4.4 Bioenergetics — coverage pack

6 specification leaves · notes, questions, answers and worked methods

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

1. State the word equation for photosynthesis.[2 marks]
Answer
- Carbon dioxide and water are the reactants.
- Glucose and oxygen are the products.
Method: Place the substances taken in by the plant on the left and the substances made on the right: carbon dioxide + water $\rightarrow$ glucose + oxygen.

Tier 2 · Standard

1. 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.[4 marks]
Answer
- The reactants are $\mathrm{CO_2}$ and $\mathrm{H_2O}$ .
- The products are $\mathrm{C_6H_{12}O_6}$ and $\mathrm{O_2}$ .
- Light transfers energy from the environment.
- The energy is transferred to the chloroplasts, so the reaction is endothermic.
Method: Match the formulae to the word equation carbon dioxide + water $\rightarrow$ glucose + oxygen. Award one point for both reactant formulae, one for both product formulae, one for light transferring energy from the environment, and one for that energy entering the chloroplasts.

Tier 3 · Hard

1. 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 marks]
Answer
- Carbon dioxide is a reactant in photosynthesis, so it is used up.
- Oxygen is a product of photosynthesis, so it is released.
- Water is also a reactant and glucose is also a product.
- Light transfers the energy needed for the endothermic reaction to the chloroplasts.
Method: Start with carbon dioxide + water $\rightarrow$ glucose + oxygen. A reactant falls as it is consumed, whereas a product rises as it is formed. Then identify light as the energy transfer into the chloroplasts rather than as a reactant.

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

1. A pondweed shoot releases $18\,\mathrm{cm^3}$ of oxygen in $6\,\mathrm{min}$ . Calculate its mean rate of oxygen production.[2 marks]
Answer
- $\text{rate}=\text{oxygen volume}/\text{time}$
- $3\,\mathrm{cm^3\,min^{-1}}$
Method: Divide the measured oxygen volume by the elapsed time: $18\,\mathrm{cm^3}/6\,\mathrm{min}=3\,\mathrm{cm^3\,min^{-1}}$ .

Tier 2 · Standard

1. 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.[4 marks]
Answer
- Change light intensity, for example by changing the lamp's distance from the pondweed.
- Measure oxygen volume produced in a fixed time, or measure the time taken to produce a fixed volume.
- Calculate a rate from oxygen volume and time.
- Control two suitable variables, such as temperature, pondweed length or carbon dioxide concentration.
Method: Change only the factor being investigated. Use oxygen production per unit time as the dependent measure, then name at least two other conditions that must remain constant so that a change in rate can be attributed to light intensity.

Tier 3 · Hard

1. 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.[6 marks]
Answer
- Doubling distance makes intensity one quarter as large by the inverse-square law.
- Predicted intensity $=96/4=24$ arbitrary units.
- Carbon dioxide is not limiting under these conditions because adding it has no effect.
- The low light intensity is the limiting factor, so extra light could increase the rate.
- The grower should compare the value of the increased yield with the cost of lighting.
- Extra lighting is worthwhile only if the extra income exceeds its cost, with other factors kept sufficient.
Method: Use $I\propto1/d^2$ : $I_2=96\times(20/40)^2=96\times1/4=24$ . No response to added carbon dioxide means that carbon dioxide is not the current limit; the fall in light identifies light as limiting. The economic decision depends on whether the added crop value is greater than the lighting cost and whether another factor would then limit the rate.

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

1. State four uses of glucose made by a plant in photosynthesis.[4 marks]
Answer
- Respiration
- Conversion to insoluble starch for storage
- Production of fats or oils for storage
- Production of cellulose or amino acids
Method: Choose four distinct destinations for glucose from the specification: respiration, starch storage, fat or oil storage, cellulose formation, or amino-acid formation.

Tier 2 · Standard

1. Explain why a plant converts glucose to starch for storage and describe how the plant uses glucose to make proteins.[4 marks]
Answer
- Starch is insoluble, so it is suitable for storage.
- The plant absorbs nitrate ions from the soil.
- It uses glucose and nitrate ions to make amino acids.
- Amino acids are joined to synthesise proteins.
Method: Treat storage and protein synthesis as separate chains. Link glucose to insoluble starch, then link glucose plus absorbed nitrate ions to amino acids and finally to proteins.

Tier 3 · Hard

1. 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.[5 marks]
Answer
- Protein production will be reduced.
- Nitrate ions are needed with glucose to make amino acids.
- Amino acids are needed to synthesise proteins.
- Starch, cellulose and oils can all be made using glucose from photosynthesis.
- Their formation does not require nitrate ions as a raw material in the stated pathways.
Method: Follow the pathway glucose + nitrate ions $\rightarrow$ amino acids $\rightarrow$ proteins. A shortage of nitrate interrupts this route. The specified routes from glucose to starch, cellulose and fats or oils do not use nitrate ions, so those conversions can still occur if glucose is available.

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

1. State the word equation for aerobic respiration.[2 marks]
Answer
- Glucose and oxygen are the reactants.
- Carbon dioxide and water are the products.
Method: Write the substances used before the arrow and the substances formed after it: glucose + oxygen $\rightarrow$ carbon dioxide + water.

Tier 2 · Standard

1. Compare aerobic respiration in human cells with anaerobic respiration in human muscle cells. Refer to oxygen, products and energy transfer.[4 marks]
Answer
- Aerobic respiration requires oxygen, whereas anaerobic respiration does not.
- Aerobic respiration produces carbon dioxide and water.
- Anaerobic respiration in muscles produces lactic acid.
- Aerobic respiration transfers much more energy because glucose oxidation is complete; anaerobic oxidation is incomplete.
Method: Compare the same three features in turn: oxygen requirement, reaction products and relative energy transfer. Link the smaller anaerobic energy transfer to incomplete oxidation of glucose.

Tier 3 · Hard

1. 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.[6 marks]
Answer
- The yeast respires anaerobically because oxygen is absent.
- Glucose is converted into ethanol and carbon dioxide.
- This process in yeast is called fermentation.
- Glucose is only partly oxidised, so much less energy is transferred than in aerobic respiration.
- Carbon dioxide can make bread dough rise.
- Ethanol is used in the manufacture of alcoholic drinks.
Method: Use the yeast anaerobic word equation: glucose $\rightarrow$ ethanol + carbon dioxide. Name this fermentation, connect incomplete oxidation with lower energy transfer, then link each product to one specified economic use.

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

1. State three changes that occur in the human body during exercise.[3 marks]
Answer
- Heart rate increases.
- Breathing rate increases.
- Breath volume increases.
Method: Recall the three named responses that deliver more oxygenated blood to active muscles: a faster heart, more frequent breaths and a larger volume per breath.

Tier 2 · Standard

1. Explain why a runner's muscles may become fatigued during a long period of vigorous exercise.[4 marks]
Answer
- The muscles have an increased demand for energy.
- The oxygen supply may be insufficient, so anaerobic respiration occurs.
- Glucose is incompletely oxidised and lactic acid builds up.
- The muscles become fatigued and stop contracting efficiently.
Method: Build a cause-and-effect chain from high energy demand to inadequate oxygen supply, then anaerobic respiration, lactic-acid accumulation and less efficient muscle contraction.

Tier 3 · Hard

1. 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.[5 marks]
Answer
- Anaerobic respiration during the sprint caused lactic acid to accumulate.
- The athlete has an oxygen debt after exercise.
- Oxygen debt is the extra oxygen needed to react with accumulated lactic acid and remove it from cells.
- Blood transports lactic acid from the muscles to the liver.
- In the liver, the lactic acid is converted back into glucose.
Method: Connect the post-exercise breathing response to the extra oxygen required after anaerobic respiration. Then trace lactic acid in order: it enters the blood, travels from muscle to liver and is converted back into glucose.

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

1. Define metabolism and name one metabolic process.[2 marks]
Answer
- Metabolism is the sum of all reactions in a cell or the body.
- One valid process is respiration, synthesis of a named larger molecule, or breakdown of excess protein to form urea.
Method: Give the complete definition, including all reactions rather than only energy-releasing ones, then select one process from the specified examples.

Tier 2 · Standard

1. Describe how the smaller molecules used to make a lipid and a protein differ in metabolism.[4 marks]
Answer
- A lipid is formed from one molecule of glycerol.
- It also requires three molecules of fatty acids.
- Amino acids are used to synthesise proteins.
- Plants form amino acids using glucose and nitrate ions.
Method: State the building blocks for each large molecule separately: glycerol plus fatty acids for a lipid, and amino acids for a protein. Then give the specified plant sources used to form amino acids.

Tier 3 · Hard

1. Explain how respiration, synthesis and excretion are linked within metabolism. Your answer should refer to glucose, amino acids, proteins and urea.[6 marks]
Answer
- Respiration of glucose transfers energy.
- This energy supports continual enzyme-controlled metabolic processes.
- In plants, glucose and nitrate ions are used to make amino acids.
- Amino acids are assembled to synthesise proteins.
- Excess proteins are broken down.
- This breakdown forms urea, which is excreted.
Method: Link the processes in sequence: respiration transfers energy for enzyme-controlled reactions; glucose and nitrate ions supply amino acids; amino acids build proteins; and excess protein is broken down to urea for removal. Together these synthesis and breakdown reactions form part of metabolism.