PhotosynthesisCambridge IGCSE Biology: Flashcards

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Define photosynthesis.

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Define photosynthesis.: Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light.
State the word equation for photosynthesis.: Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
Write the balanced chemical equation for photosynthesis.: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
What is chlorophyll and where is it found?: Chlorophyll is a green pigment found in chloroplasts. It transfers energy from light into chemical energy for the synthesis of carbohydrates.
Name five uses of the carbohydrates produced by photosynthesis.: Starch (energy store), cellulose (cell walls), glucose (respiration), sucrose (phloem transport), and nectar (attract insects)
What is the importance of nitrate ions in plants?: Nitrate ions are needed for making amino acids.
What is the importance of magnesium ions in plants?: Magnesium ions are needed for making chlorophyll.
Describe an investigation to determine if chlorophyll is needed for photosynthesis.: Use variegated leaves with green and non-green areas. Keep in light, then test with iodine solution. Only the green areas containing chlorophyll will turn blue-black, indicating starch production. Control: use a leaf that received no light to show no starch is produced in darkness.
Describe an investigation to test the need for carbon dioxide in photosynthesis.: Set up two identical plants in the same light conditions. Place one in a sealed container with soda lime (which absorbs CO₂) and one without. After several days, test both leaves with iodine. Only the control plant without soda lime will show starch (blue-black with iodine), proving CO₂ is needed.
Name three factors that affect the rate of photosynthesis.: Light intensity, carbon dioxide concentration, and temperature.
Describe how to investigate the effect of light intensity on photosynthesis rate using an aquatic plant.: Place an aquatic plant (e.g. Elodea) in a water bath at constant temperature. Vary the distance of a light source from the plant (changing light intensity). Count the number of oxygen bubbles released per minute at each distance. Use a control in darkness to show that light is required.
Explain how hydrogencarbonate indicator can be used to investigate gas exchange in aquatic plants in light and dark conditions.: Hydrogencarbonate indicator is yellow in acidic conditions (high CO₂), red in neutral/alkaline conditions (low CO₂). In light, the indicator turns red because photosynthesis removes CO₂. In dark, the indicator turns yellow because respiration releases CO₂. This shows net gas exchange differs between light and dark conditions.
What is a limiting factor of photosynthesis?: A limiting factor is any variable that restricts the rate of photosynthesis. The main limiting factors are light intensity, carbon dioxide concentration, and temperature.
Explain how light intensity acts as a limiting factor of photosynthesis at low light levels.: At low light intensity, there is insufficient light to transfer energy to chlorophyll molecules, so fewer carbohydrates are synthesised. As light intensity increases, the rate of photosynthesis increases proportionally until another factor becomes limiting.
Explain how temperature acts as a limiting factor of photosynthesis.: Temperature affects enzyme activity in photosynthesis. At low temperatures, enzyme activity is slow so the rate is low. As temperature increases, enzyme activity increases and the rate increases. However, at very high temperatures, enzymes denature and the rate decreases.
In a greenhouse on a cloudy winter day, which factor is most likely to be limiting photosynthesis?: Light intensity, because there is insufficient natural light on a cloudy day, and this will have the greatest effect on the rate of photosynthesis at that time.