Energy Metabolism and Photosynthesis Practice Questions

Flashcards testing knowledge of cellular energy metabolism, glycolysis, TCA cycle, oxidative phosphorylation, photosynthesis, and toxic uncouplers/inhibitors based on lecture review questions.

Which one of these is the most enery-rich molecule under normal cellular conditions?

$$\text{NADH}(\text{H}^+)$$

The glycolytic enzyme phosphofructokinase is inhibited when ATP binds at a site other than the active site of the enzyme. This regulation is

allosteric regulation

The composition of the inner mitochondrial membrane is most similar to

the plasma membrane of an aerobic bacterium

In Kearns-Sayre syndrome, vision (among many other systems heavily dependent on aerobic metabolism) is impaired due to a mutation in the mitochondrial gene for Complex II in the ETS. Electron transport from which of the following is impaired?

Succinate

In what form does the product of glycolysis enter the TCA cycle?

acetyl CoA

Which of the following is not a feature of oxidative phosphorylation?

direct transfer of phosphate from an organic substrate to ADP

If the reaction center chlorophyll absorbs light at a wavelength of $700\,\text{nm}$, then the antenna pigment molecules must absorb

light with wavelengths shorter than $700\,\text{nm}$

Which of the following does not directly contribute to the proton gradient across the thylakoid membrane?

plastocyanin (PC)

Which of the following is not part of cyclic photophosphorylation?

PS II

If you wanted to isolate the enzyme phosphoenolpyruvate carboxylase from plants, you would purify it from

mesophyll cells of $\text{C}_4$ plants

What is/are the first products) we see when RUBISCO is operating under normal conditions in a $\text{C}_3$ plant.

3-phosphoglycerate

What is/are the first product(s) we see when RUBISCO is operating under hot, bright, dry conditions in a $\text{C}_4$ plant?

oxaloacetate

If you wanted to isolate a large quantity of malate from a CAM plant, when would you

crack of dawn

Catabolic reactions

have a negative $\Delta G$

Hydrolysis of the phosphoanhydride bond on the 3rd phosphate of ATP is energetically favored because

negative charges on phosphates repel, favoring hydrolysis;

of resonance stabilization (electron delocalization)

The reason people can die of starvation while they still have fat stores is because

the body cannot convert fats to sugar, which the brain needs to survive

NAD and FAD are used in catabolism because they can

be reversibly oxidized and reduced, carrying high energy electrons from food to electron transport machinery

Glycolysis is a pathway which involves

Oxidizing a sugar without utilizing $\text{O}_2$

The energy of the sole oxidation reaction in glycolysis is captured in

reduced coenzyme, $\text{NADH}(\text{H}^+)$, and a high energy phosphate bond

In an experiment, kitties were fed glucose containing a radioactive $^{14}\text{C}$ atom at $\text{C}_6$. This atom was eventually found in:

carbon dioxide

The high energy thioester bond of succinyl CoA is used to

produce ATP

When six carbon fructose 1,6-bisphosphate is split in glycolysis the products are

dihydroxyacetone phosphate and glyceraldehyde 3 phosphate, of which DHP is converted into G3P, and then both G3P molecules are directly oxidized

In fermentation, $\text{NADH}(\text{H}^+)$ gives electrons to pyruvate in order to

oxidize $\text{NADH}(\text{H}^+)$ so glycolysis can continue

Glycolysis and gluconeogenesis are rarely on in the same cell because

energy would be lost, due to the 2nd Law of Thermodynamics, in this futile cycle

Which of the following statements regarding energy metabolism is true?

The glycolytic pathway includes two ATP-consuming steps, even though its function in the cell is to generate ATP.

Which of these further statements regarding energy metabolism is true?

The mitochondrial ETS requires oxygen as the e- acceptor in aerobic respiration.

In the mitochondrion (or cytoplasm of heterotrophic bacterial cells), oxygen is

as an electron acceptor, because it is very electronegative

The TCA cycle

directly produces $2\,\text{ATP}$ per glucose, capturing the remaining energy in reduced coenzymes

The electron transport system (ETS)-

pumps protons across the inner mitochondrial membrane while transferring electrons within the membrane

Protons across, e-’s within

Aerobic prokaryotic cells can derive $38$ molecules of ATP per glucose molecule, assuming that the metabolic pathways are behaving purely catabolically. Eukaryotic cells can also derive $38$ AT molecules providing that

they maintain a significantly higher [NADH]/[NAD] ratio in the cytosol than in the mitochondrial matrix.

Light harvesting complexes transfer energy to the reaction center of a photosystem by

Resonance transfer

Cyclic electron flow in the light reactions of photosynthesis results in

The generation of ATP

The frequencies of light a pigment can use to photoexcite it's electrons are called

the absorption spectrum

When light strikes chlorophyll molecules at the reaction center of a photosystem they lose electrons, and these electrons are replaced by

splitting water

Photosystems I and II couple electron transport to proton pumping into the

Thylakoid lumen

The enzyme C4 and CAM plants use for the initial carbon fixation event is

Phosphoenol pyruvate carboxylase

The correct order of flow of electrons through the thylakoid membrane during energy transduction reactions is

photosystem II, cytochrome complex, photosystem I

PSII → CytC → PSI

Only one out of every six triose molecules in the Calvin cycle feed into sugar production because

The cell needs the other trioses to regenerate ribulose 1, 5-bisphosphate

Arsenate poisoning: Arsenate ($\text{HAsO}_4^{2-}$) is a potent poison in almost all living systems. Arsenate uncouples the phosphorylation event from the oxidation of glyceraldehyde-3-phosphate. This is because the enzyme involved (glyceraldehyde-3-phosphate dehydrogenase) can utilize arsenate instead of inorganic phosphate, forming glycerate-1-arseno-3-phosphate instead of 1,3-bisphosphoglycerate. This is a highly unstable compound, which immediately undergoes nonenzymatic decomposition into 3-phosphoglycerate. Arsenate also acts as an allosteric inhibitor of pyrivate dehydrogenase, shutting it down. Explain in terms of the metabolic steps involved why arsenic is so toxic.

Arsenate causes glycolysis to skip the ATP-producing phosphoglycerate kinase step, so the ATP normally made from 1,3-bisphosphoglycerate is lost. This causes glycolysis to produce no net ATP. Arsenate also inhibits pyruvate dehydrogenase, preventing pyruvate from becoming acetyl-CoA. Therefore, glucose cannot properly feed the TCA cycle, greatly reducing NADH, FADH2, and ATP production.

Arsenate → glycolysis loses ATP + PDH stops → TCA/ETS lose fuel → major ATP loss.

Oxidative phosphorylation is tightly coupled to ATP synthesis: no electrons will flow if ATP synthesis is blocked (by low levels of ADP and Pi, e.g.). There are, however, uncoupling agents such as 2,4-dinitrophenol that allow electron flow in the absence of ATP synthesis. This agent is highly toxic to humans, causing a marked increase in metabolism, temperature, profuse sweating, collapse, and death. For a brief period in the 1950's, sublethal doses of dinitrophenol were prescribed for weight reduction! Explain in terms of what we know about mitochondrial function why this drug would cause increased metabolism and temperature, and be useful as a weight reduction drug. Also explain its lethality at higher doses.

DNP is an uncoupler that allows $\text{H}^+$ to cross the inner mitochondrial membrane without going through ATP synthase. This destroys the $\text{H}^+$ gradient, so the ETS can continue running but ATP production decreases. The cell responds by burning more glucose and fat to try to make enough ATP, increasing metabolism and causing weight loss. Because the energy from the $\text{H}^+$ gradient is released as heat instead of captured as ATP, body temperature rises. At high doses, cells cannot produce enough ATP and excessive heat is generated, which can cause organ failure and death.

DNP → H⁺ leak → less ATP + more fuel burned + more heat.

The TCA Cycle is an amphibolic pathway: often intermediates are pulled off by the organism for use in synthesis of other molecules. In a cell requiring the amino acid this can be glutamate for protein synthesis, o-ketoglutarate can be "bled" off for this anabolic transamination. What is the maximum possible yield of ATP per glucose in a cell pulling reduced carbon out at this step in TCA? (explain reasoning for credit). Assume the cell taps other sources of oxaloacetate to continue feeding in new carbons via acetyl CoA.

In this step, 8 NADH created (2 glycolysis, 2 pyruvate oxidation, 4 TCA cycle so far) and 2 ATP created (glycolysis). 1 NADH = 3 ATP so, 8 NADH x 3 ATP = 24 ATP 24 ATP + 2 ATP (gly) = $26$ total ATP per glucose

Describe the mechanistic details of how ATP synthases generate ATP.

ATP synthase uses the $\text{H}^+$ gradient across the inner mitochondrial membrane. $\text{H}^+$ flows down its gradient through the $\text{F}_0$ portion, causing the complex to rotate. This rotation changes the shape of catalytic sites in the $\text{F}_1$ portion, allowing ADP and Pi to bind and form ATP. ATP is then released.

H⁺ flows → F₀ rotates → F₁ changes shape → ADP + Pi → ATP.