How cells make ATP

Reduction is the gain of electrons. NAD+ gains two electrons (with an H+) and becomes NADH. The molecule that gave up the electrons was oxidized.

• It has been oxidized:
• Correct: It has been reduced:
• It has been hydrolyzed:

Hydrolysis splits off the outer phosphate, leaving ADP and phosphate and releasing energy for cell work. Cells then rebuild ATP from ADP and phosphate using energy from fuel.

• Correct: ATP splits into ADP and phosphate, releasing energy the cell uses for work:
• ATP joins with ADP to store energy:
• ATP gains a fourth phosphate group:

The inner membrane folds into cristae. The protein complexes that make most ATP sit in that membrane, so more folds mean more of them.

• Its smooth outer membrane:
• The ribosomes on its surface:
• Correct: The folds of its inner membrane, the cristae:

The citric acid cycle and the electron transport chain are in mitochondria, and a mature red blood cell has none. It can run only glycolysis, and it converts pyruvate to lactate to regenerate NAD+ so glycolysis can continue.

• Correct: It has no mitochondria, so pyruvate cannot be broken down with oxygen: Correct. No mitochondria means no aerobic stages, whatever oxygen is present.
• Its oxygen-carrying protein holds oxygen too tightly to use: How tightly the red cell's protein binds oxygen is not the limit. Even with free oxygen inside, the cell has no electron transport chain to use it.
• Its glycolysis enzymes work only when oxygen is absent: Glycolysis does not use oxygen, so oxygen neither turns it on nor off. It runs in cells with plenty of oxygen as well.
• It makes lactate so that it can save its oxygen for other tissues: This gives a purpose, not a mechanism. The cell makes lactate because it lacks the organelle for the aerobic stages.

With the last complex blocked, electrons cannot pass to oxygen. The chain stops, NADH cannot give up its electrons, and the citric acid cycle stalls. NAD+ runs short, so cells convert pyruvate to lactate to regenerate it, and glycolysis becomes their only ATP source. Lactate pours into the blood even though oxygen is present.

• Cyanide binds the glycolysis enzymes in his cytosol and makes them work much faster than usual: Glycolysis does speed up, but because ATP falls, not because cyanide acts on its enzymes. Cyanide's target is the electron transport chain.
• Smoke has stopped his red blood cells from delivering oxygen to every tissue in his body: His blood carries plenty of oxygen. The problem is that his cells cannot use it.
• Correct: Blocked electron transport leaves NADH unoxidized, so cells make lactate to regenerate NAD+: Correct. The block stops NADH oxidation, and lactate production takes over the job of regenerating NAD+.
• Cyanide turns pyruvate into lactate inside the mitochondrion: Pyruvate becomes lactate in the cytosol, not in the mitochondrion, and cyanide does not catalyze that reaction.

O2 is used only at the end of the electron transport chain. There each O2 accepts four electrons and four H+ and becomes two water molecules. The oxygen atoms in CO2 come from glucose and from water used in the citric acid cycle, not from the O2 you breathe.

• The carbon dioxide she breathes out: This is the common mistake. CO2 is made in the citric acid cycle and as pyruvate loses a carbon, steps that take their oxygen atoms from glucose and water, not from O2.
• Lactate in her blood: Lactate is made from pyruvate in the cytosol, a step that uses no oxygen gas.
• Pyruvate in her cells: Pyruvate comes from splitting glucose. Glycolysis uses no O2, so the label cannot enter it.
• Correct: Water made in her mitochondria: Correct. Oxygen is the final electron acceptor and ends up in water.

Oxygen is the final electron acceptor, at the end of the chain, and it becomes water. CO2 comes from the carbons of glucose, released earlier as pyruvate is broken down.

• Glycolysis in the cytosol splits glucose into two pyruvate, netting 2 ATP.: This step is right. Glycolysis runs in the cytosol and nets 2 ATP per glucose.
• In the mitochondrion, each pyruvate loses a carbon as CO2 and the citric acid cycle breaks down what remains.: This step is right. Pyruvate breakdown and the citric acid cycle release the carbons as CO2.
• NADH and reduced FAD give their electrons to the electron transport chain.: This step is right. The electron carriers feed the chain.
• Correct: Oxygen accepts the electrons at the start of the chain and is converted into CO2.: This is the error. Oxygen accepts electrons at the end of the chain and forms water, not CO2.

Glycolysis needs NAD+ to accept electrons, and a cell holds only a little. Without oxygen, NADH cannot give its electrons to the chain. Passing them to pyruvate turns NADH back into NAD+, so glycolysis can keep netting 2 ATP per glucose.

• Correct: It regenerates the NAD+ that glycolysis needs to keep running: Correct. The lactate step recycles NAD+.
• Lactate is itself a form of stored ATP: Lactate is a three-carbon fuel molecule, not ATP. It can be turned back into pyruvate later.
• It moves pyruvate into the mitochondrion faster: The lactate step happens in the cytosol and keeps pyruvate there. It does not move anything into the mitochondrion.
• It releases CO2, which drives the electron transport chain: Pyruvate and lactate both have three carbons; no CO2 is released. And CO2 does not drive the electron transport chain.

Lactate leaves muscle quickly. Heart muscle and other cells oxidize it, and the liver rebuilds glucose from it, so blood lactate is back to resting within an hour or so. Soreness that peaks one to two days later comes from small injuries inside the muscle and the repair that follows.

• Her friend is right: lactate stays in muscle until the soreness passes: Lactate does not stay trapped. It returns to resting levels within an hour or so, long before the soreness peaks.
• Correct: Lactate was cleared within an hour or so; the soreness comes from small injuries inside the muscle and their repair: Correct. The timing rules out lactate; fiber damage and repair explain delayed soreness.
• The soreness means her muscles made no lactate at all during the workout: Hard exercise does raise lactate production. It is simply cleared long before the soreness appears.
• Lactate turned into CO2 that is still held inside the thigh muscles, and this trapped gas is what now causes the pain: Lactate can be turned back into pyruvate and oxidized, but CO2 diffuses out and is breathed away; it does not cause soreness days later.

The citric acid cycle and electron transport chain run in mitochondria, and together they raise the yield from 2 ATP per glucose to about 30–32. A cell that works nonstop, like a heart cell, has many mitochondria and depends on these aerobic stages.

• Glycolysis happens inside mitochondria, so more of them split glucose faster: Glycolysis runs in the cytosol, not in mitochondria.
• Mitochondria store ready-made ATP that heart cells draw on during each beat: Cells hold only a small amount of ATP and rebuild it constantly. Mitochondria make ATP; they are not storage tanks.
• Correct: Mitochondria run the aerobic stages, which yield about 15 times more ATP per glucose: Correct. More mitochondria mean more capacity for the high-yield aerobic stages.
• Mitochondria make glucose for the heart from the oxygen in blood: Mitochondria break fuel down; they do not build glucose, and oxygen is not a raw material for glucose.