Scientific Foundations

Recall Questions

1. Which of the following macromolecules is primarily responsible for storing and transmitting genetic information?

   1. Proteins
   2. Carbohydrates
   3. Nucleic acids
   4. Lipids

2. What type of bond links amino acids in a polypeptide chain?

   1. Ionic bond
   2. Hydrogen bond
   3. Peptide bond
   4. Disulfide bond

3. Which of the following is a primary function of carbohydrates in the body?

   1. Provide a quick source of energy
   2. Form a barrier between two aqueous environments
   3. Form channels in the cell membrane
   4. Serve as genetic material

4. What is the primary structure of a protein?

   1. The overall three-dimensional shape of the protein
   2. The sequence of amino acids in a polypeptide chain
   3. The local folding into alpha-helices and beta-sheets
   4. The interactions between multiple polypeptide chains

5. Which level of protein structure is primarily determined by interactions between the side chains (R-groups) of the amino acids?

   1. Primary structure
   2. Secondary structure
   3. Tertiary structure
   4. Quaternary structure

6. What is the primary function of triglycerides in the body?

   1. Structural component of cell membranes
   2. Storage energy
   3. Act as enzyme
   4. Transport oxygen in the blood

7. Which is the most abundant lipid in cell membranes?

   1. Triglycerides
   2. Steroids
   3. Phospholipids
   4. Fatty acids

8. What is the primary role of low-density lipoprotein (LDL) in the body?

   1. Transport of cholesterol from the liver to peripheral tissues
   2. Transport of fatty acids from adipose tissue to muscles
   3. Removal of cholesterol from tissues to the liver
   4. Synthesize bile acids in the liver

Application Questions

1. You measure the voltage across the membrane of a liposome (essentially a lipid bilayer that forms a large vesicle in solution). The liposome contains calcium channels that allow calcium to flow in either direction. Chloride cannot pass across the membrane. The concentration of calcium chloride inside the liposome is 100 mM while the concentration of calcium chloride outside the liposome is 10 mM. What membrane potential would you measure across the liposome membrane at 37° C?

   1. 60 mV
   2. 30 mV
   3. -30 mV
   4. -60 mV

   Show Explanation

   Because the liposome contains a calcium channel, calcium will flow down its concentration gradient until the membrane potential reaches a point at which it counterbalances the calcium concentration gradient. To calculate that membrane potential, we use the Nerst equation.

   $$ V_m = \frac{-60 mV}{z_{Ca}} * log_{10}\frac{[Ca^{2+}]_i}{[Ca^{2+}]_o} $$ $$ V_m = \frac{-60 mV}{2} * log_{10}\frac{100 mM}{10 mM} $$ $$ V_m = -30 mV $$

2. You are working in a clinical that treats patients who suffer from lysosomal storage disease. You find a group of patients whose lysosome accumulate cellular material but appear to have all the requisite digestive enzymes. You measure the pH of the lysosomes in the patients' cells at 6.5. Normally, lysosomal pH should be between 5 and 5.5. The hydrogen ion pump in the patients' lysosome appears to work as efficiently as the pump in lysosomes from unaffected patients. Surprisingly, you discover a hydrogen ion channel in the lysosomal membrane in your patients' cells. You also measure the potential across the lysosomal membrane and hydrogen ion concentrations inside the lysosome and the cytosol of your patients' cells. Which result would youl most like find in the patients' cells?

   1. High concentration of cytosolic H⁺
   2. High concentration of lysosomal H⁺
   3. Cytosol negative potential relative to lysosome
   4. Lysosome negative potential relative to cytosol

   Show Explanation

   The presence of a hydrogen ion channel in the lysosome suggests that hydrogen ion is leaking from the lysosome into the cytosol. For that to be true, the electrochemical gradient for hydrogen ion must favor diffusion of hydrogen ion from the lysosome to the cytosol. A negative membrane potential in the cytosol relative to the inside of the lysosome would promote diffusion of positively-charged hydrogen ions (increased electrical driving force). A high concentration of hydrogen ion inside lysosome would also favor movement of hydrogen ion from inside the lysosome into the cytosol (increased chemical driving force), but the patients' lysosome are only slightly acidic so we would not expect to measure a high hydrogen ion concentration in the lysosomes.

3. A patient presents with headache, fatigue, and muscle cramps. The patient reports having diarrhea for over a day. A physical exam reveals the patient has dry skin and lips. The patient's urine is dark. You diagnose the patient as being dehydrated and start oral rehydration therapy. The oral hydration fluid contains sodium to increase plasma sodium levels which helps the patient retain water. What else is added to oral rehydration therapy to increase uptake of sodium?

   1. Fatty acid
   2. Glucose
   3. Potassium
   4. Calcium

   Show Explanation

   One way to think about the problem is how can you get sodium to enter the cells lining the GI tract. We know there is a strong electrochemical gradient that favors sodium entering the cell, but without a channel there is no route for sodium to pass across the cell membrane. There are several sodium channels in the genome but in the GI tract the main route for sodium entry is through the co-transporters (with glucose or amino acids) because the cells use the strong sodium electrochemical gradient to move glucose and amino acids against their chemical gradients. These co-transporters only open if both components are present (i.e. sodium and glucose). So we can “trick” the cells to take up sodium through the SGLT channel by including glucose in the rehydration mix. Once in the cell, sodium can be moved into the interstitial fluid by the sodium-potassium pump.

4. You are on your clerkship rotation in internal medicine, and you receive the blood test results for one of your assigned patients. The results show normal levels of protein and potassium but a serum sodium concentration of 130 mmol/liter. The attending physician asks you to calculate the sodium concentration in the patient's interstitial fluid. Which value is most accurate?

   1. 115 mmol/liter
   2. 127 mmol/liter
   3. 133 mmol/liter
   4. 147 mmol/liter

   Show Explanation

   The correct answer is 133 mmol/liter. Recall that the serum protein affects the calculation of interstitial sodium in two ways. First, you must account for the volume of serum that is occupied by protein which under normal conditions is 7%. This increase the effective concentration of serum sodium in the patient by 130/.93 = 140 mmol/liter. Second, because serum protein is negatively charged, cations tend to be retained in serum, lowering the interstitial cation concentration by about 5%. 140 mmol/liter x 0.95 = 133 mmol/liter.

5. Receptors that are bound to cargo are often internalized through clathrin-mediated endocytosis. The endocytic vesicles develop into endosomes and the pH of the lumen decreases to around 6.0. The lower pH is often sufficient to dissociate the receptor from its cargo. What change to the amino acids in the receptor likely lead to dissociation form its cargo?

   1. Become more positively charged
   2. Become more negatively charged
   3. Become more hydrophobic
   4. Become more hydrophilic

   Show Explanation

   As pH decreases and the concentration of hydrogen ion increases, charged amino acids are more likely to be found in their protonated form. This makes the overall protein more positively charged. This change in charge can weaken the association of the receptor for its cargo.

6. The sodium-amino acid co-transporter depends primarily on which of the following for its activity.

   1. Sodium-potassium-chloride co-transporter
   2. Sodium-hydrogen antiporter
   3. Sodium-potassium pump
   4. Sodium-calcium exchanger

   Show Explanation

   The sodium-potassium pump uses ATP hydrolysis to move sodium ions out of the cell. This maintains a gradient of sodium across the cell membrane that favors the movement of sodium into the cell. The sodium-amino acid co-transporter exploits this gradient to move amino acids into the cell.

7. Osteogenesis imperfecta (brittle bone disease) is caused by mutations in the genes that encode proteins for type I collagen (COL1A1 and COL1A2) that compromise the structural integrity of bone. Which type of mutation in one allele of a type I collagen gene would cause the greatest reduction in the mechanical strength of bone.

   1. Mutation in promoter region that reduces binding of TFIID
   2. Nonsense mutation in the first codon
   3. Missense mutation converts glycine to alanine
   4. Missense mutation converts glycine to aspartate

   Show Explanation

   A mutation in the promoter or the first codon might reduce the amount of type I collagen but cells could still make type I collagen from the wild-type allele. A mutation that changed glycine to a different amino would result in about half of collagen containing a different amino acid from glycine at one position. Recall that the alpha-helical region of collagen usually contains a glycine every third amino acid which allows the collagens to pack close together in trimers as glycine has the smallest side chain. Introducing an amino acid with a larger side chain would reduce the packing of the trimers. Because aspartate has a larger side group than alanine, it disrupts the trimer packing to a greater extent and results in weaker collagen.

8. A four-month old presents with muscle weakness and reduced movement of their limbs. The child appears to have lower response to visual and audio stimuli. A physical exam also reveals their head size is much larger than expected for a child their age. The attending physician makes an initial diagnosis of Tay-Sachs. A mutation in which gene would confirm the diagnosis?

   1. Hexosaminidase A
   2. Insulin
   3. Hemoglobin
   4. Phenylalanine hydroxylase

   Show Explanation

   Muscle weakness and lack of mobility could be due to a defect in skeletal muscle or the neurons (motor) that stimulate the muscle cells. The lack of response to external stimuli suggest a neurological defect. An increase in brain size could arise through over proliferation of cells or increased size of cells in the brain.

9. You are working in the ER and see a patient with a high fever, elevated heart rate and fast breathing. The patient appears somewhat disoriented. A blood test reveals an elevated white blood cell count. A diagnosis of sepsis is made and the patient is treated with IV fluids and antibiotics. The patient's history reveals they suffer from Gaucher disease which is a mutation in the gene that encodes the glucocerebrosidase, an enzyme that digests lipids. Which organelle is most affected by the mutation?

   1. Golgi
   2. Lysosome
   3. Endoplasmic Reticulum
   4. Secretory Vesicle

   Show Explanation

   Lysosomes contain many different enzymes that breakdown macromolecules, such as protein, lipid and carbohydrate. Mutations that inhibit the activity of these enzymes or the cell's ability to deliver the enzymes to the lysosome will compromise the function of the lysosome and cause the lysosome to accumulate undigested macromolecules. These mutations can affect the immune system because macrophages and neutrophils rely on the lysosome to destroy bacteria which they have phagocytosed. Consequently, patients with Gaucher disease are more susceptible to bacterial infections.

10. A patient receives an injection of glucose together with insulin. Glucose is quickly taken up via GLUT transporters into skeletal muscle cells. The ensuing increased glucose metabolism in skeletal muscle increases intracellular ATP which in turn activates the Na-K pump. Which change resulting from the increase activity of the Na-K pump would be of greatest concern?

    1. 2 mM decrease in sodium concentration inside muscle cells
    2. 2 mM increase in sodium concentration outside cells
    3. 2 mM increase in potassium concentration inside muscle cells
    4. 2 mM decrease in potassium concentration outside cells

    Show Explanation

    A change in the concentration of an ion either inside or outside of the cell changes the equilibrium potential for the ion as determined by the Nernst equation: $$ V_m = \frac{-60 mV}{Z} * log_{10}\frac{[ion]i}{[ion]o} $$ The biggest concern would be a change in potassium concentration outside the cell because that would have the largest impact on membrane potential. Recall that membrane potential is largely determined by the equilibrium potential for potassium because cell membranes have potassium channels that are leaky. If we assume the normal potassium concentration inside the cell is 120 mM and 4.5 mM outside the cell, then a 2 mM increase in potassium inside the cell would shift the equilibrium potential for potassium from ~ -85.5 mV to -86.0 mV. Whereas, a 2 mM decrease in potassium concentration outside the cell would shift the equilibrium potential for potassium to -101 mV. A decrease of 2 mM potassium outside the cell would make the membrane potential of cells more negative and make it more difficult for cells to generate action potentials.

11. Ouabain is a drug that inhibits the activity of the sodium-potassium pump. What would be the most significant change in a cell treated with ouabain?

    1. Cell swelling
    2. Cell shrinkage
    3. Increase in membrane potential
    4. Decrease in membrane potential

    Show Explanation

    One of the functions of the sodium-potassium pump is to maintain proper cell volume. Cells must ensure that solute concentrations inside the cell are similar to the fluid outside the cell. Cells have a high concentration of macromolecules in their cytosol. The fluid surrounding cells lacks most of these macromolecules and consequently, another solute(s) must exist at higher concentration outside the cell compared to inside. The sodium-potassium pump moves three sodium ions outside the cell for every two potassium ions in takes in. The net movement of solute outside the cell helps balance the higher concentration of macromolecules inside the cell. Inhibiting the sodium-potassium pump, would lead to higher concentration of solute inside cells which would draw water into the cell and cause the cell to swell.

    Although the sodium-potassium pump is electrogenic (it moves three positive ions out for every two positive ions in brings in), it is not the primary determinant of membrane potential. Membrane potential is primarily determined by the concentration difference (equilibrium potential) of ions across the membrane and the permeability of the membrane to those ions. Because membranes are most permeable to potassium, the potassium equilibrium potential is the main determinant of membrane potential.