Unit 3 - Cellular Respiration

Unknown Speaker

Alright, Grady, ready to jump into cellular respiration? I feel like this is the unit where kids start thinking, “Wait, are cells just tiny power plants?” And honestly… kinda, yeah! But instead of coal or wind, it’s glucose and oxygen fueling the whole operation.

Grady Killpack

Yeah, and don’t forget, it’s all about that ATP payout at the end—cells are just after those little energy coins. So, maybe let’s start by painting the big picture for folks: cellular respiration is basically the breakdown of organic molecules—mainly glucose—to generate ATP. And, man, this is like every cell’s day job, from bacteria to elephants.

Unknown Speaker

Exactly! And if you’re still picturing mitochondria as those bean-shaped “powerhouses,” let’s prove just how powerful they are. Let’s set the equation: glucose plus oxygen gives carbon dioxide, water, and a bunch of energy—ATP. And heat, too, but most students like to skip that part. We’re basically making cells sweat.

Grady Killpack

Yeah, can’t leave out the heat! That’s the second law of thermodynamics right there—energy transformations aren’t 100% efficient, and cells aren’t magic, so there’s always a little energy lost as heat. Like those old diesel trains we saw on that practice test, right? Only about a third of the energy from burning diesel goes into moving the wheels forward—the rest leaks out as heat. Cells face the same deal.

Unknown Speaker

And, since you brought up mitochondria—I just want to throw in a quick mitochondria structure run-down for context. We’ve got the inner and outer membranes, the cristae, and the matrix. The inner membrane is where the real magic happens: that’s where the electron transport chain is, and it’s folded into cristae to pack in more action—just like squeezing more cheerleaders on a tiny sideline, honestly.

Grady Killpack

Ha! Love the cheer analogy. So, in a nutshell, cellular respiration has three main stages—well, sometimes you’ll see four if you count the link reaction—but for classic AP Bio, we’re usually looking at glycolysis, pyruvate oxidation, citric acid cycle, and then oxidative phosphorylation with the electron transport chain and chemiosmosis. Should we walk through each?

Unknown Speaker

Yes! So first up: glycolysis. This is like the starting whistle—it happens in the cytosol, not inside the mitochondria yet. Basically, one glucose—so a six-carbon sugar—gets split in half into two pyruvate molecules, which are three carbons each. Along the way, you net two ATP and two NADH. And that’s important—because the NADH is going to matter later on.

Grady Killpack

And let’s not forget: it doesn’t need oxygen. That’s why it’s considered an anaerobic process. Whether you’re an ancient microbe or doing wind sprints—glycolysis runs no matter what. The big thing is, there’s two phases: the investment phase—where you actually use a couple ATP, and the payoff phase, which gives you four ATP overall. Net gain is just two, but hey, cells take what they can get.

Unknown Speaker

Total bargain, honestly. And students always ask, “What about the NADH?” Well, NAD+ grabs those electrons from glucose—gets reduced to NADH—and that’s going to be shuttle service for electrons later to the electron transport chain. If you’re thinking, “Wait, didn’t we just talk about electron carriers in photosynthesis last episode?”—yep! Same kind of idea, just with different players.

Grady Killpack

And, here’s the quick check for AP folks—glycolysis makes pyruvate, ATP, and NADH in the cytosol. No mitochondria needed yet. Net two ATP, and two NADH per glucose. Got it? Good. Because now it’s time for pyruvate to head into the mitochondria.

Unknown Speaker

So pyruvate enters the mitochondria—but only if oxygen’s available, right? There, it’s converted into acetyl CoA, which is just a two-carbon compound. During this conversion, you also lose a carbon as CO2 and make more NADH. Then—boom—we are officially in the matrix and ready for the citric acid cycle, also called the Krebs cycle.

Grady Killpack

I like to picture the Krebs cycle as a carousel, with acetyl CoA hopping on for a couple of loops around. Each turn, more electron carriers get loaded up: three NADH, one FADH2, one ATP, and two CO2 per turn. But since each glucose gives you two acetyl CoA, every glucose runs two loops around the carousel. Totals? Six NADH, two FADH2, two ATP, and four CO2. The ATP here comes from substrate-level phosphorylation—enzyme just hands a phosphate to ADP, nothing fancy.

Unknown Speaker

Super important: all those NADH and FADH2 molecules are about to cash in big time. It’s like filling your backpack with tokens at the arcade, and the real jackpots are in the next stage. Quick reminder for test-takers: the citric acid cycle’s in the mitochondrial matrix, and one key job is oxidizing those carbons fully to CO2—done and dusted, those carbon atoms go out as waste.

Grady Killpack

Alright, buckle up, because we’re finally at the electron transport chain—this is where most of the ATP comes from. It’s all happening in the mitochondrial inner membrane, where those cristae fold up to make room for more action. So all those NADH and FADH2 dump their electrons into the ETC—that’s a chain of proteins like cytochromes, all passing electrons down the line, kinda like a bucket brigade.

Unknown Speaker

Every time electrons move down the chain, protons are pumped from the matrix into the intermembrane space. That creates a big proton gradient—a kind of pent-up energy just begging to be released. Then, protons flow back in through the enzyme ATP synthase. The flow spins ATP synthase around like a windmill (kids love that analogy), letting it tack a phosphate onto ADP to make ATP. Fancy term for this: chemiosmosis. It’s like water turning a turbine to generate electricity—except it’s protons spinning enzymes to make ATP.

Grady Killpack

And don’t miss this: oxygen’s the final electron acceptor at the end of the chain. It grabs electrons and hydrogen ions to form water. No oxygen? The whole system backs up, the ETC stops, and you’re stuck. So, if you ever wondered why you need to breathe, this is literally it—so your cells don’t run out of ATP. That’s why animals, including us, need oxygen at all. The ETC and chemiosmosis together are called oxidative phosphorylation, and this step makes about 26–28 ATP per glucose molecule. That’s most of the energy right there.

Unknown Speaker

So, in summary: glycolysis and the citric acid cycle make a little ATP, but their main job is loading up NADH and FADH2, who pass off their electrons in the ETC, which powers the proton gradient so ATP synthase makes ATP—like, a lot of ATP. Oh, and can’t forget: that gradient is why the pH is different inside the matrix versus the intermembrane space—more protons outside, so it’s more acidic.

Grady Killpack

Let’s hit what happens if there’s no oxygen around—because, contrary to popular belief, things don’t just stop. Glycolysis chugs along, but those NADH molecules have nowhere to dump their electrons—so cells need to recycle NAD+ or glycolysis just stalls out. That’s where fermentation comes in! Think lactic acid fermentation in your muscles, or alcohol fermentation in yeast.

Unknown Speaker

Yeah, lactic acid fermentation is what gives you that burning sensation in your muscles when you’re running sprints, or doing burpees until your legs feel like Jell-O. Without enough oxygen, your muscle cells switch to fermentation, turning pyruvate into lactate. That lets NADH give its electrons to pyruvate and be recycled into NAD+, so glycolysis can keep going, even though you only get a couple ATP per glucose. Not much—but it’s better than nothing in a pinch!

Grady Killpack

Same deal with yeast, except they convert pyruvate to ethanol and CO2 during alcohol fermentation. Overall—these pathways don’t make extra ATP; they just keep glycolysis running by recycling electron carriers. That’s why fermentation ATP yield is so low—about two ATP per glucose compared to the big thirty-ish in aerobic respiration.

Unknown Speaker

I think a lot of students forget that fermentation pathways are really emergency plans—cells would much rather go full aerobic, but when it comes to survival, they’ll take what they can get. Plus, the buildup of lactic acid has to be cleared later, like by your liver. Otherwise, you’d get lactic acidosis, and that’s not fun for anyone.

Grady Killpack

Before we sign off, let’s tie this back to photosynthesis. If you listened to our last episode, you’ll remember chloroplasts making ATP and NADPH in the light reactions, then shuffling those to the Calvin cycle. The big parallel? Both photosynthesis and cellular respiration use electron carriers, electron transport chains, and build proton gradients that power ATP synthase—but the direction of energy flow is totally opposite. Photosynthesis stores energy in glucose, respiration releases it from glucose.

Unknown Speaker

Exactly! Plus, the mitochondria and chloroplasts are like rival power plants—mitochondria break down glucose to make ATP; chloroplasts capture light to build glucose. ATP production always comes down to gradients and enzyme magic—doesn’t matter if you’re a leaf or a liver cell.

Grady Killpack

Love that. And hey, if you got confused by ATP numbers or the names of stages, don’t stress—we’ll have more review episodes. Next time, maybe we’ll even dive into the wilder side of metabolism, or take some of those tricky practice test questions we saw today and break ’em down step by step.

Unknown Speaker

Let’s do it! Until then, keep your mitochondria happy, eat your carbs, and don’t fear the burn. Thanks for learning with us, Grady!

Grady Killpack

Thanks, Chloe! Catch you all next episode—bye, everybody!