Bioenergetics

Section 4.4

6 spec leafs

Notes and three levels of exam-style practice for each registered specification leaf in this section.

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4.4.1.1

Photosynthetic reaction

Photosynthesis uses light to convert carbon dioxide and water into glucose and oxygen: $6\,\mathrm{CO_2}+6\,\mathrm{H_2O}\rightarrow\mathrm{C_6H_{12}O_6}+6\,\mathrm{O_2}$ .
Photosynthesis is an endothermic reaction because light transfers energy from the environment to the chloroplasts.
In the symbol equation, $\mathrm{CO_2}$ is carbon dioxide, $\mathrm{H_2O}$ is water, $\mathrm{C_6H_{12}O_6}$ is glucose and $\mathrm{O_2}$ is oxygen.
A common error is to say that photosynthesis creates energy: energy is transferred by light and stored in the products of the reaction.

Tier 1 · Easy

2 marks

ORIGINAL

State the word equation for photosynthesis.

Tier 2 · Standard

4 marks

ORIGINAL

The formulae $\mathrm{CO_2}$ , $\mathrm{H_2O}$ , $\mathrm{O_2}$ and $\mathrm{C_6H_{12}O_6}$ occur in photosynthesis. Identify the two reactants and two products, and explain why photosynthesis is endothermic.

Tier 3 · Hard

4 marks

ORIGINAL

A student keeps an illuminated green plant in a sealed transparent container. Over time, carbon dioxide decreases and oxygen increases. Use the photosynthetic reaction to explain both changes and the role of light.

4.4.1.2

Rate of photosynthesis

Increasing light intensity or carbon dioxide concentration can increase the rate of photosynthesis; low temperature slows the reactions, very high temperature denatures enzymes, and less chlorophyll means less light is absorbed.
In the pondweed required practical, vary light intensity, measure oxygen production over a known time, calculate rate as oxygen produced divided by time, and control variables such as temperature, pondweed length and carbon dioxide availability.
Photosynthesis data may be presented numerically or graphically, so select sensible axis scales, plot points accurately and use the graph's shape to describe how one factor affects rate.
Higher tier: interacting factors can each become limiting; light intensity follows an inverse-square relationship with distance, and greenhouse growers balance the yield gained by adding heat, light or carbon dioxide against the financial cost.

Tier 1 · Easy

2 marks

ORIGINAL

A pondweed shoot releases $18\,\mathrm{cm^3}$ of oxygen in $6\,\mathrm{min}$ . Calculate its mean rate of oxygen production.

Tier 2 · Standard

4 marks

ORIGINAL

Plan an investigation using pondweed to determine how light intensity affects the rate of photosynthesis. Include the independent variable, the measurement used to calculate rate, and two control variables.

Tier 3 · Hard

6 marks

ORIGINAL

Higher tier: A lamp provides a light intensity of $96$ arbitrary units at $20\,\mathrm{cm}$ from pondweed. Predict the intensity at $40\,\mathrm{cm}$ . Moving the lamp to this distance reduces the photosynthesis rate, and adding carbon dioxide does not restore it. Explain the result and state what a greenhouse grower should consider before paying for extra lighting.

4.4.1.3

Uses of glucose from photosynthesis

Plants use some glucose in respiration to transfer energy for living processes and convert some into insoluble starch for storage.
Glucose can be converted into fats or oils for storage and into cellulose, which strengthens plant cell walls.
Plants use glucose with nitrate ions absorbed from the soil to make amino acids, which are then used to synthesise proteins.
A common error is to treat starch, cellulose and protein as substances taken directly from the soil: the plant makes them using products of photosynthesis, while the soil supplies mineral ions such as nitrate.

Tier 1 · Easy

4 marks

ORIGINAL

State four uses of glucose made by a plant in photosynthesis.

Tier 2 · Standard

4 marks

ORIGINAL

Explain why a plant converts glucose to starch for storage and describe how the plant uses glucose to make proteins.

Tier 3 · Hard

5 marks

ORIGINAL

A plant receives enough light, water and carbon dioxide but very few nitrate ions. Predict which glucose-derived substance will be made less successfully and explain why the plant can still make starch, cellulose and oils.

4.4.2.1

Aerobic and anaerobic respiration

Cellular respiration is a continuously occurring exothermic reaction in living cells that transfers energy for movement, keeping warm and reactions that build larger molecules.
Aerobic respiration uses oxygen and is represented by $\mathrm{C_6H_{12}O_6}+6\,\mathrm{O_2}\rightarrow6\,\mathrm{CO_2}+6\,\mathrm{H_2O}$ ; it transfers much more energy than anaerobic respiration.
Without sufficient oxygen, muscle cells carry out incomplete oxidation of glucose to lactic acid, while plant and yeast cells convert glucose to ethanol and carbon dioxide; both anaerobic pathways transfer less energy.
Anaerobic respiration in yeast is fermentation and is economically important in making bread and alcoholic drinks; a common error is to give ethanol and carbon dioxide as the products in human muscles.

Tier 1 · Easy

2 marks

ORIGINAL

State the word equation for aerobic respiration.

Tier 2 · Standard

4 marks

ORIGINAL

Compare aerobic respiration in human cells with anaerobic respiration in human muscle cells. Refer to oxygen, products and energy transfer.

Tier 3 · Hard

6 marks

ORIGINAL

Yeast in a sealed vessel is supplied with glucose but no oxygen. Explain the reaction that occurs, why it transfers less energy than aerobic respiration, and two ways its products are useful commercially.

4.4.2.2

Response to exercise

Exercise increases the muscles' demand for energy, so heart rate, breathing rate and breath volume increase to supply the muscles with more oxygenated blood.
If oxygen supply is insufficient, muscle cells respire anaerobically; incomplete oxidation of glucose causes lactic acid to accumulate and creates an oxygen debt.
During prolonged vigorous activity, lactic acid accumulation is associated with muscle fatigue, so the muscles stop contracting efficiently.
Higher tier: blood carries lactic acid from muscles to the liver, where it is converted back into glucose; oxygen debt is the extra oxygen needed after exercise to react with accumulated lactic acid and remove it from cells.

Tier 1 · Easy

3 marks

ORIGINAL

State three changes that occur in the human body during exercise.

Tier 2 · Standard

4 marks

ORIGINAL

Explain why a runner's muscles may become fatigued during a long period of vigorous exercise.

Tier 3 · Hard

5 marks

ORIGINAL

Higher tier: After a sprint, an athlete continues to breathe deeply and rapidly. Explain this response using oxygen debt, and describe what happens to the lactic acid made in the muscles.

4.4.2.3

Metabolism

Metabolism is the sum of all reactions in a cell or the body, and energy transferred by respiration supports its continual enzyme-controlled processes.
Metabolic reactions convert glucose into the carbohydrates starch, glycogen and cellulose, and respiration breaks down glucose to transfer energy.
One glycerol molecule combines with three fatty acid molecules to form a lipid, while glucose and nitrate ions are used to make amino acids that are assembled into proteins.
Excess proteins are broken down to form urea for excretion; a common error is to describe metabolism as only breakdown, because it includes both the synthesis and breakdown of molecules.

Tier 1 · Easy

2 marks

ORIGINAL

Define metabolism and name one metabolic process.

Tier 2 · Standard

4 marks

ORIGINAL

Describe how the smaller molecules used to make a lipid and a protein differ in metabolism.

Tier 3 · Hard

6 marks

ORIGINAL

Explain how respiration, synthesis and excretion are linked within metabolism. Your answer should refer to glucose, amino acids, proteins and urea.