AP BIOLOGY · UNIT 3 · CELLULAR ENERGETICS

How a cell turns food into ATP

Every cell in your body is pumping, building and moving right now, and every one of those jobs is paid for in ATP. Respiration remakes it, releasing energy from glucose and fat a step at a time. This is how it works with oxygen and without, where the protons come in, and how to measure it in a tube of peas.

AP Biology 3.53 lessons10 chaptersdry lab

Start learning Run the dry lab Teach it

Glucose and oxygen in, carbon dioxide and water out: the mitochondrion makes ATP, and every job in the cell spends it.

By the end you can tap a goal to go to its chapter

Explain how fermentation and cellular respiration use the energy in macromolecules to make ATP, in every form of life 3.5.A chapter 1 →
Compare aerobic respiration with fermentation: where each happens, what each needs, what each makes and how much ATP 3.5.A chapter 1 →
Measure the rate of respiration with a respirometer, and graph and interpret the data 3.5.A chapter 1 →
Describe glycolysis, the Krebs cycle and the link between them: what goes in, what comes out and where each happens 3.5.A chapter 1 →
Explain how fermentation lets glycolysis go on without oxygen by giving NAD⁺ back 3.5.A chapter 1 →
Explain how electrons passed along the electron transport chain build a proton gradient across the inner mitochondrial membrane 3.5.B chapter 8 →
Explain how chemiosmosis through ATP synthase makes ATP, why oxygen is needed at the end of the chain, and how uncoupling makes heat 3.5.B chapter 8 →
Compare lipids and carbohydrates as fuels, and say where fats and proteins join the pathway 3.5.A chapter 1 →

01Learn

Ten chapters. Read one, play its figure, and make each prediction before you open the answer. The film after chapter 2 follows one glucose through a muscle cell; then try keeping a muscle cell running yourself.

1Fuel in, ATP out
2With oxygen and without
3A rate you cannot see
4Taking hydrogen away
5Two ATP in, four out
6Giving NAD+ back
7The carbon leaves as carbon dioxide
8Down the chain
9Protons through the turbine
10Fat or sugar?

3.5.A Respiration releases the energy in food as ATP

Fuel in, ATP out

Cell respiration is how a cell makes ATP, using energy released from carbon compounds. It happens in every living cell, all the time, because ATP is used all the time and only a little is kept (ATP and coupling, in the enzymes guide). That includes plants: a leaf cell respires by day and by night.

The fuels are called respiratory substrates. The main two are glucose and fatty acids, but a cell can use a wide range of carbon compounds, amino acids among them when food is short. Glycogen in liver and muscle, and starch in a plant, are broken down to glucose first.

Respiration releases the energy a step at a time, each step helped by its own enzyme at body temperature. Burning glucose is combustion. It starts with glucose and oxygen and ends with carbon dioxide and water, as respiration does, and it releases the same total energy. But combustion releases it all at once, as heat and light, and makes no ATP. One is controlled and useful to a cell, the other is not.

Respiration is not gas exchange. Respiration is chemistry inside cells. Gas exchange is oxygen and carbon dioxide diffusing across a surface, in the lungs, the gills or a leaf, with no reaction and no energy released. The carbon dioxide you breathe out was made in the mitochondria of your cells, and it only leaves through your lungs. Breathing moves the air; that is not respiration either.

The map under this chapter is the whole of respiration on one page: where each stage happens, what goes in, what comes out and where the ATP is made. Come back to it whenever a stage is new.

Chapters 4 to 10 open up each stage, and each starts with a small copy of the map, marking where you are.

Watch out “Respiration” in everyday English means breathing. In biology it is the chemistry that makes ATP in every cell. Breathing moves air; gas exchange moves gases across a surface; neither releases energy.

Watch out Respiration does not “make energy”. Energy is released from the fuel and transferred to ATP; none is created.

A leaf in bright light takes in carbon dioxide and gives out oxygen. Is it respiring?

Three different things. What goes in, what comes out, and what is released?

Where each stage happens, what goes in and what comes out. Tap a stage to read what happens there.

3.5.A With oxygen and without

With oxygen and without

Both kinds of respiration start the same way, in the cytosol. A glucose molecule is split into two molecules of pyruvate in a process called glycolysis, which releases enough energy to make a small amount of ATP. After that the routes divide, according to whether there is oxygen, and they differ most in their yield of ATP for each glucose.

Aerobic respiration goes on with oxygen. The pyruvate goes into a mitochondrion and is broken down completely, to carbon dioxide and water, and a large amount of ATP is made. The word equation is glucose + oxygen → carbon dioxide + water. It needs mitochondria and oxygen, and it can use fatty acids and other compounds as well as glucose.

Anaerobic respiration in humans stops early. When there is not enough oxygen, the pyruvate stays in the cytosol and is turned into lactate. The word equation is glucose → lactate. It needs no mitochondria, no oxygen, and makes no carbon dioxide, but only glucose can be used: fatty acids cannot be respired without oxygen.

When does a human cell respire anaerobically? When muscles work so hard that the blood cannot bring oxygen fast enough: a short sprint, a heavy lift. Anaerobic respiration gives ATP at once, without waiting for oxygen, but only for a short time. Muscles do not switch from one route to the other. Aerobic respiration goes on, and anaerobic respiration adds to it.

Why make lactate at all, rather than stopping at pyruvate? Chapter 6 explains.

A sprinter’s leg muscles run short of oxygen. Per glucose, how much ATP does anaerobic respiration make compared with aerobic?

Remember it Anaerobic in humans is LOW

LLactatethe waste product
OOnly 2 ATPper glucose
WWithout oxygenin the cytosol

Watch out Glycolysis happens in the cytosol in both routes; only what follows it happens in the mitochondria.

Watch out The waste product in human muscle is lactate, not carbon dioxide. People often call it lactic acid. Lactate is not what makes muscles ache the next day.

One glucose in a muscle cell. Switch the oxygen supply and follow the pyruvate.

Aerobic and anaerobic respiration in humans
	Aerobic	Anaerobic
Substrates	glucose, fatty acids and other carbon compounds	glucose (carbohydrate) only
Oxygen needed?	yes	no
ATP per glucose	about 30 to 32	2
Waste products	carbon dioxide and water	lactate
Where in the cell	glycolysis in the cytosol, the rest in the mitochondria	the cytosol
Mitochondria needed?	yes	no
Word equation	glucose + oxygen → carbon dioxide + water	glucose → lactate

Watch one glucose give up its energy in a muscle cell. Then turn the oxygen off.

With oxygen, glycolysis splits each glucose in the cytosol and its two pyruvate go into the mitochondrion: about 30 ATP in all. Without oxygen, glycolysis alone: 2 ATP and lactate.
Cut the supply: how long does a working fiber's ATP last?
The sprint: the oxygen cannot keep up. Where does the extra ATP come from?
No oxygen and no lactate made: watch the NAD⁺.

3.5.A Measuring the rate of respiration

A rate you cannot see

Respiration happens inside cells, so you cannot watch it. You measure it from what goes in or comes out: oxygen taken up, carbon dioxide given out, or fuel used. The rate of respiration is the amount changed divided by the time taken, and for living material it is given per gram per minute as well, so that samples of different mass can be compared.

A class can do it three ways.

Oxygen uptake in a respirometer: germinating seeds in a sealed tube, with potassium hydroxide to absorb the carbon dioxide they give out, and a drop of colored liquid in a capillary whose movement shows the change in gas volume.
Carbon dioxide output: a gas sensor or data logger in a sealed chamber, or hydrogencarbonate indicator changing color. With yeast, the rise of a dough or the gas collected over a sugar solution works too (the dry lab's dough mode).
Fuel used: the fall in the glucose in a yeast culture, measured with test strips.

The capillary is a long thin cylinder, so the volume of gas is πr² × the distance the drop moved, where r is half the bore. A bore of 1 mm holds 0.785 mm³ for every mm.

A second tube, the same in every way but with glass beads in place of the seeds, is the control. Anything that moves its drop, such as the gas warming or cooling or the air pressure changing, is not respiration, so its movement is taken off the seeds’.

What changes the rate? Temperature: respiration is enzymes at work, so it speeds up as it warms until they begin to denature (topic 3.2). How far germination has gone: dry seeds hardly respire, and soaked ones respire more each day. The mass of the seeds, and the kind of seed. With yeast: the sugar it is given, the temperature, and how much oxygen there is. The seeds are kept in the dark so that they cannot photosynthesize and hide the gas changes.

In an experiment on any of these, the one thing you change on purpose is the independent variable (the temperature, say), the rate you measure is the dependent variable, and everything else you hold the same is a controlled variable: the mass and age of the seeds, the volume of the tube, the time. You may also be asked to work out a rate from someone else’s numbers: the steps are the same, as in the second figure.

Watch out The drop does not move because the seeds give out carbon dioxide: that is removed. It moves because of what the seeds take up.

Watch out Germinating seeds are not “breathing”. They have no lungs; their cells respire, and the gases cross their surfaces by diffusion.

In the respirometer the seeds respire and the potassium hydroxide absorbs their carbon dioxide. Which way does the drop move?

Tap a part of the respirometer.

Try it on the bench The dry lab has this respirometer set up and ready: run it, take readings and work out the rate.

Read both drops at the start and the end.
Take off the control’s movement.
Turn distance into volume with the bore.
Divide by time and by mass.

3.5.A Redox and the hydrogen carrier NAD

Taking hydrogen away

In respiration, energy is released from fuel molecules by taking hydrogen off them. To see why, you need two words. Oxidation is the loss of electrons, and reduction is the gain of electrons. They always happen together: what one molecule loses, another gains. A reaction that does both is a redox reaction.

A hydrogen atom is a proton and an electron. So when hydrogen is removed from a molecule, which is called dehydrogenation, it loses electrons too, and has been oxidized. The enzymes that do it are dehydrogenases.

The hydrogen has to go somewhere. It is accepted by NAD⁺, a molecule built from nucleotides, like ATP. When NAD⁺ accepts hydrogen it is reduced, and is then called NADH. NADH carries the hydrogen, and the energy that comes with it, to where it is used (the electron transport chain, chapters 8 and 9), and is turned back into NAD⁺ there. So NAD⁺ is a hydrogen carrier.

A cell has only a little NAD⁺, so it must be unloaded and used again and again, as ATP is. Every stage of respiration that follows either loads NAD⁺ or depends on NAD⁺ being given back (chapter 6).

Why does it matter? A large part of the energy in a fuel is in its hydrogen and the electrons that go with it. Respiration is, step by step, taking hydrogen off the fuel and passing it to oxygen: that is the role of hydrogen, and of oxygen, in the release of energy.

Remember it Oxidation and reduction

OilOxidation Is Lossof electrons
RigReduction Is Gainof electrons

Watch out NADH carries hydrogen, protons and electrons together, and the energy that comes with them. It is not ATP and it is not “energy” itself.

A dehydrogenase takes two hydrogens off a molecule of fuel. What has happened to that molecule?

A substrate, an enzyme, and NAD⁺.
Hydrogen is removed: dehydrogenation. Each hydrogen is a proton and an electron.
The substrate is oxidized; NAD⁺ is reduced. A redox reaction.
NADH carries the hydrogen away, and NAD⁺ comes back for more.

3.5.A Glycolysis

Two ATP in, four out

Glycolysis turns one glucose into two pyruvate, in the cytosol, in about ten steps. Each step has its own enzyme, and the product of one step is the substrate of the next: it is a metabolic pathway (topic 3.3). The course asks for the four stages, not for the names of the molecules in between.

Phosphorylation: two ATP each give a phosphate group to the glucose. That costs 2 ATP, but a sugar carrying two charged phosphates is made reactive and ready to split.
Lysis: the 6-carbon sugar with its two phosphates splits into two 3-carbon sugars, each with one phosphate.
Oxidation: hydrogen is taken off each 3-carbon sugar and accepted by NAD⁺, so two NADH are made for the glucose. The energy released lets each 3-carbon sugar take on a second phosphate, a free one from the cytosol, not one from ATP.
ATP formation: each 3-carbon sugar passes its two phosphates to ADP, making ATP directly, and ends as pyruvate.

Glycolysis spends 2 ATP at the start. How many does it make by the end, from one glucose?

Remember it The four stages of glycolysis, in order

PenguinsPhosphorylation2 ATP spent
LoveLysis6 carbons split into 3 and 3
OrangeOxidationhydrogen to NAD⁺
AnchoviesATP formation4 ATP made

Watch out Glycolysis does not need oxygen, and it does not happen in the mitochondrion.

One glucose, in the cytosol.
Phosphorylation: 2 ATP spent, the sugar made reactive.
Lysis: split into two 3-carbon sugars.
Oxidation: hydrogen to NAD⁺, making 2 NADH.
ATP formation: 4 ATP made, so 2 gained.
Ten steps, each with its own enzyme: two pyruvate, on to the link reaction.

3.5.A Fermentation: giving NAD back

Giving NAD⁺ back

Glycolysis needs NAD⁺. Every glucose that goes through it turns two NAD⁺ into NADH. A cell has only a little NAD⁺, so if NADH is not turned back into NAD⁺, glycolysis stops very quickly. Giving NAD⁺ back is called the regeneration of NAD⁺.

With oxygen, NADH is unloaded at the electron transport chain in the mitochondrion (chapters 8 and 9). Without oxygen it cannot be, so a cell needs another way to give NAD⁺ back.

In human muscle, pyruvate takes the hydrogen from NADH and becomes lactate. NAD⁺ is given back, so glycolysis can go on, with a net yield of 2 ATP per glucose. Making lactate releases no extra energy: its whole point is to give NAD⁺ back.

Yeast follow the same pathway as far as pyruvate. Without oxygen, each pyruvate loses a carbon as carbon dioxide, and what is left takes the hydrogen from NADH to become ethanol. NAD⁺ is given back, and the yield is again 2 ATP per glucose. Only the last step differs from humans, and so the final products do.

Bakers and brewers use what yeast make. In baking, yeast respire the sugars in the dough and the carbon dioxide forms bubbles that stretch it, so the dough rises; the ethanol evaporates in the oven. In brewing, yeast in a sugary liquid made from barley or grapes, with little air, make ethanol, which is kept, while the carbon dioxide escapes or is kept in for fizz. Early in a dough or a brew there is oxygen and the yeast respire aerobically, so carbon dioxide is given off from the start; as the oxygen runs out, anaerobic respiration takes over. Anaerobic respiration in yeast is often called fermentation.

Without oxygen, why does a muscle cell turn pyruvate into lactate instead of leaving it as pyruvate?

Watch out Yeast without oxygen make ethanol and carbon dioxide, not lactate; humans make lactate, not ethanol or carbon dioxide.

Watch out Lactate is not a poison that must be got rid of straight away. It is carried in the blood and later used as a fuel, or made back into glucose in the liver.

Glycolysis with no oxygen. Watch the NAD⁺ in the store, then see what each cell does about it.

3.5.A The link to the Krebs cycle, and the cycle

The carbon leaves as carbon dioxide

With oxygen, pyruvate is carried from the cytosol into the matrix, the fluid inside the inner membrane of the mitochondrion. Two sets of reactions follow, one after the other.

First, each pyruvate (3C) is decarboxylated, which takes a carbon off as carbon dioxide, and oxidized: hydrogen is removed and accepted by NAD⁺, making NADH. The 2-carbon acetyl group that is left goes on into the Krebs cycle. Fatty acids and other macromolecules are broken down to acetyl groups too, and join the same route (chapter 10). All of this happens twice for each glucose, once for each pyruvate.

In the Krebs cycle, the acetyl group is added to a 4-carbon compound. The 6-carbon compound that results is taken round a cycle of reactions that ends by remaking the 4-carbon compound. Round the cycle, two carbons leave as carbon dioxide, and the compound is oxidized four times: three times the hydrogen goes to NAD⁺, and once to FAD, another hydrogen carrier, making FADH₂. One ATP is made. The cycle turns twice for each glucose: 4 carbon dioxide, 6 NADH, 2 FADH₂ and 2 ATP in all. You do not need the names of the compounds or the enzymes.

Each turn of the Krebs cycle takes in a 2-carbon acetyl group. Why does the cycle not fill up with carbon?

Watch out The Krebs cycle makes little ATP directly, only one a turn. Its main product is NADH, which the chain turns into most of the ATP.

Watch out The carbon dioxide you breathe out comes from the carbons of the fuel, not from the oxygen you breathe in. The oxygen goes to water, at the end of the chain.

Pyruvate goes into the matrix.
Before the cycle: a carbon off as CO₂ (decarboxylation), hydrogen to NAD⁺ (oxidation). The acetyl group goes on.
Acetyl (2C) + a 4-carbon compound → a 6-carbon compound.
Round the cycle: two decarboxylations, four oxidations (three to NAD⁺, one to FAD), one ATP.
The 4-carbon compound is remade: a cycle.
Two turns per glucose: all six carbons gone as CO₂.

Pick a step (tap it, or Tab to it and press Enter), then pick the bin it belongs in.

0 of 8 sorted

Name the reaction: eight steps of respiration and four kinds of reaction. Which bin does each belong in, and what tells you?

3.5.B The electron transport chain

Down the chain

NADH is made in three places: glycolysis, the link reaction and the Krebs cycle. The electrons they carry all end up at the inner membrane of the mitochondrion, which is folded into cristae to give room for many copies of what happens there.

In that membrane sits the electron transport chain: a series of electron carriers, some of them proton pumps. A pair of electrons is passed from NADH to the first carrier, and the NADH is turned back into NAD⁺, which goes back for more hydrogen. That is the transfer of energy. FADH₂ from the Krebs cycle hands its electrons on too, one carrier further along, so it pumps fewer protons.

The electrons are passed from carrier to carrier, each holding them a little more tightly than the one before, so they always move on and never go back. A little energy is released at each step, like a ball dropping down a staircase. Oxygen, at the very end, holds them most tightly of all (chapter 9). You do not need to name the carriers: “the chain” and “proton pumps” are enough.

The pumps use that energy to move protons (hydrogen ions) from the matrix, across the inner membrane, into the narrow intermembrane space. They pile up there, so there are far more protons in the space than in the matrix: a proton gradient. The membrane does not let them leak back, so the gradient is a store of energy, like water held behind a dam. Chapter 9 lets it out.

Watch out The chain does not make ATP; it makes a proton gradient. And “proton” here means a hydrogen ion, not a hydrogen atom.

Electrons pass along the chain of carriers, releasing a little energy at each step. What is that energy used for?

NADH from all three stages arrives at the inner membrane.
It hands a pair of electrons to the first carrier, and becomes NAD⁺ again.
Each step down releases a little energy, used to pump protons. FADH₂ hands its electrons on one carrier later.
Protons pile up in the intermembrane space: a proton gradient.

3.5.B Chemiosmosis and ATP synthase

Protons through the turbine

Protons can return to the matrix through one protein only: ATP synthase. It spans the inner membrane, with a channel through it, a part that turns, and a head in the matrix where ATP is made.

As protons flow down their gradient through the channel, they turn the rotor. The turning changes the shape of the head, which joins ADP and phosphate into ATP. ATP synthase couples the energy released by the flow of protons to the phosphorylation of ADP. Making ATP with the energy of a proton gradient across a membrane is called chemiosmosis. In cellular respiration the whole process, the chain with the synthase, is called oxidative phosphorylation.

The chain needs one more thing at its end. Oxygen is the terminal electron acceptor: it takes the electrons from the last carrier, and protons from the matrix, and becomes water, called metabolic water. That empties the chain so that electrons keep flowing, and taking protons from the matrix helps to keep the gradient steep. About 26 to 28 of the 30 to 32 ATP from one glucose are made here, by ATP synthase.

So ATP is made in two ways. A little is made directly, in glycolysis and the Krebs cycle, when a phosphate is passed from a substrate to ADP: substrate-level phosphorylation. The rest is made by ATP synthase, with energy brought to the chain by the carriers: about 2.5 ATP for each NADH and 1.5 for each FADH₂. The ledger after the film adds it up, stage by stage.

Not every cell wants all of its energy as ATP. If protons leak back across the inner membrane by another route, without passing through ATP synthase, the electron transport chain still runs but the energy of the gradient is released as heat. This is uncoupling. Mammals that need to stay warm, such as newborns and hibernating animals, use it in their brown fat: their cells respire a lot of fuel and make a lot of heat.

This is not only a mitochondrion. The same kind of chain, with a proton gradient and ATP synthase, works in the thylakoid membranes of chloroplasts and in the plasma membrane of prokaryotes. Aerobic prokaryotes end their chain on oxygen, as we do. Anaerobic prokaryotes end it on other molecules instead, so the chain still runs and the gradient still makes ATP without oxygen.

How we know (extra) In 1961 Peter Mitchell suggested that the chain and the making of ATP were linked by a gradient of protons, not by some unknown high-energy chemical. Most biochemists doubted it for years. Experiments in the 1960s and 1970s, in which membranes given an artificial proton gradient made ATP, won them over, and he received the Nobel Prize in Chemistry in 1978.

Watch out ATP synthase is not a pump: protons flow through it, down their gradient, and that flow drives it. The pumps are in the chain.

Watch out Oxygen does not become carbon dioxide. It ends up in water. The carbon dioxide came from the fuel’s carbon, in the link reaction and the Krebs cycle.

A cell’s mitochondria run out of oxygen. What happens to the electron transport chain?

One way back for the protons: ATP synthase.
Protons flowing down the gradient turn it, and their energy joins ADP and phosphate: ATP.
Oxygen takes the electrons and protons at the end, making water.
Chemiosmosis: the gradient makes ATP, and oxygen keeps the chain flowing.

Watch the protons pile up, then turn the oxygen off and see what stops first. Then open the leak.

Every stage against the ATP it makes, directly or through the chain. Tap a stage to see it on the map.

3.5.A Lipids and carbohydrates as fuels

Fat or sugar?

Glucose and fatty acids are the two main fuels, and they are not equal. Lipids release about twice as much energy per gram as carbohydrates when they are respired: about 37 kJ in each gram against about 17.

They also take different routes. Carbohydrate goes through glycolysis, so it can be respired with oxygen or without. A fatty acid is cut into 2-carbon acetyl groups, which enter the Krebs cycle at once. Glycolysis is the one stage that works without oxygen, and a fatty acid never goes through it: no oxygen, no use for fat. Proteins can be used too: amino acids lose their nitrogen and the carbon skeleton joins the pathway as pyruvate or as an acetyl group.

In practice, muscles use fat for long, steady work, such as walking, a gentle jog or a migrating bird in flight, and carbohydrate for hard, fast work, when oxygen is short. That is why fat is the long-term store and glycogen the quick one (unit 1).

Gram for gram, fat releases about twice the energy of carbohydrate when respired. Why?

Watch out Fat cannot be used anaerobically: fatty acids enter at the Krebs cycle, which needs the chain, which needs oxygen. A sprint runs on carbohydrate.

Glucose and a fatty acid, side by side: what they are made of, and how each is respired.

Every stage in one film. Switch the fuel to a fatty acid, or the cell to yeast, then cut the oxygen.

The whole map again, with the word equations and the totals. Tap a stage to go over it.

02Dry lab: Respirometer: peas and dough

Germinating peas in a sealed tube use oxygen, and a colored drop creeps along a scale as they do. Change one thing, read the drop each minute, and work out how fast the peas respire. Or switch to dough and race five sugars.

Your questionHow does one factor change the rate at which germinating peas take up oxygen? Start with temperature. Then try the dough: which sugar makes yeast respire fastest?

What you can changeOne independent variable at a time: the water bath's temperature, how long ago the seeds were soaked, the mass of seeds, or whether the potassium hydroxide is in. In dough mode, the sugar. Everything else stays fixed unless you change it on purpose.

What to recordThe drop's position on the scale every minute, in mm, for the peas and for the glass-bead tube beside them. Then the rate in mm³/g/min, three or more trials at each value, a mean and a standard deviation, and a graph.

Yeast dough: five tubes, one sugar in each, read against a ruler every 5 minutes.
Germinating seeds over potassium hydroxide, with a glass-bead tube beside them as the control. Both drops sit at the start mark: read each one every minute.

Run it on the page: set it up, press Start, read the drops each minute, and the table and graph fill in.

Open the bench full screen: the apparatus, its controls, your readings and the graph then fit on one screen.

Find out

Temperature. Run 5 to 40 °C in 5 °C steps, with 3-day seeds and 2.0 g, three trials at each. Then try 45 °C: what happens to the drop as the run goes on?
Waking up. At 25 °C, run dry seeds, then 1 to 5 days since soaking, then boiled seeds.
Per gram. At 25 °C, run 1 to 6 g of seeds. The table gives the rate for the whole tube: work out the rate per gram for each mass too, and plot both.
No potassium hydroxide. Run 3-day mung beans with it in, then with water in its place.
Which apparatus? At 25 °C, run 3-day seeds with the capillary and its drop, then with the U-tube manometer and its control tube. Which reads more precisely, and why? (A third, an insect over soda lime with a bubble in a pipette, is in the check questions.)
The clip. At 25 °C, leave the clip open for 0, 5 and 10 minutes before starting. Watch the bead tube's drop.
Five sugars. Switch to yeast dough at 30 °C and run the rack three times, so you have three tubes of each sugar, over 30 minutes.

Record

Every run's settings: what you changed, and everything you kept the same.
Both tubes' readings every minute, the seeds' and the glass-bead tube's, in the units your apparatus reads. For the dough, each top every 5 minutes.
Change one thing at a time.
Three or more trials at each value, then a mean and its standard deviation.
The bench’s graph plots each reading as it is taken. Work out each rate yourself, then press Calculate to check it.
For the means and their spread, use Graph your results under the lab, or download the CSV and plot it yourself.
The bench keeps each variable's results: choose it again to see them.
When you are done, choose one investigation and write it up: the question, the method, a table, a graph with error bars, and a conclusion that cites your data.

With the potassium hydroxide taken out, the drop beside the mung beans hardly moves. Why?

In the first two minutes the drop beside the glass beads moved 10 mm away from its tube, then stopped. What does that tell you?

Sucrose dough rose almost as fast as glucose dough, but starch dough barely beat dough with no sugar. Why?

03Check

A fresh paper from the exam bank on this guide's topics each time: past-paper style questions, marked as you go, and saved to your Exam practice progress when you are signed in. It opens on biologybybradford.com, so it needs the internet.

Exam practice

Exam questions on 3.5, marked

Start 15 marks of exam questions Your exam practice

04Downloads

This file is the guide, saved to run without the internet.

Saved from biologybybradford.com/guides/ap-biology-unit-3-respiration, where the guide is kept up to date with the notes, the revision guide and, for the teacher, the lesson plans and slides.

05Teacher resources

The guide as three one-hour lessons. Each has a plan to print (the timings, what to project and where to stop, the activities with what they need, what students get wrong) and slides to teach it from, in the same order, with the teacher's moves in the notes.

Lesson	Covers	Focus	Chapters
1From glucose to pyruvate	3.5.A60 min	Respiration as a controlled release of energy, glycolysis and the carriers, and the respirometer	1, 2, 3, 4, 5, dry lab
2The mitochondrion	3.5.A, 3.5.B60 min	The Krebs cycle, the electron transport chain and chemiosmosis, built with a class model and finished on the ATP ledger	7, 8, 9
3Without oxygen, other fuels, free response	3.5.A, 3.5.B60 min	Fermentation, fuels and a timed free response on respiration data	6, 8, 10, dry lab

The note template, the revision guide and the offline copy are in Downloads, for you and the class.

PRACTISE AND REVISE

Make it for your class, or set it

Build for your class

Tarsia puzzle, dominoes and picture reveal on Unit 3, built from its key terms and ready to print
A pub quiz with rounds on Unit 3, to present or run with no projector
The Kelp Light Habitat escape room, or an escape room built from the question bank on Unit 3

Already on the site for Unit 3

Lesson: Cellular Respiration I: Glycolysis & Krebs
Quiz: Cellular Respiration I: Glycolysis & Krebs
Quiz: Cellular Respiration II: Electron Transport & Fermentation
Exam questions: Unit 3 in the exam maker, a paper with scoring guidelines

Card sets on splycr

Open a set to host it as a class game, or make a copy to edit.

How this was made. The course structure, the activities and the teaching choices come from my own classroom, my presentations and my years of teaching IB and AP Biology. I use Claude, an AI, to help write and build the interactive guides, lesson plans, slides and practice questions, and to draw the diagrams and code the interactives. I direct the work and review it against the IB and AP specifications before it goes live.