ScienceCBSE Class 10 · Life Processes

Respiration: Aerobic, Anaerobic and the Human Respiratory System

Updated 14 October 2026 · Fact-checked

Respiration is the breakdown of food, mainly glucose, in cells to release energy, stored as ATP. Glucose first becomes pyruvate in the cytoplasm. With oxygen, pyruvate is broken in mitochondria to carbon dioxide and water (aerobic). Without oxygen, yeast makes ethanol and carbon dioxide, and muscles make lactic acid (anaerobic).

Understand Respiration

What respiration does. Food taken in by nutrition must be broken down inside cells to release usable energy. Some organisms use oxygen to break glucose down completely; others use pathways that do not need oxygen.

Step 1 (all organisms). Glucose, a six-carbon molecule, is broken into a three-carbon molecule, pyruvate, in the cytoplasm.

Step 2: three different fates of pyruvate.

Anaerobic in yeast (fermentation): without oxygen, pyruvate is converted into ethanol and carbon dioxide. It happens in the cytoplasm. Yeast in sugar solution gives a gas that turns lime water milky.
Aerobic (in mitochondria): with oxygen, pyruvate is broken down to carbon dioxide and water. This releases far more energy than the anaerobic pathway.
Lack of oxygen in our muscle cells: during sudden heavy activity, pyruvate is converted to lactic acid (also three-carbon). Its build-up causes muscle cramps.

ATP. Energy released in cellular respiration is immediately used to make ATP, which acts like the cell's energy currency, fuelling processes such as muscle contraction, protein synthesis and nerve impulse conduction.

Gas exchange in plants. Plants exchange gases through stomata, and large intercellular spaces keep all cells in contact with air; oxygen and carbon dioxide move by diffusion. At night, with no photosynthesis, carbon dioxide release is the main exchange. In the day, the carbon dioxide made in respiration is used in photosynthesis, so the main event is oxygen release.

Aquatic animals. Water holds much less dissolved oxygen than air, so aquatic animals breathe much faster. Fish take water in through the mouth and force it over the gills, where blood takes up dissolved oxygen.

The human respiratory system. Breathing organs have a large, very thin surface for exchange, kept inside the body for protection, with passages to bring air in and a way to move air in and out.

Air enters through the nostrils, filtered by fine hairs and mucus.
It passes through the throat; rings of cartilage stop the air passage from collapsing.
In the lungs, the passages branch into smaller and smaller tubes ending in balloon-like alveoli, whose walls have an extensive network of blood vessels.
Breathing in: we lift the ribs and flatten the diaphragm, enlarging the chest cavity, so air is sucked in to fill the alveoli. Blood gives up carbon dioxide into the alveoli and takes up oxygen.
The lungs always hold a residual volume of air, so there is time for oxygen to be absorbed and carbon dioxide released.
In large animals diffusion alone cannot deliver oxygen fast enough, so a respiratory pigment carries it. In humans this is haemoglobin, in red blood corpuscles, which has a very high affinity for oxygen. Carbon dioxide, being more soluble in water than oxygen, travels mostly dissolved in blood.

Respiration: formulas and key facts

First step (cytoplasm): $glucose (6C) \to pyruvate (3C) + energy$; Common to aerobic and anaerobic pathways.
Anaerobic in yeast: $pyruvate no oxygen ethanol + C O_{2} + energy$; Fermentation; cytoplasm.
Aerobic: $pyruvate oxygen, mitochondria C O_{2} + H_{2} O + energy$; Releases much more energy.
Lack of oxygen in muscles: $pyruvate \to lactic acid + energy$; Lactic acid build-up causes cramps.
Overall aerobic respiration: $C_{6} H_{12} O_{6} + 6 O_{2} \to 6 C O_{2} + 6 H_{2} O + energy$; Same equation as in Chapter 1 (an exothermic process).

How to solve Respiration questions

For respiration questions, decide whether it is about the pathway (cell level) or breathing (organ level).

1Pathway questions: start with glucose to pyruvate in the cytoplasm for every case.
2Then branch: oxygen present (mitochondria, $C O_{2}$ + water, most energy); no oxygen in yeast (ethanol + $C O_{2}$ ); oxygen shortage in muscle (lactic acid).
3For comparisons, use a table: site, oxygen, products, energy released, examples.
4For breathing questions, trace the air path: nostrils, throat, bronchi and bronchioles, alveoli.
5Explain adaptations by function: hairs and mucus filter, cartilage rings keep the passage open, alveoli give surface area, blood vessels carry gases, residual air allows time for exchange.
6For transport of gases: oxygen by haemoglobin in RBCs; carbon dioxide mostly dissolved in plasma.

Respiration in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Anaerobic respiration in muscles

Suppose you run a 400 m race on sports day without warming up. In the last 100 m your leg muscles start to ache and cramp, and after the race you breathe heavily for a few minutes.

Explain why the cramp occurs, which molecule builds up, how this pathway differs from aerobic respiration in energy release, and why you keep breathing fast after stopping.

Try it first, then show the solutionSolution

During sudden heavy exercise, muscle cells need energy faster than oxygen can reach them.
Glucose is still broken to pyruvate in the cytoplasm, but with oxygen short, pyruvate is converted to lactic acid.
Lactic acid builds up in the muscles and causes cramps.
This pathway releases much less energy than aerobic respiration, in which pyruvate is fully broken down to carbon dioxide and water in mitochondria.
Breathing fast afterwards supplies extra oxygen so the muscles can return to aerobic respiration.

Answer: Oxygen ran short, so pyruvate became lactic acid, causing cramps; this gives far less energy than aerobic respiration, and fast breathing afterwards restores the oxygen supply.

Memory hook: Sprint too hard, oxygen runs short, lactic acid cramps the legs.

Tricks to remember Respiration

Short memory aids. Say each one aloud once, then use it on a question the same day.

One start, three ends
Glucose → pyruvate always; then Yeast: Ethanol (YE), Air: CO2 + water (A), Muscle: Lactic acid (ML).
Use it when: Drawing the breakdown-of-glucose flow chart.
Mito = More
Mitochondria (aerobic) give More energy.
Use it when: Comparing energy release.
Air path NTBA
Nostrils, Throat, Bronchi, Alveoli.
Use it when: Describing the respiratory pathway.

Respiration: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Aerobic vs anaerobic · 3 marks · Short answerBoard level

Give three differences between aerobic and anaerobic respiration and name one organism that uses the anaerobic mode.

Try it first, then show the solutionSolution

Oxygen: needed in aerobic; not needed in anaerobic (1 mark).
Site of pyruvate breakdown and products: aerobic in mitochondria giving $C O_{2}$ and water; anaerobic in cytoplasm giving ethanol and $C O_{2}$ (yeast) or lactic acid (muscles) (1 mark).
Energy: aerobic releases much more energy. Example of anaerobic: yeast (1 mark).

Answer: Oxygen needed or not; mitochondria vs cytoplasm with different products; much more vs less energy. Yeast.

Example 2: Breakdown of glucose flow chart · 3 marks · Short answerHarder board level

Draw a flow chart showing the breakdown of glucose by three pathways. Indicate the site and the condition for each step.

Try it first, then show the solutionSolution

Glucose (6-carbon) → pyruvate (3-carbon) + energy, in the cytoplasm (1 mark).
Pyruvate → ethanol + carbon dioxide + energy, in absence of oxygen, in yeast (½ mark); pyruvate → lactic acid + energy, lack of oxygen, in muscle cells (½ mark).
Pyruvate → carbon dioxide + water + energy, in presence of oxygen, in mitochondria (1 mark).

Answer: Glucose → pyruvate (cytoplasm); then ethanol + $C O_{2}$ (yeast, no $O_{2}$ ), lactic acid (muscle, less $O_{2}$ ), or $C O_{2}$ + $H_{2} O$ (mitochondria, $O_{2}$ ).

Example 3: Alveoli · 3 marks · Short answerHarder board level

(a) How are the alveoli designed to maximise exchange of gases? (b) Why do the lungs always contain some residual air? (c) What would be the consequence of haemoglobin deficiency?

Try it first, then show the solutionSolution

(a) Alveoli are balloon-like sacs giving a very large surface for exchange; their walls are thin and have an extensive network of blood vessels (1 mark).
(b) So that there is enough time for oxygen to be absorbed into blood and for carbon dioxide to be released (1 mark).
(c) Less oxygen would be carried to body cells, so cells get less energy from respiration; the person would feel tired and breathless (1 mark).

Answer: (a) Large thin surface with dense blood supply. (b) Time for exchange. (c) Less oxygen to tissues, less energy.

Example 4: Fish versus land animals · 3 marks · Short answerHarder board level

(a) Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms? (b) How does a fish obtain oxygen? (c) What advantage does a terrestrial organism have over an aquatic one in obtaining oxygen?

Try it first, then show the solutionSolution

(a) The amount of oxygen dissolved in water is fairly low compared with the oxygen in air, so they must move more water over their breathing surfaces (1 mark).
(b) It takes water in through the mouth and forces it past the gills, where blood takes up the dissolved oxygen (1 mark).
(c) Air contains much more oxygen, so a terrestrial animal can get enough oxygen with a slower breathing rate (1 mark).

Answer: (a) Low dissolved oxygen. (b) Water over gills. (c) Oxygen-rich air allows slower breathing.

Example 5: Lime water and fermentation · 5 marks · Long answerHOTS / competency

Two activities are set up: (i) air is blown through lime water by mouth and, separately, room air is pumped through lime water with a syringe; (ii) yeast in sugar solution is connected by a bent tube to lime water. (a) In (i), which tube turns milky faster and why? (b) What does (ii) show? (c) Why should the yeast set-up be kept stoppered, and what would change if oxygen were bubbled in freely?

Try it first, then show the solutionSolution

(a) The tube with exhaled air turns milky faster, because exhaled air contains more carbon dioxide than room air (1½ marks).
(b) Yeast in sugar solution (fermentation) produces carbon dioxide, which turns lime water milky; fermentation also gives ethanol (1½ marks).
(c) A stoppered tube keeps out fresh air so the yeast respires anaerobically, and makes sure the gas reaching the lime water comes from the yeast (1 mark).
(c) With plenty of oxygen, pyruvate would be broken down aerobically to carbon dioxide and water instead of ethanol; lime water would still turn milky, but the ethanol from fermentation would no longer be the main product (1 mark).

Answer: (a) Exhaled air; it has more $C O_{2}$ . (b) Fermentation releases $C O_{2}$ . (c) To keep conditions anaerobic; with oxygen, aerobic breakdown would replace ethanol formation.

Where students lose marks in Respiration

The slips examiners see most often, and what to write instead.

Mistake: Wrong site for the first step
Students put all of respiration in mitochondria.
Wrong working
Glucose is broken into pyruvate in mitochondria.
Right working
Glucose → pyruvate happens in the cytoplasm; only aerobic breakdown of pyruvate is in mitochondria.
Do this: Learn the two sites with the flow chart.
Usually costs: ½ to 1 mark.
Mistake: Mixing up anaerobic products
Yeast and muscle pathways are swapped.
Wrong working
Muscles make ethanol during exercise.
Right working
Muscles make lactic acid; yeast makes ethanol and carbon dioxide.
Do this: Yeast–ethanol (brewing); muscle–lactic acid (cramps).
Usually costs: 1 mark.
Mistake: Confusing breathing with respiration
The words are used loosely in daily speech.
Wrong working
Respiration is taking in oxygen and giving out carbon dioxide.
Right working
Breathing moves air in and out; respiration breaks down food in cells to release energy.
Do this: Use 'breathing' for the lungs and 'respiration' for the cell process.
Usually costs: 1 mark in definition questions.
Mistake: Vague alveoli answer
Students say only 'they are small'.
Wrong working
Alveoli are small air sacs.
Right working
Alveoli give a very large, thin surface with a rich network of blood vessels for gas exchange.
Do this: Mention surface area, thin walls and blood supply.
Usually costs: 1 mark.

Exam tips

Draw the glucose breakdown flow chart for any 'pathways of respiration' question; it carries most of the marks.
Say 'pyruvate (three-carbon)' and 'glucose (six-carbon)' to show precision.
Link cramps to lactic acid and lack of oxygen, not to tiredness alone.
For the human respiratory system diagram, label nostrils, pharynx or throat, trachea, bronchi, bronchioles, alveoli, lungs, diaphragm, ribs.
Facts in 'Do You Know?' boxes, such as alveolar area figures, are not assessed.

Practice questions on Respiration

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In which part of the cell is pyruvate fully broken down, with oxygen, into carbon dioxide and water?
(a) nucleus (b) chloroplast (c) cytoplasm (d) mitochondria
Hint
This is the aerobic step.
Answer
(d) mitochondria.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Plants release carbon dioxide mainly at night.
Reason (R): During the day, the carbon dioxide produced in respiration is used up in photosynthesis.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
What happens to respiratory $C O_{2}$ when photosynthesis is going on?
Answer
(a). At night there is no photosynthesis, so $C O_{2}$ from respiration is released.
Q3 · 2 marks · Very short answerBoard level
A student says: 'Haemoglobin carries both oxygen and carbon dioxide, so a person who is short of haemoglobin will find it just as hard to get rid of carbon dioxide as to get oxygen.' Evaluate the claim, and say why large animals need a respiratory pigment at all.
Hint
Compare how soluble the two gases are in water.
Answer
The claim is wrong. Oxygen is carried by haemoglobin in red blood corpuscles, but carbon dioxide, being more soluble in water, is carried mostly dissolved in blood; so a shortage of haemoglobin mainly cuts the oxygen supply (1 mark). In a large body, diffusion alone is too slow to bring oxygen to every cell, so a pigment with a high affinity for oxygen picks it up in the lungs and carries it to tissues that lack it (1 mark).
Q4 · 5 marks · Long answerHarder board level
Describe the mechanism of breathing in (inhalation) in human beings and trace the path of air from the nostrils to the blood. Mention two features of the passage that protect or support it.
Hint
Ribs and diaphragm; hairs, mucus and cartilage rings.
Answer
Inhalation: ribs are lifted and the diaphragm flattens, enlarging the chest cavity; air is sucked into the lungs and fills the alveoli (2 marks). Path: nostrils → throat → lungs, where the passage branches into smaller and smaller tubes → alveoli; oxygen diffuses into blood in the alveolar capillaries (2 marks). Features: fine hairs and mucus filter the air in the nostrils; rings of cartilage in the throat prevent the air passage from collapsing (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a baker mixes yeast into dough with some sugar and leaves it in a warm, covered bowl. After a few hours the dough has risen and has small holes inside, and there is a faint smell like that of alcohol.
(a) Name the process carried out by yeast. (1 mark)
(b) Write the two products of this process in yeast and say which one makes the dough rise. (1 mark)
(c) Where in the yeast cell does this happen, and why does this pathway release less energy than the one used by our cells when oxygen is plentiful? (2 marks)
Hint
Anaerobic breakdown stops at a three-carbon product's conversion, not complete breakdown.
Answer
(a) Fermentation (anaerobic respiration). (b) Ethanol and carbon dioxide; the carbon dioxide gas forms bubbles that make the dough rise. (c) In the cytoplasm. Without oxygen, glucose is only partly broken down (to ethanol and $C O_{2}$ ); aerobic respiration in mitochondria breaks pyruvate down completely to $C O_{2}$ and water, releasing much more energy.
Q6 · 3 marks · Short answerHOTS / competency
A student claims: 'Plants do not respire during the day, because they only give out oxygen then.' Evaluate this claim using the direction of gas exchange in plants by day and night.
Hint
Respiration never stops in living cells.
Answer
The claim is wrong. Plants respire all the time. In the day, the carbon dioxide produced by respiration is used up in photosynthesis, which also releases oxygen, so the net exchange is oxygen release. At night, with no photosynthesis, the carbon dioxide of respiration is released and is the main exchange. The direction of diffusion depends on conditions and the plant's needs, not on whether respiration happens.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where is glucose broken into pyruvate?Show answer
Cytoplasm.
2.Products of fermentation in yeast?Show answer
Ethanol and carbon dioxide.
3.What causes muscle cramps during sudden activity?Show answer
Build-up of lactic acid.
4.Energy currency of the cell?Show answer
ATP.
5.Why do fish breathe faster than humans?Show answer
Dissolved oxygen in water is much lower than oxygen in air.
6.Function of cartilage rings in the throat?Show answer
Keep the air passage from collapsing.

Respiration: frequently asked questions

What is the difference between aerobic and anaerobic respiration?

Aerobic respiration uses oxygen, occurs in mitochondria and breaks pyruvate into carbon dioxide and water, releasing much more energy. Anaerobic respiration happens without oxygen and gives ethanol and carbon dioxide in yeast or lactic acid in muscles.

Why do we get muscle cramps after running?

When oxygen is short in muscle cells, pyruvate is converted to lactic acid, and its build-up causes cramps.

How is respiration different from breathing?

Breathing is moving air in and out of the lungs. Respiration is the breakdown of food in cells to release energy.

What is ATP?

A molecule made using the energy released in respiration. Cells break it down to power activities such as muscle contraction and protein synthesis.

Do plants respire at night only?

No. Plants respire all the time; at night the carbon dioxide from respiration is released because there is no photosynthesis to use it.