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4.E: How Cells Obtain Energy (Exercises)

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    4.1: Energy and Metabolism

    Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the combination of chemical reactions that take place within it. Catabolic reactions break down complex chemicals into simpler ones and are associated with energy release. Anabolic processes build complex molecules out of simpler ones and require energy. In studying energy, the term system refers to the matter and environment involved in energy transfers.


    Review Questions

    Which of the following is not an example of an energy transformation?

    A. Heating up dinner in a microwave
    B. Solar panels at work
    C. Formation of static electricity
    D. None of the above


    Which of the following is not true about enzymes?

    A. They are consumed by the reactions they catalyze.
    B. They are usually made of amino acids.
    C. They lower the activation energy of chemical reactions.
    D. Each one is specific to the particular substrate(s) to which it binds.


    Free Response

    Does physical exercise to increase muscle mass involve anabolic and/or catabolic processes? Give evidence for your answer.

    Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism.

    Explain in your own terms the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.

    A spontaneous reaction is one that has a negative ∆G and thus releases energy. However, a spontaneous reaction need not occur quickly or suddenly like an instantaneous reaction. It may occur over long periods of time due to a large energy of activation, which prevents the reaction from occurring quickly.

    With regard to enzymes, why are vitamins and minerals necessary for good health? Give examples.

    Most vitamins and minerals act as cofactors and coenzymes for enzyme action. Many enzymes require the binding of certain cofactors or coenzymes to be able to catalyze their reactions. Since enzymes catalyze many important reactions, it is critical to obtain sufficient vitamins and minerals from diet and supplements. Vitamin C (ascorbic acid) is a coenzyme necessary for the action of enzymes that build collagen.


    4.2: Glycolysis

    ATP functions as the energy currency for cells. It allows cells to store energy briefly and transport it within itself to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphate groups attached. As ATP is used for energy, a phosphate group is detached, and ADP is produced. Energy derived from glucose catabolism is used to recharge ADP into ATP. Glycolysis is the first pathway used in the breakdown of glucose to extract energy.


    Multiple Choice

    Energy is stored long-term in the bonds of _____ and used short-term to perform work from a(n) _____ molecule.

    A. ATP : glucose
    B. an anabolic molecule : catabolic molecule
    C. glucose : ATP
    D. a catabolic molecule : anabolic molecule


    The energy currency used by cells is _____.

    A. ATP
    B. ADP
    C. AMP
    D. adenosine


    The glucose that enters the glycolysis pathway is split into two molecules of _________.

    A. ATP
    B. phosphate
    C. NADH
    D. pyruvate


    Free Response

    Both prokaryotic and eukaryotic organisms carry out some form of glycolysis. How does that fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?

    If glycolysis evolved relatively late, it likely would not be as universal in organisms as it is. It probably evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate metabolism that evolved later.


    4.3: Citric Acid Cycle and Oxidative Phosphorylation

    The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. One molecule of ATP (or an equivalent) is produced per each turn of the cycle. The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor for electrons removed from the intermediate compounds in glucose catabolism.

    Multiple Choice

    What do the electrons added to NAD+ do?

    A. They become part of a fermentation pathway.
    B. They go to another pathway for ATP production.
    C. They energize the entry of the acetyl group into the citric acid cycle.
    D. They are converted into NADP.


    Chemiosmosis involves

    A. the movement of electrons across the cell membrane
    B. the movement of hydrogen atoms across a mitochondrial membrane
    C. the movement of hydrogen ions across a mitochondrial membran
    D. the movement of glucose through the cell membrane


    Free Response

    We inhale oxygen when we breathe and exhale carbon dioxide. What is the oxygen used for and where does the carbon dioxide come from?

    The oxygen we inhale is the final electron acceptor in the electron transport chain and allows aerobic respiration to proceed, which is the most efficient pathway for harvesting energy in the form of ATP from food molecules. The carbon dioxide we breathe out is formed during the citric acid cycle when the bonds in carbon compounds are broken.


    4.4: Fermentation

    If NADH cannot be metabolized through aerobic respiration, another electron acceptor is used. Most organisms will use some form of fermentation to accomplish the regeneration of NAD+, ensuring the continuation of glycolysis. The regeneration of NAD+ in fermentation is not accompanied by ATP production; therefore, the potential for NADH to produce ATP using an electron transport chain is not utilized.

    Review Questions

    Which of the following fermentation methods can occur in animal skeletal muscles?

    A. lactic acid fermentation
    B. alcohol fermentation
    C. mixed acid fermentation
    D. propionic fermentation


    Free Response

    When muscle cells run out of oxygen, what happens to the potential for energy extraction from sugars and what pathways do the cell use?

    Without oxygen, oxidative phosphorylation and the citric acid cycle stop, so ATP is no longer generated through this mechanism, which extracts the greatest amount of energy from a sugar molecule. In addition, NADH accumulates, preventing glycolysis from going forward because of an absence of NAD+. Lactic acid fermentation uses the electrons in NADH to generate lactic acid from pyruvate, which allows glycolysis to continue and thus a smaller amount of ATP can be generated by the cell.


    4.5: Connections to Other Metabolic Pathways

    Metabolic pathways should be thought of as porous—that is, substances enter from other pathways, and other substances leave for other pathways. These pathways are not closed systems. Many of the products in a particular pathway are reactants in other pathways.

    Multiple Choice

    The cholesterol synthesized by cells uses which component of the glycolytic pathway as a starting point?

    A. glucose
    B. acetyl CoA
    C. pyruvate
    D. carbon dioxide


    Beta oxidation is ________.

    A. the breakdown of sugars
    B. the assembly of sugars
    C. the breakdown of fatty acids
    D. the removal of amino groups from amino acids


    Free Response

    Would you describe metabolic pathways as inherently wasteful or inherently economical, and why?

    They are very economical. The substrates, intermediates, and products move between pathways and do so in response to finely tuned feedback inhibition loops that keep metabolism overall on an even keel. Intermediates in one pathway may occur in another, and they can move from one pathway to another fluidly in response to the needs of the cell.