Bioenergetics

Bioenergetics

:::info Board Coverage AQA Paper 1 | Edexcel Paper 1 | OCR A Gateway B2 & B3 | WJEC B2 :::

1. Photosynthesis

1.1 The Equation

Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water Into glucose and oxygen.

6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O \xrightarrow{\mathrm{light energy} \mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + 6\mathrm{O_2

Photosynthesis is an endothermic reaction — it absorbs energy from the environment (in the form Of light). This energy is stored in the chemical bonds of glucose, which can later be released Through respiration.

Why this reaction matters. Photosynthesis is the primary source of energy for nearly all life on Earth. It produces the oxygen in the atmosphere (essential for aerobic respiration) and fixes carbon From \mathrm{CO_2 into organic molecules that form the base of all food chains. Without Photosynthesis, there would be no food, no oxygen, and no complex life.

Balancing the equation. To check that the equation is balanced, count the atoms on each side:

Atom	Left side	Right side
Carbon (C)	6	6
Hydrogen (H)	12	12
Oxygen (O)	6 + 6 = 12	6 + 6 = 12

Both sides have the same number of each type of atom, so the equation is balanced.

1.2 Where Photosynthesis Occurs

Photosynthesis takes place in the chloroplasts, which contain the green pigment chlorophyll That absorbs light energy (mainly red and blue wavelengths; green light is reflected, which is why Plants appear green).

Adaptations of the leaf for photosynthesis:

Adaptation	Function
Large surface area	Maximises light absorption
Thin	Short diffusion distance for gases
Chlorophyll in chloroplasts	Absorbs light energy
Stomata (pores on the underside)	Allow \mathrm{CO_2 to enter and \mathrm{O_2 to leave
Palisade mesophyll (near the top)	Packed with chloroplasts for maximum light absorption
Spongy mesophyll (near the bottom)	Air spaces allow gas circulation
Waxy cuticle	Reduces water loss by evaporation
Network of veins (xylem and phloem)	Xylem brings water; phloem removes products

Stomata and gas exchange. Stomata are pores on the underside of the leaf, surrounded by guard Cells that can open and close them. When guard cells are turgid (full of water), the stomata are Open, allowing \mathrm{CO_2 to enter for photosynthesis. When guard cells are flaccid (water has Been lost), the stomata close, reducing water loss. This is a trade-off: open stomata allow Photosynthesis but also allow water to evaporate (transpiration).

1.3 Factors Affecting the Rate of Photosynthesis

Factor	Effect	Reason
Light intensity	Rate increases, then plateaus	More light = more energy, but other factors become limiting
\mathrm{CO_2 concentration	Rate increases, then plateaus	\mathrm{CO_2 is a raw material; limited supply limits the rate
Temperature	Rate increases up to optimum, then decreases	Enzymes work faster; too hot and they denature

Limiting factors: The factor that is in shortest supply limits the rate of photosynthesis. At Any given time, only ONE factor is limiting. This is known as the law of limiting factors (Blackman, 1905).

Understanding the graphs. A graph of photosynthesis rate against any single factor shows the Same characteristic shape: the rate increases linearly at first (because the factor being increased Is the limiting factor), then levels off (because a different factor has become limiting). This Plateau tells you that increasing the original factor further will have no effect until the new Limiting factor is also increased.

Worked Example: Interpreting a limiting factor graph.

A graph shows the effect of light intensity on the rate of photosynthesis at two different \mathrm{CO_2 concentrations (low and high). Both curves show the rate increasing with light Intensity and then levelling off. However, the high \mathrm{CO_2 curve levels off at a higher rate.

Step-by-step interpretation:

1. At low light intensity, both curves rise steeply. Light is the limiting factor. Increasing light increases the rate.
2. At moderate light intensity, the low \mathrm{CO_2 curve begins to level off while the high \mathrm{CO_2 curve is still rising. This means that at this point, \mathrm{CO_2 is limiting for the low \mathrm{CO_2 treatment, but light is still limiting for the high \mathrm{CO_2 treatment.
3. At high light intensity, both curves have levelled off. Neither is limited by light anymore. The low \mathrm{CO_2 curve is limited by \mathrm{CO_2 concentration; the high \mathrm{CO_2 curve may be limited by temperature.
4. The high \mathrm{CO_2 curve reaches a higher maximum rate, showing that increasing \mathrm{CO_2 concentration raises the maximum possible rate of photosynthesis.

1.4 Required Practical: Investigating Photosynthesis

Using pondweed (Elodea):

1. Set up a lamp at a specific distance from a beaker containing pondweed in water.
2. Ensure the pondweed is submerged and the cut end is facing upwards (so oxygen bubbles rise straight up and can be counted).
3. Leave the apparatus for 5 minutes to acclimatise.
4. Count the number of oxygen bubbles produced per minute (or measure the volume of oxygen collected using a gas syringe or inverted measuring cylinder).
5. Repeat at different distances from the lamp (different light intensities).
6. Plot a graph of rate of photosynthesis against light intensity.

Expected graph:

• At low light intensity: rate increases linearly (light is the limiting factor).
• At higher light intensity: rate levels off (\mathrm{CO_2 or temperature is now the limiting factor).

Controls:

• Use sodium hydrogencarbonate solution to ensure a constant \mathrm{CO_2 concentration (it provides dissolved \mathrm{CO_2).
• Keep temperature constant (use a water bath if necessary).
• Use the same piece of pondweed and the same volume of water throughout.

Variables:

• Independent variable: light intensity (distance from lamp, or use a light meter to measure lux).
• Dependent variable: rate of oxygen production (bubbles per minute or volume per minute).
• Control variables: temperature, \mathrm{CO_2 concentration, volume of water, piece of pondweed.

Worked Example: Calculating light intensity.

Light intensity follows an inverse square law: intensity $\propto 1/d^2$Where $d$ is the distance From the lamp.

If the lamp is 10 cm from the pondweed and produces 100 bubbles per minute, what rate would you Expect at 20 cm?

At 20 cm, the distance is doubled, so the light intensity decreases by a factor of $2^2 = 4$. The Rate should decrease by a factor of 4: $100 / 4 = 25$ bubbles per minute (assuming light is still The limiting factor).

1.5 Uses of Glucose from Photosynthesis

Plants use the glucose produced by photosynthesis for:

Use	Description
Respiration	Releases energy for cellular processes
Making cellulose	For cell walls (strengthens plant structure)
Making amino acids	Combined with nitrate ions from the soil; used to make proteins
Stored as starch	Insoluble storage carbohydrate; does not affect water potential
Stored as lipids	In seeds; for energy storage (oils in seeds are energy-dense)

Why plants store starch rather than glucose. Starch is insoluble, so it does not dissolve in the Cell sap and does not affect the water potential of the cell. If plants stored glucose (which is Soluble), water would enter the cells by osmosis, potentially causing them to swell and burst. Starch is also a more compact energy store than glucose because it is a polymer (many glucose Molecules joined together in a small space).

Worked Example: Why nitrate ions are needed.

A farmer notices that a crop of wheat has pale leaves and stunted growth. The soil is deficient in Nitrate ions.

Without nitrate ions, the plant cannot make amino acids (amino acids contain nitrogen, which comes From nitrates). Without amino acids, the plant cannot make proteins. Proteins are needed for growth (new cells require proteins for their structure and enzymes), so the plant cannot grow properly. The Pale leaves are because the plant cannot make enough chlorophyll (chlorophyll contains nitrogen in Its structure), reducing the rate of photosynthesis.

1.6 Higher Tier: Limiting Factors and Greenhouse Management

Commercial greenhouse operators manipulate limiting factors to maximise crop yield:

• Light: Artificial lighting extends the hours of photosynthesis during winter months.
• \mathrm{CO_2: Burning propane heaters or adding \mathrm{CO_2 gas directly increases the \mathrm{CO_2 concentration beyond atmospheric levels ( 0.04%). Greenhouse \mathrm{CO_2 levels can be raised to 0.1% or higher.
• Temperature: Heating systems maintain the optimum temperature year-round.

The economic argument. The cost of providing additional light, \mathrm{CO_2And heat must be Weighed against the value of the increased crop yield. There is a point of diminishing returns where The cost of increasing a factor exceeds the value of the additional growth.

Worked Example: Evaluating greenhouse management decisions.

A tomato grower is considering adding \mathrm{CO_2 enrichment to their greenhouse. The current Yield is 10 kg per plant per year. Adding \mathrm{CO_2 enrichment is expected to increase the yield To 15 kg per plant per year (a 50% increase). The cost of the \mathrm{CO_2 enrichment system is 2000 pounds per year. The tomatoes sell for 2 pounds per kg.

Current revenue per plant: $10 \times 2 = 20$ pounds.

Revenue with \mathrm{CO_2 enrichment: $15 \times 2 = 30$ pounds.

Additional revenue per plant: 10 pounds.

If the greenhouse has 500 plants, the additional revenue is $500 \times 10 = 5000$ pounds per year. Since the cost is 2000 pounds per year, the net gain is $5000 - 2000 = 3000$ pounds per year. The Investment is worthwhile.

1.7 Higher Tier: The Light-Dependent and Light-Independent Reactions

Photosynthesis occurs in two stages:

Light-dependent reactions (in the thylakoid membranes):

• Chlorophyll absorbs light energy.
• Water is split by photolysis: 2\mathrm{H_2\mathrm{O \to 4\mathrm{H^+ + 4e^- + \mathrm{O_2.
• Light energy is used to produce ATP and reduced NADP (NADPH).

Light-independent reactions (Calvin cycle, in the stroma):

• \mathrm{CO_2 is fixed by combining with a 5-carbon compound called RuBP.
• ATP and NADPH from the light-dependent reactions provide energy to convert the products into glucose.
• RuBP is regenerated so the cycle can continue.

The two stages are linked: the light-dependent reactions produce the ATP and NADPH that the Calvin Cycle needs. Without light, the light-dependent reactions stop, and the Calvin cycle gradually runs Out of ATP and NADPH.

Summary table: comparing the two stages of photosynthesis.

Feature	Light-Dependent Reactions	Light-Independent Reactions
Location	Thylakoid membranes	Stroma
Does it need light?	Yes, directly	No (but needs products of LDR)
Inputs	Water, light, ADP, NADP+	\mathrm{CO_2ATP, NADPH
Outputs	Oxygen, ATP, NADPH	Glucose, ADP, NADP+
Key process	Photolysis of water	Carbon fixation by RuBisCO

2. Respiration

2.1 Aerobic Respiration

Aerobic respiration uses oxygen to break down glucose, releasing a large amount of energy.

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + 6\mathrm{O_2 \to 6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O \quad \mathrm{(energy released)

Aerobic respiration occurs in the mitochondria. It is an exothermic reaction.

A common misconception. Many students think that respiration is “breathing.” Respiration is a Chemical process that occurs inside cells; breathing (ventilation) is the physical process of moving Air in and out of the lungs. Breathing supplies the oxygen needed for respiration and removes the \mathrm{CO_2 produced, but it is not respiration itself.

Energy released is used for:

• Muscle contraction (including heartbeat)
• Protein synthesis (building new proteins from amino acids)
• Cell division (DNA replication, mitosis)
• Active transport (moving substances against the concentration gradient)
• Maintaining body temperature (in mammals and birds)
• Transmitting nerve impulses
• Building large molecules (e.g., cellulose, DNA)

2.2 Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen. It releases much less energy than aerobic Respiration because glucose is only partially broken down.

In animals (including humans):

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 \to 2\mathrm{C_3\mathrm{H_6\mathrm{O_3

Glucose is broken down into lactic acid. This builds up in muscles during vigorous exercise, Causing fatigue and cramp. The lactic acid lowers the pH in the muscle cells, which inhibits enzyme Activity and contributes to the feeling of muscle fatigue.

Oxygen debt: After exercise, extra oxygen is needed to break down the lactic acid. This is why You continue to breathe heavily after exercise — your body is taking in extra oxygen to repay the Oxygen debt. The lactic acid is transported to the liver, where it is converted back to pyruvate and Then either oxidised further (via the Krebs cycle) or converted back to glucose (via Gluconeogenesis).

2\mathrm{C_3\mathrm{H_6\mathrm{O_3 + 6\mathrm{O_2 \to 6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O

In yeast (fermentation):

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 \to 2\mathrm{C_2\mathrm{H_5\mathrm{OH + 2\mathrm{CO_2

Yeast converts glucose into ethanol (alcohol) and carbon dioxide. This process is used in:

• Brewing: Ethanol makes alcoholic drinks (beer, wine, spirits). The ethanol concentration eventually becomes toxic to the yeast, which is why fermented drinks have a limited alcohol content ( up to about 15%).
• Baking: \mathrm{CO_2 causes bread to rise. The ethanol evaporates during baking.

2.3 Response to Exercise

During exercise, the body’s demand for energy increases. Several changes occur to meet this demand:

• Breathing rate increases (to take in more \mathrm{O_2 and remove more \mathrm{CO_2)
• Heart rate increases (to pump oxygenated blood faster to muscles)
• Arteries supplying muscles dilate (widen) to increase blood flow
• Glycogen stores in muscles and the liver are converted to glucose for respiration

Why these changes happen. Muscles need more ATP for contraction during exercise. ATP is produced By aerobic respiration, which requires oxygen and glucose. Increasing breathing rate and heart rate Delivers more oxygen and glucose to the muscles and removes more \mathrm{CO_2. Dilating the Arteries increases blood flow, further improving delivery.

Worked Example: Calculating oxygen debt.

A student runs 400 metres and builds up 120 mg of lactic acid in their muscles. After the race, they Breathe heavily to repay the oxygen debt.

To oxidise lactic acid fully:

2\mathrm{C_3\mathrm{H_6\mathrm{O_3 + 6\mathrm{O_2 \to 6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O

The molar mass of lactic acid is 90 g/mol. So 120 mg = 0.12 g = $0.12/90 = 0.00133$ mol.

From the equation, 2 moles of lactic acid require 6 moles of \mathrm{O_2.

So 0.00133 mol of lactic acid requires $0.00133 \times 3 = 0.004$ mol of \mathrm{O_2.

At room temperature and pressure, 1 mole of gas occupies approximately 24 litres.

Volume of \mathrm{O_2 needed: $0.004 \times 24000 = 96$ mL of extra oxygen.

This is why the student continues to breathe heavily for several minutes after the race — they need To take in this extra oxygen to fully oxidise the accumulated lactic acid.

2.4 Metabolism

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

• Building larger molecules from smaller ones (anabolism): e.g. Proteins from amino acids, cellulose from glucose, lipids from fatty acids and glycerol.
• Breaking down larger molecules into smaller ones (catabolism): e.g. Glucose in respiration, proteins into amino acids, glycogen into glucose.

The liver plays a central role in metabolism:

• Detoxification: Breaks down harmful substances (alcohol is broken down into ethanal, then into ethanoate, which can be used in respiration).
• Glycogen storage: Converts excess glucose to glycogen for storage (glycogenesis) and converts glycogen back to glucose when needed (glycogenolysis).
• Deamination: Converts excess amino acids (which cannot be stored) into ammonia (a toxic by-product), which is then converted to urea in the ornithine cycle. Urea is excreted by the kidneys.
• Bile production: Produces bile, which emulsifies fats and neutralises stomach acid.

Summary of liver functions.

Function	Process	Description
Detoxification	Oxidation	Alcohol $\to$ ethanal $\to$ ethanoic acid
Glycogenesis	Glucose $\to$ glycogen	Stores excess glucose
Glycogenolysis	Glycogen $\to$ glucose	Releases glucose when blood sugar is low
Deamination	Amino acid $\to$ urea	Removes amino group; urea excreted by kidneys
Bile production	—	Emulsifies fats; neutralises stomach acid

3. Required Practical: Respiration

3.1 Investigating the Rate of Respiration

Method (using respirometer):

1. Place germinating peas (actively respiring) in a test tube connected to a capillary tube containing coloured liquid.
2. Use a control tube with dead (boiled) peas.
3. Measure the distance the coloured liquid moves along the capillary tube in a set time.
4. The movement indicates oxygen consumption.
5. Place soda lime in the apparatus to absorb \mathrm{CO_2 (so that the change in volume is due to \mathrm{O_2 consumption only).

Why soda lime is essential. Respiration produces \mathrm{CO_2 as well as consuming \mathrm{O_2. If the \mathrm{CO_2 were not absorbed, the volume change would be the difference Between \mathrm{O_2 consumed and \mathrm{CO_2 produced, not the true rate of oxygen consumption. Since soda lime absorbs \mathrm{CO_2The measured volume change reflects only \mathrm{O_2 Consumption.

3.2 Interpreting Results

• Living peas: liquid moves towards the peas (\mathrm{O_2 consumed, reducing gas volume in the tube).
• Dead peas: no movement (no respiration).
• The rate of respiration = distance moved / time.

Worked Example: Calculating the rate of respiration.

A respirometer containing germinating peas shows that the coloured liquid moves 12 mm along the Capillary tube in 5 minutes. The capillary tube has an internal diameter of 1 mm.

Step 1: Calculate the cross-sectional area of the capillary tube.

A = \pi r^2 = \pi \times (0.5)^2 = 0.785 \mathrm{ mm^2

Step 2: Calculate the volume of oxygen consumed.

V = A \times d = 0.785 \times 12 = 9.42 \mathrm{ mm^3 = 0.00942 \mathrm{ mL

Step 3: Calculate the rate.

\mathrm{Rate = 0.00942 / 5 = 0.00188 \mathrm{ mL/min

4. Higher Tier: Aerobic and Anaerobic Respiration Compared

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen required?	Yes	No
Location	Mitochondria	Cytoplasm
Products	\mathrm{CO_2 and \mathrm{H_2\mathrm{O	Lactic acid (animals) or ethanol + \mathrm{CO_2 (yeast)
Energy released	Large (approximately 38 ATP per glucose)	Small (2 ATP per glucose)
Speed	Slower	Faster (but less efficient)
Toxicity of products	Non-toxic	Lactic acid / ethanol can be toxic

Why anaerobic respiration is faster but less efficient. Aerobic respiration involves the Complete oxidation of glucose through glycolysis, the Krebs cycle, and the electron transport chain. This produces many ATP but requires oxygen and involves many steps. Anaerobic respiration only Involves glycolysis (the first stage of respiration), which is fast because it involves fewer steps, But it produces only 2 ATP per glucose because the rest of the energy remains locked in the lactic Acid or ethanol.

5. Higher Tier: Investigating the Effect of Temperature on Respiration Rate

Method:

1. Set up respirometers with germinating peas at different temperatures (e.g., 10$^{\circ}$C, 20$^{\circ}$C, 30$^{\circ}$C, 40$^{\circ}$C, 50$^{\circ}$C).
2. Measure the rate of oxygen consumption at each temperature.
3. Plot a graph of respiration rate against temperature.

Expected graph:

• Rate increases from 10$^{\circ}$C to approximately 40$^{\circ}$C (increased kinetic energy, more enzyme-substrate collisions).
• Rate peaks at approximately 40$^{\circ}$C (the optimum temperature for the enzymes involved).
• Rate decreases sharply above 40$^{\circ}$C (enzymes denature).

This graph has the same characteristic shape as the enzyme activity graph, because respiration is Controlled by enzymes.

6. Higher Tier: Photosynthesis vs. Respiration — A Detailed Comparison

Feature	Photosynthesis	Respiration
Organisms	Plants, algae, some bacteria	All living organisms
Location in plant	Chloroplasts	Cytoplasm and mitochondria
Raw materials	\mathrm{CO_2 and \mathrm{H_2\mathrm{O	Glucose and \mathrm{O_2
Products	Glucose and \mathrm{O_2	\mathrm{CO_2 and \mathrm{H_2\mathrm{O
Energy change	Endothermic (stores energy)	Exothermic (releases energy)
When it occurs	Only in light	24 hours a day
Reactants/Products	They are essentially reverse reactions	—

Key point about the relationship. In the dark, plants only respire. In bright light, the rate of Photosynthesis exceeds the rate of respiration, so the plant has a net uptake of \mathrm{CO_2 and Net release of \mathrm{O_2. At a certain light intensity (the compensation point), the rate of Photosynthesis exactly equals the rate of respiration, so there is no net gas exchange.

Common Pitfalls

• Confusing photosynthesis and respiration. Photosynthesis: \mathrm{CO_2 + \mathrm{H_2\mathrm{O $\to$ glucose + \mathrm{O_2 (requires light, endothermic). Respiration: glucose + \mathrm{O_2 $\to$ \mathrm{CO_2 + \mathrm{H_2\mathrm{O (releases energy, exothermic).
• Writing the word “energy” in the respiration equation. Energy is not a substance — it is released. The word equation should only list chemical substances.
• Thinking plants only photosynthesise. Plants respire 24 hours a day; photosynthesis only occurs in light. In the dark, plants only respire. In bright light, the rate of photosynthesis exceeds the rate of respiration, so the plant has a net uptake of \mathrm{CO_2 and net release of \mathrm{O_2.
• Confusing the products of anaerobic respiration in animals and yeast. Animals produce lactic acid; yeast produces ethanol and \mathrm{CO_2. If you mix these up in an exam, you will lose marks.
• Stating that anaerobic respiration releases more energy than aerobic. It releases much LESS energy (2 ATP vs. Approximately 38 ATP per glucose) because glucose is only partially broken down.
• Forgetting that the oxygen debt must be repaid. Lactic acid must be broken down using extra oxygen after exercise. This is why breathing rate remains elevated after exercise stops.
• Confusing the roles of the light-dependent and light-independent reactions. The light-dependent reactions need light directly and produce ATP and NADPH. The Calvin cycle (light-independent) does not need light directly but needs the products of the light-dependent reactions.
• Thinking that respiration only happens in animals. All living organisms respire, including plants. Respiration is not the same as breathing.
• Confusing chlorophyll and chloroplast. Chlorophyll is the green pigment that absorbs light; the chloroplast is the organelle that contains chlorophyll.
• Forgetting that the inverse square law applies to light intensity. Doubling the distance from a light source reduces the light intensity to one quarter, not one half.

Practice Questions

1. Explain how the structure of a leaf is adapted for efficient photosynthesis.

2. A student investigates the effect of light intensity on the rate of photosynthesis using pondweed. Describe how they would carry out this investigation and sketch the expected graph.

3. Explain the difference between aerobic and anaerobic respiration in terms of the products formed and the amount of energy released.

4. Describe the changes that occur in the body during exercise and explain why these changes are necessary.

5. Explain why plants store glucose as starch rather than as a liquid sugar.

6. Describe how anaerobic respiration in yeast is used in the baking and brewing industries.

7. Explain what is meant by oxygen debt and how it is repaid after vigorous exercise.

8. A gardener wants to increase the rate of photosynthesis in a greenhouse. Describe two methods they could use and explain the science behind each.

9. Describe the role of the liver in metabolism.

10. Explain why a sprinter breathes heavily for several minutes after completing a 100 m race.

11. (Higher Tier) Explain the concept of a limiting factor in photosynthesis and describe how a farmer could use this knowledge to increase crop yield in a greenhouse.

12. (Higher Tier) Explain why the rate of respiration increases with temperature up to an optimum and then decreases, with reference to enzyme activity.

13. A student sets up a respirometer to investigate the effect of temperature on the rate of respiration in germinating peas. Describe how the student would ensure the results are valid and reliable.

14. Explain why photosynthesis and respiration are described as opposite processes in terms of their chemical equations, but both are essential for the survival of a plant.

15. Describe and explain the effect of increasing \mathrm{CO_2 concentration on the rate of photosynthesis at (a) low light intensity and (b) high light intensity.

16. Explain why a plant with yellow leaves (lacking chlorophyll) would struggle to survive, with reference to both photosynthesis and respiration.

17. A student measures the rate of oxygen production by pondweed at different temperatures. The results show that the rate peaks at 35$^{\circ}$C and drops to zero at 50$^{\circ}$C. Explain these results.

18. Explain the process of deamination in the liver and describe what happens to the products.

19. Compare and contrast aerobic respiration and fermentation in yeast, including the products, energy yield, and conditions required.

20. A farmer adds nitrate fertiliser to a field of wheat. Explain why this increases the growth rate of the wheat, with reference to the uses of glucose in plants.

21. Explain why soda lime is used in respirometers and what would happen if it were omitted.

22. (Higher Tier) Explain what is meant by the compensation point and describe how the net gas exchange of a plant changes as light intensity increases from zero to bright sunlight.

23. Describe the process of glycogenesis and glycogenolysis in the liver, and explain why these processes are important for maintaining blood glucose levels.

24. A student claims that “plants release oxygen during the day and carbon dioxide at night.” Evaluate this claim, explaining when it is correct and when it is not.

25. Explain why athletes who train at high altitudes may have an advantage when competing at sea level, with reference to respiration and oxygen transport.

7. Higher Tier: Investigating the Rate of Photosynthesis with Algal Balls

A practical method for measuring the rate of photosynthesis uses immobilised algae (algal balls) Suspended in a hydrogen carbonate indicator solution.

Method:

1. Mix algae with sodium alginate solution and drop the mixture into calcium chloride solution to form small, spherical algal balls (each ball contains many algal cells).
2. Place a known number of algal balls into a test tube containing hydrogen carbonate indicator (which changes colour depending on the pH, which changes with \mathrm{CO_2 concentration).
3. Expose the test tube to light of a specific intensity for a set time.
4. As photosynthesis occurs, the algae absorb \mathrm{CO_2Raising the pH. The indicator changes from red/orange (high \mathrm{CO_2) to purple (low \mathrm{CO_2).
5. Use a colorimeter to measure the exact colour change, which corresponds to the amount of \mathrm{CO_2 absorbed.
6. Repeat at different light intensities, \mathrm{CO_2 concentrations, or temperatures.

Advantages of using algal balls:

• Each ball contains approximately the same number of algal cells, making results more reproducible.
• The balls are easy to handle and can be reused.
• The method is quantitative (produces numerical data rather than subjective bubble counts).

Worked Example: Interpreting algal ball results.

A student finds that at 20$\degree$C with algal balls in hydrogen carbonate indicator, the colour Changes from red to purple in 8 minutes. At 5$\degree$C, no colour change occurs after 20 minutes.

Explanation: At 5$\degree$C, the kinetic energy of the enzyme molecules (including RuBisCO, Which catalyses carbon fixation) is very low. There are fewer successful enzyme-substrate Collisions, so the rate of photosynthesis is too slow to produce a detectable change in \mathrm{CO_2 concentration within 20 minutes. At 20$\degree$C, the enzymes are working at a rate Sufficient to absorb enough \mathrm{CO_2 to cause the colour change within 8 minutes.

8. Higher Tier: The Importance of Minerals in Plant Nutrition

Plants require a range of mineral ions, absorbed from the soil by active transport in the root Hairs, for healthy growth:

Mineral Ion	Function	Deficiency Symptom
Nitrate (\mathrm{NO_3^-)	Making amino acids and proteins, chlorophyll	Stunted growth, yellowing of older leaves
Magnesium (\mathrm{Mg^{2+})	Component of chlorophyll molecule	Yellowing between leaf veins (chlorosis)
Phosphate (\mathrm{PO_4^{3-})	Making DNA, cell membranes, ATP	Poor root growth, purple leaves
Potassium (\mathrm{K^+)	Regulating stomatal opening and closing, enzyme activation	Wilting, yellow leaves with dead spots
Calcium (\mathrm{Ca^{2+})	Making cell walls (calcium pectate)	Stunted growth, leaves curl

Why mineral deficiencies affect photosynthesis. Both nitrates and magnesium are needed for Chlorophyll production. Without chlorophyll, leaves cannot absorb light energy, and photosynthesis Cannot occur. Without nitrates, the plant cannot make proteins, so new cells cannot be built and Growth stops. Without phosphate, the plant cannot make ATP (the energy currency of the cell) or DNA, So cell division cannot occur.

Worked Example: Diagnosing a mineral deficiency.

A tomato plant has leaves that are yellow between the veins but the veins themselves remain green. The older leaves are affected first.

Diagnosis: This is a classic symptom of magnesium deficiency. Magnesium is a central Component of the chlorophyll molecule. Without magnesium, the plant cannot synthesise chlorophyll, So the leaves lose their green colour (chlorosis). The yellowing appears between the veins because The veins still contain some chlorophyll. Older leaves are affected first because the plant Redistributes its limited magnesium supply to younger, growing leaves.

9. Higher Tier: Respirometer Calculations in Context

Worked Example: Comparing respiration rates.

A student sets up two respirometers. Respirometer A contains 5 g of germinating peas at 20$\degree$C. Respirometer B contains 5 g of germinating peas at 10$\degree$C. After 10 minutes, the Liquid in A moves 15 mm and the liquid in B moves 8 mm. The capillary tube has an internal diameter Of 0.8 mm.

Calculate the rate of oxygen consumption per gram per minute for each respirometer.

Respirometer A:

Cross-sectional area $= \pi \times (0.4)^2 = 0.503$ $mm^2$.

Volume consumed in 10 minutes $= 0.503 \times 15 = 7.54$ $mm^3$.

Rate per minute $= 7.54 / 10 = 0.754$ $mm^3$/min.

Rate per gram per minute $= 0.754 / 5 = 0.151$ $mm^3$/g/min.

Respirometer B:

Volume consumed in 10 minutes $= 0.503 \times 8 = 4.02$ $mm^3$.

Rate per minute $= 4.02 / 10 = 0.402$ $mm^3$/min.

Rate per gram per minute $= 0.402 / 5 = 0.080$ $mm^3$/g/min.

Conclusion: The respiration rate at 20$\degree$C is approximately 1.9 times higher than at 10$\degree$C. This is because the enzymes involved in respiration have higher kinetic energy at 20$\degree$C, leading to more frequent enzyme-substrate collisions and a faster rate of reaction.

10. Higher Tier: Fermentation in Industry

Yeast fermentation is used in several important industrial processes:

Baking: Yeast is added to bread dough, where it ferments sugars to produce \mathrm{CO_2 and Ethanol. The \mathrm{CO_2 gets trapped in the dough, causing it to rise (leavening). The ethanol Evaporates during baking (temperatures in an oven reach 180—220$\degree$C, well above Ethanol’s boiling point of 78$\degree$C). Bread makers control the temperature and humidity during Proving (the initial fermentation stage) to optimise yeast activity.

Brewing: Yeast ferments sugars in malted barley (or other grains) to produce ethanol. The Process involves:

1. Malting: Barley grains are soaked in water and allowed to germinate. Amylase enzymes break down starch into maltose sugar.
2. Mashing: The malted barley is mixed with hot water to extract the sugars.
3. Fermentation: Yeast is added to the sugary liquid (wort). Anaerobic conditions are maintained so the yeast produces ethanol.
4. Conditioning: The beer is stored to allow flavours to develop.

The ethanol concentration in fermented drinks is limited to about 15% because ethanol is Toxic to yeast at higher concentrations. Stronger alcoholic drinks require distillation.

Biofuel production: Yeast can ferment sugars from plant material (e.g., corn, sugarcane) to Produce bioethanol, which can be used as a renewable fuel. The advantage of bioethanol is that it is Carbon-neutral in principle: the \mathrm{CO_2 released when it is burned was originally absorbed by The plants during photosynthesis. However, growing biofuel crops requires land that could otherwise Be used for food production, creating a conflict between energy and food security.

11. Higher Tier: Investigating the Effect of Carbon Dioxide Concentration on Photosynthesis

A student investigates how \mathrm{CO_2 concentration affects the rate of photosynthesis using Pondweed.

Method:

1. Set up five boiling tubes, each containing pondweed in water with sodium hydrogencarbonate at different concentrations (0%, 1%, 2%, 3%, 4%).
2. Place all tubes at the same distance from a lamp (same light intensity).
3. Maintain all tubes at the same temperature using a water bath.
4. Count the number of oxygen bubbles produced per minute in each tube.
5. Plot a graph of rate of photosynthesis against \mathrm{CO_2 concentration.

Expected results:

• At 0% \mathrm{CO_2: no photosynthesis (no carbon dioxide available for carbon fixation).
• From 0% to approximately 3%: the rate increases as \mathrm{CO_2 concentration increases.
• Above approximately 3%: the rate plateaus because another factor (light or temperature) has become limiting.

Controlled variables: light intensity, temperature, volume of water, piece of pondweed, time Allowed for acclimatisation.

Worked Example: Interpreting the results.

At 1% \mathrm{CO_2The rate is 5 bubbles/min. At 2% \mathrm{CO_2The rate is 9 bubbles/min. At 3% \mathrm{CO_2The rate is 10 bubbles/min. At 4% \mathrm{CO_2The rate is 10 bubbles/min.

Between 1% and 2%, the rate increases from 5 to 9 bubbles/min (an increase of 4). Between 2% and 3%, The rate increases from 9 to 10 (an increase of only 1). Between 3% and 4%, there is no increase at All. This shows that above 3% \mathrm{CO_2, \mathrm{CO_2 is no longer the limiting factor. Another Factor (light intensity or temperature) is now limiting, and increasing \mathrm{CO_2 further has no Effect.

12. Higher Tier: Anaerobic Respiration in Sport

Understanding anaerobic respiration is important for sports science. During intense exercise, the Body’s demand for energy exceeds what aerobic respiration can supply, even with increased heart rate And breathing rate. The muscles switch to anaerobic respiration to produce additional ATP.

The lactate threshold: The exercise intensity at which lactate begins to accumulate in the blood Faster than it can be removed. Above this threshold, the athlete is relying increasingly on Anaerobic respiration.

Training effects: Endurance training increases the lactate threshold by improving the body’s Ability to deliver oxygen to muscles (increased capillary density, increased myoglobin content) and By increasing the number and size of mitochondria in muscle cells. This allows trained athletes to Sustain higher exercise intensities before reaching their lactate threshold.

Recovery: After intense exercise, the accumulated lactate is transported to the liver, where it Is converted back to pyruvate (by the enzyme lactate dehydrogenase). The pyruvate can then be Oxidised through the Krebs cycle (if oxygen is available) or converted back to glucose (gluconeogenesis). This process requires oxygen, explaining the continued heavy breathing after Exercise stops.

Practice Problems

Question 1: Photosynthesis rate factors

A student investigates the effect of light intensity on the rate of photosynthesis in pond weed. Describe the shape of the graph produced and explain why the rate plateaus.

Answer

The graph shows the rate of photosynthesis increasing linearly at low light intensity, then curving and eventually plateauing. At low light, light is the limiting factor and more light means more energy for the light-dependent reactions. The rate plateaus when another factor (such as \mathrm{CO_2 concentration or temperature) becomes limiting — increasing light intensity further has no effect because the Calvin cycle cannot work any faster.

Question 2: Respiration equations

Write the word equation for aerobic respiration. A student claims that plants do not carry out respiration. Evaluate this claim.

Answer

Glucose + oxygen $\to$ carbon dioxide + water (+ energy released).

The claim is incorrect. Plants carry out both photosynthesis (during the day) and respiration (continuously, day and night). During the day, the rate of photosynthesis exceeds respiration, so there is a net release of oxygen. At night, only respiration occurs, so plants take in oxygen and release carbon dioxide.

Question 3: Investigating respiration rate

Describe an experiment using respirometers to measure the rate of aerobic respiration in germinating seeds. Explain the purpose of the soda lime and the control.

Answer

Germinating seeds are placed in a respirometer with soda lime (to absorb \mathrm{CO_2). As seeds respire, they consume \mathrm{O_2 and release \mathrm{CO_2. The soda lime absorbs the \mathrm{CO_2So the only gas change is a reduction in \mathrm{O_2. The coloured liquid in the capillary tube moves towards the seeds as oxygen is consumed. The rate of movement gives the respiration rate.

A control respirometer contains dead seeds (boiled). Any movement in the control accounts for non-biological changes in gas volume (e.g., temperature or pressure changes). The experimental result is corrected by subtracting the control movement.

Question 4: Uses of glucose in plants

List three ways that plants use the glucose produced by photosynthesis, and for each use, explain why it is important for the plant.

Answer

1. Respiration: glucose is broken down to release energy (ATP) for cellular processes.
2. Storage: glucose is converted to starch for storage in leaves (temporarily) and in storage organs like potatoes.
3. Cell walls: glucose is converted to cellulose, which makes up the cell wall for structural support.
4. Making other substances: glucose can be converted to lipids for seed oils, amino acids for proteins, and chlorophyll.

(Any three with explanations.)

Question 5: Fermentation in yeast

Describe how yeast is used in the production of bread and beer, explaining the role of fermentation in each process.

Answer

In bread-making, yeast carries out anaerobic respiration (fermentation), converting glucose to ethanol and carbon dioxide. The \mathrm{CO_2 gets trapped in the dough, causing it to rise, giving bread its light, airy texture. The ethanol evaporates during baking.

In beer production, yeast ferments sugars in the wort (malted barley extract), producing ethanol and \mathrm{CO_2. The ethanol is the desired product (alcohol in the beer), and the \mathrm{CO_2 provides carbonation. The process is carried out in anaerobic conditions to ensure fermentation rather than aerobic respiration.

Worked Examples

Example 1:

A typical exam question on Bioenergetics requires you to apply your knowledge to an unfamiliar context. Read the question carefully, identify the key concept being tested, and structure your answer using the appropriate terminology.

Example 2:

Multi-step problems in Bioenergetics often combine two or more concepts. Break the problem down: identify what you need to find, recall the relevant formula or principle, substitute values, and state your answer with correct units or formatting.

Summary

This topic covers the biological principles of bioenergetics, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

• photosynthesis (light-dependent and independent)
• transpiration and water transport
• plant hormones and tropisms
• reproduction in flowering plants
• plant responses to the environment

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.