1. D. Enzymes act as catalysts for biochemical reactions. A catalyst is not consumed in a reaction, but, rather, lowers the activation energy for that reaction. The potential energy of the substrate and the product remain the same, but the activation energy, which is the energy needed to make the reaction progress, “can be lowered with the help of an enzyme.
2. A. A cofactor is an inorganic substance that is required for an enzymatic reaction to occur. Cofactors bind to the active site of the enzyme and enable the substrate to fit properly. Many cofactors are metal ions, such as zinc, iron, and copper.
3. D. Enzyme inhibitors attach to an enzyme and block substrates from entering the active site, thereby preventing enzyme activity. As stated in the question, cyanide is a poison that irreversibly binds to an enzyme and blocks its active site, thus fitting the definition of an enzyme inhibitor.
4. A. Because the energy of the products is less than the energy of the substrate, the reaction releases energy and is an exergonic reaction.
5. D. Enzymes are substrate-specific. Most enzymes catalyze only one biochemical reaction. Their active sites are specific for a certain type of substrate and do not bind to other substrates and catalyze other reactions.
6. C. Lactic acid fermentation converts pyruvate into lactate using high-energy electrons from NADH. This process allows ATP production to continue in anaerobic conditions by providing NAD+ so that ATP can be made in glycolysis.
7. B. Proteins in the inner membrane of the mitochondrion accept high-energy electrons from NAD and FADH2, and in turn transport protons from the matrix to the intermembrane space. The high proton concentration in the intermembrane space creates a gradient which is harnessed by ATP synthase to produce ATP.
8. E. Electrons trapped by the chlorophyll P680 molecule in photosystem II are energized by light. They are then transferred to electron acceptors in an electron transport chain.
9. D. CAM photosynthesis occurs in plants that grow where water loss must be minimized, such as cacti. These plants open their stomata and fix CO2 at night. During the day, stomata are closed, reducing water loss. Thus, photosynthesis can proceed without water loss.
10. B. DNA wrapped around histone proteins is called chromatin. In a eukaryotic cell, DNA is always associated with protein; it is not “naked” as with prokaryotic cells.