Campbell Biology Chapter 9 Review: Cellular Respiration and Fermentation

9.1 Redox reactions and the stages of respiration

pp. 164–168

On the AP exam? Yes

Topics 3.3 and 3.5 expect you to follow electrons from food to NADH to oxygen and to know where each stage happens; you won't need free-energy values like kcal per mole.

In the course: Topic 3.3 Cellular Energy, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Catabolic pathways release energy by breaking large fuel molecules into small, low-energy products such as CO₂ and water. Cells trap part of that energy in ATP, and the remainder escapes as heat.
Aerobic respiration uses O₂ and breaks fuel all the way down to CO₂ and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Fermentation only partly breaks fuel down and needs no O₂. Some prokaryotes run anaerobic respiration, which uses something other than O₂ at the end.
In a redox reaction, one substance loses electrons (it's oxidized) and another gains them (it's reduced). The two always happen together. Sometimes electrons don't move all the way over; they just get pulled closer to a strongly electronegative atom such as oxygen.
In respiration, glucose is oxidized and O₂ is reduced. Electrons in C–H bonds sit at high energy, and as they end up closer to oxygen's nuclei they drop to lower energy. That drop is the energy cells harvest.
Cells don't burn fuel in one blast. Enzymes take it apart in many small steps. Electrons stripped from fuel usually go first to the carrier NAD⁺, making NADH, and NADH hands them to the electron transport chain, which lets them fall to O₂ a little at a time.
There are three stages: glycolysis in the cytosol; pyruvate oxidation and the Krebs cycle in the mitochondrial matrix (the cytosol in prokaryotes); and oxidative phosphorylation on the inner mitochondrial membrane (the plasma membrane in prokaryotes).
Cells make ATP in two ways. In substrate-level phosphorylation, an enzyme moves a phosphate straight from an organic molecule onto ADP, giving a few ATP. In oxidative phosphorylation, ATP synthase is powered by an H⁺ gradient, and this makes roughly 90% of the ATP, for a total of about 30–32 ATP per glucose.

Key terms (15)

catabolic pathway: A series of reactions that breaks complex molecules into simpler ones and releases energy, like the breakdown of glucose.
cellular respiration: The set of pathways cells use to break down fuel molecules and capture their energy as ATP. Usually it means the version that uses O₂.
aerobic respiration: Respiration in which O₂ is the last stop for electrons from food. It breaks glucose all the way to CO₂ and water and gives the most ATP.
anaerobic respiration: Respiration with an electron transport chain that ends with a substance other than O₂, such as nitrate or sulfate. Only some prokaryotes do it.
fermentation: A way to make a little ATP without O₂ or an electron transport chain: glycolysis plus a step that recycles NAD⁺.
redox reaction: A reaction in which electrons move from one substance to another, or shift toward a more electronegative atom. Short for oxidation-reduction.
oxidation: Losing electrons. When glucose is oxidized in your cells, its electrons are passed on to carriers and finally to oxygen.
reduction: Gaining electrons. The name fits because each added electron makes the atom's charge more negative, lowering it.
reducing agent: The substance that gives away electrons in a redox reaction. It gets oxidized as it reduces something else.
oxidizing agent: The substance that takes electrons in a redox reaction. It gets reduced. O₂ is a very strong one.
NAD⁺ / NADH: An electron carrier made from the vitamin niacin. NAD⁺ picks up electrons (and a hydrogen) from fuel to become NADH, which delivers them to the electron transport chain.
dehydrogenase: An enzyme that pulls hydrogen atoms (electrons plus protons) off a fuel molecule and usually hands the electrons to NAD⁺.
electron transport chain: A line of carriers, mostly proteins, in a membrane. Electrons pass down it step by step to a final acceptor, releasing energy in small amounts.
substrate-level phosphorylation: Making ATP by having an enzyme move a phosphate group directly from an organic molecule to ADP. No membrane or H⁺ gradient is needed.
oxidative phosphorylation: Making ATP with energy from electron transport: the chain builds an H⁺ gradient, and ATP synthase uses it. It makes most of a cell's ATP.

Check yourself: 9.1 Redox reactions and the stages of respiration

4 questions on 9.1 Redox reactions and the stages of respiration. Pick an answer to see if you got it, and why.

Question 1 of 4

Some bacteria living in acidic mine drainage get their energy by turning Fe²⁺ ions into Fe³⁺ ions. They pass the electrons they remove down a transport chain to O₂, which becomes water. Which statement correctly describes the redox roles in this process?

Question 2 of 4

Which statement best explains why moving electrons from glucose to oxygen releases energy that a cell can use?

Question 3 of 4

A 1 g sample of sucrose burned in a calorimeter gives off the same total energy as 1 g of sucrose fully oxidized by yeast cells. Burning releases it all at once as heat and light. What is the main advantage of the cell's slower, multistep route?

Question 4 of 4

A researcher purifies an enzyme that moves a phosphate group directly from a phosphorylated three-carbon sugar acid onto ADP, forming ATP. Which statement about this way of making ATP is correct?

0 of 4 answered

9.2 Glycolysis: splitting glucose in the cytosol

pp. 168–169

On the AP exam? Yes

Topic 3.5 tests glycolysis's inputs, outputs and location, and Topic 3.3 uses it as evidence of common ancestry; you don't need the ten steps, their intermediates or enzyme names.

In the course: Topic 3.3 Cellular Energy, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Glycolysis means "sugar splitting." It turns one six-carbon glucose into two three-carbon molecules of pyruvate, and it happens in the cytosol.
First the cell invests: it spends 2 ATP adding phosphate groups, which makes the sugar reactive and ready to split. Then comes the payoff: 4 ATP are made by substrate-level phosphorylation, and 2 NAD⁺ are reduced to NADH.
The net result per glucose is 2 pyruvate, 2 ATP and 2 NADH.
No carbon is lost as CO₂ in glycolysis. All six of glucose's carbons are still in the two pyruvates.
Glycolysis doesn't use O₂, so it runs with or without oxygen. It does need NAD⁺ on hand to take electrons, so it stalls if NAD⁺ runs out.
Glycolysis releases only a small share of glucose's energy. Most of the energy is still in pyruvate (and in the NADH), waiting for the later stages.
Nearly every living thing runs glycolysis with similar enzymes. That makes it strong evidence that all life shares a common ancestor (Topic 3.3).

Key terms (8)

glycolysis: The pathway in the cytosol that splits one glucose into two pyruvates, with a net gain of 2 ATP and 2 NADH. It doesn't need oxygen.
pyruvate: The three-carbon molecule glycolysis ends with. With O₂ it goes on to the mitochondrion; without O₂ it's used in fermentation.
cytosol: The fluid part of the cytoplasm, outside the organelles. Glycolysis's enzymes are dissolved here.
energy investment phase: The first half of glycolysis, where the cell spends 2 ATP adding phosphates to the sugar to get it ready to split.
energy payoff phase: The second half of glycolysis, where the split sugars are oxidized, making 4 ATP and 2 NADH per glucose.
net yield: What you gain after paying back what you spent. Glycolysis makes 4 ATP but used 2, so its net yield is 2 ATP.
phosphorylation: Adding a phosphate group to a molecule. It can make a molecule more reactive, or, when the target is ADP, it makes ATP.
ATP: The cell's main short-term energy carrier. Breaking off its last phosphate releases energy that powers cell work.

Check yourself: 9.2 Glycolysis: splitting glucose in the cytosol

4 questions on 9.2 Glycolysis: splitting glucose in the cytosol. Pick an answer to see if you got it, and why.

Question 1 of 4

A cell breaks down 5 molecules of glucose by glycolysis alone. Which list gives the net products?

Question 2 of 4

Researchers remove all mitochondria from a cell extract and add glucose labeled with radioactive carbon (¹⁴C) at all six positions, along with ATP, ADP, phosphate and plenty of NAD⁺. After glycolysis is complete, where is the ¹⁴C found?

Question 3 of 4

Why does glycolysis spend ATP in its early steps even though the pathway's purpose is to make ATP?

Question 4 of 4

A cell-free extract contains all the glycolysis enzymes, plus excess glucose, ADP and phosphate, but only a small, fixed amount of NAD⁺ and no way to turn NADH back into NAD⁺. Pyruvate is made quickly for 4 minutes, then production stops. Adding more NAD⁺ at 10 minutes restarts it. Which explanation best fits these results?

0 of 4 answered

9.3 Pyruvate oxidation and the Krebs cycle

pp. 170–172

On the AP exam? Yes

Topic 3.5 covers what goes in and comes out of the Krebs cycle and where it happens; the exam leaves out its intermediates (like citrate) and enzyme names.

In the course: Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

When O₂ is around, pyruvate is actively transported into the mitochondrion's matrix (in prokaryotes, the next steps happen in the cytosol).
Pyruvate oxidation: each pyruvate loses one carbon as CO₂, the leftover two-carbon piece is oxidized (making NADH), and it's attached to coenzyme A. The result is acetyl CoA.
In the Krebs cycle (also called the citric acid cycle), the two-carbon acetyl group joins a four-carbon molecule. Over the next steps two carbons leave as CO₂ and the four-carbon molecule is rebuilt, so the cycle can take in another acetyl group.
Per glucose, the two pyruvates make two acetyl groups, so the cycle turns twice. Pyruvate oxidation and the Krebs cycle together release all six of glucose's carbons as 6 CO₂.
Most of the energy goes into electron carriers: per glucose, these steps make 8 NADH and 2 FADH₂, but only 2 ATP, made through substrate-level phosphorylation.
The CO₂ you breathe out comes from these steps, carved off your food's carbon skeletons. The O₂ you breathe in ends up in water, not CO₂.
The Krebs cycle doesn't use O₂ directly, but it stops without it, because the electron transport chain is the only place NADH and FADH₂ can drop off electrons to become NAD⁺ and FAD again.

Key terms (8)

mitochondrial matrix: The fluid inside a mitochondrion's inner membrane. Pyruvate oxidation and the Krebs cycle happen here.
pyruvate oxidation: The step linking glycolysis to the Krebs cycle: pyruvate loses a carbon as CO₂, gives electrons to NAD⁺ and becomes acetyl CoA.
acetyl CoA: A two-carbon acetyl group attached to coenzyme A. It's the form in which carbon from sugars, fats and many amino acids enters the Krebs cycle.
coenzyme A: A carrier molecule, made from a B vitamin, that holds the two-carbon acetyl group and hands it to the Krebs cycle.
Krebs cycle: Also called the citric acid cycle. A cycle of reactions in the mitochondrial matrix that releases CO₂ and loads electrons onto NADH and FADH₂, with a little ATP.
citrate: The six-carbon molecule formed when an acetyl group joins the cycle's four-carbon starter. The cycle's other name, the citric acid cycle, comes from it.
FAD / FADH₂: An electron carrier made from riboflavin (vitamin B₂). FAD picks up two electrons and two protons to become FADH₂, which delivers them to the electron transport chain.
GTP: A molecule much like ATP but with the base guanine. In some cells, the Krebs cycle makes GTP, which can then be used to make ATP.

Check yourself: 9.3 Pyruvate oxidation and the Krebs cycle

4 questions on 9.3 Pyruvate oxidation and the Krebs cycle. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers break open liver cells and separate three parts: the cytosol, the mitochondrial matrix, and pieces of inner mitochondrial membrane. Each part is given pyruvate, coenzyme A and plenty of NAD⁺, with no O₂. Which part would release CO₂ from the pyruvate?

Question 2 of 4

Isolated mitochondria are given pyruvate as their only fuel, and the products of the matrix reactions are measured. For each pyruvate, the mitochondria make 3 CO₂, 4 NADH, 1 FADH₂ and 1 ATP. Which conclusion do these results best support?

Question 3 of 4

In each turn of the Krebs cycle, a two-carbon group enters and two carbons leave as CO₂. What allows the cycle to keep accepting new two-carbon groups turn after turn?

Question 4 of 4

One glucose molecule is completely oxidized by aerobic respiration. How many of its carbon atoms leave as CO₂ during pyruvate oxidation, and how many leave during the Krebs cycle?

0 of 4 answered

9.4 Electron transport, chemiosmosis and ATP synthase

pp. 172–177

On the AP exam? Yes

This is the most heavily tested part of Topic 3.5: expect to predict what happens when the chain, the H⁺ gradient or ATP synthase is blocked. You won't need the names of the complexes or cytochromes.

In the course: Topic 3.5 Cellular Respiration, Topic 2.1 Cell Structure and Function (notes, videos and more questions)

Key points

After glycolysis and the Krebs cycle, a cell has made only 4 ATP per glucose. Most of the captured energy is riding in NADH and FADH₂.
The chain is a set of protein complexes, plus a couple of small mobile carriers, embedded in the mitochondrion's inner membrane. Folding that membrane into cristae packs many more chains into each mitochondrion.
Electrons pass from carrier to carrier, each one pulling harder on electrons than the last, and lose a little energy at each step. At the end, O₂ takes the electrons and combines with H⁺ to make water.
The chain makes no ATP itself. It uses the energy from falling electrons to pump H⁺ from the matrix into the intermembrane space, so that space ends up with more H⁺ (a lower pH) than the matrix.
H⁺ can get back into the matrix only through ATP synthase. The flow spins part of the enzyme like a turbine, and it builds ATP from ADP and phosphate. Using an H⁺ gradient this way is chemiosmosis. Electron transport plus chemiosmosis is oxidative phosphorylation.
FADH₂'s electrons join the chain farther down, with less energy left than NADH's, so fewer H⁺ get pumped: about 1.5 ATP per FADH₂ versus 2.5 per NADH. The total is about 30–32 ATP per glucose, roughly a third of glucose's energy, and the rest becomes heat.
Chemiosmosis also makes ATP in chloroplasts (powered by light) and in prokaryotes (across the plasma membrane). Some cells have uncoupling proteins that let H⁺ leak back without ATP synthase, turning the gradient's energy into heat to keep the body warm.

Key terms (12)

inner mitochondrial membrane: The folded inner membrane of a mitochondrion. It holds the electron transport chain and ATP synthase, and it doesn't let H⁺ leak through.
cristae: The folds of the inner mitochondrial membrane. More folds mean more surface for electron transport chains and ATP synthases.
intermembrane space: The narrow space between a mitochondrion's two membranes. H⁺ is pumped into it, so it becomes more acidic than the matrix.
cytochrome: A protein in the electron transport chain with an iron-containing heme group that picks up and passes on electrons.
ubiquinone: A small carrier that isn't a protein. It drifts through the inner membrane, ferrying electrons from one protein complex to the next. Also called coenzyme Q.
final electron acceptor: The molecule at the very end of an electron transport chain. In aerobic respiration it's O₂, which becomes water.
proton gradient: A difference in H⁺ concentration across a membrane. It stores energy, like water behind a dam.
proton-motive force: The push on H⁺ to flow back across a membrane, created by the proton gradient. It can power ATP synthesis, transport or flagella.
ATP synthase: A membrane enzyme that lets H⁺ flow down its gradient and uses that flow to join ADP and phosphate into ATP. It works like a tiny rotating motor.
chemiosmosis: Using the energy stored in a membrane's H⁺ gradient to do work, especially to make ATP through ATP synthase.
uncoupling protein: A channel in the inner mitochondrial membrane that lets H⁺ leak back without passing through ATP synthase, so the energy comes out as heat.
thermogenesis: Making heat to keep warm. Cells can do it by letting the H⁺ gradient run down without making ATP.

Check yourself: 9.4 Electron transport, chemiosmosis and ATP synthase

4 questions on 9.4 Electron transport, chemiosmosis and ATP synthase. Pick an answer to see if you got it, and why.

Question 1 of 4

In a classic 1970s experiment, researchers built artificial lipid vesicles containing two purified proteins: ATP synthase from a mammal and a light-driven H⁺ pump from an archaeon. The vesicles had no electron transport chain. With ADP and phosphate present, ATP was made in light but not in darkness, and no ATP was made in light when a compound that makes membranes leaky to H⁺ was added. Which conclusion is best supported?

Question 2 of 4

In mitochondria, the electrons from one NADH cause about 10 H⁺ to be pumped across the inner membrane. Suppose a newly discovered organism's ATP synthase needs 5 H⁺ to flow back through it (including transport costs) to make each ATP. About how many ATP could this organism make per NADH?

Question 3 of 4

Why do electrons move along the electron transport chain toward O₂ rather than backward?

Question 4 of 4

Electron micrographs show that mitochondria in hummingbird flight muscle have much more tightly packed cristae than mitochondria in the bird's fat-storage cells. Which explanation best links this structure to its function?

0 of 4 answered

9.5 Making ATP without oxygen

pp. 177–179

On the AP exam? Yes

Topic 3.5 tests why fermentation is needed (to recycle NAD⁺) and how its yield compares with aerobic respiration; Topic 3.3 uses glycolysis's spread across all life as evidence of common ancestry.

In the course: Topic 3.5 Cellular Respiration, Topic 3.3 Cellular Energy (notes, videos and more questions)

Key points

Without O₂, electrons back up in the transport chain and oxidative phosphorylation stops. Cells then have two other options: anaerobic respiration or fermentation.
Anaerobic respiration, found only in some prokaryotes, still uses an electron transport chain, but it ends with an acceptor such as nitrate, sulfate or iron ions instead of O₂. These pull on electrons less strongly than O₂, so less energy is released, but there's still an H⁺ gradient and ATP synthase.
Fermentation is glycolysis plus one extra step that passes NADH's electrons to pyruvate (or a molecule made from it). That step recycles NAD⁺, so glycolysis can keep making its 2 ATP per glucose. The extra step makes no ATP itself.
In alcohol fermentation (yeasts and many bacteria), pyruvate gives off CO₂, and the two-carbon leftover is reduced to ethanol. In lactic acid fermentation (some bacteria and fungi, and your muscle cells when O₂ runs short), pyruvate is reduced straight to lactate, with no CO₂.
Aerobic respiration gives about 30–32 ATP per glucose, compared with 2 for fermentation, so a cell that switches to fermenting has to burn sugar about 15 times faster to keep up.
Obligate anaerobes can't use O₂ and are often harmed by it. Facultative anaerobes, like yeast and E. coli, can respire or ferment, with pyruvate as the branch point. Lactate from your muscles isn't wasted: your heart and other tissues burn it, and your liver can turn it back into glucose.
Glycolysis needs no O₂ and no organelles, and almost every organism has it. Early prokaryotes probably relied on it before cyanobacteria filled the air with O₂ (around 2.4 billion years ago), which is why it's seen as an ancient, shared pathway.

Key terms (9)

anaerobic: Without oxygen. Anaerobic processes, like fermentation, don't use O₂.
alcohol fermentation: Fermentation in which pyruvate releases CO₂ and the two-carbon leftover is reduced to ethanol, recycling NAD⁺. Yeasts and many bacteria do it.
lactic acid fermentation: Fermentation in which pyruvate is reduced directly to lactate, recycling NAD⁺ without releasing CO₂. Your muscles do it when O₂ runs short.
ethanol: The two-carbon alcohol made by alcohol fermentation. It still holds a lot of energy, since fermentation breaks sugar down only partway.
lactate: The three-carbon end product of lactic acid fermentation; it's lactic acid after losing an H⁺. Other tissues can take it up and use it as fuel.
obligate anaerobe: A microbe that grows only where O₂ is absent. It gets ATP by fermenting or by anaerobic respiration, and O₂ often harms it.
facultative anaerobe: An organism that can make ATP by aerobic respiration when O₂ is present and switch to fermentation (or anaerobic respiration) when it isn't.
obligate aerobe: An organism that needs O₂ to survive because it depends on aerobic respiration.
NAD⁺ regeneration: Turning NADH back into NAD⁺ so glycolysis can keep going. Fermentation's whole job is to do this when O₂ is missing.

Check yourself: 9.5 Making ATP without oxygen

4 questions on 9.5 Making ATP without oxygen. Pick an answer to see if you got it, and why.

Question 1 of 4

A student writes: "Cells ferment so that they can squeeze extra ATP out of pyruvate when oxygen runs out." Which is the best correction?

Question 2 of 4

A facultatively anaerobic bacterium is grown in a sealed flask of broth with glucose. Measurements (invented data): Time (h) | Dissolved O₂ (% of start) | Lactate (mmol/L) 0 | 100 | 0.0 2 | 45 | 0.0 4 | 0 | 1.5 6 | 0 | 7.8 8 | 0 | 14.6 Which conclusion is best supported?

Question 3 of 4

A bacterium isolated from a pickle brine grows without O₂. As it grows, the medium becomes acidic with a three-carbon acid, and no gas is produced. Which process is it most likely using?

Question 4 of 4

In waterlogged soil with no O₂, some bacteria pass electrons from their food to nitrate (NO₃⁻) at the end of an electron transport chain. Which feature do these bacteria share with aerobic organisms but not with organisms that only ferment?

0 of 4 answered

9.6 Other fuels, building blocks and feedback control

pp. 179–181

On the AP exam? Background

Not tested directly: the exam won't ask about beta oxidation or which enzymes control respiration, but allosteric control is part of Topic 3.2 and breaking polymers into usable monomers is part of Topic 1.3.

In the course: Topic 3.2 Environmental Impacts on Enzyme Function, Topic 3.5 Cellular Respiration, Topic 1.3 Introduction to Macromolecules, Topic 4.4 Feedback (notes, videos and more questions)

Key points

Respiration can run on any of the three main fuels in food: carbohydrates, fats and proteins. Starch and glycogen are broken down by hydrolysis to glucose, and other sugars are converted so they can enter glycolysis.
Proteins are broken down to amino acids. Extra amino acids have their amino group removed (deamination), and the nitrogen is excreted as ammonia, urea or uric acid. The leftover carbon skeletons enter glycolysis or the Krebs cycle at different points.
Fats are split into glycerol, which enters glycolysis, and fatty acids. Beta oxidation chops fatty acids into two-carbon pieces that enter the Krebs cycle as acetyl CoA, making NADH and FADH₂ along the way.
Fats have lots of C–H bonds and little oxygen, so their electrons have far to fall. Gram for gram, fat yields over twice the ATP that carbohydrate does.
The same pathways supply building blocks. Cells pull intermediates out of glycolysis and the Krebs cycle to make amino acids, fats and sugars. These anabolic pathways use ATP. Extra sugar can be turned into fat by way of acetyl CoA.
Respiration follows supply and demand. When ATP is used up, respiration speeds up; when ATP is plentiful, it slows. Allosteric enzymes at key steps act as the switches. An enzyme early in glycolysis (phosphofructokinase) is slowed by ATP and citrate and sped up by AMP.
In feedback inhibition, a pathway's end product binds an allosteric site on an early enzyme and turns it down. This is negative feedback, and it keeps cells from wasting materials (Topics 3.2 and 4.4).

Key terms (10)

deamination: Removing the amino group (–NH₂) from an amino acid so its carbon skeleton can be used as fuel. The nitrogen is excreted.
beta oxidation: The pathway that breaks fatty acids into two-carbon pieces, which enter the Krebs cycle as acetyl CoA. It also makes NADH and FADH₂.
glycerol: The three-carbon backbone of a fat. Once freed from the fatty acids, it can be changed into a glycolysis intermediate and used as fuel.
anabolic pathway: A series of reactions that builds complex molecules from simpler ones, using energy, often from ATP.
carbon skeleton: The chain of carbon atoms in an organic molecule. Food supplies these skeletons for building new molecules as well as for fuel.
essential amino acid: An amino acid your body can't make from other molecules, so it has to come from your diet.
feedback inhibition: When the end product of a pathway shuts down an enzyme near the start of that pathway, so the cell doesn't make more than it needs.
allosteric regulation: Control of an enzyme by a molecule that binds somewhere other than the active site, changing the enzyme's shape and activity.
phosphofructokinase: An allosteric enzyme early in glycolysis that sets the pace of respiration. High ATP slows it, and AMP speeds it up.
AMP: Adenosine monophosphate, ATP with only one phosphate. A rise in AMP signals that the cell is low on energy.

Check yourself: 9.6 Other fuels, building blocks and feedback control

4 questions on 9.6 Other fuels, building blocks and feedback control. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

The respiratory quotient (RQ) is the volume of CO₂ released divided by the volume of O₂ used. Burning only glucose gives an RQ of 1.0; burning only a typical fat gives about 0.7. A rat's gas exchange was measured in two conditions (invented data): Condition | O₂ used (mL/min) | CO₂ released (mL/min) Just after a high-starch meal | 10.0 | 9.6 After 3 days without food | 8.0 | 5.7 Which conclusion is best supported?

Question 2 of 4

A teenager eats far more protein than her body needs for growth and repair. What happens to the extra amino acids that are used for fuel?

Question 3 of 4

Cultured human liver cells are given glucose labeled with ¹⁴C. Later, the label is found in some of the amino acids in their proteins, such as alanine and glutamate, but never in others, such as leucine and lysine. Which explanation best fits these results?

Question 4 of 4

A bacterium makes the amino acid histidine through a pathway of about ten enzyme-catalyzed steps. When histidine is added to its growth medium, the activity of the enzyme for the first step drops sharply within seconds, even though the amount of that enzyme stays the same. Which explanation is most likely?

0 of 4 answered