SubjectsAI Test GeneratorRevision PlanMock ExamsPricing
Back to Biology

Respiration

Cambridge

9 subtopics

Subtopics

Unlock all subtopics

Upgrade

Revision Method

Study Plan

Aerobic Respiration

Revision Notes

Key Points

  • Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy (ATP), occurs in mitochondria
  • Exothermic process releasing energy for movement, active transport, synthesis, temperature maintenance, and cell processes
  • Produces 32-38 ATP molecules per glucose - much more efficient than anaerobic respiration (2 ATP)
  • Active cells contain many mitochondria to meet high energy demands; mitochondria have folded inner membrane for large surface area
  • Occurs continuously in all living cells; rate affected by temperature, oxygen availability, glucose, and activity level

Overview

Aerobic respiration is the process by which cells release energy from glucose using oxygen. It occurs continuously in all living cells, providing energy for essential life processes. Understanding aerobic respiration is fundamental to biology, explaining how organisms obtain energy from food and why we breathe.

Definition

Aerobic respiration is the chemical process that uses oxygen to break down glucose and release energy in cells.

Word equation:

Glucose + Oxygen → Carbon dioxide + Water + Energy

Symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

This is an exothermic reaction - it releases energy as heat and ATP.

Location of Respiration

Aerobic respiration occurs in the mitochondria - the "powerhouses" of the cell.

Mitochondria structure:

  • Rod-shaped or spherical organelles
  • Double membrane: outer membrane and folded inner membrane (cristae)
  • Inner compartment contains enzymes for respiration
  • Matrix (fluid interior) where many reactions occur

Why mitochondria?

  • Large surface area (folded inner membrane) for enzyme attachment
  • Contain enzymes needed for respiration reactions
  • Isolated compartment allows control of conditions

Active cells (e.g., muscle, liver, nerve cells) have many mitochondria because they need lots of energy. Less active cells (e.g., fat storage cells) have fewer mitochondria.

Why Do Cells Need Energy?

Energy from respiration (as ATP molecules) is used for:

  1. **Movement**
  • Muscle contraction
  • Cell movement
  • Moving substances across cell membranes
  1. **Active transport**
  • Moving molecules against concentration gradient
  • Requires energy (unlike diffusion)
  • E.g., absorbing glucose in intestines, mineral uptake in roots
  1. **Synthesis of large molecules**
  • Making proteins from amino acids
  • Making starch/glycogen from glucose
  • Making lipids from fatty acids and glycerol
  • DNA replication
  • Cell division
  1. **Maintaining body temperature**
  • In mammals and birds (endotherms)
  • Heat released during respiration keeps body warm
  • Especially important in cold environments
  1. **Transmitting nerve impulses**
  • Electrical signals along neurons require energy
  1. **Cell division and growth**
  • Mitosis and meiosis require energy
  • Growth involves making new cells and molecules

ATP - The Energy Currency

ATP (Adenosine Triphosphate) is the molecule that stores and transfers energy in cells.

  • Energy from respiration is stored in ATP molecules
  • ATP is the **immediate energy source** for cellular processes
  • When energy is needed, ATP → ADP + P + Energy
  • ATP can be quickly remade: ADP + P + Energy (from respiration) → ATP
  • This allows rapid energy release when required

Advantages of ATP:

  • Universal energy carrier in all living organisms
  • Releases energy quickly when needed
  • Can be remade rapidly
  • Small, soluble, easily transported in cells

The Respiration Process

Aerobic respiration involves multiple stages:

1. Glycolysis (in cytoplasm)

  • Glucose (6-carbon) broken into two pyruvate molecules (3-carbon each)
  • Small amount of ATP produced
  • Does not require oxygen

2. Krebs Cycle and Electron Transport Chain (in mitochondria)

  • Pyruvate broken down completely
  • Oxygen used
  • Large amounts of ATP produced
  • Carbon dioxide and water released

Overall, one glucose molecule produces approximately 32-38 ATP molecules through aerobic respiration.

Comparing Aerobic and Anaerobic Respiration

| Feature | Aerobic | Anaerobic |

|---------|---------|-----------|

| Oxygen needed? | Yes | No |

| Location | Mitochondria (mainly) | Cytoplasm |

| Products (animals) | CO₂ + H₂O | Lactic acid |

| Products (yeast/plants) | CO₂ + H₂O | Ethanol + CO₂ |

| Energy released | Large (32-38 ATP) | Small (2 ATP) |

| Glucose breakdown | Complete | Incomplete |

| When occurs | Most of the time | During intense exercise, low oxygen |

Aerobic respiration is much more efficient - releases about 19 times more energy per glucose molecule than anaerobic.

Evidence for Respiration

Experiments can demonstrate respiration by detecting:

1. Carbon dioxide production

  • Use hydrogen carbonate indicator or limewater
  • Limewater turns milky/cloudy with CO₂
  • Hydrogen carbonate indicator: red → yellow with CO₂

2. Oxygen consumption

  • Use oxygen sensor or respirometer
  • Oxygen levels decrease as organisms respire
  • Can measure rate of oxygen uptake

3. Heat production

  • Respiration releases heat energy
  • Temperature increases around respiring organisms
  • E.g., germinating seeds warm up

4. Water production

  • Difficult to measure directly
  • Can collect and test for water

Factors Affecting Respiration Rate

1. Temperature

  • Increases temperature → faster respiration (up to optimum)
  • Enzymes work faster at higher temperatures
  • Too high → enzymes denature → respiration stops

2. Oxygen availability

  • More oxygen → faster aerobic respiration
  • No oxygen → switch to anaerobic respiration

3. Glucose availability

  • More glucose → faster respiration
  • Limited glucose → less energy released

4. Cell/Organism activity

  • Active cells/organisms respire faster
  • Exercise increases respiration rate dramatically

Respiration in Different Organisms

Animals:

  • Continuous aerobic respiration in all cells
  • Switch to anaerobic during intense exercise (oxygen debt)
  • Breathe to obtain oxygen and remove carbon dioxide

Plants:

  • Respire 24 hours a day (day and night)
  • During day: photosynthesis produces oxygen, respiration uses some
  • During night: only respiration occurs, oxygen from air needed
  • Plants respire slower than animals generally

Microorganisms:

  • Bacteria and fungi respire
  • Some use aerobic, some anaerobic, some both
  • Yeast switches between aerobic and anaerobic depending on oxygen availability

Exam Tips

  • Always write the full balanced equation including "energy" as a product
  • Distinguish clearly between aerobic (with oxygen) and anaerobic (without oxygen)
  • Link mitochondria to respiration - more mitochondria in active cells
  • Explain that respiration is not the same as breathing (breathing supplies oxygen for respiration)
  • Remember respiration occurs in ALL living cells ALL the time
  • Use specific examples of energy uses: muscle contraction, active transport, protein synthesis
  • Don't confuse respiration with photosynthesis - they are opposite processes
  • In exam calculations, use the equation to work out reactant/product quantities