativity than hydrogen, which means that the shared electrons spend more time in the vicinity of the oxygen nucleus than they do near the nucleus of the hydrogen atoms.
Polar and Nonpolar Covalent Bonds : Whether a molecule is polar or nonpolar depends both on bond type and molecular shape. Both water and carbon dioxide have polar covalent bonds, but carbon dioxide is linear, so the partial charges on the molecule cancel each other out.
Nonpolar Covalent Bonds
Nonpolar covalent bonds form between two atoms of the same element or between different elements that share electrons equally. For example, molecular oxygen (O 2 ) is nonpolar because the electrons will be equally distributed between the two oxygen atoms. The four bonds of methane are also considered to be nonpolar because the electronegativies of carbon and hydrogen are nearly identical.
Hydrogen Bonds and Van Der Waals Interactions
Not all bonds are ionic or covalent; weaker bonds can also form between molecules. Two types of weak bonds that frequently occur are hydrogen bonds and van der Waals interactions. Without these two types of bonds, life as we know it would not exist.
Hydrogen bonds provide many of the critical, life-sustaining properties of water and also stabilize the structures of proteins and DNA, the building block of cells. When polar covalent bonds containing hydrogen are formed, the hydrogen atom in that bond has a slightly positive charge (δ+) because the shared electrons are pulled more strongly toward the other element and away from the hydrogen atom. Because the hydrogen has a slightly positive charge, it’s attracted to neighboring negative charges. The weak interaction between the δ+ charge of a hydrogen atom from one molecule and the δ- charge of a more electronegative atom is called a hydrogen bond. Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers in water and in organic polymers, and the additive force can be very strong. For example, hydrogen bonds are responsible for zipping together the DNA double helix.
Adenosine Triphosphate, ATP : Adenosine Triphosphate, or ATP, is the most commonly used cofactor in nature. Its biosynthesis involves the fixation of nitrogen to provide feedstocks that eventually produce the carbon-nitrogen bonds it contains.
Like hydrogen bonds, van der Waals interactions are weak interactions between molecules. Van der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations of the electron densities, which can lead to slight temporary dipoles around a molecule. For these attractions to happen, the molecules need to be very close to one another. These bonds, along with hydrogen bonds, help form the three-dimensional structures of the proteins in our cells that are required for their proper function.
Interactions between different types of molecules : In this interactive, you can explore how different types of molecules interact with each other based on their bonds.


12.6: Spinal Cord and Spinal Nerves
By the end of this section, you will be able to:
- Describe the structures that protect and supply the spinal cord
- Describe the structures of the spinal cord and their functions
- Explain the arrangement of gray and white matter in the spinal cord
- Compare and contrast how white and gray matter process information
- Describe the sensory and motor components of spinal nerves and the plexuses that they pass through
- Explain what a dermatome is and its clinical significance
The spinal cord is continuous with the brain and transmits sensory information from the periphery of the body to the brain, and motor information from the brain to the periphery. It is also a center for coordination of reflexes, as well as control of reflexes independently from the brain. Along the spinal cord, spinal nerves emerge that carry both sensory and motor information.
Protective Coverings of the Spinal Cord
The spinal cord, as the brain, is protected by meninges, which are continuous with the cranial meninges. From outermost to innermost, the spinal meninges are: dura mater , arachnoid mater and pia mater . The spinal meninges are similar to the cranial ones, except for the dura mater. The spinal dura mater consists of a single layer of connective tissue, differing from the cranial dura mater. Moreover, at each intervertebral foramen, the dura mater extends and fuses with the connective tissue that covers spinal nerves.
One space between the spinal meninges also differs slightly from the ones present around the brain. The epidural space between the periosteum and dura mater is a real space around the spinal cord and houses areolar and adipose connective tissues, and blood vessels (shown in Figure \(\PageIndex{2}\)). Epidural anesthesia is injected in this space. The subarachnoid space is filled with circulating CSF, which also provides a liquid cushion to the brain and spinal cord. Similar to clinical blood work, a sample of CSF can be withdrawn to find chemical evidence of neuropathology or metabolic traces of the biochemical functions of nervous tissue. Because the spinal cord does not extend through the lower lumbar region of the vertebral column, a needle can be inserted through the dura and arachnoid layers to withdraw CSF. This procedure is called a lumbar puncture and avoids the risk of damaging the central tissue of the spinal cord.
Blood Supply to the Spinal Cord
The spinal cord is supported by an extensive collateral supply and drainage of blood. Three longitudinal spinal arteries supply the anterior and posterior regions of the spinal cord. The anterior and posterior spinal arteries are connected by cross-connections of pia mater called the arterial vasocorona which encircles the cord and supplies the lateral portion of the spinal cord. The blood supply is reinforced by numerous arteries that pass through the intervertebral foramina and divide into anterior and posterior radicular arteries and have variable connections with the spinal arteries. Numerous veins and venous plexuses surround the spinal cord and drain the blood from it. These veins share the same names as the arteries that supply the spinal cord.
Spinal Cord
The description of the CNS is concentrated on the structures of the brain, but the spinal cord is another major organ of the system. The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. As shown in Figure \(\PageIndex{1}\), immediately inferior to the medulla oblongata is the cervical region of the spinal cord that contains motor neurons whose axons, together with sensory axons, extend into cervical spinal nerves. Inferior to the cervical region is the thoracic region , that contains neurons for the thoracic spinal nerves. The lumbar region is a short segment that contains neurons for the lumbar spinal nerves. Finally the sacral region contain neurons for the sacral nerves. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. For this reason, the names of the spinal cord regions and vertebral regions to not match up. The lumbar and sacral spinal cord segments are found between vertebral levels T9 and L2. The spinal cord ends around the L1/L2 vertebral level, forming a tapered structure known as the conus medullaris , which corresponds to the sacral spinal cord. The nerves that emerge from the spinal cord pass through the intervertebral foramina at the corresponding vertebral segments. For example, lumbar nerves exit the vertebral canal at the respective lumbar vertebrae. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves inferior to the conus medullaris that resembles a horse’s tail and therefore is named the cauda equina . Within the cauda equina, there is a thin strand of pia mater called filum terminale that helps anchoring the conus medullaris to the coccyx. At each intervertebral foramen, the spinal dura mater extends between vertebrae and surrounds the spinal nerves.
The spinal cord has a diameter of approximately 3/4 of an inch (just less than 2 cm) and its length ranges between 16 to 18 inches (41-45 cm). However, its diameter changes between the different regions because the ratio of white and gray matter changes to reflect the functions of the different regions. The cervical and lumbar regions of the spinal cord show a larger diameter compared to the rest of the regions. The cervical enlargement and lumbar enlargement represent an increased amount of neurons in the gray matter and axons in the white matter that serve the upper limbs and lower limbs, respectively.
Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. As the spinal cord continues to develop in the newborn, anatomical features mark its surface. Viewed in cross section, the anterior midline is marked by the anterior median fissure , and the posterior midline is marked by the posterior median sulcus . As shown in Figure \(\PageIndex{2}\), axons coming from the dorsal (posterior) root ganglion enter the posterior side through the dorsal (posterior) nerve root , which marks the posterolateral sulcus on either side. The axons emerging from the anterior side do so through the ventral (anterior) nerve root . Note that it is common to see the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord. You should learn to be comfortable with both. The posterior and anterior nerve roots fuse together to form the spinal nerves . In the central region of the spinal cord, a central canal is the continuation of the fourth ventricle of the brain and contains cerebrospinal fluid (CSF).
On the whole, the posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions. This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate. The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives rise to motor neurons. The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery.
Gray Horns
In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure \(\PageIndex{3}\), the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing and contain the axons of sensory neurons and the cell bodies of interneurons. The axons of sensory neurons are bundled into the posterior nerve root and are coming directly from the posterior root ganglion which contains cell bodies of sensory neurons. The lateral horn , which is only found in the thoracic, upper lumbar, and sacral regions, is a central component of the autonomic nervous system and contains cell bodies of autonomic motor neurons which innervate cardiac muscle, smooth muscle and glands.The anterior horn contains the cell bodies of somatic motor neurons that control skeletal muscles. The somatic motor neurons are the largest neurons of the body and are classified as multipolar neurons. The fibers that cause contraction of skeletal muscles are the axons of these neurons, which exit the spinal cord through the anterior nerve root. The posterior and anterior nerve roots fuse to form the spinal nerve, which contains mixed sensory and motor information.The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body. The neurons of the spinal cord are organized into functional groups called nuclei representing their respective functions (somatic sensory, visceral sensory, somatic motor and autonomic motor). Surrounding the central canal, a horizontal line of gray matter called the gray commissure connects the left and right sides of the spinal cord.
White Columns
Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. Between the two posterior horns of gray matter are the posterior columns . The posterior columns are composed of axons of ascending tracts. Between the two anterior horns, and bounded by the axons of motor neurons emerging from that gray matter area, are the anterior columns . The white matter on either side of the spinal cord, between the posterior horn and the axons of the anterior horn neurons, are the lateral columns . The anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts—the latter carrying motor commands down from the brain to the spinal cord to control output to the periphery. The anterior columns are connected by a bundle of axons called white commissure that crosses the central region of the spinal cord.
Dorsal Root Ganglion
A ganglion (ganglia for plural) is a group of neuron cell bodies in the peripheral nervous system. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (Figure \(\PageIndex{4}\)). The cells of the dorsal root ganglion are unipolar cells. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies (Figure \(\PageIndex{5}\)).
Spinal Nerves
The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root.
There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.
Spinal nerves extend outward from the vertebral column to innervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a systemic nerve . This occurs at four places along the length of the vertebral column, each identified as a nerve plexus , whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies.
After exiting the intervertebral foramen, each spinal nerve splits into two branches called rami (ramus for singular) (Figure \(\PageIndex{6}\)). On the posterior side, the dorsal ramus is a short nerve that innervates the deep muscle and skin of the back. On the anterior side, the ventral ramus splits into multiple branches that eventually can form nerve plexuses. The ventral ramus innervates the anterior and lateral portions of the trunk, the upper and the lower limbs. Spinal nerves are also associated with other branches called rami communicantes which are formed by the gray and white rami and connect the spinal nerve to the autonomic ganglia.
Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (Figure \(\PageIndex{7}\)). The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve , which connects to the diaphragm at the base of the thoracic cavity.
The other plexus from the cervical level is the brachial plexus . Spinal nerves C5 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. The ventral rami of the brachial plexus rearrange to form the superior, middle and inferior trunks . At the clavicle, the trunks split into an anterior division and posterior division , which innervate the anterior and posterior parts of the upper limbs, respectively. At the axilla, the divisions converge to form the posterior, medial and lateral cords . Finally, the cords split into five terminal branches : the axillary, median, musculocutaneous, radial and ulnar nerves. The axillary nerve innervates the deltoid and teres minor muscles, and receives sensation from the superolateral side of the arm. The median nerve innervates the most anterior forearm muscles as well as muscles that move the thumb. It receives sensation from the palmar region and dorsal tips of the lateral 3 and a half digits. The musculocutaneous nerve innervate the anterior arm muscles and receives sensation from the lateral region of the forearm. The radial nerve innervates muscles and receives sensation from the posterior arm and forearm, including the dorsal region of the lateral three digits (not including their tips). The ulnar nerve innervates anterior forearm muscles as well as intrinsic hand muscles. It receives sensation from the dorsal and palmar regions of the medial one and a half digits.
The lumbar plexus arises from all the lumbar spinal nerves (L1 to L5) and gives rise to nerves innervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a branch that extends through the anterior lower leg. The femoral nerve innervates anterior thigh muscles and receives sensory information from the anterior and medial aspects of the thigh.
The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic nerve , which is a combination of the tibial nerve and the fibular nerve . The sciatic nerve extends across the hip joint and is the largest and longest nerve of the body. Through the tibial and fibular nerves, the sciatic nerve innervates the muscles and receives sensation of the posterior thigh and lower leg. It is most commonly associated with the condition sciatica , which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it.
Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve. The intercostal nerves innervate the intercostal muscles and receive sensation from the skin of the anterior and lateral sides of the thoracic region. The last nerves (T7 to T12) also innervate the abdominal muscles and their overlying skin.
Dermatomes
The spinal nerves, which contain sensory fibers with endings in the skin, connect with the skin in a topographically organized manner, illustrated as dermatomes (Figure \(\PageIndex{8}\)). For example, the fibers of eighth cervical nerve innervate the medial surface of the forearm and extend out to the fingers. All spinal nerves except C1 innervates a region on the skin. Dermatomes are important in a clinical setting as they can help identify potential damage to spinal nerves. In a sensory exam, the dermatome of a specific area is tested for sensation (for example the stroking of a cotton ball) to evaluate the functioning of the spinal nerve connected to that dermatome. Dermatomes are also associated with referred visceral pain, as the spinal nerves innervated the regions of the skin also innervate internal organs. Pain from the internal organ can then be mistakenly referred to a dermatome. For example, myocardial infarction is perceived as referred pain in the chest, left shoulder and left arm. This is because the sensory fibers that innervate the cardiac tissue travel through the T1-T4 spinal nerves, which also innervate the medial and anterior side of the body.
Concept Review
The spinal cord is covered and protected by the spinal meninges, similar to the cranial ones. However, the spinal dura mater presents only one layer of tissue. Moreover, the epidural space is a real space that houses areolar and adipose connective tissues, and blood vessels.
The spinal cord is divided into segments that take the same name as the vertebral regions. However, the sacral spinal cord ends at the vertebral lumbar level (L1/L2) with the conus medullaris, while the spinal nerves continue exiting at their respective region of the vertebral column. Inferior to the conus medullaris, the spinal nerves form a bundle called the cauda equina. The spinal cord presents two enlargements at the cervical and lumbar level.
In cross section, the gray matter is deep within the spinal cord and is separated into horns. The sensory axons coming from the posterior root ganglia enter the spinal cord through the posterior nerve root and synapse in the posterior horn, where interneurons are found. Autonomic motor neurons are found in the lateral horns, while somatic motor neurons are found in the anterior horns. Motor neurons send their axons out of the spinal cord through the anterior nerve root. The posterior and anterior root fuse to form the spinal nerve, which contains mixed sensory and motor information. The white matter is separated into columns which contain axons of sensory and motor neurons. The posterior columns contain ascending sensory tracts which are bundles of axons going towards the brain. The lateral and anterior column contain ascending sensory tracts as well as descending motor tracts, which carry the motor responses from the brain. Overall, the posterior regions of the spinal cord are responsible for ascending sensory information, while the anterior regions are responsible for descending motor information.
Spinal nerves are all mixed nerves with both sensory and motor fibers. Spinal nerves emerge from the spinal cord and reorganize through plexuses, which then give rise to systemic nerves. Thoracic spinal nerves are not part of any plexus, but give rise to the intercostal nerves directly.
Review Questions
Q. What region of the spinal cord contains motor neurons that direct the movement of skeletal muscles?
A. anterior horn
B. posterior horn
C. lateral horn
D. alar plate
- Answer
-
A
Glossary
- anterior column
- white matter between the anterior horns of the spinal cord composed of many different groups of axons of both ascending and descending tracts
- anterior division
- anterior branches of the brachial plexus originating from the trunks
- anterior horn
- gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn
- anterior median fissure
- deep midline feature of the anterior spinal cord, marking the separation between the right and left sides of the cord
- arachnoid mater
- middle layer of the meninges named for the spider-web–like trabeculae that extend between it and the pia mater
- arterial vasocorona
- blood supply to the spinal cord formed by pial anastomoses between the anterior and posterior spinal arteries on the surface of the spinal cord
- ascending tract
- central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain
- axillary nerve
- systemic nerve of the arm that arises from the brachial plexus
- brachial plexus
- nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve
- cauda equina
- bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse's tail
- central canal
- hollow space within the spinal cord that is the remnant of the center of the neural tube
- cervical enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb
- cervical plexus
- nerve plexus associated with the upper cervical spinal nerves
- cervical region
- region of the spinal cord where cervical spinal nerves originate from
- conus medullaris
- tapered end of the spinal cord at the lumbar vertebral level, corresponding to the sacral region of the spinal cord
- cord
- branch of the brachial plexus originating from the divisions
- dermatome
- topographical area of skin in which sensory nerves end
- descending tract
- central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery
- dorsal (posterior) nerve root
- axons entering the posterior horn of the spinal cord
- dorsal ramus
- dorsal branch of a spinal nerve that forms from the dorsal root of the nerve after it emerges from the spinal cord
- dorsal root ganglion
- cluster of cell bodies in the dorsal root of a spinal nerve containing sensory neurons
- dura mater
- tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS
- epidural space
- area between the dura mater and the above structures
- femoral nerve
- systemic nerve of the anterior leg that arises from the lumbar plexus
- fibular nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- filum terminale
- thin strand of pia mater that helps anchoring the conus medullaris to the coccyx
- gray commissure
- horizontal line of gray matter that connects the left and right sides of the spinal cord
- intercostal nerve
- systemic nerve in the thoracic cavity that is found between two ribs
- lateral column
- white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying motor commands to and from the brain
- lateral horn
- region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the autonomic nervous system
- lumbar enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb
- lumbar plexus
- nerve plexus associated with the lumbar spinal nerves
- lumbar puncture
- procedure of taking fluid from the spine in the lumbar vertebral region through a hollow needle
- lumbar region
- region of the spinal cord where lumbar spinal nerves originate from
- median nerve
- systemic nerve of the arm, located between the ulnar and radial nerves
- musculocutaneous nerve
- systemic nerve of the arm, located lateral to the other nerves of the brachial plexus
- nerve plexus
- network of nerves without neuronal cell bodies included
- phrenic nerve
- systemic nerve from the cervical plexus that innervates the diaphragm
- pia mater
- thin, innermost membrane of the meninges that directly covers the surface of the CNS
- posterior columns
- white matter of the spinal cord that lies between the posterior horns of the gray matter, sometimes referred to as the dorsal column; composed of axons of ascending tracts that carry sensory information up to the brain
- posterior division
- posterior branches of the brachial plexus originating from the trunks
- posterior horn
- gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn
- posterior median sulcus
- midline feature of the posterior spinal cord, marking the separation between right and left sides of the cord
- posterolateral sulcus
- feature of the posterior spinal cord marking the entry of posterior nerve roots and the separation between the posterior and lateral columns of the white matter
- radial nerve
- systemic nerve of the arm, the distal component of which is located near the radial bone
- radicular arteries
- blood supply to the spinal cord that runs along the posterior and anterior roots of the spinal nerves
- ramus
- branch of a spinal nerve
- rami communicantes
- structures that provide a short connection from an autonomic ganglion to the spinal nerve
- sacral plexus
- nerve plexus associated with the lower lumbar and sacral spinal nerves
- sacral region
- region of the spinal cord where sacral spinal nerves originate from
- saphenous nerve
- systemic nerve of the lower anterior leg that is a branch from the femoral nerve
- sciatic nerve
- systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg
- sciatica
- painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it
- spinal arteries
- blood supply to the anterior and posterior regions of the spinal cord
- spinal nerve
- one of 31 nerves connected to the spinal cord
- systemic nerve
- nerve in the periphery distal to a nerve plexus or spinal nerve
- terminal branch
- branch of the brachial plexus originating from the cords
- thoracic region
- region of the spinal cord where intercostal nerves originate from
- tibial nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- trunk
- branch of the brachial plexus originating from the spinal roots
- ulnar nerve
- systemic nerve of the arm located close to the ulna, a bone of the forearm
- ventral (anterior) nerve root
- axons emerging from the anterior or lateral horns of the spinal cord
- ventral ramus
- ventral branch of a spinal nerve that forms from the ventral root of the nerve after it emerges from the spinal cord
- white commissure
- bundle of myelinated axons that crosses the central region of the spinal cord
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology


30.4: X Rays - Atomic Origins and Applications
Learning Objectives
By the end of this section, you will be able to:
- Define x-ray tube and its spectrum.
- Show the x-ray characteristic energy.
- Specify the use of x rays in medical observations.
- Explain the use of x rays in CT scanners in diagnostics.
Each type of atom (or element) has its own characteristic electromagnetic spectrum. X rays lie at the high-frequency end of an atom’s spectrum and are characteristic of the atom as well. In this section, we explore characteristic x rays and some of their important applications.
We have previously discussed x rays as a part of the electromagnetic spectrum in Photon Energies and the Electromagnetic Spectrum . That module illustrated how an x-ray tube (a specialized CRT) produces x rays. Electrons emitted from a hot filament are accelerated with a high voltage, gaining significant kinetic energy and striking the anode.
There are two processes by which x rays are produced in the anode of an x-ray tube. In one process, the deceleration of electrons produces x rays, and these x rays are called bremsstrahlung , or braking radiation. The second process is atomic in nature and produces characteristic x rays , so called because they are characteristic of the anode material. The x-ray spectrum in Figure is typical of what is produced by an x-ray tube, showing a broad curve of bremsstrahlung radiation with characteristic x-ray peaks on it.
The spectrum in Figure is collected over a period of time in which many electrons strike the anode, with a variety of possible outcomes for each hit. The broad range of x-ray energies in the bremsstrahlung radiation indicates that an incident electron’s energy is not usually converted entirely into photon energy. The highest-energy x ray produced is one for which all of the electron’s energy was converted to photon energy. Thus the accelerating voltage and the maximum x-ray energy are related by conservation of energy. Electric potential energy is converted to kinetic energy and then to photon energy, so that \(E_{max} = hf_{max} = q_eV\). Units of electron volts are convenient. For example, a 100-kV accelerating voltage produces x-ray photons with a maximum energy of 100 keV.
Some electrons excite atoms in the anode. Part of the energy that they deposit by collision with an atom results in one or more of the atom’s inner electrons being knocked into a higher orbit or the atom being ionized. When the anode’s atoms de-excite, they emit characteristic electromagnetic radiation. The most energetic of these are produced when an inner-shell vacancy is filled—that is, when an \(n = 1\) or \(n = 2\) shell electron has been excited to a higher level, and another electron falls into the vacant spot. A characteristic x ray (see Photon Energies and the Electromagnetic Spectrum ) is electromagnetic (EM) radiation emitted by an atom when an inner-shell vacancy is filled. Figure shows a representative energy-level diagram that illustrates the labeling of characteristic x rays. X rays created when an electron falls into an \(n = 1\) shell vacancy are called \(K_a\) when they come from the next higher level; that is, an \(n = 2\) to \(n = 1\) transition. The labels \(K, \, L, \, M, . . . \) come from the older alphabetical labeling of shells starting with \(K\) rather than using the principal quantum numbers 1, 2, 3, …. A more energetic \(K_{\beta}\) x ray is produced when an electron falls into an \(n = 1\) shell vacancy from the \(n = 3\) shell; that is, an the \(n = 3\) to the \(n = 1\) transition. Similarly, when an electron falls into the the \(n = 2\) shell from the the \(n = 3\) shell, an \(L_{\alpha}\) x ray is created. The energies of these x rays depend on the energies of electron states in the particular atom and, thus, are characteristic of that element: every element has it own set of x-ray energies. This property can be used to identify elements, for example, to find trace (small) amounts of an element in an environmental or biological sample.
Example \(\PageIndex{1}\): Characteristic X-Ray Energy
Calculate the approximate energy of a \(K_{\alpha}\)
x ray from a tungsten anode in an x-ray tube.
Strategy
How do we calculate energies in a multiple-electron atom? In the case of characteristic x rays, the following approximate calculation is reasonable. Characteristic x rays are produced when an inner-shell vacancy is filled. Inner-shell electrons are nearer the nucleus than others in an atom and thus feel little net effect from the others. This is similar to what happens inside a charged conductor, where its excess charge is distributed over the surface so that it produces no electric field inside. It is reasonable to assume the inner-shell electrons have hydrogen-like energies, as given by \(E_n = -\frac{Z^2}{n^2} E_0 (n = 1, \, 2, \, 3, . . .)\). As noted, a \(K_a\) x ray is produced by an \(n = 2\) to \(n = 1\) transition. Since there are two electrons in a filled \(K\) shell, a vacancy would leave one electron, so that the effective charge would be \(Z - 1\) rather than \(Z\). For tungsten, \(Z = 74\), so that the effective charge is 73.
Solution
\(E_n = - \frac{Z^2}{n^2}E_0(n = 1, \, 2, \, 3, . . .)\) gives the orbital energies for hydrogen-like atoms to be \(E_n = -(Z^2/n^2)E_0\), where \(E_0 = 13.6 \, eV\). As noted, the effective \(Z\) is 73. Now the \(K_{\alpha}\) x-ray energy is given by \[E_{K_{\alpha}} = \Delta E = E_i - E_f = E_2 - E_1,\] where \[E_1 = -\dfrac{Z^2}{1^2}E_0 = - \dfrac{73^2}{1} \left(13.6 \, eV\right) = - 72.5 \, keV\] and \[E_2 = - \dfrac{Z^2}{2^2} E_0 = - \dfrac{73^2}{4}\left(13.6 \, eV\right) = -18.1 \, keV.\] Thus, \[E_{K_{\alpha}} = -18.1 \, keV - (- 72.5 \, keV) = 54.4 \, keV.\]
Discussion
This large photon energy is typical of characteristic x rays from heavy elements. It is large compared with other atomic emissions because it is produced when an inner-shell vacancy is filled, and inner-shell electrons are tightly bound. Characteristic x ray energies become progressively larger for heavier elements because their energy increases approximately as \(Z^2\). Significant accelerating voltage is needed to create these inner-shell vacancies. In the case of tungsten, at least 72.5 kV is needed, because other shells are filled and you cannot simply bump one electron to a higher filled shell. Tungsten is a common anode material in x-ray tubes; so much of the energy of the impinging electrons is absorbed, raising its temperature, that a high-melting-point material like tungsten is required.
Medical and Other Diagnostic Uses of X-rays
All of us can identify diagnostic uses of x-ray photons. Among these are the universal dental and medical x rays that have become an essential part of medical diagnostics. (See Figure and Figure .) X rays are also used to inspect our luggage at airports, as shown in Figure , and for early detection of cracks in crucial aircraft components. An x ray is not only a noun meaning high-energy photon, it also is an image produced by x rays, and it has been made into a familiar verb—to be x-rayed.
The most common x-ray images are simple shadows. Since x-ray photons have high energies, they penetrate materials that are opaque to visible light. The more energy an x-ray photon has, the more material it will penetrate. So an x-ray tube may be operated at 50.0 kV for a chest x ray, whereas it may need to be operated at 100 kV to examine a broken leg in a cast. The depth of penetration is related to the density of the material as well as to the energy of the photon. The denser the material, the fewer x-ray photons get through and the darker the shadow. Thus x rays excel at detecting breaks in bones and in imaging other physiological structures, such as some tumors, that differ in density from surrounding material. Because of their high photon energy, x rays produce significant ionization in materials and damage cells in biological organisms. Modern uses minimize exposure to the patient and eliminate exposure to others. Biological effects of x rays will be explored in the next chapter along with other types of ionizing radiation such as those produced by nuclei.
As the x-ray energy increases, the Compton effect (see Photon Momentum ) becomes more important in the attenuation of the x rays. Here, the x ray scatters from an outer electron shell of the atom, giving the ejected electron some kinetic energy while losing energy itself. The probability for attenuation of the x rays depends upon the number of electrons present (the material’s density) as well as the thickness of the material. Chemical composition of the medium, as characterized by its atomic number \(Z\), is not important here. Low-energy x rays provide better contrast (sharper images). However, due to greater attenuation and less scattering, they are more absorbed by thicker materials. Greater contrast can be achieved by injecting a substance with a large atomic number, such as barium or iodine. The structure of the part of the body that contains the substance (e.g., the gastro-intestinal tract or the abdomen) can easily be seen this way.
Breast cancer is the second-leading cause of death among women worldwide. Early detection can be very effective, hence the importance of x-ray diagnostics. A mammogram cannot diagnose a malignant tumor, only give evidence of a lump or region of increased density within the breast. X-ray absorption by different types of soft tissue is very similar, so contrast is difficult; this is especially true for younger women, who typically have denser breasts. For older women who are at greater risk of developing breast cancer, the presence of more fat in the breast gives the lump or tumor more contrast. MRI (Magnetic resonance imaging) has recently been used as a supplement to conventional x rays to improve detection and eliminate false positives. The subject’s radiation dose from x rays will be treated in a later chapter.
A standard x ray gives only a two-dimensional view of the object. Dense bones might hide images of soft tissue or organs. If you took another x ray from the side of the person (the first one being from the front), you would gain additional information. While shadow images are sufficient in many applications, far more sophisticated images can be produced with modern technology. Figure shows the use of a computed tomography (CT) scanner, also called computed axial tomography (CAT) scanner. X rays are passed through a narrow section (called a slice) of the patient’s body (or body part) over a range of directions. An array of many detectors on the other side of the patient registers the x rays. The system is then rotated around the patient and another image is taken, and so on. The x-ray tube and detector array are mechanically attached and so rotate together. Complex computer image processing of the relative absorption of the x rays along different directions produces a highly-detailed image. Different slices are taken as the patient moves through the scanner on a table. Multiple images of different slices can also be computer analyzed to produce three-dimensional information, sometimes enhancing specific types of tissue, as shown in Figure . G. Hounsfield (UK) and A. Cormack (US) won the Nobel Prize in Medicine in 1979 for their development of computed tomography.
X-Ray Diffraction and Crystallography
Since x-ray photons are very energetic, they have relatively short wavelengths. For example, the 54.4-keV \(K_{\alpha}\) x ray of Example has a wavelength \(\lambda = hc/E = 0.0228 \, nm\). Thus, typical x-ray photons act like rays when they encounter macroscopic objects, like teeth, and produce sharp shadows; however, since atoms are on the order of 0.1 nm in size, x rays can be used to detect the location, shape, and size of atoms and molecules. The process is called x-ray diffraction , because it involves the diffraction and interference of x rays to produce patterns that can be analyzed for information about the structures that scattered the x rays. Perhaps the most famous example of x-ray diffraction is the discovery of the double-helix structure of DNA in 1953 by an international team of scientists working at the Cavendish Laboratory—American James Watson, Englishman Francis Crick, and New Zealand–born Maurice Wilkins. Using x-ray diffraction data produced by Rosalind Franklin, they were the first to discern the structure of DNA that is so crucial to life. For this, Watson, Crick, and Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine. There is much debate and controversy over the issue that Rosalind Franklin was not included in the prize.
Figure shows a diffraction pattern produced by the scattering of x rays from a crystal. This process is known as x-ray crystallography because of the information it can yield about crystal structure, and it was the type of data Rosalind Franklin supplied to Watson and Crick for DNA. Not only do x rays confirm the size and shape of atoms, they give information on the atomic arrangements in materials. For example, current research in high-temperature superconductors involves complex materials whose lattice arrangements are crucial to obtaining a superconducting material. These can be studied using x-ray crystallography.
Historically, the scattering of x rays from crystals was used to prove that x rays are energetic EM waves. This was suspected from the time of the discovery of x rays in 1895, but it was not until 1912 that the German Max von Laue (1879–1960) convinced two of his colleagues to scatter x rays from crystals. If a diffraction pattern is obtained, he reasoned, then the x rays must be waves, and their wavelength could be determined. (The spacing of atoms in various crystals was reasonably well known at the time, based on good values for Avogadro’s number.) The experiments were convincing, and the 1914 Nobel Prize in Physics was given to von Laue for his suggestion leading to the proof that x rays are EM waves. In 1915, the unique father-and-son team of Sir William Henry Bragg and his son Sir William Lawrence Bragg were awarded a joint Nobel Prize for inventing the x-ray spectrometer and the then-new science of x-ray analysis. The elder Bragg had migrated to Australia from England just after graduating in mathematics. He learned physics and chemistry during his career at the University of Adelaide. The younger Bragg was born in Adelaide but went back to the Cavendish Laboratories in England to a career in x-ray and neutron crystallography; he provided support for Watson, Crick, and Wilkins for their work on unraveling the mysteries of DNA and to Max Perutz for his 1962 Nobel Prize-winning work on the structure of hemoglobin. Here again, we witness the enabling nature of physics—establishing instruments and designing experiments as well as solving mysteries in the biomedical sciences.
Certain other uses for x rays will be studied in later chapters. X rays are useful in the treatment of cancer because of the inhibiting effect they have on cell reproduction. X rays observed coming from outer space are useful in determining the nature of their sources, such as neutron stars and possibly black holes. Created in nuclear bomb explosions, x rays can also be used to detect clandestine atmospheric tests of these weapons. X rays can cause excitations of atoms, which then fluoresce (emitting characteristic EM radiation), making x-ray-induced fluorescence a valuable analytical tool in a range of fields from art to archaeology.
Summary
- X rays are relatively high-frequency EM radiation. They are produced by transitions between inner-shell electron levels, which produce x rays characteristic of the atomic element, or by accelerating electrons.
- X rays have many uses, including medical diagnostics and x-ray diffraction.
Glossary
- x rays
- a form of electromagnetic radiation
- x-ray diffraction
- a technique that provides the detailed information about crystallographic structure of natural and manufactured materials


1.17: MSA, MacConkey, EMB Plates
The student will:
- Use aseptic techniques in the safe inoculation of various forms of media.
- Follow oral and written instructions and manage time in the lab efficiently.
- Apply correct terminology regarding microbiological techniques, instruments, microbial growth, biochemical testing, and media types when making observations.
- Correctly perform various inoculation techniques and describe each technique’s purpose.
- Make accurate observations and appropriate interpretations of biochemical test results and use them in the identification of potentially disease causing microbes.
In this course you will encounter many types of growth media. Most of the types, TSA/TSB (Tryptic Soy Agar/Broth) and NA/NB (Nutrient Agar/Broth) for example, are all purpose or general media . They contain a wide variety of complex carbon, nitrogen and sugar compounds. You will often see terms like beef extract, peptone, tryptone, soytone in the list of ingredients. These components are slurries of animal and plant tissue that have been partially hydrolyzed (broken down) so that the components are more readily available. These media contain many different complex molecules, however the exact amount and types of each is unknown. These media will support a wide range of nonfastidious microbes with differing nutritional requirements.
Media that inhibit the growth of unwanted microorganisms and support the growth of the organism of interest by supplying nutrients and reducing competition are called selective media . (OpenStax CNX, 2018) Selective media are formulated with inhibitors such as antibiotics or high NaCl concentration. When studying a mixed sample, selective media can be helpful. For example, if you suspect a patient is carrying Salmonella (a pathogenic Gram negative bacillus), you may plate a stool sample on a selective medium containing an antibiotic effective against Gram positive bacteria. By eliminating the Gram positive organisms, the range of organisms growing on the plate will be narrowed to Gram negatives. Thus, the variety of bacteria you will need to study is reduced.
The fact that a medium does not grow every microbe, does not make it selective. For example, TSA is an all-purpose medium and a wide range of organisms grow on it. Certain fastidious organisms, however, will fail to grow or grow poorly on TSA because it lacks the specific nutrients required by those bacteria. Even so, TSA is not classified as selective. To be selective, a medium must contain a specific substance intentionally added to inhibit certain microbes and not others.
Differential media contain substrates and indicators (often pH indicators) that make a certain biochemical process visible. Differential media allow one to differentiate between types of organisms growing on the plate because each has a distinct appearance based on whether or not it is carrying out a particular biochemical reaction. “Color changes are the result of end products created by interaction of bacterial enzymes with differential substrates in the medium or, in the case of hemolytic reactions, the lysis of red blood cells in the medium” (OpenStax CNX, 2018). Differential media can be used to distinguish between bacteria that can ferment a specific type of sugar and those that cannot or between bacteria that utilize a certain electron acceptor and those that do not.
Some media are selective, some are differential and some are both. We will study Mannitol Salt Agar, MacConkey agar, and EMB as examples.
Mannitol Salt Agar (MSA) can be used to presumptively isolate and identify Staphylococci from human samples. Refer to the compositions of MSA and MacConkey agar below. MSA contains 75 g/L NaCl (7.5%) compared to the 5 g/L found in TSA and other all-purpose media. MSA favors the growth of salt tolerant microbes, namely Staphylococci , because other bacteria from a human sample, are inhibited by the high NaCl component. In addition, to distinguish pathogenic Staphylococci, namely S. aureus from other common Staphylococci, the substrate mannitol (a sugar) and the pH indicator phenol red are added. If the organism ferments mannitol, acids will be produced as byproducts. These acids will lower the pH changing the indicator from pink to yellow. S. aureus can ferment mannitol, while other common Staphylococci found in humans cannot.
| MS Agar | ||
| Selectivity | Interpretation | Identification |
| Growth | Organism not inhibited by NaCl | E.g., Staphylococcus , Micrococcus |
| No growth | Organism inhibited by NaCl | Not Staphylococcus |
| Differentiation | ||
| Yellow halo | Organism ferments mannitol | Probable S. aureus |
| No yellow halo |
Organism does not ferment
mannitol |
Staphylococcus species (other than S. em>aureus); Micrococcus (yellow colonies) |
MacConkey agar contains bile salts and crystal violet, which interfere with the growth of many gram-positive bacteria and favor the growth of gram-negative bacteria, particularly the Enterobacteriaceae . These species, commonly named enterics, reside in the intestine, and are adapted to the presence of bile salts. Enterics can be further characterized by their ability to ferment lactose. In MacConkey agar, the lactose fermenters ( coliforms ) utilize lactose in the medium producing acid, lowering the pH. The medium is supplemented with the pH indicator neutral red, which turns to hot pink at low pH. (OpenStax CNX, 2018) Thus, lactose fermenters are observed as bright pink colonies or with a bright pink halo surrounding the growth. Non-lactose fermenters (noncoliforms) include some notable human pathogens, such as Salmonella spp., Shigella spp., and Yersinia pestis . (OpenStax CNX, 2018)
Eosin-methylene blue agar (EMB) contains peptone, lactose, sucrose and the dyes eosin Y and methylene blue. Gram positive organisms are inhibited by the dyes, so this medium is selective for Gram negative bacteria. The medium differentiates based on the ability to ferment lactose (and/or sucrose.) Organisms that cannot ferment either of the sugars produce colorless colonies. Organisms that ferment the sugars with some acid production produce pink or purple colonies; organisms that ferment the sugars and produce large amounts of acid form colonies with a green metallic sheen. This medium is commonly used to detect the presence of fecal coliforms (like E. coli )—bacteria that grow in the intestines of warm-blooded animals. Fecal coliforms produce large amounts of acid when fermenting lactose and/or sucrose; non-fecal coliforms will produce less acid and appear as pink or purple colonies.
| EMB Agar | ||
| Result | Interpretation | Identification |
| No or poor growth | Organism inhibited by dyes | Organism is Gram-positive |
| Good growth |
Organism not inhibited by
dyes |
Organism is Gram-negative |
| Colorless growth |
Organism does not ferment
sucrose or lactose |
Non-coliform |
| Growth is pink and mucoid |
Organism ferments lactose
and/or sucrose with some acid production |
Coliform bacteria |
|
Growth is dark (purple to black with or without
green metallic sheen) |
Organism ferments lactose and/or sucrose, with large
amounts of acid production |
Possible fecal coliform ( E. coli ) |
Coliform bacteria are microbes found in the digestive systems of warm-blooded animals, in soil, on plants, and in surface water. (Note their ability to assist mammals in the digestion of milk sugar, lactose.) These microbes typically do not make you sick; however, because microbes that do cause disease are hard to test for in the water, “total coliforms” are tested instead. If the total coliform count is high, then it is very possible that harmful germs like viruses, bacteria, and parasites might also be found in the water. Thus, they are considered one of several a water quality indicators. (U.S. Centers for Disease Control and Prevention, 2019)
MacConkey Agar (MacC) Pancreatic digest of Gelatin 17g/L Peptones (meat and Casein) 3g/L Lactose 10g/L
Bile Salts 1.5g/L Sodium Chloride 5g/L Agar 13.5g/L
Neutral red 0.03g/L Crystal Violet 1mg/L
Mannitol Salts Agar (MSA) Pancreatic digest of Casein 5g/L Peptic digest of Animal Tissue 5g/L Beef extract 1g/L
Sodium Chloride 75g/L D-Mannitol 10g/L Phenol red 25mg/L Agar 15g/L
Tryptic Soy Agar (TSA)
Tryptone 17g/L Soytone 3g/L
Sodium Chloride 5g/L Dipotassium Phosphate 2.5g/L Agar 15g/L
Materials per student
1 EMB plate*
1 MSA plate*
1 MacConkey Agar plate*
* Be sure to label each as you take it. They look very similar!
Cultures
Fresh overnight broth cultures
E. coli
Staphylococcus epidermidis
Staphylococcus aureus
Pseudomonas aeruginosa
Procedure Lab 1
- Obtain one EMB plate, one MSA plate and one MacConkey agar plate. On the plate base write the medium abbreviation when you remove it..
- Add the following to the bottom of each plate around the edge: your name, date.
- Divide each plate into 4 sections. Label each section with an abbreviation for each organism. Write small but legibly! Each plate will be inoculated with each of the 4 organisms.
- Aseptically spot inoculate each sector with the corresponding microbe. A spot inoculation is a short (1 cm) streak line as shown. (DO NOT STAB the agar.)
- Keep the inoculation lines short and away from the other inoculations on the plate.
- Be sure to make each inoculation separately and refrain from “double dipping.”
- Be sure to hold the lid of the plate above the plate surface to protect it from airborne contaminants.
- It may help to set the plate on a piece of white scratch paper so that you can see the sector lines.
- Place the plates upside-down in the location designated for cultures to be incubated.
- They will be incubated for 24-48 hours at 37 o C.
Procedure Lab 2
- Obtain your plates and make observations in the data table.
- Use + for growth and – for no growth. If growth is poor, simply write “poor growth” in the table.
- In the appearance column, describe any color change in the growth and/or the surrounding medium. If the organism did not grow, you will not be able to describe an appearance. In this case write “N/A” in the table.
- Be sure to use the organism’s full scientific name written correctly.
- After making observations, dispose of your plates in the hazardous waste.
References:
OpenStax CNX. (2018, Mar 19). OpenStax Microbiology. Retrieved from<EMAIL_ADDRESS>U.S. Centers for Disease Control and Prevention. (2019, June 30). Well Testing . Retrieved from Healthy Water: https:// www.cdc.gov/healthywater/drinking/private/wells/testing.html


31.7: Glomeromycota- Asexual Plant Symbionts
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Learning Objectives
- Describe the ecology and reproduction of Glomeromycetes
In the kingdom Fungi, the Glomeromycota is a newly-established phylum comprised of about 230 species that live in close association with the roots of trees and plants. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that most members of this family form arbuscular mycorrhizae: the hyphae interact with the root cells forming a mutually-beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus while the fungus supplies essential minerals from the soil to the plant. This association is termed biotrophic. The Glomeromycota species that have arbuscular mycorrhizal are terrestrial and widely distributed in soils worldwide where they form symbioses with the roots of the majority of plant species. They can also be found in wetlands, including salt-marshes, and are associated with epiphytic plants.
The glomeromycetes do not reproduce sexually and cannot survive without the presence of plant roots. They have coenocytic hyphae and reproduce asexually, producing glomerospores. The biochemical and genetic characterization of the Glomeromycota has been hindered by their biotrophic nature, which impedes laboratory culturing. This obstacle was eventually surpassed with the use of root cultures. With the advent of molecular techniques, such as gene sequencing, the phylogenetic classification of Glomeromycota has become clearer. The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA). This gene is highly conserved and commonly used in phylogenetic studies so it was isolated from spores of each taxonomic group. Using a molecular clock approach based on the substitution rates of SSU sequences, scientists were able to estimate the time of divergence of the fungi. This analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago, making them a monophyletic lineage. A long-held theory is that Glomeromycota were instrumental in the colonization of land by plants.
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- Glomeromycota. Provided by : Wikipedia. Located at : http://en.Wikipedia.org/wiki/Glomeromycota . License : CC BY: Attribution
Key Points
- Most glomeromycetes form arbuscular mycorrhizae, a type of symbiotic relationship between a fungus and plant roots; the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant.
- Glomeromycota that have arbuscular mycorrhizal are mostly terrestrial, but can also be found in wetlands.
- The glomeromycetes reproduce asexually by producing glomerospores and cannot survive without the presence of plant roots.
- DNA analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago.
- The classification of fungi as Glomeromycota has been redefined with adoption of molecular techniques.
Key Terms
- biotrophic : describing a parasite that needs its host to stay alive
- arbuscular mycorrhizae : a type of symbiotic relationship between a fungus and the roots of a plant where the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant
- glomeromycete : an organism of the phylum Glomeromycota


1
Which nutritional deficiency listed increases the risk for bleeding?
-
Vitamin K
-
Iron
-
Vitamin B
12
-
Folate
2
The nurse is providing nutritional counseling to a client planning pregnancy. The nurse should emphasize the importance of consuming what micronutrients to avoid macrocytic anemia?
-
Vitamin D
-
Vitamin E
-
Folic acid
-
Vitamin C
3
A middle-aged client with a history of alcohol use disorder is admitted to an acute care setting with symptoms of severe anemia. The nurse anticipates what nutrient(s) to be deficient?
-
Iron
-
Vitamin B
12
-
Calcium
-
Copper
4
The nurse notices that a client has excessive bruising and brittle nails. The client reports frequent infections. Which nutritional deficiency would the nurse expect for this client?
-
Iron
-
Protein
-
Vitamin C
-
Copper
5
A nurse is preparing discharge instructions for a client who has just been diagnosed with iron deficiency anemia and will be starting oral iron replacement therapy. Which of the following is an important nursing consideration to include in the teaching plan?
-
Hemoglobin should be checked in 6 months.
-
Always take iron with milk or food to increase absorption.
-
Always take iron with a citrus beverage, 1 hour before or 2 hours after meals, to increase absorption.
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Increase diet in fatty fish.
6
Which nursing diagnosis is a priority for an older adult client just diagnosed with nutritional anemia?
-
Risk for falls
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Risk for imbalanced nutrition, less than body requirements
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Frail elderly syndrome, related to malnutrition
-
Risk for bleeding
7
The nurse is preparing discharge instructions for a client with anemia related to vitamin B
12
deficiency. The nurse suggests which of the following food items be increased in the client’s diet?
-
Animal meat, fish, eggs, and dairy
-
Beans, seeds, and nuts
-
Dark green leafy vegetables
-
Citrus fruits and acidic raw vegetables
8
Which of the following psychosocial influences impacting nutritional goals for hematologic wellness is the best target for an
initial
nursing intervention?
-
Health literacy
-
Transportation
-
Social support
-
Cognitive function
9
Assessing a client’s readiness to learn about nutrition-related hematologic illness, utilizing a holistic approach, should include all of the following except:
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Providing written materials regarding vitamin C deficiency to the client immediately before discharge
-
Determining the client’s preferred language for learning
-
Discussing the client’s preferred learning style
-
Delaying discharge education until the client’s significant other is present
10
When evaluating the effectiveness of a nutritional plan for iron deficiency anemia, the nurse considers which of the following factors?
-
Behavior change typically takes 6 months to achieve, according to the Transtheoretical Model (TTM).
-
Hemoglobin is expected to increase as a response to oral ferrous sulfate within 2 to 3 months of starting.
-
An early sign suggesting improved anemia may include the client’s report of sleeping more.
-
Shortness of breath may take several months to improve.


6.4: Religious Conflict
Human beings have struggled against one another for a variety of reasons. Religious disagreements can be particularly intense. Sectarian violence involves differences based on interpretations of religious doctrine or practice. Thestruggles between the Catholic and Orthodox churches, or the wars associated with the Protestant Reformation and Counter Reformation, are examples of this form of conflict. The current violence seen between Sunni and Shia Muslims is also in this category. Closely associated with this kind of conflict is religious fundamentalism . Religious fundamentalism rests on a literal interpretation and strict and intense adherence to the basic principles of a religion. The conflict arises when religious fundamentalists see their coreligionists as being insufficiently pious. Extremism is the idea that the end of a religious goal can be justified by almost any means. Some groups that are convinced that they have divine blessing have few limits to their behavior, including resorting to violence.
Another form of religious violence is between completely different religions. Wars between Muslims and Christians or Hindus and Buddhists have been framed as wars for the benefit or detriment of particular religions. What is described as religious strife, however, is often not. Although some religions are fighting over doctrinal differences, most conflict stems from more secular causesa desire for political power, a struggle for resources, ethnic rivalries, and economic competition.
The Israel/Palestine conflict is a struggle over territory, resources, and political recognition. The Rohingya crisis in Myanmar has less to do with religion and more to do with differences in ethnicity, national origin, and post-colonial identity. Massacres in Sahelian Africa are better framed as farmers versus herders. The long running violence between Protestants and Catholics in Northern Ireland is better framed as a violent dispute between one group who holds allegiance with the Republic of Ireland and the other who holds allegiance with the United Kingdom.
This is not to say that religious violence does not exist. It does. The most obvious example of this in recent years has been the emergence of Islamic State.