IGCSE Biology – 2e: Nutrition – Photosynthesis
Photosynthesis
o Photosynthesis is the chemical process by which plants make their own food.
o They convert light energy to chemical energy (in the form of glucose).
o This glucose can then be stored as starch or used in respiration.
o Carbon dioxide + water –light--> glucose + oxygen
o 6CO₂ + 6H₂o C₆H₁₂O₆ + 6O₂
Where does Photosynthesis Happen?
o Photosynthesis happens wherever there is chlorophyll (the green pigment that traps
light energy).
o Most photosynthesis happens in the palisade cells – they are packed full of
chloroplasts that contain chlorophyll.
Requirements of Photosynthesis Practical
Method
o Place the leaves into the boiling water for 2 minutes.
o Remove and place into the water Ethanol for 5 minutes.
o Remove the leaves carefully and place them back into the boiling water for 1 minute.
o Take the leaves out and place them on a white tile, making they are spread flat.
o Pipette iodine on.
Results
o Where the iodine turned the leaf dark blue/black, chlorophyll was present, and that
photosynthesis was taking place there.
o The experiment tells you that chlorophyll is required for photosynthesis as where
there was no chlorophyll, there was no photosynthesis
o Iodine can be used to show that photosynthesis has occurred because the glucose
produced in photosynthesis can be stored as starch.
Factors Affecting Photosynthesis
Limiting Factors
o The principal of limiting factors says, ‘when more than one factor affects how fast a
process works, the factor that actually slows the process at any time is the one that’s
in shortest supply.’
Light as a Limiting Factor
o If there is plenty of light, lost of photosynthesis will take place.
o If there is very little light, photosynthesis will stop, regardless of the other conditions
around the plant.
o For most plants, the brighter the light, the faster the rate of photosynthesis.
, Carbon Dioxide as a Limiting Factor
o Plants need carbon dioxide to make glucose.
o The atmosphere only contains 0.04% carbon dioxide, so the atmosphere often limits
the amount of photosynthesis that takes place.
o Increasing carbon dioxide levels will increase the rate of photosynthesis.
o The graph for carbon dioxide looks the same as the one for the light intensity.
Temperature as a Limiting Factor
o Temperature effects all chemical reactions, including photosynthesis.
o As the temperature rises, the rate of photosynthesis will increase as the reaction
speeds up.
o However, photosynthesis is controlled by enzymes and enzymes are destroyed
(denatured) once the temperature rises to around 40-50°C.
Evolution of Oxygen in Elodea Practical
Method
o Make sure to use a fresh piece of Elodea – cut its stem cleanly at 45° using a sharp
scalpel.
o Then place the Elodea with this fresh cut facing upwards, using a paper clip to weight
it down.
o Set up the equipment and place the lamp directly next to the Elodea.
o Leave for 20 minutes to acclimatise.
o After 20 minutes, take readings of the number of bubbles per minute. Repeat 3
times.
o After this, repeat 3 times at 10cm away, 20cm away and 30cm away.
o R ecord the results.
Photosynthesis
o Photosynthesis is the chemical process by which plants make their own food.
o They convert light energy to chemical energy (in the form of glucose).
o This glucose can then be stored as starch or used in respiration.
o Carbon dioxide + water –light--> glucose + oxygen
o 6CO₂ + 6H₂o C₆H₁₂O₆ + 6O₂
Where does Photosynthesis Happen?
o Photosynthesis happens wherever there is chlorophyll (the green pigment that traps
light energy).
o Most photosynthesis happens in the palisade cells – they are packed full of
chloroplasts that contain chlorophyll.
Requirements of Photosynthesis Practical
Method
o Place the leaves into the boiling water for 2 minutes.
o Remove and place into the water Ethanol for 5 minutes.
o Remove the leaves carefully and place them back into the boiling water for 1 minute.
o Take the leaves out and place them on a white tile, making they are spread flat.
o Pipette iodine on.
Results
o Where the iodine turned the leaf dark blue/black, chlorophyll was present, and that
photosynthesis was taking place there.
o The experiment tells you that chlorophyll is required for photosynthesis as where
there was no chlorophyll, there was no photosynthesis
o Iodine can be used to show that photosynthesis has occurred because the glucose
produced in photosynthesis can be stored as starch.
Factors Affecting Photosynthesis
Limiting Factors
o The principal of limiting factors says, ‘when more than one factor affects how fast a
process works, the factor that actually slows the process at any time is the one that’s
in shortest supply.’
Light as a Limiting Factor
o If there is plenty of light, lost of photosynthesis will take place.
o If there is very little light, photosynthesis will stop, regardless of the other conditions
around the plant.
o For most plants, the brighter the light, the faster the rate of photosynthesis.
, Carbon Dioxide as a Limiting Factor
o Plants need carbon dioxide to make glucose.
o The atmosphere only contains 0.04% carbon dioxide, so the atmosphere often limits
the amount of photosynthesis that takes place.
o Increasing carbon dioxide levels will increase the rate of photosynthesis.
o The graph for carbon dioxide looks the same as the one for the light intensity.
Temperature as a Limiting Factor
o Temperature effects all chemical reactions, including photosynthesis.
o As the temperature rises, the rate of photosynthesis will increase as the reaction
speeds up.
o However, photosynthesis is controlled by enzymes and enzymes are destroyed
(denatured) once the temperature rises to around 40-50°C.
Evolution of Oxygen in Elodea Practical
Method
o Make sure to use a fresh piece of Elodea – cut its stem cleanly at 45° using a sharp
scalpel.
o Then place the Elodea with this fresh cut facing upwards, using a paper clip to weight
it down.
o Set up the equipment and place the lamp directly next to the Elodea.
o Leave for 20 minutes to acclimatise.
o After 20 minutes, take readings of the number of bubbles per minute. Repeat 3
times.
o After this, repeat 3 times at 10cm away, 20cm away and 30cm away.
o R ecord the results.
