4.4.1 – Photosynthesis // 4.4.1.1 – Photosynthesis Reaction
o Photosynthesis is represented by the equation:
carbon dioxide + water –(light, chlorophyll)→ glucose + oxygen
6CO2 + 6H2O –(light, chlorophyll)→ C6H12O6 + 6O2
o Photosynthesis takes place inside chloroplasts found in plants & algae.
o The reaction is endothermic – it requires energy taken from sunlight, which is trapped by the chemical chlorophyll, a green pigment
found inside the chloroplasts of plant cells → i.e. Energy is transferred from the environment to the chloroplasts by light.
4.4.1.2 – Rate of Photosynthesis
There are four key factors that affect the rate of photosynthesis:
o Temperature – increasing the temperature increases the rate of photosynthesis because more energy is provided to the particles via
heat.
• If the temperature is increased to above ~45 degrees Celsius, the enzymes that catalyse (speed up) the reaction begin to denature,
causing the rate of the reaction to drop sharply until it stops altogether.
o Carbon dioxide concentration – increasing the carbon dioxide concentration increases the rate of photosynthesis as carbon dioxide
is a reactant.
• Above a certain threshold, further increases don’t increase the rate of photosynthesis because another factor (eg. Light intensity) is
limiting the rate of reaction.
o Light intensity – greater light intensity = greater rate of photosynthesis as more energy is being provided.
• Above a certain threshold, the rate of photosynthesis will not increase because another factor (eg. Temperature) is limiting the rate
of reaction.
o Chlorophyll concentration – high chlorophyll concentration allows more light energy to be absorbed, giving a higher rate of
photosynthesis.
o A limiting factor restricts the rate of a reaction. The identity of the limiting factor in photosynthesis depends on the condition faced by
the plant.
• At night, light intensity is usually the limiting reactant.
• If plants are grown in mineral-deficient soil, they may not absorb enough minerals to produce lots of chlorophyll (magnesium ions
are especially required to produce chlorophyll) so chlorophyll concentration can be the limiting factor in this case.
• In warm and bright conditions, carbon dioxide concentration is usually the limiting factor (as there’s already lots of light and
oxygen around).
• In winter, temperature is usually the limiting factor.
o An understanding of limiting factors is important for plant production in greenhouses.
o Farmers must evaluate the trade-off (balance) between the costs of lighting & heating, and the rate of photosynthesis that’s possible.
• Increasing yield – lighting and heating in greenhouses increases the rate of photosynthesis that’s possible → high rate of
photosynthesis = crops grow faster + bigger total yield.
• Cost – if lighting and heating is expensive, it may not be possible to spend money on increasing the light intensity and temperature
of a greenhouse, so farmers must evaluate the trade-off between the costs of both in this case.
o There’s an inverse relationship between distance and light intensity – as distance increases, light intensity decreases.
o This is because as distance away from a light source increases, photons of light become spread over a wider area.
o The light energy at twice the distance away is spread over four times the area / the light energy at three times the distance away is
spread over nine times the area, etc.
o The inverse square law states that light intensity is inversely proportional to the square of the distance.
o For each distance of the plant from the lamp, light intensity will be proportional to the inverse of distance squared.
• Eg. For a lamp 10cm away from the plant, 1/d² would be 1/100 = 0.01.
• A graph of rate/frequency would be linear against 1/d², which light intensity is proportional to.
Light intensity at source (1)/distance² = light intensity [lumen/cm²]
Distance = 1/sq. root of light intensity
o You can investigate the effect of light intensity on the rate of photosynthesis by using an aquatic plant like pondweed –
• Change the distance between the lamp and the pondweed and count the number of bubbles produced per unit time.
o Photosynthesis is represented by the equation:
carbon dioxide + water –(light, chlorophyll)→ glucose + oxygen
6CO2 + 6H2O –(light, chlorophyll)→ C6H12O6 + 6O2
o Photosynthesis takes place inside chloroplasts found in plants & algae.
o The reaction is endothermic – it requires energy taken from sunlight, which is trapped by the chemical chlorophyll, a green pigment
found inside the chloroplasts of plant cells → i.e. Energy is transferred from the environment to the chloroplasts by light.
4.4.1.2 – Rate of Photosynthesis
There are four key factors that affect the rate of photosynthesis:
o Temperature – increasing the temperature increases the rate of photosynthesis because more energy is provided to the particles via
heat.
• If the temperature is increased to above ~45 degrees Celsius, the enzymes that catalyse (speed up) the reaction begin to denature,
causing the rate of the reaction to drop sharply until it stops altogether.
o Carbon dioxide concentration – increasing the carbon dioxide concentration increases the rate of photosynthesis as carbon dioxide
is a reactant.
• Above a certain threshold, further increases don’t increase the rate of photosynthesis because another factor (eg. Light intensity) is
limiting the rate of reaction.
o Light intensity – greater light intensity = greater rate of photosynthesis as more energy is being provided.
• Above a certain threshold, the rate of photosynthesis will not increase because another factor (eg. Temperature) is limiting the rate
of reaction.
o Chlorophyll concentration – high chlorophyll concentration allows more light energy to be absorbed, giving a higher rate of
photosynthesis.
o A limiting factor restricts the rate of a reaction. The identity of the limiting factor in photosynthesis depends on the condition faced by
the plant.
• At night, light intensity is usually the limiting reactant.
• If plants are grown in mineral-deficient soil, they may not absorb enough minerals to produce lots of chlorophyll (magnesium ions
are especially required to produce chlorophyll) so chlorophyll concentration can be the limiting factor in this case.
• In warm and bright conditions, carbon dioxide concentration is usually the limiting factor (as there’s already lots of light and
oxygen around).
• In winter, temperature is usually the limiting factor.
o An understanding of limiting factors is important for plant production in greenhouses.
o Farmers must evaluate the trade-off (balance) between the costs of lighting & heating, and the rate of photosynthesis that’s possible.
• Increasing yield – lighting and heating in greenhouses increases the rate of photosynthesis that’s possible → high rate of
photosynthesis = crops grow faster + bigger total yield.
• Cost – if lighting and heating is expensive, it may not be possible to spend money on increasing the light intensity and temperature
of a greenhouse, so farmers must evaluate the trade-off between the costs of both in this case.
o There’s an inverse relationship between distance and light intensity – as distance increases, light intensity decreases.
o This is because as distance away from a light source increases, photons of light become spread over a wider area.
o The light energy at twice the distance away is spread over four times the area / the light energy at three times the distance away is
spread over nine times the area, etc.
o The inverse square law states that light intensity is inversely proportional to the square of the distance.
o For each distance of the plant from the lamp, light intensity will be proportional to the inverse of distance squared.
• Eg. For a lamp 10cm away from the plant, 1/d² would be 1/100 = 0.01.
• A graph of rate/frequency would be linear against 1/d², which light intensity is proportional to.
Light intensity at source (1)/distance² = light intensity [lumen/cm²]
Distance = 1/sq. root of light intensity
o You can investigate the effect of light intensity on the rate of photosynthesis by using an aquatic plant like pondweed –
• Change the distance between the lamp and the pondweed and count the number of bubbles produced per unit time.
