GCSE Biology Bioenergetics

Photosynthesis

Definition: The process by which plants use light energy to make glucose from carbon dioxide and water.

Photosynthesis Word Equation

Carbon dioxide + water → glucose + oxygen

Photosynthesis Symbol Equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Where Photosynthesis Happens

In the chloroplasts of plant cells (mainly in the leaves).

Role of Chlorophyll

Chlorophyll absorbs light energy needed for photosynthesis.

It is found in the chloroplasts and gives plants their green colour.

Limiting Factors of Photosynthesis

Factors that can restrict the rate of photosynthesis when in short supply:

•  Light intensity

•  Carbon dioxide concentration

•  Temperature

•  Amount of chlorophyll

Temperature as a Limiting Factor

Rate of photosynthesis increases with temperature up to the optimum.

Above the optimum, enzymes denature and the rate falls.

Aerobic Respiration

Definition: The process of releasing energy from glucose using oxygen.

Aerobic Respiration Word Equation

Glucose + oxygen → carbon dioxide + water (+ energy)

Aerobic Respiration Symbol Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

Where Aerobic Respiration Happens

In the mitochondria of cells.

Anaerobic Respiration in Animals

Definition: Respiration without oxygen.

Equation: Glucose → lactic acid (+ energy)

Lactic Acid and Muscle Fatigue

During intense exercise, lactic acid builds up in muscles.

This causes muscle fatigue and pain (oxygen debt).

Anaerobic Respiration in Plants and Yeast

Definition: Respiration without oxygen in plants and yeast.

Equation: Glucose → ethanol + carbon dioxide (+ energy)

Also called fermentation.

Comparison: Aerobic vs Anaerobic

Aerobic: needs oxygen, lots of energy, products = CO₂ + water

Anaerobic: no oxygen, little energy, products = lactic acid (animals) or ethanol + CO₂ (plants/yeast)

Oxygen Debt

Definition: The extra oxygen needed after exercise to break down the lactic acid that has built up.

How Oxygen Debt is Repaid

Breathing rate and heart rate stay high after exercise.

Lactic acid is transported to the liver and converted back to glucose using oxygen.

Metabolism

Definition: The sum of all the chemical reactions that take place in a cell or the whole organism.

Examples of Metabolic Reactions

•  Respiration

•  Building proteins from amino acids

•  Converting glucose to starch/glycogen/cellulose

•  Breaking down excess proteins (forming urea)

Investigating Rate of Photosynthesis

Common method: Count bubbles of oxygen produced by pondweed, or measure volume of oxygen.

Independent variable: light intensity / CO₂ / temperature

Dependent variable: rate of oxygen production

Effect of Temperature on Enzyme-Controlled Reactions

Both photosynthesis and respiration are controlled by enzymes.

Rate increases with temperature up to the optimum, then decreases as enzymes denature.

Energy Released in Respiration

Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration.

This is why aerobic respiration is preferred when oxygen is available.