Respiration

What you'll learn

Respiration is the process by which all living cells release energy from glucose. This comprehensive guide covers both aerobic and anaerobic respiration, their locations within cells, word and symbol equations, and practical applications. You'll master the key differences between these processes and understand how organisms use them in different conditions.

Key terms and definitions

Respiration — the chemical process that occurs in all living cells to release energy from glucose; this is not the same as breathing

Aerobic respiration — respiration that requires oxygen and produces carbon dioxide, water, and large amounts of ATP

Anaerobic respiration — respiration without oxygen that produces different products depending on the organism and releases less energy than aerobic respiration

Mitochondria — organelles in the cytoplasm where most aerobic respiration occurs

ATP (adenosine triphosphate) — the molecule that stores and transfers energy in cells for all cellular processes

Lactic acid — the product of anaerobic respiration in animal cells and some bacteria

Ethanol — one of the products of anaerobic respiration in plant cells and yeast (along with carbon dioxide)

Oxygen debt — the amount of extra oxygen needed after anaerobic respiration to break down accumulated lactic acid

Core concepts

The purpose of respiration

Respiration releases energy from glucose in a controlled, step-by-step manner. This energy is not released as heat (as it would be if glucose was simply burned) but is stored in ATP molecules.

Cells use ATP to power all energy-requiring processes:

• Muscle contraction for movement
• Active transport of molecules across cell membranes
• Building large molecules from smaller ones (protein synthesis, DNA replication)
• Cell division
• Maintaining body temperature in mammals
• Transmission of nerve impulses

The process occurs continuously in all living cells, whether plant, animal, fungal, or bacterial.

Aerobic respiration

Aerobic respiration occurs when oxygen is available. It is the most efficient way to release energy from glucose.

Location: Aerobic respiration begins in the cytoplasm but most of the reactions occur in the mitochondria. Cells with high energy demands (like muscle cells and sperm cells) contain more mitochondria.

Word equation: glucose + oxygen → carbon dioxide + water (+ energy)

Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

Key points:

• Produces approximately 38 ATP molecules per glucose molecule
• This is a series of enzyme-controlled reactions, not a single reaction
• The process is exothermic (releases energy)
• Carbon dioxide is produced as a waste product and must be removed from the body
• Water is also produced (metabolic water)

Anaerobic respiration

Anaerobic respiration occurs when insufficient oxygen is available. It releases much less energy than aerobic respiration because glucose is not completely broken down.

In animal cells and bacteria

Word equation: glucose → lactic acid (+ energy)

Symbol equation: C₆H₁₂O₆ → 2C₃H₆O₃ (+ energy)

Key points:

• Produces only 2 ATP molecules per glucose molecule
• Occurs entirely in the cytoplasm
• Lactic acid accumulates in muscle cells during vigorous exercise
• Lactic acid buildup causes muscle fatigue and pain
• The lactic acid must be broken down after exercise, requiring oxygen

In plant cells and yeast

Word equation: glucose → ethanol + carbon dioxide (+ energy)

Symbol equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (+ energy)

Key points:

• Also produces only 2 ATP molecules per glucose molecule
• This process is called fermentation when it occurs in yeast
• Used commercially in brewing and bread-making
• Plants respire anaerobically when waterlogged (roots cannot get oxygen from the soil)

Comparing aerobic and anaerobic respiration

Feature	Aerobic	Anaerobic
Oxygen required?	Yes	No
Location	Cytoplasm and mitochondria	Cytoplasm only
Products (animals)	CO₂ + H₂O	Lactic acid
Products (plants/yeast)	CO₂ + H₂O	Ethanol + CO₂
Energy released	Large amount (~38 ATP)	Small amount (2 ATP)
Glucose breakdown	Complete	Incomplete

Respiration during exercise

During exercise, muscle cells require more energy for contraction. The body responds in several ways:

Before vigorous exercise begins:

• Heart rate increases to deliver more oxygen and glucose to muscles
• Breathing rate and depth increase to take in more oxygen and remove more carbon dioxide
• Glycogen stores in muscles are converted to glucose

During moderate exercise:

• Aerobic respiration increases to meet energy demands
• Heart rate and breathing rate remain elevated
• The body maintains oxygen supply to muscles

During vigorous exercise:

• Oxygen cannot be supplied fast enough for aerobic respiration alone
• Anaerobic respiration begins in muscle cells
• Lactic acid accumulates
• An oxygen debt builds up

After vigorous exercise:

• Heavy breathing continues (panting)
• Heart rate remains elevated
• Extra oxygen is used to break down lactic acid in the liver
• Lactic acid is converted back to glucose or broken down completely to CO₂ and water
• This repays the oxygen debt

Investigating respiration

Several practical investigations demonstrate respiration:

Germinating seeds experiment:

• Germinating seeds respire rapidly to provide energy for growth
• Place seeds in a vacuum flask with a thermometer
• Temperature increases, showing respiration releases energy as heat
• Include a control flask with dead (boiled) seeds
• Use a data logger to record temperature changes over time

Limewater test:

• Organisms respiring aerobically produce carbon dioxide
• Exhaled breath or gas from respiring organisms turns limewater cloudy
• This works with small animals, germinating seeds, or yeast solutions

Yeast fermentation:

• Add yeast to a glucose solution in a test tube
• Attach a delivery tube leading to limewater
• Keep at warm temperature (30-40°C)
• Bubbles of CO₂ appear and limewater turns cloudy
• Ethanol can be detected by smell or using chemical tests

Respirometers:

• Measure oxygen uptake by organisms
• Small organisms (insects, woodlice, germinating seeds) are placed in a sealed container
• Soda lime absorbs CO₂ produced
• A capillary tube with coloured liquid shows oxygen consumption as the liquid moves
• Control for temperature and pressure changes using a second respirometer with dead organisms

Worked examples

Example 1: Calculating energy efficiency

Question: A student investigates respiration in yeast. She adds 1.8g of glucose to a yeast suspension. Calculate the maximum number of ATP molecules that could be produced if all the glucose underwent: (a) aerobic respiration [2 marks] (b) anaerobic respiration [2 marks]

(Relative formula mass of glucose = 180)

Answer:

(a)

• Number of moles of glucose = 1.8 ÷ 180 = 0.01 moles [1 mark]
• Aerobic respiration produces 38 ATP per glucose molecule
• Number of ATP molecules = 0.01 × 38 = 0.38 moles of ATP (or 38 × 10⁻² moles) [1 mark]

(b)

• Number of moles of glucose = 0.01 moles (from part a)
• Anaerobic respiration produces 2 ATP per glucose molecule [1 mark]
• Number of ATP molecules = 0.01 × 2 = 0.02 moles of ATP (or 2 × 10⁻² moles) [1 mark]

Example 2: Interpreting data on exercise

Question: The graph shows a student's heart rate before, during, and after exercise.

[Graph shows heart rate rising from 70 bpm at rest to 160 bpm during exercise, then falling back to 70 bpm over 10 minutes]

(a) Explain why heart rate increases during exercise. [3 marks] (b) Explain why heart rate remains elevated for several minutes after exercise stops. [3 marks]

Answer:

(a)

• Muscles are contracting more / muscle cells need more energy / more respiration occurring [1 mark]
• More oxygen and glucose need to be delivered to muscle cells [1 mark]
• Increased heart rate delivers more blood to muscles / more carbon dioxide needs to be removed [1 mark]

(b)

• Anaerobic respiration occurred during exercise / lactic acid accumulated in muscles [1 mark]
• Lactic acid needs to be broken down / oxygen debt needs to be repaid [1 mark]
• Extra oxygen is needed to convert lactic acid back to glucose in the liver / oxidise lactic acid [1 mark]

Example 3: Practical investigation

Question: A student set up two flasks containing peas as shown below:

Flask A: Living germinating peas with thermometer Flask B: Dead (boiled) peas with thermometer

(a) State why Flask B is necessary in this investigation. [1 mark] (b) Predict which flask will show a temperature increase. Explain your answer. [3 marks] (c) Suggest one way to improve this investigation. [1 mark]

Answer:

(a) Flask B is a control / to show that any temperature change is due to respiration not other factors [1 mark]

(b)

• Flask A will show a temperature increase [1 mark]
• Because the germinating peas are respiring / releasing energy [1 mark]
• Respiration is exothermic / releases energy as heat [1 mark]

(c)

• Use a vacuum flask / insulated container to reduce heat loss
• OR use a data logger to measure temperature more accurately
• OR repeat the experiment and calculate a mean
• OR use the same mass/number of peas in each flask [1 mark for any suitable improvement]

Common mistakes and how to avoid them

• Confusing respiration with breathing: Respiration is a chemical reaction in cells that releases energy from glucose. Breathing (ventilation) is the physical movement of air in and out of the lungs. Always use "respiration" when referring to the chemical process.

• Thinking anaerobic respiration produces the same products in all organisms: Animals produce lactic acid only. Plants and yeast produce ethanol and carbon dioxide. Learn both equations separately.

• Forgetting that respiration occurs in plants: Plants respire 24 hours a day, just like animals. Photosynthesis only occurs during daylight, but respiration is continuous.

• Writing that respiration produces energy: Respiration releases energy that is stored in ATP molecules. Energy cannot be created or destroyed, only transferred between stores.

• Mixing up reactants and products in equations: Glucose and oxygen are reactants (left side); carbon dioxide and water are products (right side) in aerobic respiration. Practice writing equations until you can recall them perfectly.

• Not understanding oxygen debt: The oxygen debt is not the oxygen you didn't get during exercise. It's the extra oxygen needed afterwards to break down accumulated lactic acid in the liver.

Exam technique for "Respiration"

• Command words matter: "State" requires a simple answer without explanation (1 mark). "Explain" requires a reason using because/therefore (usually 2-3 marks). "Compare" means identify similarities and differences, not just describe one type.

• Learn all three equations: You must be able to write word equations for aerobic respiration and both types of anaerobic respiration. For higher tier, learn the symbol equations too. Check your chemical formulae carefully.

• Use data effectively: When given graphs or tables, quote specific values in your answers. For example, "heart rate increased from 70 bpm to 160 bpm" scores more marks than "heart rate increased."

• Link structure to function: When discussing mitochondria or muscle cells, connect the high number of mitochondria to high energy requirements. Examiners reward this type of biological reasoning with marks for application.

Quick revision summary

Respiration releases energy from glucose in all living cells. Aerobic respiration requires oxygen and produces CO₂ and water, releasing lots of energy (38 ATP). Anaerobic respiration occurs without oxygen, producing lactic acid in animals or ethanol and CO₂ in plants/yeast, releasing little energy (2 ATP). During exercise, muscles may respire anaerobically, creating an oxygen debt that must be repaid afterwards. Mitochondria are the site of most aerobic respiration reactions.