Cambridge IGCSE0610

Photosynthesis

Biology 0610 Chapter Notes

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In this chapter

  1. 1The Photosynthesis Equation
  2. 2Site of Photosynthesis: Chloroplasts
  3. 3Limiting Factors of Photosynthesis
  4. 4Investigating Photosynthesis
  5. 5Photosynthesis vs Respiration
  6. 6The Fate of Glucose

1. The Photosynthesis Equation

Photosynthesis is the fundamental process plants use to create their own food. They are 'autotrophs', meaning 'self-feeders'. Using energy from sunlight, they convert simple inorganic molecules, carbon dioxide (from the air) and water (from the soil), into a sugar called glucose. This process also releases oxygen as a by-product. Think of it as a plant's way of cooking, where the sun is the oven, and carbon dioxide and water are the ingredients.

carbon dioxide + water → glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Key term

Photosynthesis: The process by which plants manufacture carbohydrates from raw materials using energy from light.

Examiner insight

Examiners require the full word equation, including both reactants and both products, for full marks. Simply writing 'makes food' is not sufficient.

Common pitfall

Confusing photosynthesis with respiration. Remember, photosynthesis builds up food (glucose) and respiration breaks it down.

Worked example 12 marks

A student writes the word equation for photosynthesis but makes two mistakes. Their equation is: carbon dioxide + oxygen → glucose + water. Identify and correct the two mistakes.

  1. 1

    Step 1: Identify the reactants and products in the correct equation. Reactants are carbon dioxide and water. Products are glucose and oxygen.

  2. 2

    Step 2: Compare this to the student's equation. The student has listed oxygen as a reactant, but it is a product.

  3. 3

    Step 3: The student has listed water as a product, but it is a reactant.

  4. 4

    Step 4: The corrected equation is: carbon dioxide + water → glucose + oxygen.

Worked example 23 marks

State the balanced chemical equation for photosynthesis.

  1. 1

    Step 1: Recall the chemical formulas for the reactants and products. Carbon Dioxide is CO₂, Water is H₂O, Glucose is C₆H₁₂O₆, and Oxygen is O₂.

  2. 2

    Step 2: Write the unbalanced equation: CO₂ + H₂O → C₆H₁₂O₆ + O₂.

  3. 3

    Step 3: Balance the atoms on both sides. To get 6 carbons in glucose, you need 6CO₂. To get 12 hydrogens in glucose, you need 6H₂O. This gives 18 oxygen atoms on the left (12 from 6CO₂ and 6 from 6H₂O).

  4. 4

    Step 4: Balance the oxygen on the right. C₆H₁₂O₆ has 6 oxygens, so you need 12 more. This means you need 6O₂ molecules (6 x 2 = 12).

  5. 5

    Step 5: The final balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Recap

  • Photosynthesis converts light energy into chemical energy stored in glucose.
  • The raw materials (reactants) are carbon dioxide and water.
  • The useful product is glucose, and the by-product is oxygen.
  • The word equation is carbon dioxide + water → glucose + oxygen.
  • The balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Quick check

  1. Name the gas that is a waste product of photosynthesis.1 mark
  2. What is the chemical formula for glucose?1 mark

2. Site of Photosynthesis: Chloroplasts

Photosynthesis doesn't happen just anywhere in a plant. It occurs inside specialised organelles called chloroplasts. These are tiny green discs found mainly in the cells of leaves, specifically the palisade mesophyll and spongy mesophyll layers. Chloroplasts contain a green pigment called chlorophyll, which is brilliant at absorbing light energy. Chlorophyll is what gives plants their characteristic green colour. Essentially, a chloroplast acts as a tiny solar-powered factory, converting light energy into the chemical energy of glucose.

Key term

Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis.

Common pitfall

Stating that the whole plant or all plant cells photosynthesise. It's only the parts with chloroplasts, primarily the leaves and sometimes green stems.

Fun fact

A single square millimetre of a leaf can contain around 500,000 chloroplasts.

Worked example 13 marks

Explain the role of chlorophyll and chloroplasts in photosynthesis.

  1. 1

    Step 1: State the location. Chlorophyll is a pigment located inside organelles called chloroplasts.

  2. 2

    Step 2: Describe the function of chlorophyll. Its primary role is to absorb or trap light energy from the sun.

  3. 3

    Step 3: Describe the function of the chloroplast. It is the site of photosynthesis; it provides the enzymes and membranes needed for the chemical reactions to convert the trapped light energy, CO₂ and water into glucose.

Recap

  • Photosynthesis takes place in chloroplasts.
  • Chloroplasts are mainly found in leaf cells.
  • Chloroplasts contain the green pigment chlorophyll.
  • Chlorophyll's job is to absorb light energy.
  • Chloroplasts convert light energy to chemical energy.

Quick check

  1. In which organelle does photosynthesis occur?1 mark
  2. What is the name of the green pigment that traps light?1 mark

3. Limiting Factors of Photosynthesis

The rate of photosynthesis isn't always constant. It's controlled by several factors. The rate is limited by whichever factor is in shortest supply. This is called the 'limiting factor'. Imagine an assembly line: if you run out of one part, the whole line stops, no matter how many of the other parts you have. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration, and temperature.

Key term

Limiting Factor: A factor that restricts the rate of a process because it is in the shortest supply.

Examiner insight

When interpreting graphs of limiting factors, always state which factor is limiting in each section of the curve and explain why the rate changes or plateaus.

Fun fact

Commercial greenhouses in the Netherlands, a major vegetable exporter, often enrich the air to 3 times the normal atmospheric CO₂ level to boost plant growth.

Worked example 14 marks

The graph shows how light intensity affects the rate of photosynthesis. Explain what is happening in section A and section B of the graph.

  1. 1

    Step 1 (Section A): Describe the trend. In section A, as light intensity increases, the rate of photosynthesis also increases.

  2. 2

    Step 2 (Section A): Explain the trend. This is because light intensity is the limiting factor. More light provides more energy for the reaction, so the rate increases.

  3. 3

    Step 3 (Section B): Describe the trend. In section B, the graph flattens. Increasing the light intensity further has no effect on the rate of photosynthesis.

  4. 4

    Step 4 (Section B): Explain the trend. This is because light is no longer the limiting factor. Something else, such as carbon dioxide concentration or temperature, is now in short supply and is limiting the rate.

Worked example 23 marks

A farmer wants to grow tomatoes in a greenhouse in winter. List three factors they could control to increase the rate of photosynthesis and maximise their crop yield.

  1. 1

    Step 1: Identify the limiting factors. The main limiting factors are light, CO₂, and temperature.

  2. 2

    Step 2: State how to control light. Use artificial lamps to increase light intensity and extend the 'daylight' hours.

  3. 3

    Step 3: State how to control CO₂. Use a paraffin heater or release pure CO₂ from a cylinder to increase carbon dioxide concentration.

  4. 4

    Step 4: State how to control temperature. Use heaters to maintain the optimal temperature for the plant's enzymes (e.g., around 25°C).

Recap

  • A limiting factor is a factor that restricts the rate of photosynthesis.
  • The three main limiting factors are light intensity, CO₂ concentration, and temperature.
  • Increasing a limiting factor will increase the rate of photosynthesis, but only up to a point.
  • Once a factor is no longer limiting, another factor will take its place.
  • At high temperatures, enzymes can denature, causing the rate to drop sharply.

Quick check

  1. At night, what is the most likely limiting factor for photosynthesis?1 mark
  2. Why might a greenhouse owner use a paraffin heater?2 marks

4. Investigating Photosynthesis

Scientists can't just take a plant's word for it; they need to test if photosynthesis has happened. There are several key methods. The simplest is testing for starch (the storage form of glucose) using iodine solution. A blue-black colour indicates starch is present. To measure the rate, we can measure a product. A common experiment uses aquatic plants like Elodea (pondweed) and measures the rate of oxygen production by counting bubbles or collecting the gas produced over time. We can also measure the consumption of a reactant, carbon dioxide. Hydrogencarbonate indicator is used for this; it changes colour depending on the CO₂ concentration.

Key term

Destarch: To remove starch from a plant, usually by leaving it in complete darkness for 24-48 hours, forcing it to use up its starch reserves for respiration.

Examiner insight

For questions about experimental design, examiners look for an understanding of variables. You must state what you are changing (independent variable), what you are measuring (dependent variable), and what you are keeping constant (control variables).

Common pitfall

Forgetting to turn off the Bunsen burner before using ethanol. Ethanol is highly flammable, and heating it directly with a flame is extremely dangerous.

Worked example 15 marks

Describe the steps you would take to test a leaf for the presence of starch.

  1. 1

    Step 1: Safety first. Use tongs and wear safety goggles. Place the leaf in a beaker of boiling water for 1 minute to kill the cells and break down the cell walls.

  2. 2

    Step 2: Turn off the Bunsen burner. Place the leaf in a boiling tube containing ethanol and place the tube in the beaker of hot water (a water bath). This removes the chlorophyll.

  3. 3

    Step 3: Once the leaf is pale, remove it and dip it briefly in the hot water to soften it.

  4. 4

    Step 4: Spread the leaf out on a white tile and add a few drops of iodine solution.

  5. 5

    Step 5: Observe the colour change. If starch is present, the iodine will turn from brown/yellow to blue-black.

Worked example 23 marks

A student places a piece of pondweed in a beaker of water with hydrogencarbonate indicator and shines a bright light on it. What colour change would you expect and why?

  1. 1

    Step 1: State the initial process. The bright light will cause the pondweed to photosynthesise at a high rate.

  2. 2

    Step 2: Relate photosynthesis to CO₂. Photosynthesis uses up carbon dioxide from the water.

  3. 3

    Step 3: Describe the effect on the indicator. As CO₂ is removed, the solution becomes less acidic (more alkaline).

  4. 4

    Step 4: State the colour change. Hydrogencarbonate indicator turns from red/orange to purple in low CO₂/alkaline conditions.

Recap

  • To test for starch, boil the leaf in water, then ethanol, then add iodine.
  • A positive test for starch is a blue-black colour.
  • The rate of photosynthesis can be measured by counting oxygen bubbles from pondweed.
  • Hydrogencarbonate indicator can show changes in CO₂ concentration.
  • A plant must be destarched before an experiment to ensure any starch found was made during the experiment.

Quick check

  1. Why is a leaf boiled in ethanol during the starch test?1 mark
  2. What colour is hydrogencarbonate indicator in high CO₂ levels?1 mark

5. Photosynthesis vs Respiration

It's a common misconception that plants only photosynthesise. In fact, like all living things, plant cells respire 24/7 to release energy from the food they've made. Photosynthesis and respiration are opposing processes in terms of gas exchange. Photosynthesis uses CO₂ and produces O₂, while respiration uses O₂ and produces CO₂. During the day, when light is available, the rate of photosynthesis is usually much higher than the rate of respiration. This means there is a net intake of CO₂ and a net output of O₂. At night, photosynthesis stops, but respiration continues, so the plant takes in O₂ and releases CO₂.

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Key term

Compensation Point: The point at which the rate of photosynthesis is exactly equal to the rate of respiration, resulting in no net exchange of gases with the environment.

Examiner insight

Clear answers distinguish between the rate of a single process (e.g. photosynthesis) and the 'net' gas exchange, which is the overall result of both photosynthesis and respiration happening at the same time.

Common pitfall

Thinking that plants photosynthesise during the day and respire at night. They respire ALL the time.

Worked example 13 marks

A healthy potted plant is kept in a sealed, transparent container in a dark room for 24 hours. A CO₂ sensor is placed inside. Predict whether the CO₂ level will increase, decrease, or stay the same. Explain your answer.

  1. 1

    Step 1: Identify the processes occurring. In the dark, the plant cannot photosynthesise.

  2. 2

    Step 2: Identify the other process. However, the plant's cells are still living and must respire to release energy.

  3. 3

    Step 3: State the gas exchange for respiration. Respiration releases carbon dioxide.

  4. 4

    Step 4: Conclude the net effect. Since only respiration is occurring, the plant will release CO₂ into the sealed container, so the CO₂ level will increase.

Worked example 22 marks

Explain what is meant by the 'compensation point' in terms of glucose.

  1. 1

    Step 1: Define the compensation point in terms of processes. It is the light intensity at which the rate of photosynthesis equals the rate of respiration.

  2. 2

    Step 2: Relate processes to glucose. Photosynthesis produces glucose, while respiration consumes glucose.

  3. 3

    Step 3: Synthesise the explanation. Therefore, at the compensation point, the amount of glucose produced by photosynthesis is exactly equal to the amount of glucose used up by respiration. There is no net gain or loss of glucose for the plant.

Recap

  • Plants respire all the time, day and night.
  • Plants only photosynthesise when there is light.
  • During the day, photosynthesis is usually faster than respiration, leading to a net uptake of CO₂.
  • At night, only respiration occurs, leading to a net release of CO₂.
  • The compensation point is when the rates of photosynthesis and respiration are equal.

Quick check

  1. Which process, photosynthesis or respiration, occurs in a plant root cell?1 mark
  2. At dawn, a plant passes its compensation point. Will it now start to show a net uptake or a net release of CO₂?1 mark

6. The Fate of Glucose

Making glucose is just the start. A plant is a master chemist and uses the glucose from photosynthesis for several vital purposes. Firstly, some glucose is used immediately in respiration to release energy for all life processes, like growth and active transport. Secondly, glucose can be converted into starch for storage. Starch is ideal for storage because it is insoluble (so it doesn't affect the water balance of cells) and it's a large molecule that can't diffuse out of the cell. This stored starch acts as a food reserve for when light is unavailable. Thirdly, glucose is a building block. It can be converted into other essential molecules: cellulose to build strong cell walls, lipids (fats and oils) for storage in seeds, and, with the addition of nitrates from the soil, amino acids which are then built into proteins for growth and making enzymes.

Key term

Starch: A large, insoluble carbohydrate made from many glucose units, used as the main energy storage molecule in plants.

Examiner insight

Examiners often ask for more than one use of glucose. Be prepared to give at least three distinct uses (respiration, storage as starch, building cellulose/proteins/lipids).

Fun fact

The massive trunk of a giant sequoia tree is primarily made of cellulose, which was all originally synthesised from glucose made during photosynthesis.

Worked example 13 marks

List three uses of the glucose produced during photosynthesis.

  1. 1

    Step 1: Recall the most immediate use. Glucose is used for respiration to provide energy for the plant's metabolic processes.

  2. 2

    Step 2: Recall the storage molecule. Glucose is converted into insoluble starch for storage, typically in leaves, stems, or roots.

  3. 3

    Step 3: Recall the building block uses. Glucose is used to synthesise other essential molecules, such as cellulose for cell walls, or proteins (when combined with nitrates).

Worked example 24 marks

Explain why it is advantageous for a plant to store carbohydrate as starch rather than as glucose.

  1. 1

    Step 1: Consider the properties of glucose. Glucose is soluble in water.

  2. 2

    Step 2: Explain the problem with storing a soluble substance. If stored as glucose, it would dissolve in the cytoplasm, increasing the solute concentration and causing water to enter the cell by osmosis, potentially damaging the cell.

  3. 3

    Step 3: Consider the properties of starch. Starch is a large, insoluble molecule.

  4. 4

    Step 4: Explain the advantage of this. Because it is insoluble, it has no osmotic effect and does not disturb the water balance of the cell. It is also a large molecule, so it is unable to diffuse out of the storage cells.

Recap

  • Glucose is used for immediate energy via respiration.
  • Glucose is converted to starch for storage because starch is insoluble.
  • Glucose is converted to cellulose to build strong cell walls.
  • Glucose is used to make lipids for energy storage in seeds.
  • With nitrates, glucose is used to make amino acids for protein synthesis.

Quick check

  1. What mineral is required, in addition to glucose, to make amino acids?1 mark
  2. Name the structural carbohydrate made from glucose that forms plant cell walls.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the word equation for photosynthesis and name the plant organelle where this process occurs.2 marks
  2. A variegated leaf (with green and white parts) is destarched and then left in the light for several hours. Describe the results you would expect after testing the leaf for starch, and explain these results.4 marks
  3. Explain why temperature, light intensity, and carbon dioxide concentration are described as 'limiting factors' for photosynthesis.3 marks
  4. Describe an experiment to investigate the effect of light intensity on the rate of photosynthesis using pondweed.6 marks
  5. Explain why a plant has a net intake of carbon dioxide during the day but a net output of carbon dioxide at night.4 marks
  6. The graph shows the rate of photosynthesis of a crop plant at two different carbon dioxide concentrations (0.04% and 0.1%) at different light intensities. Explain why a greenhouse owner might choose to grow their plants at 0.1% CO₂ and high light intensity.3 marks
  7. Apart from respiration, state two other ways a plant uses the glucose it produces from photosynthesis. For one of these uses, name the substance produced.3 marks
  8. What is the compensation point? Explain why it is important for a commercial grower to ensure their plants are above the compensation point for most of the day.4 marks
  9. Explain why storing carbohydrate as starch is more advantageous for a plant than storing it as glucose.3 marks
  10. Three test tubes are set up with hydrogencarbonate indicator. Tube A contains indicator and a leaf in the light. Tube B contains indicator and a leaf in the dark. Tube C contains only indicator as a control. Predict the final colour in Tube A and Tube B, explaining your reasoning for each.5 marks

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