Cells at work: where the energy comes from
By the end of this lesson you will be able to
- State the word equation for aerobic respiration and describe where in the cell it happens.
- Explain why every living cell — plant, animal, fungus or bacterium — must respire all the time, day and night.
- Outline what the energy released is used for: movement, warmth, growth and repair, and moving substances across membranes.
- Interpret evidence that living things respire — germinating peas, yeast, your own breath — and distinguish a control experiment from a control variable.
- Design an investigation into the rate of respiration in yeast, with a hypothesis that says “because”, and control variables that carry values.
The puzzle
A farmer near Serowe stores a sack of maize that was caught in the rain. Two days later the sack is warm to the touch — warmer than the air around it, warm enough to feel through the cloth. Nothing has been lit; the sack sits in the shade. A sack of dry maize next to it is cool. Where is the heat coming from?
Predict: if the farmer sealed a handful of the wet maize in a jar with a small dish of limewater (which turns milky in carbon dioxide), what would happen to the limewater after a day? Then predict what a jar of the same maize, first boiled for ten minutes, would do.
Compare your answer
The wet maize is germinating: every grain is alive and its cells are working hard, and working cells release energy from food. Some of that energy leaves as heat, which is why the sack is warm, and the cells release carbon dioxide, which turns the limewater milky. Boiled maize does neither — the heat has killed the cells, so no reactions run, the limewater stays clear and the jar stays cool. The boiled jar is not wasted; it is the comparison that proves the living grains are the source. That reaction inside every living cell is the subject of this lesson.
The science — structured notes
1.1 Every cell needs energy, all the time
In Year 1 you learned that all living things are made of cells and that cells are the smallest units that are alive. Being alive is work. A cell must build new molecules, move things in and out through its membrane, repair damage, copy itself and, in animals, contract, send signals or keep the body warm. None of that happens without a supply of energy — and the only supply a cell has is the food it takes in. Respiration is the chemical reaction, inside every living cell, that releases the energy stored in food so that the cell can use it.
| What the energy is used for | Example cell | What you would see if it stopped |
|---|---|---|
| Movement — muscle contraction, beating cilia, a sperm’s tail | Muscle cell, ciliated cell | No heartbeat, no breathing, no swimming |
| Warmth — mammals and birds keep their bodies at a steady temperature | Every cell, especially liver and muscle | Body temperature falls towards the air temperature |
| Growth and repair — building proteins, membranes and new cells | Cells at a root tip, skin cells, cells at a wound | No growth, no healing |
| Transport — pumping substances across the membrane against their natural direction (active transport) | Root hair cell, kidney cell, nerve cell | Roots cannot take in minerals; nerves cannot fire |
| Division — copying DNA and splitting into two (Lesson 3.2) | Any cell that divides | No replacement of worn-out cells |
1.2 Aerobic respiration — the word equation
Aerobic means “with air”: the reaction uses oxygen. It is the same in a yeast cell, a maize seed, a muscle fibre and a bacterium.
glucose + oxygen → carbon dioxide + water (energy released)
A student says: “A running athlete breathes harder to get rid of heat.” Use the equation to correct her.
Running muscles contract many times a second, and each contraction spends energy. To release more energy the muscle cells respire faster, so they use glucose and oxygen faster and make carbon dioxide faster.
Breathing harder does two things the equation demands: it supplies the extra oxygen and it removes the extra carbon dioxide. The heat is a by-product that the blood carries to the skin, where sweat deals with it — breathing is not the main route for heat.
So: faster respiration → more oxygen needed and more carbon dioxide produced → faster breathing. The equation predicts the panting; the sweat is a separate story.
1.3 The mitochondrion — the organelle built for the job
Most of the energy in aerobic respiration is released inside the mitochondria (singular: mitochondrion), sausage-shaped organelles about a thousandth of a millimetre long — far too small to see with a school light microscope, and visible only in an electron micrograph (Lesson 3.4). Their form fits their function: each has two membranes, and the inner one is folded into shelves called cristae. The folding packs a large surface area into a tiny space, and it is on that surface that the final, energy-releasing steps of respiration happen. More folds, more surface, more energy released per second.
| Cell | Mitochondria per cell (approx.) | Why |
|---|---|---|
| Human red blood cell | 0 | Carries oxygen for other cells; releases little energy itself and must not use up what it carries |
| Skin cell | a few hundred | Low energy demand |
| Heart muscle cell | several thousand — up to a third of the cell’s volume | Contracts about 100 000 times a day and never rests |
| Sperm cell | 50–80, wound round the base of the tail | All the energy goes to one job: swimming |
| Liver cell | 1000–2000 | Hundreds of chemical reactions to run |
| Yeast cell | a few to a few dozen; more with oxygen present | A single cell doing everything for itself |
For each cell, state whether you expect many or few mitochondria and give the reason in terms of the energy the cell spends.
(a) A cell in the flight muscle of a bee.
(b) A fat-storage cell under the skin.
(c) A root hair cell that takes in mineral ions from the soil against their natural direction of movement.
Reveal answer
(a) Many — the wing muscles contract about 200 times a second in flight and every contraction spends energy; the cells are among the most mitochondria-rich known.
(b) Few — the cell’s job is to store fat, which needs little energy once the fat is in place; most of the cell is a single droplet of oil.
(c) Many — moving ions into the cell against their natural direction (active transport) is work that must be paid for with energy from respiration; root hair cells and the cells around them are rich in mitochondria, and roots in waterlogged soil, starved of oxygen, stop absorbing minerals.
1.4 The evidence: how do we know cells respire?
The equation makes three testable predictions: living things use oxygen, release carbon dioxide and release energy (some of it as heat). Each has a classic experiment.
| Prediction | Experiment | What is seen | The comparison that makes it fair |
|---|---|---|---|
| Carbon dioxide is released | Germinating peas (or maize) in a sealed jar with limewater, or breath bubbled through limewater | Limewater turns milky (a white cloud of calcium carbonate) | A jar of boiled peas: limewater stays clear |
| Heat is released | Germinating peas in a vacuum flask with a thermometer through the bung | Temperature rises by several degrees over two days | A flask of boiled peas: no rise. Both sets rinsed in disinfectant so that microbes on the seed coats are not the source |
| Oxygen is used | Woodlice or seeds in a tube with soda lime (absorbs CO₂) and a drop of coloured liquid in a capillary | The drop moves towards the organism as the gas volume falls | An identical tube with glass beads instead of the organism: the drop does not move |
| Cells respire, not just whole organisms | Yeast (single cells) in sugar solution with a delivery tube into water | A steady stream of bubbles; limewater goes milky; the flask warms slightly | The same flask with no sugar: no bubbles |
Two vacuum flasks are set up on Monday at 22 °C: flask A holds 100 g of soaked, germinating peas rinsed in disinfectant; flask B holds 100 g of peas that were boiled for ten minutes, cooled and rinsed in the same disinfectant. Thermometers read: Monday 22 °C and 22 °C; Tuesday 26 °C and 22 °C; Wednesday 29 °C and 22 °C.
The germinating peas warmed their flask by 7 °C in two days while the boiled peas warmed theirs by 0 °C — so the heat came from the living cells, not from the room, the disinfectant or the flask.
Flask B is the control experiment: the same set-up with the living factor removed. It is not a control variable. The control variables are what was kept the same for both — 100 g of peas, the same disinfectant, the same starting temperature of 22 °C, identical flasks.
Why disinfectant? Seeds carry bacteria and fungi on their coats, which also respire. Without the rinse, a sceptic could say the warmth came from microbes, not peas — and in flask B, dead peas would be an excellent meal for them. The rinse removes the alternative explanation.
“Plants respire only at night, when they cannot photosynthesise.”
Reveal answer
False. Every plant cell respires all the time — leaf cells included, in full sunlight. During the day a leaf photosynthesises faster than it respires, so overall it takes in carbon dioxide and gives out oxygen, and the respiration is hidden inside the bigger flow. At night photosynthesis stops, respiration continues, and the leaf gives out carbon dioxide like an animal. A root, with no chloroplasts, respires exactly the same way at noon and at midnight.
1.5 Without oxygen — the emergency version
If oxygen runs out, cells can still release a little energy from glucose without it: anaerobic respiration. It releases much less energy per glucose molecule — less than a twentieth — and it leaves a waste product that cannot be breathed away.
| Where | Word equation | What you notice |
|---|---|---|
| Yeast (and some bacteria) | glucose → ethanol + carbon dioxide | Bread dough rises on the carbon dioxide; the ethanol bakes off. Traditional sorghum beer and ginger beer ferment the same way — fermentation is this reaction with a different name. |
| Your muscles in a sprint | glucose → lactic acid | The blood cannot deliver oxygen fast enough for 10 seconds of flat-out effort; lactic acid builds up and the muscle aches and slows. Afterwards you pant to pay back the oxygen and clear it. |
| Plant roots in waterlogged soil | glucose → ethanol + carbon dioxide | Roots make ethanol, which poisons them; flooded crops die from the roots up. |
1.6 Rate: why temperature makes a hill, not a line
Respiration is not one reaction but a chain of about thirty, each controlled by an enzyme — a protein whose shape fits its reaction. Warm the cell and its particles move faster and collide more often, so every step speeds up: the rate roughly doubles for each 10 °C rise from 10 °C to about 35 °C. Above about 40 °C the enzymes begin to lose their shape (denature) and the chain breaks; by 60 °C almost nothing happens. A graph of rate against temperature is therefore a hill with its peak near 37–40 °C — which is the body temperature of mammals and the temperature bakers choose for proving dough. A fever of 41 °C is dangerous for exactly this reason.
A flask of yeast at 30 °C produced 54 bubbles in 3 minutes. Give the rate, with unit and sensible precision, and predict the count for one minute at 40 °C.
Rate = 54 ÷ 3 = 18 bubbles per minute. A bubble count is a whole number, so the rate is written to the nearest whole bubble, not “18.00”.
From 30 °C to 40 °C the rate roughly doubles, so about 35–40 bubbles per minute — provided 40 °C is still below the temperature at which this yeast’s enzymes start to denature. The prediction must be checked, not assumed; that is what the lab is for.
A student holds the yeast flask at 60 °C for ten minutes, sees no bubbles, then cools it back to 30 °C. It still does not bubble. Explain both observations at the level of the cell.
Reveal answer
At 60 °C the enzymes that control each step of respiration in the yeast cells lose their shape — they are denatured — so the chain of reactions stops and no carbon dioxide is made. Denaturing is not reversible: cooling the flask does not refold the proteins, and the cells are dead. The flask at 30 °C afterwards is a flask of dead yeast, so it stays silent. Compare this with a flask cooled to 10 °C, where the reactions merely slow down and recover on warming — slowing is reversible, denaturing is not.
A student writes: “Respiration: oxygen + carbon dioxide → glucose + energy. It happens in the lungs, and it produces energy for the body.” Find three errors.
Reveal answer
The equation is reversed and mixed: the reactants are glucose and oxygen, the products carbon dioxide and water. It happens in every cell of the body (mostly in the mitochondria), not in the lungs — the lungs only exchange the gases. Energy is released from the glucose, not produced; energy cannot be made, only transferred from the store in the glucose to the cell’s uses and to heat.
What if a poison stopped every mitochondrion in a person’s muscle cells but nothing else? Predict what the person could and could not do in the first minute, and why the effect on heart muscle would be worst.
Reveal answer
The muscle cells could still release a little energy anaerobically — glucose → lactic acid — so the person could move briefly, but with less than a twentieth of the normal energy supply and rising lactic acid, the muscles would weaken within seconds and ache. The heart is the worst case because it can never rest: it needs energy for every beat, its cells are packed with mitochondria precisely because anaerobic respiration cannot supply enough, and without them it would fail within a minute. Cyanide does something close to this, which is why it kills so fast.
Try it — yeast respiration and temperature
The flask holds 1 g of dried yeast in 100 cm³ of water with 5 g of glucose, standing in a water bath. Set the bath temperature, run, and count the bubbles of carbon dioxide leaving the delivery tube in one minute. Take three counts at each of at least five temperatures, then plot the mean against temperature. Then remove the sugar — the control experiment — and swap glucose for sucrose.
Suppose your means with glucose and 1 g of yeast were: 20 °C → 9, 30 → 18, 40 → 35, 50 → 8, 60 → 0 bubbles per minute; with no sugar at 40 °C → 0.
(a) Describe the relationship, quoting values, and state whether the rate is proportional to temperature.
(b) Explain the shape between 40 and 60 °C at the level of the cell.
(c) State what the no-sugar run shows and why the investigation would be weaker without it.
Reveal answer
(a) The rate rises with temperature from 9 bubbles per minute at 20 °C to a peak of 35 at 40 °C, then falls steeply to 8 at 50 °C and 0 at 60 °C. It is not proportional: doubling the temperature from 20 to 40 °C gives about four times the rate, not twice, and above 40 °C the rate falls as temperature rises — a straight line through the origin cannot describe a hill.
(b) Up to about 40 °C warmer particles collide more often and the enzyme-controlled reactions of respiration run faster. Above that, the enzymes’ shapes are changed by the heat — they denature — so each step of the chain fails; at 50 °C most are damaged and at 60 °C the cells are dead, so no carbon dioxide is made.
(c) With no sugar there is nothing for the cells to respire, and the count is zero: the carbon dioxide comes from the glucose being broken down by living yeast, not from air trapped in the flask, water boiling or gas dissolved in the solution. Without this control experiment someone could argue the bubbles were not respiration at all.
Common mistakes to avoid
- “Respiration is breathing” — breathing moves air; respiration is the chemical reaction inside cells.
- “Respiration produces energy” — it releases energy stored in glucose; energy is never made.
- “Plants respire only at night” — every plant cell respires day and night; photosynthesis hides it by day.
- “Respiration happens in the lungs” — it happens in every cell, mostly in the mitochondria.
- Boiled peas called a control variable — they are a control experiment; control variables are the things kept the same.
- A straight line through a rate-against-temperature graph — enzymes make it a hill; draw the curve the points show.
How we found out — a guinea pig in an ice bucket and a chemist who refused to believe in yeast
In the winter of 1782–83 the French chemist Antoine Lavoisier and the mathematician Pierre-Simon Laplace put a guinea pig in a chamber surrounded by ice and measured how much ice it melted in ten hours. They also measured the carbon dioxide it breathed out, and burned charcoal to release the same mass of carbon dioxide, measuring the ice that melted. The two amounts of melting were close. Lavoisier’s conclusion, published in 1789, was that “respiration is therefore a combustion” — a slow burning of food with oxygen inside the body, giving out heat and carbon dioxide exactly as a candle does. It was the first time anyone had shown that a living process obeys the same chemistry as a dead one. Lavoisier was executed in the Revolution in 1794; his idea survived him.
Where in the body the burning happened was argued about for seventy years. In 1857 Louis Pasteur, then a young professor in Lille, was asked by a distiller why his beet-sugar vats sometimes soured. Under the microscope Pasteur found living yeast cells budding in every healthy vat and rod-shaped bacteria in every sour one, and over the next few years he proved that fermentation is what living yeast cells do — not, as the leading chemists insisted, a chemical decay that yeast merely happened to be present for. In 1861 he added that yeast could live and ferment without air at all, and grew more of it with air than without: the first description of aerobic and anaerobic respiration. The chemists were partly right, though. In 1897 Eduard Buchner ground yeast cells with sand, filtered out every fragment of cell, and found that the cell-free juice still fermented sugar — the reaction is run by enzymes, which are chemicals, inside living cells. Both sides won a Nobel Prize’s worth of truth. The final piece came in 1961, when Peter Mitchell, working in a converted farmhouse in Cornwall, proposed how the mitochondrion’s inner membrane captures the energy — an idea so strange that it took the field seventeen years to accept it and give him the Nobel Prize in 1978.
Lavoisier burned charcoal in the same ice apparatus as the guinea pig. Explain what that comparison was for, and why the result was more convincing than the temperature of the guinea pig alone.
Reveal answer
Charcoal burning is a known chemical reaction: carbon + oxygen → carbon dioxide, releasing heat. By burning enough charcoal to make the same mass of carbon dioxide the animal had breathed out, and finding that it melted about the same mass of ice, Lavoisier showed that the animal’s heat matched the heat of the chemistry — the same reaction gives the same heat. A warm guinea pig on its own shows only that animals are warm; the matched charcoal shows why, and turns “living warmth” into a measurable combustion. It is a control experiment in the modern sense: a known reaction run beside the unknown one for comparison.
Where it is used now
- Malting and brewing in Botswana. Sorghum for traditional beer is soaked and allowed to germinate: the sprouting grain respires, warms the heap and makes the enzymes that turn its starch to sugar. Maltsters turn the heap to keep it from overheating — the same hill-shaped curve as the yeast lab, managed by hand.
- Bread. A baker proves dough at about 35–40 °C because yeast respiration peaks there; a fridge slows it; an oven kills it and the bubbles are set in place.
- Sport. Botswana’s sprinters — Letsile Tebogo’s 200 m is under 20 seconds — run largely on anaerobic respiration in the muscles; middle-distance runners depend on mitochondria, and training increases the number of mitochondria in muscle cells within weeks.
- Grain storage. A silo of wet grain respires, heats, and can catch fire; moisture meters and ventilation exist because grain is alive. The farmer’s warm sack is a warning, not a curiosity.
- Medicine. A fever of 40 °C is treated because enzymes begin to fail; hypothermia is dangerous because respiration slows; cyanide and carbon monoxide kill by stopping mitochondria or the oxygen supply to them.
- Compost and hospital waste. A compost heap steams for the same reason as the sack; incubators for premature babies and for bacterial cultures are set to the temperature at which cells work best.
Science now — and the scientist you need to be
What is happening in 2026
Mitochondria have become a medical target. In 2025 doctors in Newcastle reported the first eight babies born in the United Kingdom after mitochondrial donation — the nucleus of the mother’s egg was moved into a donor egg with healthy mitochondria, so that a disease carried in the mitochondria’s own small piece of DNA was not passed on. In laboratories, instruments now measure the oxygen used by a few thousand living cells in a dish, minute by minute, so that a drug’s effect on respiration can be read as a curve in an afternoon rather than guessed from a whole animal. Sports scientists count mitochondria in muscle biopsies to tailor training, and the same measurements are used to study ageing, because mitochondria wear out. And artificial-intelligence models trained on millions of measured reaction rates are being used to predict how a cell’s whole network of respiration reactions will respond to a change in temperature or food — a prediction that a student with a flask of yeast can then test. The reaction Lavoisier called a combustion is now something engineers try to control one organelle at a time.
The mindshift
What you need to be able to do
Three examinable skills from this lesson: write the word equation and say where in the cell it happens; explain an observation (a warm sack, a milky limewater, a panting runner) by reaching the cell and the reaction; and plan a rate investigation in which the dependent variable is a count read from a stopwatch and a tube — not “the rate” — and every control variable carries a value. The Think task below is the rehearsal for Task 2 of the unit test.
See an example
Warm water with sugar: the balloon stood up after about 20 minutes and was firm after 40 — carbon dioxide from the yeast. Cold water with sugar: a slight bulge after 40 minutes, so the reactions ran but slowly. Warm water with no sugar: nothing — the control experiment; the gas comes from the sugar being used. The AI’s first answer said “breathing takes in oxygen for respiration, which releases energy from food in the cells”; asked where in the cell, it added the mitochondria and the folded inner membrane — correct, and the version that earns marks.
Key terms
| Respiration | The chemical reaction in every living cell that releases energy from glucose; not breathing. |
| Aerobic respiration | Respiration using oxygen: glucose + oxygen → carbon dioxide + water, energy released. Mostly in the mitochondria. |
| Anaerobic respiration | Respiration without oxygen: in yeast glucose → ethanol + carbon dioxide (fermentation); in muscle glucose → lactic acid. Releases much less energy. |
| Mitochondrion | The organelle where most of aerobic respiration happens; a folded inner membrane gives a large surface area. Numbers match the cell’s energy demand. |
| Cristae | The folds of the mitochondrion’s inner membrane. |
| Enzyme | A protein that speeds up one reaction; works faster when warm, is destroyed (denatured) by heat above about 40–45 °C. |
| Denature | To lose shape and stop working, permanently, because of heat or the wrong pH. |
| Limewater | Calcium hydroxide solution; turns milky in carbon dioxide — the test for the gas. |
| Control experiment | A run with the factor under test removed (boiled peas; no sugar), for comparison. |
| Control variable | A factor kept the same, stated with its value and how it is checked. |
| Rate | How much happens per unit time — bubbles per minute; calculated from a count and a time, not read directly. |
Think like a scientist
Criterion B A bakery in Francistown wants to know whether adding more sugar to its dough makes the yeast work faster, or whether there is a point beyond which extra sugar is wasted. You are asked to design a laboratory investigation of the question: does the concentration of glucose affect the rate of respiration in yeast? You have dried yeast, glucose, distilled water, a balance, measuring cylinders, conical flasks with bungs and delivery tubes, test tubes, a thermostatically controlled water bath, a thermometer and a stopwatch.
(a) Write a research question that names both variables and the range you will test. [2]
(b) Write a hypothesis with scientific reasoning that reaches the cell. [3]
(c) Identify the independent variable and the dependent variable, each with its unit and the instrument used, and explain why “the rate of respiration” is not an acceptable statement of the dependent variable. [4]
(d) Identify three control variables. For each, give the value you will use and how you will keep it the same. [6]
(e) Write a numbered method another student could follow, including the five values of the independent variable, the number of trials, what is recorded, what is calculated, and the control experiment. [8]
(f) State one safety precaution specific to this investigation and the hazard it addresses. [2]
Model answer
(a) How does the concentration of glucose solution (0, 2, 4, 6, 8 and 10 g per 100 cm³ of water) affect the number of bubbles of carbon dioxide released per minute by 1 g of dried yeast at 35 °C?
(b) If the glucose concentration is increased, then the number of bubbles per minute will increase, because glucose is the reactant the yeast cells break down in respiration, so with more glucose molecules in the solution more enter each cell every second and the enzyme-controlled reactions that release carbon dioxide run faster; but above some concentration every enzyme is already working as fast as it can, so the rate will level off — and at very high concentrations water may be drawn out of the cells by osmosis, so the rate may even fall.
(c) Independent variable: the concentration of glucose, in g per 100 cm³, set by weighing the glucose on a balance to 0.1 g and dissolving it in 100 cm³ of water measured with a measuring cylinder. Dependent variable: the number of bubbles of gas leaving the delivery tube in one minute, counted by eye while the stopwatch runs — a whole number. “The rate of respiration” is not measured by any instrument; it is a calculated quantity (bubbles ÷ time) and does not say what is counted, over what time, or with what — so it cannot be checked or repeated.
(d) Temperature of the yeast suspension: 35 °C, set by standing the flask in the thermostatic water bath and checked with a thermometer in the flask before each count, because enzyme activity depends on temperature. Mass of dried yeast: 1.0 g, weighed on the balance for every flask, because the count depends on how many cells are respiring. Volume of water: 100 cm³ from a measuring cylinder, because concentration depends on volume and dissolved gas depends on the volume of liquid. (Also acceptable: the same batch and age of dried yeast; the same delivery-tube diameter and depth in the water, since bubble size depends on it; five minutes of standing time before counting, so every flask has used up its dissolved oxygen.)
(e) 1. Weigh 2.0 g of glucose on the balance and dissolve it in 100 cm³ of distilled water measured with a measuring cylinder in a conical flask. 2. Add 1.0 g of dried yeast, swirl, fit the bung and delivery tube, and stand the flask in the water bath at 35 °C. 3. Put the free end of the delivery tube into a test tube of water. 4. Wait 5 minutes for the yeast to activate and for the temperature to settle; check the thermometer reads 35 °C. 5. Start the stopwatch and count the bubbles leaving the tube for exactly 60 s; record the count. 6. Repeat the count twice more from the same flask, 1 minute apart, to give three trials. 7. Repeat steps 1–6 with 4, 6, 8 and 10 g of glucose — five concentrations — using a fresh flask and fresh yeast each time. 8. Run a control experiment with 0 g of glucose, following exactly the same steps. 9. Record all counts in a table with the unit in the column heading; calculate the mean for each concentration to the nearest whole bubble, excluding any anomalous count and stating why. 10. Plot mean bubbles per minute (y) against glucose concentration (x) and draw a smooth curve of best fit.
(f) The water bath and the flasks are at 35 °C, which is not a scald hazard, but the glassware is wet and the bench is close to electrical sockets for the bath, so keep the bench dry, lift flasks with both hands and dry them before moving them, and switch the bath off before emptying it. Also: yeast suspension must not be tasted or left open — fermenting flasks build up pressure if the delivery tube blocks, so check the tube is clear before fitting the bung. (Generic laboratory rules such as “wear goggles” score nothing on their own.)
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State the word equation for aerobic respiration and name the organelle where most of it takes place.
Reveal answer
Glucose + oxygen → carbon dioxide + water, with energy released [1]; the mitochondrion [1].
Explain why a person’s breathing rate rises during exercise, using the equation.
Reveal answer
Muscle cells contract more often and need more energy, so they respire faster [1]; faster respiration uses oxygen faster, which must be supplied by breathing more air in [1]; and produces carbon dioxide faster, which must be removed by breathing more out [1].
A student writes: “The boiled peas are the control variable. They show the heat in the other flask came from the peas, because boiled peas have no energy.” Identify three errors.
Reveal answer
The boiled peas are the control experiment, not a control variable — control variables are the mass of peas, the starting temperature and the flask type kept the same [1]; the heat came from respiration in the living cells, not from “the peas” in general [1]; boiled peas still contain the energy stored in their starch — what they lack is living cells with working enzymes to release it [1].
Heart muscle cells contain thousands of mitochondria, each with a highly folded inner membrane. Relate both features to the function of the cell.
Reveal answer
Heart muscle contracts continuously — about 100 000 times a day — and each contraction needs energy from respiration [1]; many mitochondria mean many sites releasing energy at once, so the supply matches the demand [1]; the folded inner membrane gives a large surface area on which the energy-releasing steps of respiration happen, so each mitochondrion releases more energy per second than a smooth one could [1].
True or false, with a reason: “A yeast flask that has stopped bubbling at 60 °C will start again when it is cooled to 35 °C.”
Reveal answer
False [1]: at 60 °C the enzymes in the yeast cells have been denatured — their shapes are permanently changed — so cooling does not restore them and the cells are dead; only a flask that was merely chilled recovers on warming [1].
A farmer’s flooded maize field turns yellow and the plants die within days, although the leaves are in full sun. Explain, using respiration in the roots.
Reveal answer
Root cells respire aerobically using oxygen from the air spaces in the soil; flooding fills those spaces with water, so oxygen cannot reach them [1]; the roots switch to anaerobic respiration, which releases far less energy — too little to take in minerals by active transport — and produces ethanol, which poisons the root cells [1]; the roots die, water and minerals stop reaching the leaves, and the plant dies even though its leaves could still photosynthesise [1].
Summary
- Every living cell respires all the time to release the energy stored in glucose: glucose + oxygen → carbon dioxide + water, energy released. It is not breathing and it does not make energy.
- Most of the energy is released in the mitochondria, whose folded inner membranes give a large surface area; cells that work hardest have the most.
- The energy pays for movement, warmth, growth and repair, transport across membranes and cell division.
- Without oxygen, cells respire anaerobically: yeast → ethanol + carbon dioxide (fermentation, bread, brewing); muscle → lactic acid. Far less energy.
- Rate against temperature is a hill peaking near 37–40 °C, because enzymes speed up with warmth and denature above it. A control experiment (boiled peas, no sugar) is not a control variable.
Next time → Cells dividing: the nucleus and its DNA, why cells copy themselves, why they stay small, and what happens when division runs out of control.