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GCSE BiologyAQA, Edexcel and OCR

Cell biology: GCSE questions with answers

40 biology questions on cell biology. Pick your answer, then tap to see if you were right and why. Every answer comes from the Show Working science bank, checked against the GCSE content every board in England teaches.

Quiz yourself: 40 questions 3 6-mark questions

01 / Quiz yourselfTry it, then tap to check

Cell biology questions to try

Choose your answer before you open it. Then read why, so it sticks.

01 / Cell biologyBoth tiers

What is the job of the nucleus in an animal cell?

  1. AIt is where proteins are made
  2. BIt contains the genetic material
  3. CIt controls what enters and leaves
  4. DIt releases energy from glucose
Show the answer

Answer: B It contains the genetic material

The nucleus holds the genetic material: DNA arranged in chromosomes. The genes on this DNA carry the instructions that control what the cell does and which proteins it makes.

Checked against OpenStax Biology 2e, 4.3 Eukaryotic Cells

02 / Cell biologyBoth tiers

Where is most of the genetic material in a bacterial cell?

  1. AStored inside a nucleus
  2. BInside mitochondria
  3. CFree in the cytoplasm
  4. DInside the ribosomes
Show the answer

Answer: C Free in the cytoplasm

Prokaryotic cells such as bacteria have no nucleus. Their main DNA is a single loop free in the cytoplasm. They still have a cell membrane, cytoplasm, ribosomes and a cell wall.

Checked against OpenStax Biology 2e, 4.2 Prokaryotic Cells

03 / Cell biologyBoth tiers

What are plasmids?

  1. ASmall chloroplasts found in algae
  2. BPores in a bacterial cell wall
  3. CTiny structures that release energy
  4. DSmall rings of DNA in bacteria
Show the answer

Answer: D Small rings of DNA in bacteria

Plasmids are small extra rings of DNA in the cytoplasm of many bacteria, separate from the main DNA loop. They carry a few extra genes, such as genes for resistance to an antibiotic.

Checked against OpenStax Biology 2e, 4.2 Prokaryotic Cells

04 / Cell biologyBoth tiers

What happens in the mitochondria?

  1. AMaking lactic acid
  2. BPhotosynthesis
  3. CAerobic respiration
  4. DProtein synthesis
Show the answer

Answer: C Aerobic respiration

Mitochondria are where most of the reactions of aerobic respiration happen, releasing energy from glucose. Cells that need a lot of energy, such as muscle cells and sperm cells, have many mitochondria.

Checked against OpenStax Biology 2e, 4.3 Eukaryotic Cells

05 / Cell biologyBoth tiers

What do chloroplasts contain that absorbs light?

  1. ACellulose
  2. BGlycogen
  3. CHaemoglobin
  4. DChlorophyll
Show the answer

Answer: D Chlorophyll

Chloroplasts are where photosynthesis happens. They contain chlorophyll, a green pigment that absorbs light. They are found in the green parts of plants and in algae, but not in animal cells or in roots, which get no light.

Checked against OpenStax Biology 2e, 4.3 Eukaryotic Cells

06 / Cell biologyBoth tiers

What does the cell membrane do?

  1. AControls what goes in and out
  2. BMakes the cell rigid and strong
  3. CHolds the genetic material
  4. DReleases energy from glucose
Show the answer

Answer: A Controls what goes in and out

Every cell, including a bacterium, is surrounded by a cell membrane. It is partially permeable, so it controls which substances move into and out of the cell.

Checked against OpenStax Biology 2e, 5.1 Components and Structure

07 / Cell biologyBoth tiers

What is made at the ribosomes?

  1. AProteins
  2. BDNA
  3. CStarch
  4. DGlucose
Show the answer

Answer: A Proteins

Ribosomes are tiny structures where proteins are made by joining amino acids together. They are found in the cytoplasm of animal, plant and bacterial cells.

Checked against OpenStax Biology 2e, 4.3 Eukaryotic Cells

08 / Cell biologyBoth tiers

How does the size of a typical bacterial cell compare with an animal cell?

  1. AIt is much larger
  2. BIt is exactly twice as big
  3. CIt is about the same size
  4. DIt is much smaller
Show the answer

Answer: D It is much smaller

Bacterial cells are much smaller than plant and animal cells. Most bacteria are about 1 to 5 micrometres long, while most animal and plant cells are about 10 to 100 micrometres across.

Checked against OpenStax Biology 2e, 4.2 Prokaryotic Cells

09 / Cell biologyBoth tiers

Why can an electron microscope show more detail than a light microscope?

  1. AIt has a much higher resolution
  2. BIt shines a brighter lamp on the cell
  3. CIt uses better glass lenses
  4. DIt only looks at living cells
Show the answer

Answer: A It has a much higher resolution

Resolution is how well a microscope can show two points that are close together as separate. Electron microscopes have much higher resolution and magnification than light microscopes, so they show far finer detail inside cells.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

10 / Cell biologyBoth tiers

Which of these is too small to see with a light microscope?

  1. AA plant cell wall
  2. BA nucleus
  3. CA ribosome
  4. DA chloroplast
Show the answer

Answer: C A ribosome

Ribosomes are far too small to see with a light microscope. Electron microscopes revealed them, and the inner membranes of mitochondria and chloroplasts, which helped biologists work out what these structures do.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

11 / Cell biologyBoth tiers

Why are light microscopes still widely used in schools?

  1. AThey are cheap and can view living cells
  2. BThey show ribosomes very clearly
  3. CThey have a higher resolution
  4. DThey magnify more than any other kind of microscope
Show the answer

Answer: A They are cheap and can view living cells

Electron microscopes are large and expensive, and specimens must be dead because they are viewed in a vacuum. Light microscopes are cheap and portable and can show living cells, but with much less detail.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

12 / Cell biologyBoth tiers

What happens to the DNA before a cell divides by mitosis?

  1. AIt is halved
  2. BIt moves into the mitochondria
  3. CIt is digested by enzymes
  4. DIt is copied
Show the answer

Answer: D It is copied

Before dividing, a cell grows and makes more sub-cellular structures, then copies its DNA. In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides. Finally the cytoplasm and membrane divide.

Checked against OpenStax Biology 2e, 10.2 The Cell Cycle

13 / Cell biologyBoth tiers

A human body cell with 46 chromosomes divides by mitosis. How many chromosomes does each new cell have?

  1. A23
  2. B46
  3. C47
  4. D92
Show the answer

Answer: B 46

Mitosis makes two cells that are genetically identical to the parent cell, each with the full set of chromosomes. The DNA was copied first, so nothing is halved or doubled.

Checked against OpenStax Biology 2e, 10.2 The Cell Cycle

14 / Cell biologyBoth tiers

Which of these happens by mitosis?

  1. AHalving the number of chromosomes
  2. BMaking sperm cells
  3. CMaking egg cells
  4. DRepairing a cut in the skin
Show the answer

Answer: D Repairing a cut in the skin

Mitosis makes new, genetically identical cells. Multicellular organisms use it to grow, to repair damaged tissue and to replace old cells. Gametes are made by meiosis instead.

Checked against OpenStax Biology 2e, 10.2 The Cell Cycle

15 / Cell biologyBoth tiers

What is cell differentiation?

  1. AA cell becoming specialised for a job
  2. BA cell growing larger
  3. CA cell dividing into two
  4. DA cell copying its DNA
Show the answer

Answer: A A cell becoming specialised for a job

As an organism develops, its cells differentiate: they gain the structures needed for one job and become specialised. This lets cells work together in tissues and organs, which makes the organism efficient. Their genes do not change.

Checked against AQA GCSE Biology (8461) specification, 4.1.1.4 Cell differentiation, page 17

16 / Cell biologyBoth tiers

How does differentiation in plants differ from animals?

  1. AMany plant cells can differentiate all their life
  2. BPlant cells never become specialised
  3. CMost animal cells keep differentiating all their life
  4. DOnly animal cells become specialised
Show the answer

Answer: A Many plant cells can differentiate all their life

Most animal cells differentiate at an early stage, and in adult animals cell division is mainly for repair and replacement. Many plant cells keep the ability to differentiate throughout the life of the plant.

Checked against AQA GCSE Biology (8461) specification, 4.1.1.4 Cell differentiation, page 17

17 / Cell biologyBoth tiers

Why does a sperm cell have many mitochondria?

  1. ATo carry extra genes to the egg
  2. BTo release energy for swimming
  3. CTo photosynthesise on the way
  4. DTo make its tail grow longer
Show the answer

Answer: B To release energy for swimming

A sperm cell is a specialised cell. Its tail lets it swim, and its many mitochondria release the energy for swimming by aerobic respiration. These features come from differentiation.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, statement 1.2

18 / Cell biologyBoth tiers

Which describes cancer?

  1. ACells dividing more slowly than normal
  2. BCells stopping all division
  3. CA disease always caught from others
  4. DUncontrolled cell division
Show the answer

Answer: D Uncontrolled cell division

Cancer is caused by changes in a cell that make it grow and divide out of control. These changes are in the genes that normally control the cell cycle, so the cell keeps dividing when it should stop.

Checked against NHS, Cancer

19 / Cell biologyBoth tiers

What is a mass of cells made by uncontrolled cell division called?

  1. AA meristem
  2. BA gamete
  3. CA tumour
  4. DAn organelle
Show the answer

Answer: C A tumour

When cells divide out of control they can build up into a lump called a tumour. Some tumours are cancers that can spread into other parts of the body.

Checked against OpenStax Biology 2e, 10.4 Cancer and the Cell Cycle

20 / Cell biologyBoth tiers

What is a stem cell?

  1. AA cell with no nucleus
  2. BA specialised cell found only in plant stems
  3. CA gamete made by meiosis
  4. DAn unspecialised cell that can differentiate
Show the answer

Answer: D An unspecialised cell that can differentiate

A stem cell is undifferentiated. It can divide to make more cells like itself, and some of those cells can differentiate into other, specialised types of cell.

Checked against AQA GCSE Biology (8461) specification, 4.1.2.3 Stem cells, page 21

21 / Cell biologyBoth tiers

Which stem cells can become almost any type of human cell?

  1. AAdult bone marrow stem cells
  2. BNerve cells
  3. CEmbryonic stem cells
  4. DRed blood cells
Show the answer

Answer: C Embryonic stem cells

Stem cells in an early embryo can differentiate into almost every type of cell, so they build every tissue of the developing body. Adult stem cells can make only a smaller range of cell types.

Checked against AQA GCSE Biology (8461) specification, 4.1.2.3 Stem cells, page 21

22 / Cell biologyBoth tiers

Where in an adult are the stem cells that make new blood cells?

  1. ABone marrow
  2. BThe kidneys
  3. CThe heart
  4. DThe lungs
Show the answer

Answer: A Bone marrow

Adult stem cells replace cells that wear out and repair damaged tissue. Stem cells in adult bone marrow can make several types of cell, including all the types of blood cell.

Checked against NHS, Stem cell and bone marrow transplants

23 / Cell biologyBoth tiers

How might stem cells help a person with type 1 diabetes in the future?

  1. ABy breaking down extra glucose
  2. BBy replacing the cells that make insulin
  3. CBy acting as antibodies
  4. DBy killing the bacteria that cause it
Show the answer

Answer: B By replacing the cells that make insulin

Stem cells could be made to differentiate into the cells a patient has lost, such as insulin-making cells in type 1 diabetes or nerve cells after paralysis. Many of these treatments are still being researched.

Checked against AQA GCSE Biology (8461) specification, 4.1.2.3 Stem cells, page 21

24 / Cell biologyBoth tiers

Which is a risk of a stem cell treatment?

  1. APassing on a viral infection
  2. BThe cells all turning into gametes
  3. CThe patient becoming immune to food
  4. DThe cells being unable to divide
Show the answer

Answer: A Passing on a viral infection

Stem cells grown in a lab could carry a virus into the patient, and cells from a donor may be rejected. Some people have ethical or religious objections, because collecting embryonic stem cells destroys an embryo.

Checked against AQA GCSE Biology (8461) specification, 4.1.2.3 Stem cells, page 21

25 / Cell biologyBoth tiers

A human body cell has 46 chromosomes. How many are in a human egg cell?

  1. A22
  2. B23
  3. C46
  4. D92
Show the answer

Answer: B 23

Meiosis halves the chromosome number, so human gametes have 23 chromosomes, one from each pair, instead of 46. Body cells are made by mitosis and keep the full number.

Checked against OpenStax Biology 2e, 11.1 The Process of Meiosis

26 / Cell biologyBoth tiers

Why must gametes have half the normal number of chromosomes?

  1. ASo the gametes can swim faster
  2. BSo the embryo has double the number
  3. CSo the fertilised egg has the full number
  4. DSo all the gametes are identical
Show the answer

Answer: C So the fertilised egg has the full number

At fertilisation two gametes join. Because each has half the chromosomes, the fertilised egg has the full number, with one set from each parent. Without halving, the number would double every generation.

Checked against OpenStax Biology 2e, 11.1 The Process of Meiosis

27 / Cell biologyBoth tiers

What does meiosis make from one cell?

  1. AFour genetically different cells
  2. BTwo genetically identical cells
  3. CTwo genetically different cells
  4. DFour genetically identical cells
Show the answer

Answer: A Four genetically different cells

In meiosis the DNA is copied once and the cell divides twice, making four gametes. Each has a single set of chromosomes, and every gamete is genetically different.

Checked against OpenStax Biology 2e, 11.1 The Process of Meiosis

28 / Cell biologyBoth tiers

What is an enzyme?

  1. AA type of hormone
  2. BA biological catalyst
  3. CA type of carbohydrate
  4. DA product of digestion
Show the answer

Answer: B A biological catalyst

Enzymes are proteins that act as biological catalysts. They speed up reactions in living things without being used up, so one enzyme molecule can be used again and again.

Checked against OpenStax Biology 2e, 6.5 Enzymes

29 / Cell biologyBoth tiers

What is the active site of an enzyme?

  1. AThe part where the substrate binds
  2. BThe cell where the enzyme is made
  3. CThe temperature it works best at
  4. DThe product the enzyme makes
Show the answer

Answer: A The part where the substrate binds

Each enzyme has an active site with a particular shape. Only a substrate with the matching shape fits, like a key in a lock, so each enzyme catalyses one type of reaction.

Checked against OpenStax Biology 2e, 6.5 Enzymes

30 / Cell biologyBoth tiers

What happens to an enzyme at a temperature well above its optimum?

  1. AIt turns into the substrate
  2. BIts active site changes shape
  3. CIt becomes a different enzyme
  4. DIt works faster and faster
Show the answer

Answer: B Its active site changes shape

Above its optimum temperature, the bonds holding an enzyme in shape break and its active site changes shape. The substrate no longer fits, so the enzyme is denatured and the reaction slows and stops. This cannot be undone.

Checked against OpenStax Biology 2e, 6.5 Enzymes

31 / Cell biologyBoth tiers

Why does warming an enzyme reaction up to the optimum speed it up?

  1. AThe enzyme is denatured
  2. BMolecules move faster and collide more often
  3. CThe cell makes more substrate
  4. DThe active site gets larger
Show the answer

Answer: B Molecules move faster and collide more often

Up to the optimum temperature, warming gives the enzyme and substrate molecules more kinetic energy. They move faster and collide more often, so the rate of reaction increases.

Checked against OpenStax Biology 2e, 6.5 Enzymes

32 / Cell biologyBoth tiers

What happens to an enzyme at a pH far from its optimum?

  1. AIt gains more kinetic energy
  2. BIts active site changes shape
  3. CIt turns into its substrate
  4. DIt speeds the reaction up
Show the answer

Answer: B Its active site changes shape

Each enzyme has an optimum pH. Far from it, the active site changes shape and the substrate no longer fits, so the enzyme is denatured. The stomach is strongly acidic, so its protease works best at a low pH.

Checked against OpenStax Biology 2e, 6.5 Enzymes

33 / Cell biologyBoth tiers

Respiration is exothermic. What does this mean?

  1. AIt transfers energy to the surroundings
  2. BIt takes in energy from the surroundings
  3. CIt needs light to happen
  4. DIt only happens in muscles
Show the answer

Answer: A It transfers energy to the surroundings

Respiration is an exothermic reaction: it transfers energy from glucose to the surroundings. The cell uses some of this energy for its life processes, and some is lost as heat.

Checked against OpenStax Biology 2e, 7.1 Energy in Living Systems

34 / Cell biologyBoth tiers

Which cells carry out respiration?

  1. AOnly plant cells at night
  2. BOnly animal cells
  3. COnly muscle cells
  4. DAll living cells
Show the answer

Answer: D All living cells

Respiration happens all the time in every living cell, in plants as well as animals, because cells always need energy. Plants respire day and night, even while they photosynthesise in the light.

Checked against OpenStax Biology 2e, 7.1 Energy in Living Systems

35 / Cell biologyBoth tiers

Which of these needs energy from respiration?

  1. AWater entering roots by osmosis
  2. BOxygen diffusing into the blood
  3. CWater evaporating from leaves
  4. DMuscles contracting
Show the answer

Answer: D Muscles contracting

Energy from respiration is used for movement, for building large molecules from small ones, for active transport, and in mammals and birds for keeping warm. Diffusion and osmosis need no energy from respiration.

Checked against AQA GCSE Biology (8461) specification, 4.4.2.1 Aerobic and anaerobic respiration, page 39

36 / Cell biologyBoth tiers

What are the products of aerobic respiration?

  1. ALactic acid only
  2. BEthanol and carbon dioxide
  3. CGlucose and oxygen
  4. DCarbon dioxide and water
Show the answer

Answer: D Carbon dioxide and water

Aerobic respiration uses oxygen: glucose + oxygen gives carbon dioxide + water. It releases the most energy from each glucose molecule.

Checked against OpenStax Biology 2e, 7.1 Energy in Living Systems

37 / Cell biologyBoth tiers

What does anaerobic respiration in human muscles produce?

  1. ALactic acid
  2. BEthanol and carbon dioxide
  3. CGlucose
  4. DCarbon dioxide and water
Show the answer

Answer: A Lactic acid

When muscles cannot get enough oxygen, for example during hard exercise, they respire anaerobically: glucose gives lactic acid. No carbon dioxide is made.

Checked against OpenStax Biology 2e, 7.5 Metabolism without Oxygen

38 / Cell biologyBoth tiers

What does anaerobic respiration in yeast produce?

  1. AOxygen and glucose
  2. BLactic acid
  3. CEthanol and carbon dioxide
  4. DCarbon dioxide and water
Show the answer

Answer: C Ethanol and carbon dioxide

In yeast and plant cells, anaerobic respiration turns glucose into ethanol and carbon dioxide. In yeast this is called fermentation; the carbon dioxide makes bread rise and the ethanol is the alcohol in drinks.

Checked against OpenStax Biology 2e, 7.5 Metabolism without Oxygen

39 / Cell biologyBoth tiers

How does anaerobic respiration compare with aerobic respiration?

  1. AIt transfers much more energy
  2. BIt transfers much less energy
  3. CIt transfers no energy at all
  4. DIt transfers exactly the same energy
Show the answer

Answer: B It transfers much less energy

Aerobic respiration uses oxygen and breaks glucose down completely. Anaerobic respiration needs no oxygen, but glucose is only partly broken down, so much less energy is transferred from each glucose molecule.

Checked against OpenStax Biology 2e, 7.5 Metabolism without Oxygen

40 / Cell biologyBoth tiers

Starch, glycogen and cellulose are built from which small molecules?

  1. AAmino acids
  2. BFatty acids
  3. CGlycerol
  4. DGlucose
Show the answer

Answer: D Glucose

Large carbohydrates such as starch, glycogen and cellulose are chains of glucose. Cells join sugars together to build them, and carbohydrase enzymes such as amylase break them back down into sugars.

Checked against OpenStax Biology 2e, 3.2 Carbohydrates

02 / Longer answersMarked by levels

6-mark questions on this topic

Write your answer first, on paper. Then open it and tick off the points you made. How 6-mark questions are marked.

6 marks / The cell cycle and mitosisBoth tiers

Describe what happens to a body cell during the cell cycle, and explain why mitosis is important.

What a full answer includes
  • In interphase the cell grows and makes more sub-cellular structures, such as ribosomes and mitochondria.
  • Also in interphase the DNA is copied, so each chromosome now has two identical copies.
  • In mitosis the chromosomes condense and become visible (prophase), then line up across the middle of the cell (metaphase).
  • One copy of each chromosome is pulled to each end of the cell (anaphase).
  • A new nucleus forms at each end, so the nucleus has divided (telophase).
  • The cytoplasm and cell membrane divide to make two cells (cytokinesis).
  • The two new cells are genetically identical to each other and to the parent cell, with the same number of chromosomes.
  • Mitosis is needed for growth, to repair or replace damaged cells, and for asexual reproduction.

Model answer

The cell cycle starts with interphase. The cell grows and makes more sub-cellular structures, such as ribosomes and mitochondria, so there will be enough for two cells. The DNA is then copied, so that each chromosome has an identical copy of itself. Next comes mitosis. In prophase the chromosomes condense and become visible, and in metaphase they line up across the middle of the cell. In anaphase one copy of each chromosome is pulled to each end of the cell. In telophase a new nuclear membrane forms around each set of chromosomes, so the nucleus has divided into two. Finally, in cytokinesis, the cytoplasm and the cell membrane divide, producing two daughter cells. Because the DNA was copied exactly, the two daughter cells are genetically identical to each other and to the parent cell, with the same number of chromosomes. Mitosis is important because it lets multicellular organisms grow, by increasing the number of cells, and it replaces damaged or worn-out cells, for example when skin heals after a cut. Some organisms also use mitosis for asexual reproduction.

Where marks slip

Saying the cell divides before its DNA is copied, or that the new cells have double the chromosomes. The DNA is copied first, and each new cell ends up with the same number of chromosomes as the parent. Some boards also ask for the stage names, so learn them in order.

6 marks / How temperature affects an enzymeBoth tiers

A pupil measures how fast a protease enzyme digests egg white at temperatures from 10 °C to 70 °C. The rate rises steadily to a peak at 40 °C, then falls quickly, and by 60 °C there is almost no reaction.

Explain these results.

What a full answer includes
  • From 10 °C to 40 °C the rate rises because the enzyme and substrate molecules have more kinetic energy.
  • They move faster and collide more often, so more enzyme-substrate complexes form each second.
  • 40 °C is the optimum temperature, where the rate is fastest.
  • Above the optimum, the bonds holding the enzyme in its shape start to break.
  • The shape of the active site changes.
  • The substrate no longer fits the active site, so fewer enzyme-substrate complexes form and the rate falls.
  • By 60 °C the enzyme is denatured, and this change is permanent, so there is almost no reaction.

Model answer

As the temperature rises from 10 °C to 40 °C, the enzyme and the protein in the egg white gain kinetic energy, so they move faster and collide more often. This means more substrate molecules fit into the active site each second and more enzyme-substrate complexes form, so the rate of reaction increases. The rate is fastest at 40 °C, which is the optimum temperature for this protease. Above 40 °C, the extra energy makes the enzyme molecule vibrate so much that some of the bonds holding it in shape break. This changes the shape of the active site, so the substrate no longer fits it. Fewer enzyme-substrate complexes can form, so the rate falls quickly. By 60 °C the active site of almost every enzyme molecule has changed shape, so the enzyme is denatured and there is almost no reaction. Denaturing is permanent, so cooling the enzyme down again would not make it work.

Where marks slip

Writing that the enzyme is “killed” or “destroyed” at high temperature. Enzymes are not alive: say it is denatured because the active site changes shape, so the substrate no longer fits.

6 marks / Aerobic and anaerobic respiration comparedBoth tiers

Compare aerobic respiration with anaerobic respiration in human muscle cells.

What a full answer includes
  • Aerobic respiration needs oxygen, whereas anaerobic respiration happens when there is not enough oxygen.
  • Both use glucose and both are exothermic, releasing energy for the cell, for example for muscle contraction.
  • Aerobic respiration produces carbon dioxide and water, whereas anaerobic respiration in muscles produces only lactic acid.
  • Aerobic respiration releases much more energy from each glucose molecule than anaerobic respiration.
  • This is because in anaerobic respiration glucose is only partly broken down (incomplete oxidation).
  • Aerobic respiration takes place mainly in the mitochondria, whereas anaerobic respiration takes place in the cytoplasm.
  • Lactic acid builds up in anaerobic respiration and can cause muscle fatigue, and extra oxygen is needed afterwards to remove it; aerobic respiration leaves no lactic acid.

Model answer

Both types of respiration break down glucose and are exothermic, so both release energy that the muscle cells can use to contract. Aerobic respiration needs oxygen, whereas anaerobic respiration happens when the muscles cannot get enough oxygen, for example during a sprint. The products are different. Aerobic respiration produces carbon dioxide and water, but anaerobic respiration in muscle cells produces only lactic acid. Aerobic respiration releases much more energy from each glucose molecule than anaerobic respiration does, because in anaerobic respiration the glucose is only partly broken down, so energy is left in the lactic acid. Aerobic respiration takes place mostly in the mitochondria, while anaerobic respiration happens in the cytoplasm. Another difference is that the lactic acid made in anaerobic respiration builds up in the muscles and can make them tire and ache. After exercise the person keeps breathing hard because extra oxygen is needed to deal with the lactic acid, whereas aerobic respiration leaves no lactic acid to remove.

Where marks slip

Writing that respiration “produces” or “makes” energy: energy is released (or transferred). And a compare answer needs both sides in each point, using words like “whereas” or “more than”.

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