4.E: Cell Energy (Exercises)

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

Answer

D

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.

Answer

A

Free Response

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

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.

Answer

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.

Answer

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

Answer

C

The energy currency used by cells is _____.

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

Answer

A

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

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

Answer

D

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?

Answer

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.

Answer

B

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

Answer

C

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?

Answer

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

Answer

A

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?

Answer

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.

Free Response

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

Answer

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.

4.6: Overview of Photosynthesis

All living organisms on earth consist of one or more cells. Each cell runs on the chemical energy found mainly in carbohydrate molecules (food), and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy used to hold these molecules together is released when an organism breaks down food.

Multiple Choice

What two products result from photosynthesis?

A. water and carbon dioxide
B. water and oxygen
C. glucose and oxygen
D. glucose and carbon dioxide

Answer

C

From where does a heterotroph directly obtain its energy?

A. the sun
B. the sun and eating other organisms
C. eating other organisms
D. simple chemicals in the environment

Answer

C

Free Response

What is the overall purpose of the light reactions in photosynthesis?

Answer

To convert solar energy into chemical energy that cells can use to do work.

Why are carnivores, such as lions, dependent on photosynthesis to survive?

Answer

Because lions eat animals that eat plants.

4.7: The Light-Dependent Reactions of Photosynthesis

How can light be used to make food? It is easy to think of light as something that exists and allows living organisms, such as humans, to see, but light is a form of energy. Like all energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is transformed into chemical energy, which autotrophs use to build carbohydrate molecules. However, autotrophs only use a specific component of sunlight .

Multiple Choice

What is the energy of a photon first used to do in photosynthesis?

A. split a water molecule
B. energize an electron
C. produce ATP
D. synthesize glucose

Answer

B

Which molecule absorbs the energy of a photon in photosynthesis?

A. ATP
B. glucose
C. chlorophyll
D. water

Answer

C

Plants produce oxygen when they photosynthesize. Where does the oxygen come from?

A. splitting water molecules
B. ATP synthesis
C. the electron transport chain
D. chlorophyll

Answer

A

Which color(s) of light does chlorophyll a reflect?

A. red and blue
B. green
C. red
D. blue

Answer

B

Free Response

Describe the pathway of energy in light-dependent reactions.

Answer

The energy is present initially as light. A photon of light hits chlorophyll, causing an electron to be energized. The free electron travels through the electron transport chain, and the energy of the electron is used to pump hydrogen ions into the thylakoid space, transferring the energy into the electrochemical gradient. The energy of the electrochemical gradient is used to power ATP synthase, and the energy is transferred into a bond in the ATP molecule. In addition, energy from another photon can be used to create a high-energy bond in the molecule NADPH.

4.8: The Calvin Cycle

Carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

Multiple Choice

Where in plant cells does the Calvin cycle take place?

A. cytoplasm
B. mitochondria
C. chloroplasts
D. extracellular matrix

Answer

C

Which statement correctly describes carbon fixation?

A. the conversion of CO₂ to an organic compound
B. the use of RuBisCO to form 3-PGA
C. the production of carbohydrate molecules from G3P
D. the formation of RuBP from G3P molecules
E. the use of ATP and NADPH to reduce CO₂

Answer

A

Free Response

Explain the reciprocal nature of the net chemical reactions for photosynthesis and respiration.

Answer

Photosynthesis takes the energy of sunlight and combines water and carbon dioxide to produce sugar and oxygen as a waste product. The reactions of respiration take sugar and consume oxygen to break it down into carbon dioxide and water, releasing energy. Thus, the reactants of photosynthesis are the products of respiration, and vice versa.