What you'll learn
This revision guide covers how organisms release energy from nutrients and use it for essential life processes. You'll understand the structure and role of ATP, compare aerobic and anaerobic respiration, and explain how energy from food powers activities like muscle contraction, protein synthesis and maintaining body temperature. These concepts are fundamental to understanding how all living organisms function.
Key terms and definitions
ATP (adenosine triphosphate) — the energy-transfer molecule that releases energy when broken down, used to power cellular processes in all living cells
Respiration — an exothermic reaction that occurs continuously in living cells to release energy from glucose
Aerobic respiration — respiration using oxygen, producing carbon dioxide and water as waste products; releases large amounts of energy
Anaerobic respiration — respiration without oxygen, producing different waste products in animals (lactic acid) and plant cells/yeast (ethanol and carbon dioxide); releases much less energy than aerobic respiration
Metabolism — the sum of all chemical reactions in a cell or organism, including energy-requiring and energy-releasing processes
Glycolysis — the first stage of respiration occurring in the cytoplasm, breaking down glucose into smaller molecules
Mitochondria — cellular organelles where most of the reactions of aerobic respiration take place
Oxygen debt — the amount of extra oxygen the body needs after exercise to react with accumulated lactic acid and remove it from cells
Core concepts
The role of ATP in cells
Energy from respiration cannot be used directly by cells. Instead, energy released from glucose breakdown is transferred to ATP molecules. When cells need energy for biological processes, ATP is broken down to release the stored energy.
ATP is formed when ADP (adenosine diphosphate) joins with an inorganic phosphate group (Pi) using energy from respiration:
ADP + Pi → ATP
When energy is needed, this reaction reverses:
ATP → ADP + Pi (+ energy)
This energy transfer happens constantly in all living cells. ATP acts as a temporary energy store, making it ideal for transferring energy to where it's needed immediately.
Cells use energy transferred by ATP for:
- Muscle contraction — ATP provides energy for muscle fibres to contract during movement
- Protein synthesis — building proteins from amino acids requires energy from ATP
- Cell division — mitosis and meiosis are energy-requiring processes
- Active transport — moving substances against concentration gradients across cell membranes
- Transmission of nerve impulses — maintaining electrical gradients in nerve cells
- Maintaining constant body temperature — particularly important in mammals and birds
Aerobic respiration
Aerobic respiration occurs when oxygen is available. It's the most efficient way to transfer energy from glucose.
The word equation for aerobic respiration:
glucose + oxygen → carbon dioxide + water
The balanced symbol equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration occurs in multiple stages:
- Glycolysis takes place in the cytoplasm, where glucose is partially broken down
- Further reactions occur in the mitochondria, where the majority of ATP is produced
- Oxygen is used in the final stages to accept electrons and combine with hydrogen ions, forming water
A single glucose molecule can produce approximately 32 ATP molecules through aerobic respiration, though the exact number varies. This makes aerobic respiration highly efficient at transferring energy.
Most of the reactions occur inside mitochondria. This explains why cells requiring large amounts of energy (such as muscle cells and sperm cells) contain many mitochondria.
Anaerobic respiration in animals
When insufficient oxygen reaches respiring cells, anaerobic respiration occurs. This happens during vigorous exercise when muscles cannot receive oxygen quickly enough through the bloodstream.
The word equation for anaerobic respiration in muscle cells:
glucose → lactic acid
Anaerobic respiration transfers much less energy than aerobic respiration because glucose is not completely broken down. Only 2 ATP molecules are produced per glucose molecule — approximately 5% of the energy released by aerobic respiration.
During intense exercise:
- Muscles contract vigorously, requiring rapid ATP production
- Heart rate and breathing rate increase to supply more oxygen and glucose
- If oxygen supply remains insufficient, muscles respire anaerobically
- Lactic acid accumulates in muscle tissue
- Lactic acid buildup causes muscle fatigue and pain
After exercise, the body must remove lactic acid. This requires extra oxygen, creating an oxygen debt. You continue breathing heavily after exercise to:
- Take in additional oxygen
- Oxidise lactic acid to carbon dioxide and water
- Convert some lactic acid back to glucose in the liver
Anaerobic respiration in plants and fungi
Plant cells and yeast (a fungus) also respire anaerobically when oxygen is unavailable, but they produce different waste products.
The word equation for anaerobic respiration in plant cells and yeast:
glucose → ethanol + carbon dioxide
This process is called fermentation. Yeast fermentation has important economic applications:
Bread making:
- Yeast is mixed with flour and water
- Yeast respires anaerobically in the dough
- Carbon dioxide gas produces bubbles, making the bread rise
- Ethanol evaporates during baking
Alcoholic drink production:
- Yeast ferments sugars in fruit juice (wine) or grain extracts (beer)
- Ethanol remains in the liquid
- Carbon dioxide escapes or creates carbonation
- Fermentation continues until ethanol concentration becomes too high and kills the yeast
Like animal anaerobic respiration, fermentation produces only 2 ATP molecules per glucose molecule.
Response to exercise
The body responds to exercise in several coordinated ways to meet increased energy demands:
During exercise:
- Muscle cells require more energy for contraction
- Heart rate increases to pump blood faster, delivering more oxygen and glucose to muscles
- Breathing rate increases and breath depth increases to take in more oxygen and expel more carbon dioxide
- Blood flow to muscles increases through vasodilation
- Blood diverts from less essential organs (like the digestive system) to muscles
Short-term vigorous exercise:
When oxygen supply cannot meet demand, anaerobic respiration supplements energy production. This allows temporary bursts of intense activity but cannot be sustained long-term due to lactic acid accumulation.
Recovery after exercise:
- Elevated heart rate and breathing rate continue
- Extra oxygen repays the oxygen debt
- Lactic acid is removed from muscles via the bloodstream
- The liver processes lactic acid, converting it to useful compounds
- Breathing and heart rate gradually return to resting levels
Metabolism and energy
Metabolism encompasses all chemical reactions occurring in organisms. These reactions are organized into pathways and can be categorized as:
Energy-requiring reactions (anabolic):
- Protein synthesis from amino acids
- Building glucose into starch, glycogen or cellulose
- Photosynthesis (converting carbon dioxide and water into glucose)
- Forming lipid molecules from fatty acids and glycerol
Energy-releasing reactions (catabolic):
- Respiration breaking down glucose
- Breaking down excess proteins to form urea for excretion
- Digesting large molecules into smaller ones
The metabolic rate is the rate at which energy stored in food is transferred by all the reactions occurring in the body. Several factors affect metabolic rate:
- Muscle to fat ratio — muscle tissue requires more energy than fat tissue, even at rest
- Exercise — physical activity significantly increases energy demands
- Age — metabolic rate typically decreases with age
- Body size — larger organisms generally have higher total metabolic rates
- Gender — males typically have higher metabolic rates than females due to greater muscle mass
Worked examples
Example 1: Comparing respiration types (4 marks)
Compare aerobic and anaerobic respiration in muscle cells.
Mark scheme answer:
- Both break down glucose to transfer energy (1 mark)
- Aerobic respiration uses oxygen; anaerobic respiration does not (1 mark)
- Aerobic respiration produces carbon dioxide and water; anaerobic respiration produces lactic acid (1 mark)
- Aerobic respiration transfers much more energy / produces approximately 32 ATP compared to 2 ATP from anaerobic respiration (1 mark)
Example 2: Explaining oxygen debt (3 marks)
Explain why a person continues to breathe heavily after finishing a sprint race.
Mark scheme answer:
- During the sprint, muscles respired anaerobically (1 mark)
- This produced lactic acid that accumulated in the muscles (1 mark)
- Extra oxygen is needed after exercise to break down/oxidise the lactic acid (1 mark)
Example 3: ATP and muscle contraction (2 marks)
Describe the role of ATP in muscle contraction.
Mark scheme answer:
- ATP is broken down into ADP and phosphate (1 mark)
- This releases energy that is used for muscle fibres to contract / enables muscles to contract (1 mark)
Common mistakes and how to avoid them
Confusing respiration with breathing — Respiration is a chemical reaction in cells that releases energy; breathing is the physical process of moving air in and out of lungs. Always refer to respiration as a "chemical reaction" in your answers.
Saying ATP is energy — ATP is not energy itself but an energy-transfer molecule. Write that ATP "releases energy when broken down" or "transfers energy" to processes, not that it "is energy."
Thinking anaerobic respiration only happens during exercise — While muscle cells commonly respire anaerobically during intense exercise, plant cells and microorganisms also respire anaerobically when oxygen is unavailable. Context determines which type of anaerobic respiration occurs.
Forgetting carbon dioxide in fermentation — The fermentation equation must include carbon dioxide as well as ethanol: glucose → ethanol + carbon dioxide. Missing either product loses marks.
Mixing up lactic acid and ethanol production — Animal cells produce lactic acid; plant cells and yeast produce ethanol and carbon dioxide. Learn which organism produces which products.
Vague statements about oxygen debt — Don't just say "oxygen is needed to remove lactic acid." Be specific: oxygen is needed to oxidise/break down lactic acid into carbon dioxide and water, or to convert it back to glucose in the liver.
Exam technique for "ATP and energy in biological processes"
Command words matter — "Describe" requires you to state features or processes; "Explain" requires reasons or mechanisms. For "explain" questions about respiration, link cause and effect: oxygen is unavailable therefore anaerobic respiration occurs.
Use chemical equations accurately — Word equations are often acceptable at GCSE, but if you write symbol equations, ensure they're balanced correctly. Practice writing both the word equation and symbol equation for aerobic respiration.
Show your understanding of energy transfer — Rather than saying "ATP stores energy," demonstrate understanding by writing "energy from respiration is transferred to ATP" or "ATP releases energy when broken down to ADP and phosphate."
Link structure to function — When discussing mitochondria, explain that cells with high energy requirements contain many mitochondria because aerobic respiration occurs there. This shows you understand why structure relates to function.
Quick revision summary
Respiration is an exothermic reaction transferring energy from glucose to ATP in all living cells. Aerobic respiration requires oxygen and produces carbon dioxide, water and approximately 32 ATP molecules per glucose. Anaerobic respiration occurs without oxygen, producing lactic acid in animals or ethanol and carbon dioxide in plants/yeast, releasing only 2 ATP molecules. During exercise, heart rate and breathing rate increase to meet energy demands. Oxygen debt occurs when lactic acid accumulates and extra oxygen is required for its removal. Metabolism encompasses all chemical reactions in organisms, with ATP powering energy-requiring processes like muscle contraction, protein synthesis and active transport.