Bioenergetics — AQA Combined Science: Trilogy

Bioenergetics is about how organisms get and use energy through photosynthesis (making food) and respiration (releasing energy from food).

Photosynthesis

Photosynthesis takes place in chloroplasts, which contain the green pigment chlorophyll that absorbs light. It is an endothermic reaction — energy is transferred from the environment (light) to the plant.

Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light)

Symbol equation: $$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$

Limiting factors

The rate of photosynthesis is affected by:

• Light intensity — more light, faster rate (until another factor limits it).
• Carbon dioxide concentration — more CO₂, faster rate.
• Temperature — increases rate up to an optimum; too hot and enzymes denature, so the rate falls.
• Amount of chlorophyll — affected by disease or lack of minerals.

A limiting factor is the one in shortest supply that holds back the rate. On a graph, the rate increases then levels off when a different factor becomes limiting.

Light intensity and the inverse square law

Light intensity is inversely proportional to the square of the distance from the source: $$\text{light intensity} \propto \frac{1}{\text{distance}^2}$$ So doubling the distance reduces the light intensity to a quarter.

Required practical: investigating the effect of light intensity on the rate of photosynthesis using pondweed (e.g. Elodea) and counting bubbles of oxygen, varying the distance of a lamp.

Using limiting factors commercially

Greenhouse growers can increase yield by controlling conditions: adding artificial light, using heaters, and increasing CO₂ (e.g. by burning fuel). The cost must be balanced against the increased profit — an example of an economic decision.

Uses of glucose

The glucose made in photosynthesis is used for:

• Respiration — to release energy.
• Converting into starch for storage (insoluble, so it does not affect osmosis).
• Producing cellulose to strengthen cell walls.
• Producing lipids (oils/fats) for storage in seeds.
• Combined with nitrate ions (absorbed from the soil) to make amino acids for proteins.

Respiration

Respiration is an exothermic reaction that occurs continuously in all living cells to release energy from glucose. It is not the same as breathing.

The energy released is used for:

• Movement (muscle contraction).
• Keeping warm (maintaining body temperature in mammals/birds).
• Building larger molecules from smaller ones (e.g. proteins from amino acids).

Energy is transferred and stored in a molecule called ATP.

Aerobic respiration

Uses oxygen and releases the most energy. Occurs in the mitochondria.

word equation: glucose + oxygen → carbon dioxide + water $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

Anaerobic respiration

Occurs without oxygen, releasing much less energy because glucose is not fully broken down.

• In muscles: glucose → lactic acid. This happens during vigorous exercise when oxygen cannot be supplied fast enough.
• In plants and yeast: glucose → ethanol + carbon dioxide. In yeast this is called fermentation and is used to make bread and alcoholic drinks.

Response to exercise

During exercise the body needs more energy, so:

• Heart rate, breathing rate and breath volume increase to supply more oxygen and glucose to muscles and remove CO₂.
• If insufficient oxygen reaches the muscles, they respire anaerobically, producing lactic acid.

Oxygen debt

After exercise, the body needs extra oxygen to react with the built-up lactic acid and remove it (converting it back to glucose in the liver). This extra oxygen needed is the oxygen debt, which is why you keep breathing hard after stopping.

Required practical: investigating the effect of temperature on the rate of respiration in yeast (or measuring how exercise changes heart rate).

Metabolism

Metabolism is the sum of all the chemical reactions in a cell or the body. It includes:

• Converting glucose to starch, glycogen and cellulose.
• Forming lipids from glycerol and fatty acids.
• Using glucose and nitrate ions to form amino acids, then proteins.
• Respiration.
• Breaking down excess proteins to form urea for excretion.

These reactions are controlled by enzymes and require energy from respiration.

Exam tips

• Learn both photosynthesis and respiration equations (word and symbol).
• Remember photosynthesis is endothermic and respiration is exothermic.
• For limiting-factor graphs, explain which factor is limiting at each part of the curve.
• Compare aerobic vs anaerobic respiration: oxygen used, energy released, and products.
• Don't confuse respiration (in cells) with breathing (gas exchange in the lungs).