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Bioenergetics

AQA · GCSE · Biology · Topic 4

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4.1

Bioenergetics: photosynthesis and respiration

Plants capture the Sun's energy; every cell releases it again. This reference covers AQA GCSE Biology 8461, topic 4.4 Bioenergetics.

How the exam treats this topic:

  • Paper 1 carries Bioenergetics. RP6: light intensity and the rate of photosynthesis using pondweed.
  • Limiting-factor graphs with two or three factors, the inverse square law and greenhouse economics are HT only.
  • Equations are word equations — know the symbol forms too (CO₂, H₂O, O₂, C₆H₁₂O₆).
4.1

Photosynthesis (4.4.1.1)
Syllabus

Photosynthesis (AQA 8461 statements 4.4.1.1-4.4.1.2, RP6).

  1. Write the photosynthesis word and symbol equations, and describe it as endothermic.
  2. Explain how light intensity, CO2 concentration and temperature limit the rate of photosynthesis, including interpreting limiting-factor graphs.
  3. (HT) Use the inverse-square relation between light intensity and distance, and reasons for greenhouse economics.
  4. Describe RP6: light intensity and the rate of photosynthesis with pondweed, its controls and the rate calculation.

Source: Cambridge International syllabus

$$\text{carbon dioxide} + \text{water} \xrightarrow{\ \text{light}\ } \text{glucose} + \text{oxygen}$$
$$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$$

Photosynthesis is an endothermic 吸热 reaction: energy is transferred from the environment to the chloroplasts by light.

Vocabulary Train
English
endothermic/ˌendəʊˈθɜːmɪk/
4.1

Rate of photosynthesis and limiting factors (4.4.1.2)
Syllabus

Photosynthesis (AQA 8461 statements 4.4.1.1-4.4.1.2, RP6).

  1. Write the photosynthesis word and symbol equations, and describe it as endothermic.
  2. Explain how light intensity, CO2 concentration and temperature limit the rate of photosynthesis, including interpreting limiting-factor graphs.
  3. (HT) Use the inverse-square relation between light intensity and distance, and reasons for greenhouse economics.
  4. Describe RP6: light intensity and the rate of photosynthesis with pondweed, its controls and the rate calculation.

Source: Cambridge International syllabus

Rate of photosynthesis against one limiting factor at a time.
Each curve rises then flattens where another factor limits.

Rate increases with: light intensity, CO₂ concentration, temperature (to the enzyme optimum), and the amount of chlorophyll.

Reading single-factor graphs: the flat top means something else has become the limiting factor 限制因素. (HT) On two- or three-factor graphs, decide which factor is limiting at each point.

(HT) Inverse square law: doubling the distance from the lamp quarters the light intensity:

$$\text{light intensity} \propto \frac{1}{\text{distance}^2}$$

(HT) Greenhouse economics: adding heat, light or CO₂ raises the rate — but each costs money; the grower adds the cheapest factor that is limiting, up to the point where extra yield no longer pays.

RP6 set-up: pondweed under an inverted funnel in a beaker, with a lamp at a measured distance.

RP6: pondweed in water at different lamp distances; count the oxygen bubbles per minute (or collect the gas); control temperature and CO₂ (sodium hydrogencarbonate); repeat and mean; rate = bubbles ÷ time.

Worked example. Bubbles: 30 per minute at 10 cm; at 20 cm the light intensity is a quarter, so expect about 8 per minute (if light is still limiting).

Vocabulary Train
English
limiting factor/ˈlɪmɪtɪŋ ˈfæktə/
4.1

Uses of glucose (4.4.1.3)
Syllabus

Photosynthesis (AQA 8461 statements 4.4.1.1-4.4.1.2, RP6).

  1. Write the photosynthesis word and symbol equations, and describe it as endothermic.
  2. Explain how light intensity, CO2 concentration and temperature limit the rate of photosynthesis, including interpreting limiting-factor graphs.
  3. (HT) Use the inverse-square relation between light intensity and distance, and reasons for greenhouse economics.
  4. Describe RP6: light intensity and the rate of photosynthesis with pondweed, its controls and the rate calculation.

Source: Cambridge International syllabus

Glucose from photosynthesis is used for:

  • respiration;
  • converted to insoluble starch for storage;
  • fat or oil for storage;
  • cellulose for cell walls;
  • amino acids for protein — this also needs nitrate ions from the soil.
4.2

Respiration (4.4.2.1)
Syllabus

Respiration and metabolism (AQA 8461 statements 4.4.1.3, 4.4.2.1-4.4.2.3).

  1. Name five uses of glucose from photosynthesis.
  2. Compare aerobic and anaerobic respiration in muscle and yeast, with word equations, and fermentation's economic importance.
  3. Explain the body's response to exercise and (HT) oxygen debt and the liver's role.
  4. Define metabolism and list the reactions it includes.

Source: Cambridge International syllabus

Respiration is an exothermic 放热 reaction occurring continuously in living cells, transferring the energy for: building larger molecules, movement, and keeping warm.

Aerobic Anaerobic (muscle) Anaerobic (plant/yeast)
oxygen needed not needed not needed
equation glucose + oxygen → CO₂ + water glucose → lactic acid glucose → ethanol + CO₂
energy much more much less much less

Anaerobic respiration in yeast is fermentation 发酵, economically important in bread (CO₂ makes it rise) and alcoholic drinks.

Vocabulary Train
English
exothermic/eɡzəˈðɜːmɪk/
fermentation/fɜːmənˈteɪʃn/
4.2

Response to exercise (4.4.2.2)
Syllabus

Respiration and metabolism (AQA 8461 statements 4.4.1.3, 4.4.2.1-4.4.2.3).

  1. Name five uses of glucose from photosynthesis.
  2. Compare aerobic and anaerobic respiration in muscle and yeast, with word equations, and fermentation's economic importance.
  3. Explain the body's response to exercise and (HT) oxygen debt and the liver's role.
  4. Define metabolism and list the reactions it includes.

Source: Cambridge International syllabus

During exercise the heart rate, breathing rate and breath volume increase to deliver more oxygenated blood to muscles. If oxygen supply is insufficient, muscles switch to anaerobic respiration: incomplete glucose oxidation builds up lactic acid 乳酸 → fatigue; an oxygen debt 氧债 builds.

(HT) Blood carries lactic acid to the liver, which converts it back to glucose. The oxygen debt is the extra oxygen needed after exercise to react with and remove the accumulated lactic acid.

Vocabulary Train
English
oxygen debt/ˈɒksɪdʒn det/
lactic acid/ˈlæktɪk ˈæsɪd/
4.2

Metabolism (4.4.2.3)
Syllabus

Respiration and metabolism (AQA 8461 statements 4.4.1.3, 4.4.2.1-4.4.2.3).

  1. Name five uses of glucose from photosynthesis.
  2. Compare aerobic and anaerobic respiration in muscle and yeast, with word equations, and fermentation's economic importance.
  3. Explain the body's response to exercise and (HT) oxygen debt and the liver's role.
  4. Define metabolism and list the reactions it includes.

Source: Cambridge International syllabus

Metabolism 代谢 is the sum of all reactions in a cell or body; respiration fuels them. It includes:

  • glucose → starch, glycogen, cellulose;
  • glycerol + 3 fatty acids → lipids;
  • glucose + nitrate ions → amino acids → proteins;
  • respiration;
  • breakdown of excess protein → urea for excretion.
Vocabulary Train
English
Metabolism/məˈtæbəlɪzəm/
4.2

Checklist before you call this topic done
  • Write the photosynthesis word and symbol equations; say endothermic + light→chloroplasts.
  • Explain the four rate factors; read flattening single-factor graphs; (HT) inverse square law and greenhouse economics.
  • Describe RP6 with its controls and the rate calculation.
  • List five uses of glucose (with nitrate for protein).
  • Compare aerobic with both anaerobic equations; name fermentation's two products and uses.
  • Exercise: three increases; lactic acid and oxygen debt; (HT) the liver's role.
  • Define metabolism with its five strands.

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