Lecture: Cellular Respiration
Lecture script · about 15–20 minutes spoken · Week 6
Opening
Picture two runs on the same afternoon. In the first, you sprint up a long flight of stairs, and by the top your thighs burn and you're gasping. In the second, you jog slowly around a park, and you could talk the whole way. Both runs were paid for in ATP. So why does one burn out fast while the other feels like it could last all afternoon? By the end of this lecture, you'll know.
These ideas show up well beyond this course. A nursing student wondering why someone breathes harder during exercise is looking at them. So is a baker waiting for dough to rise, and so is an athlete planning a sprint workout against a long slow run. Each one is watching cells turn sugar into ATP.
Here's where we are. In Week 1 you learned to read a controlled experiment. Week 3 gave you glucose. Week 4 toured the cell and its mitochondrion. Week 5 showed you ATP and the enzymes that run every reaction. In this lecture I'll follow one glucose molecule through four stages, in order, and name the place each stage happens. Then we'll see what a cell does when oxygen runs out, and we'll finish with the yeast balloons from your lab. Next week, in Week 7, you meet photosynthesis, the near-mirror image of this pathway.
1. Respiration is not breathing
When you gasp at the top of the stairs, it's natural to call that respiration. In everyday speech, it is. Biologists use the word for something deeper inside you.
Here's the term. Cellular respiration is the chemistry inside cells that breaks down glucose with oxygen, releasing carbon dioxide and water and using the released energy to recharge ATP. It is not breathing, and nearly every living thing, plants included, does it day and night.
Think of your lungs as a delivery truck and your cells as the factory. Breathing moves gas between the air and your blood. Cellular respiration happens inside each cell. It uses the oxygen breathing delivers, and it makes the carbon dioxide breathing carries away. Two more words help here. Aerobic describes a process that uses oxygen. Anaerobic describes a process that runs without oxygen.
Pause and try this. A houseplant sits on a windowsill at midnight. Is it doing cellular respiration?
Think about whether its cells still need ATP in the dark.
Here's the answer. Yes, it is, because plants have mitochondria as well as chloroplasts, and their cells respire around the clock to keep their ATP charged.
2. The recipe card
A recipe card tells you what goes in and what comes out before it explains the steps. Cellular respiration has one too. In words, glucose plus oxygen becomes carbon dioxide plus water plus ATP. Written out with numbers, one glucose and 6 oxygen molecules become 6 carbon dioxide molecules, 6 water molecules, and ATP.
Read it forward. The glucose comes from your food, and the oxygen comes from the air you breathe in. Now read the products. The carbon dioxide is what you breathe out. The water joins the water already in you. And the ATP is the point.
Every atom that goes in comes out. Glucose brings in 6 carbons, and the 6 carbon dioxide molecules carry 6 carbons out. Glucose brings in 12 hydrogens, and 6 waters with 2 hydrogens each carry 12 out. The oxygen atoms balance too, 18 on each side.
Here's a surprising use for that recipe. When someone loses a pound of fat, where does it go? Most of it leaves through the lungs, as the carbon dioxide breathed out.
Pause and try this. In that recipe, which product is the reason a cell bothers to run it?
Think about what a cell spends on every job.
Here's the answer. It's the ATP, because that's the charged energy currency the cell spends; the carbon dioxide and water are what's left over.
3. The mitochondrion, read for its job
Picture a balloon pushed inside a slightly bigger balloon, with the inner one crumpled into deep folds. That's a mitochondrion. There's a smooth outer membrane on the outside. Inside it sits a folded membrane. And inside that is a fluid-filled space.
Here are the terms. The inner mitochondrial membrane is the folded inner membrane of a mitochondrion; it carries the electron transport chain, and its folds add surface for that job. Those folds are the cristae you met in Week 4. The mitochondrial matrix is the fluid-filled space inside the inner membrane, where pyruvate oxidation and the citric acid cycle run.
Structure decides function here, as it does every week. More folds mean more membrane, and more membrane means more room for the chain.
Pause and try this. Which part of the mitochondrion carries the electron transport chain, and why does folding it help?
Picture the crumpled inner balloon.
Here's the answer. The inner membrane carries the chain, and folding it packs more surface into a small space, so there's room for more chains.
4. Glycolysis: split the sugar
Picture a log too big for a small stove. Before you burn it, you split it. A cell splits glucose the same way, before the sugar ever reaches a mitochondrion.
Here's the term. Glycolysis is the first stage of cellular respiration, in the cytoplasm, which splits one glucose into two pyruvate molecules and nets 2 ATP and 2 NADH without using oxygen. Pyruvate is the three-carbon molecule glycolysis makes, two from each glucose.
That NADH needs a word. An electron carrier is a molecule that picks up electrons in one step of a pathway and delivers them to a later step. NADH is the loaded form of the carrier, and NAD-plus is the same carrier, empty. Think of an empty delivery van and the same van, loaded.
Hold onto three facts. Glycolysis runs in the cytoplasm. It needs no oxygen. And it runs in nearly every cell, with or without a mitochondrion.
Pause and try this. Glycolysis splits one glucose. How many pyruvate come out, and does the cell need oxygen for it?
Count the halves of the split log.
Here's the answer. Two pyruvate come out, and no oxygen is needed, because glycolysis is anaerobic and oxygen only acts at the very end of the pathway.
5. Pyruvate oxidation: crossing in
Think of a relay race. The first runner hands off the baton, and the baton crosses into the next lane. Pyruvate is the baton, and the next lane is inside the mitochondrion.
Here's the term. Pyruvate oxidation is the second stage of cellular respiration, in which each pyruvate enters the mitochondrion and is converted to acetyl-CoA while a carbon leaves as carbon dioxide. Acetyl-CoA is the molecule pyruvate becomes, and it carries the remaining carbon into the next stage.
This stage is short, so it's easy to skip. Please don't. It's where the fuel crosses into the mitochondrion, and it's where the first carbon dioxide of the pathway leaves.
Pause and try this. One glucose made 2 pyruvate. How many carbon dioxide molecules leave in pyruvate oxidation?
One carbon walks out of each pyruvate.
Here's the answer. Two leave, because each of the 2 pyruvate gives off 1 carbon dioxide as it becomes acetyl-CoA.
6. The citric acid cycle: loading the vans
Picture a merry-go-round. Riders climb on, ride around, and step off, and the ride is ready for the next rider. The third stage works like that.
Here's the term. The citric acid cycle is the third stage of cellular respiration, also called the Krebs cycle, which runs in the mitochondrial matrix, releases carbon dioxide, makes 2 ATP or GTP per glucose, and loads the carriers NADH and FADH-two. It's named after Hans Krebs, who worked out its steps with Johnson in 1937. FADH-two is a second loaded electron carrier, filled in this cycle.
Notice how little ATP the cycle makes. Its real job is loading the vans. One glucose turns the cycle twice. By the end, every carbon has left as carbon dioxide: 2 in pyruvate oxidation and 4 in the cycle, so 6 in all, one for each carbon in glucose.
Pause and try this. A classmate places the citric acid cycle in the cytoplasm, right next to glycolysis. Where does it really run?
Think inside the inner balloon.
Here's the answer. It runs in the mitochondrial matrix, inside the inner membrane; only glycolysis runs in the cytoplasm.
7. The electron transport chain: cashing in
Picture loaded vans pulling up to a warehouse dock. They unload, the boxes ride a conveyor, and at the end of the line someone has to take the last box, or the whole line jams. The fourth stage is that dock.
Here's the term. The electron transport chain is the fourth stage of cellular respiration, a chain of carriers built into the inner mitochondrial membrane that receives electrons from NADH and FADH-two and makes the most ATP of any stage. The final electron acceptor is the molecule waiting at the end to take the electrons. In cellular respiration it's oxygen, which joins with hydrogen to form water.
That's where the oxygen from your last breath goes. It ends up in water, not in carbon dioxide. Now say the whole order with me: glycolysis in the cytoplasm, then pyruvate oxidation in the mitochondrion, then the citric acid cycle in the matrix, then the electron transport chain on the inner membrane.
Pause and try this. Which stage makes the most ATP, glycolysis or the electron transport chain?
Glycolysis is taught first, but is it the biggest?
Here's the answer. The electron transport chain makes the most by far, because glycolysis nets only 2 ATP per glucose.
8. The payoff, in words
So how much ATP does one glucose buy? The honest answer is a range. Current estimates say about 30 to 32 ATP per glucose. Older estimates say 36 to 38. The total depends on the cell and on how the chain runs, so it's an estimate, and you learn it as a range in words.
What you should hold firmly is the shape. A small amount comes from glycolysis, a little from the cycle, and the most from the chain. Think of it like a commute time. Nobody gives you one exact number for a commute; they give you a range, and that's honest, not vague.
Pause and try this. One source says 36 to 38 ATP per glucose and another says about 30 to 32. Is one of them wrong?
Think about older and newer estimates.
Here's the answer. Not exactly; the count is an estimate that has been revised over time, so the older range and the current range are both reports of the same uncertain total.
9. Fermentation: the backup
Back to the sprint. Halfway up the stairs, your muscle cells burn ATP faster than your blood can bring oxygen. With no oxygen at the end of the chain, the vans can't unload, and the empty NAD-plus that glycolysis needs runs short.
Here's the term. Fermentation is a low-energy backup for when oxygen runs out: glycolysis plus a side path that turns NADH back into NAD-plus, so glycolysis can keep running and making its 2 ATP per glucose. There are two kinds. Lactic acid fermentation turns pyruvate and NADH into lactic acid and NAD-plus. It runs in hard-working muscle short of oxygen and in red blood cells, which have no mitochondria. Alcoholic fermentation turns pyruvate into ethanol and carbon dioxide. Yeast use it, and its carbon dioxide is the gas that raises bread.
Each kind yields only glycolysis's 2 ATP, against about 30 to 32 for the full pathway. That's why the sprint can't last, and why the slow jog, which stays aerobic, can.
Pause and try this. A red blood cell has no mitochondria. Where does its ATP come from?
Which stages need a mitochondrion?
Here's the answer. It comes from glycolysis, kept running by lactic acid fermentation, because without mitochondria the cell has no cycle and no chain.
10. The yeast balloons
Now let's read your lab's experiment, with illustrative numbers made up for this course. Four bottles hold the same yeast in the same warm water, with 0, 1, 2, and 4 teaspoons of sugar. A balloon covers each bottle, and at 30 minutes you measure its circumference.
The readings are 0, 8, 14, and 18 centimeters. The no-sugar bottle reads 0. That's the control: no fuel, no gas. Now subtract each reading from the next. 8 minus 0 is 8. 14 minus 8 is 6. 18 minus 14 is 4. The gains shrink from 8 to 6 to 4. Doubling the sugar from 2 teaspoons to 4 adds only 4 centimeters. That's diminishing returns, a pattern in which each added amount of an input produces a smaller gain than the one before.
The gas is carbon dioxide from alcoholic fermentation. The sugar is the independent variable, the balloon size is the dependent variable, and the evidence supports the hypothesis that more sugar makes more gas. It doesn't prove it. What matters is the trend and the zero control, never any one centimeter.
Pause and try this. Why is the 0-teaspoon bottle, the one with no gas at all, the most important bottle on the table?
Think about what it rules out.
Here's the answer. It's the control, because it shows the gas comes from fermenting the sugar and not from the yeast or the water alone.
Closing
Let me pull it together. Cellular respiration is chemistry in your cells, not air in your lungs, and nearly every living thing does it. Glucose and oxygen go in; carbon dioxide, water, and ATP come out. The folded inner membrane carries the chain, and the matrix runs the cycle. Glycolysis splits glucose in the cytoplasm, without oxygen. Pyruvate crosses into the mitochondrion, the cycle loads the carriers, and the chain cashes them in, with oxygen waiting at the very end. The payoff is about 30 to 32 ATP, an estimate. When oxygen runs out, fermentation keeps glycolysis going for only 2 ATP. And the yeast balloons rise with more sugar, but with shrinking gains.
Next in this module come the slide deck and the Readings & Resources page, then the lecture tutorial, the practice set, and the quiz. Next week, in Week 7, Photosynthesis, you'll watch a plant build the very sugar this pathway takes apart.
Built with The Course Maker · samples.thecoursemaker.com