2. The “active site” of an enzyme is:
c. The region of an enzyme that attaches to a substrate
By definition, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of residues that form temporary bonds with the substrate (binding site) and residues that catalyze a reaction of that substrate (catalytic site).
3. A molecule that has the same shape as the substrate of an enzyme would tend to:
e. slow metabolism by blocking the enzymes active site.
An enzyme has an active site where substrates bind. The shape of the active site is specific for particular binding substrates. Therefore, a molecule that has the same structure as the substrate will compete with the substrate for the active sites of the enzyme causing competitive inhibition ( Alberts et al., 2013) . Consequently, the metabolism of the substrate by the enzyme will be slow because of the reduced number of active sites for the substrate to bind to be metabolized by the enzyme.
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4. A plant cell is placed in a solution whose solute concentration is twice as hig h as the concentration of the cell cytoplasm. The cell membrane is selectively permeable, allowing water but not solutes to pass through. What will happen to the cell?
c. The cell membrane will shrivel up because of osmosis, and it will become flaccid.
The water concentration of the plant cell is higher than that of the solution in which it is placed in. It means that the medium is a hypertonic solution, therefore, the number of water molecules, since they are free to pass across the selectively permeable cell membrane, diffusing out will be more than water molecules entering which will cause the cell to shrink and shrivel due to osmosis ( Alberts et al., 2013) .
5. Passive movement of water from areas of high concentration to areas of low concentration is called:
b. Osmosis
According to the law of osmosis, water molecules move from lower solute concentration to a higher solute concentration across a semi-permeable membrane until an equilibrium is achieved when the two media acquire the same concentration.
6. Which of the following is true of active transport?
b. Energy is required
In active transport, solute particles or molecules dissolved in a solution move from across a semi-permeable membrane from a region of lower concentration to a region of higher concentration ( Alberts et al., 2013) . Since the process is against the concentration gradient, it requires energy to enable the molecules cross the barrier.
16. Of the metabolic pathways listed below, the only pathway considered to be universal among organisms is _______.
d. Glycolysis
Glycolysis is the process through which enzymes in an organism break down glucose to pyruvic acid in both anaerobic and aerobic conditions and in the process produce energy. All living organisms carry out the process to generate energy from food. However, cellular respiration and Kreb’s Cycle only occurs in organisms that carry out metabolism in aerobic conditions ( Alberts et al., 2013) . Electron transport occurs only in organisms that have mitochondria yet some prokaryotic organisms lack mitochondria. Fermentation is a process that occurs in yeasts and bacteria, however, it can occur in other animals but only in anaerobic conditions.
22. CAM plants, such as cacti and agave, can photosynthesize in hot, dry environments because they:
b. Store carbon dioxide during the night for use during the day
CAM plants are adapted to arid conditions because they carry out Crassulacean acid metabolism that helps in fixing the limited amount of carbon dioxide available ( Alberts et al., 2013) . The leaves of CAM plants have stomata that reduce evapotranspiration by shutting during the day, and open at night to collect carbon dioxide.
23. In respiration, the final destination (product) of the energy (usually found in Hydrogen) in glucose is:
b. ATP
Cellular respiration involves three stages; glycolysis that convert oxygen to pyruvates; Kreb’s Cycle that converts pyruvates to acetyl-CoA, NADH, and sometimes FADH2; Electron transport where NADH and FADH2 give up their electrons which release energy that pumps hydrogen ions through ATP synthase enzyme and in the process form ATP as the final product of oxygen ( Alberts et al., 2013) .
24. A chemist has discovered a drug that blocks glucose phosphate isomerase, an enzyme that catalyzes the second reaction in glycolysis . He wants to use the drug to kill bacteria in humans with infections. However, he cannot do this because
c. Human cells also do glycolysis, the drug may also poison them
Glycolysis is a universal metabolic pathway used to generate energy and is present in all living organisms. Therefore, a drug that blocks glucose phosphate isomerase will inhibit glycolysis which will deny both the bacteria and the host (humans) from generating energy that is essential for many physiological processes in the body ( Alberts et al., 2013) . Therefore, the drug will harm both the bacteria and humans because it does not have selective toxicity.
25. The carbon dioxide we produce (exhale) is produced in
b. Kreb’s Cycle
The molecules of pyruvates generated in the first stage of cell respiration (glycolysis), enter the mitochondria where they are converted via Kreb’s Cycle into acetyl-CoA, NADH, and carbon dioxide is released in the process.
26. Lactic acid is produced by
d. human muscle cells during anaerobic respiration
During intense exercise, the oxygen supply is usually lower than the demand for respiration to produce the energy required. Therefore, the body compensates by carrying out anaerobic glycolysis in the muscles producing lactate which acts as a fuel to produce energy for muscles during prolonged exercise ( Alberts et al., 2013) . And in the process it is converted to lactic acid which acts as a defense mechanism for the body by causing pain in the muscle which makes the person to relax and regain normal respiration.
28. Which of the following is not a difference between photosynthesis and respiration?
c. Photosynthesis consumes carbon dioxide and produces oxygen
Photosynthesis is a process that occurs in plants where carbon dioxide is combined with light energy in the leaves to produce chemical energy that can provide nutrients to plants or as a fuel for other processes in the plants. Combination of carbon dioxide and light energy produces oxygen which is released through the stomata into the environment ( Alberts et al., 2013) . On the other hand, respiration uses oxygen to convert glucose into energy and in the process release carbon dioxide.
29. Most of the ATP produced in cellular respiration comes from
c. Electron Transport system
Electron transport occurs in the mitochondria of eukaryotes during oxidative phosphorylation. And both aerobic and anaerobic respiration require electron transport system for ATP production. And the two processes are the main sources of energy production via cellular respiration.
30. Some friends are trying to make wine in their basement. They’ve added yeast to a sweep grape juice mixture and have allowed the yeast to grow ( Alberts et al., 2013) . After several days they find that the sugar levels in the grape juice have dropped, but there’s no alcohol in the mixture. The most likely explanation is
c. The mixture needs less oxygen. Yeast only produce alcohol in the absence of oxygen.
Yeast can produce alcohol from juice through a process known as fermentation. Fermentation involves the chemical breakdown of sugars to give alcohol in the absence of oxygen ( Alberts et al., 2013) . The process cannot occur in aerobic conditions like the ones provided in the question.
32. Which of the following processes is the most efficient (produces the most ATP per molecule of glucose oxidized)?
c. Aerobic respiration
Aerobic respiration gives 38 ATP molecules per oxygen molecule while anaerobic respiration gives 2 ATP molecules for every glucose molecule. Photosynthesis uses energy to produce nutrients from carbon dioxide. It requires 18 ATPs to make one glucose molecule. Also, lactic acid fermentation produces 2 ATPs per oxygen molecule ( Alberts et al., 2013) .
35. A mutant protist is found in which mitochondria lack an inner mitochondrial membrane. Which of the following pathways would be completely disrupted in these mitochondria?
a. Electron transport chain
Electron transport chain is a cellular respiratory process that occurs only in the mitochondria. Kreb’s cycle occurs in the mitochondria of eukaryotes and cytoplasm of prokaryotic cells. Glycolysis takes place in the cytoplasm of the cell. Alcoholic fermentation occurs in the cytosol of yeasts under anaerobic conditions ( Alberts et al., 2013) . Photosynthesis occurs in plants in chloroplasts found in the mesophyll layer of the leaves.
37. If you expose photosynthesizing plants to radioactive carbon dioxide, which of the products of photosynthesis will be radioactive?
a. Glucose
Photosynthesis can be broadly categorized into two biochemical reactions. The first reaction involves chlorophyll found in chloroplasts trapping sunlight energy and concentrating it in the leaves. The second process combination of water and carbon dioxide with the help of the sunlight energy to make sugars such as glucose ( Alberts et al., 2013) . Therefore, the radioactive carbon dioxide will be incorporated in the glucose formed.
40. The reactions of the dark reaction are not directly dependent on light, yet they usually do not occur in the dark. Why?
c. the dark reaction depends on hydrogen from the light reaction to take place
Dark reactions utilize organic energy obtained from ATP and electrons from NADPH to fix carbon dioxide into carbohydrates. The process occurs in the stroma where light does not reach, but since it is dependent on a previous step, they all occur during the day ( Alberts et al., 2013) .
42. Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left in a salt solution become limp and soft. From this, we can deduce that the cells of the celery stalks.
c. Hypertonic to fresh water but hypotonic to the salt solution
Plant cells, like cells of celery stalk, have a cytoplasm who concentration of solutes is higher than that of fresh water thus creating a concentration gradient which allows the cell to take in water by osmosis and swell thus becoming stiff and hard ( Alberts et al., 2013) . However, a salt solution has more solutes than the cytoplasm, and therefore water molecules move from the cytoplasm to the salt solution until their concentrations are equal.
43. The movement of a substance across a biological membrane against its concentration gradient with the help of energy input is which of the following?
b. Active transport
In active transport, solute particles or molecules dissolved in a solution move from across a semi-permeable membrane from a region of lower concentration to a region of higher concentration ( Alberts et al., 2013) . Since the process is against the concentration gradient, it requires energy. Diffusion and osmosis do not require energy, and they are not against the concentration gradient. Facilitated diffusion is against the gradient, but it does require energy because the process is spontaneous. Exocytosis is a form of active transport, but it specifically involves transport of molecules out of the cells ( Alberts et al., 2013) .
44. The oxygen consumed during cellular respiration (breathing) is directly involved in
b. Accepting hydrogen at the end of the electron transport chain
Electron transport chain (ETC) is the final stage of cellular respiration. Once the products of Kreb’s cycle enter the cycle, they donate electrons which are passed from one molecule to another and finally react with protons and oxygen to form water ( Alberts et al., 2013) . Oxygen is the rate-limiting component of the final stage.
46. Glycolysis takes place in which location?
a. Cytoplasm
Glycolysis involves breaking down of glucose molecule into two pyruvate or lactate molecules, a process that occurs in the cytoplasm. In the chloroplast is where photosynthesis takes place. Kreb's cycle in eukaryotes and electron transport chain takes place in the mitochondria.
48. What are the products of the light reaction that are used by the dark reaction?
c. NADPH and hydrogen
Photosynthesis is divided into two stages namely light and dark reactions. During the light reaction which is the first stage, light is absorbed by chlorophyll which splits water molecules releasing oxygen. And I the process ATP molecules are produced. Via movement of electrons generated, a hydrogen-gradient is created which re-energizes electrons causing them to produce NADPH (energy carrier) ( Alberts et al., 2013) . Therefore, NADPH and ATP in the form of hydrogen are the light reaction products that enter the dark stage reaction.
49. Under arid conditions, C4 plants can photosynthesize which C3 plants cannot because C4 plants
a. Use an enzyme to find unused carbon dioxide in the leaf.
C4 plants are those plants that are efficient in hot, dry climates while C3 plants are the most efficient at photosynthesis in cold, wet environments. During the day when photosynthesis can take place, the stomata of C4 plants is usually closed ( Alberts et al., 2013) . Therefore, the plants use fixed carbon and wastes energy.
References
Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., ... & Walter, P. (2013). Essential cell biology . New York, NY: Garland Science.
