Photosynthesis (OCR GCSE Combined Science A (Gateway): Biology): Flashcards
Exam code: J250
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Define photosynthesis.
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Define photosynthesis.
The process in which plants and algae make glucose from carbon dioxide and water, using energy from light.
Why are photosynthetic organisms called producers?
They make their own food from simple molecules, and that food supports the rest of the food chain.
Define biomass.
The mass of living material in an organism.
How do photosynthetic organisms produce biomass?
The glucose they make is used to build more complex molecules, which the organism grows from.
What are the reactants of photosynthesis?
Carbon dioxide and water.
What are the products of photosynthesis?
Glucose and oxygen.
Write the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen
Light is needed for the reaction to happen.
Write the balanced symbol equation for photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where in the cell does photosynthesis take place?
In the chloroplasts, which contain chlorophyll to absorb light.
Describe the two stages of photosynthesis.
Energy from light splits water into oxygen and hydrogen ions.
The hydrogen ions then combine with carbon dioxide to make glucose.
Define endothermic reaction.
A reaction that takes in energy from the surroundings.
Why is photosynthesis described as endothermic?
Energy is transferred from the environment to the chloroplasts by light.
＿＿＿＿＿＿＿＿ is the green pigment in chloroplasts that absorbs light for photosynthesis.
Chlorophyll is the green pigment in chloroplasts that absorbs light for photosynthesis.
The glucose made in photosynthesis can be broken down again in ＿＿＿＿＿＿＿＿ to transfer energy.
The glucose made in photosynthesis can be broken down again in respiration to transfer energy.
True or False?
Plants respire as well as photosynthesise.
True.
Plants respire continuously in every cell, just as animals do.
True or False?
Photosynthesis is an exothermic reaction.
False.
It is endothermic — it takes energy in from light. Respiration is the exothermic one.
Name the three factors that affect the rate of photosynthesis.
Temperature, light intensity and carbon dioxide concentration.
How does increasing light intensity affect photosynthesis?
The rate increases, because more energy is available for the reaction.
Why does the rate stop rising at very high light intensity?
Another factor needed for photosynthesis is now in short supply, so it holds the rate back instead.
How does carbon dioxide concentration affect photosynthesis?
Carbon dioxide is a raw material, so the more there is, the faster the rate — until another factor runs short.
Why does raising the temperature increase the rate at first?
Particles gain kinetic energy and move faster, so successful collisions happen more often.
Why does the rate fall at very high temperatures?
The enzymes controlling photosynthesis are denatured, so their active sites no longer fit their substrates.
Describe the shape of a graph of rate against temperature.
The rate rises to a peak at the optimum temperature, then falls sharply as the enzymes denature.
Describe the shape of a graph of rate against light intensity.
The rate rises as light intensity increases, then levels off when another factor becomes limiting.
Enzymes are ＿＿＿＿＿＿＿＿ at very high temperatures, so the rate of photosynthesis falls.
Enzymes are denatured at very high temperatures, so the rate of photosynthesis falls.
As light intensity increases, the rate of photosynthesis ＿＿＿＿＿＿＿＿.
As light intensity increases, the rate of photosynthesis increases.
Why is water not usually treated as a factor limiting the rate?
There is normally enough water available for photosynthesis.
Suggest how a grower could raise the rate in a greenhouse.
Increase the light intensity, raise the temperature to the optimum, or increase the carbon dioxide concentration.
True or False?
Raising the temperature always increases photosynthesis.
False.
Above the optimum the enzymes are denatured and the rate falls.
True or False?
Carbon dioxide is a raw material for photosynthesis.
True.
It combines with hydrogen from water to make glucose, so its concentration affects the rate.
Define limiting factor. (Higher Tier Only)
The factor in shortest supply, which is holding back the rate of photosynthesis.
What does a levelling-off graph line tell you? (Higher Tier Only)
The factor plotted on the x-axis is no longer limiting the rate — something else has become the limiting factor.
What does a still-rising graph line tell you? (Higher Tier Only)
The factor on the x-axis is the limiting factor, because increasing it is still increasing the rate.
Two curves at 15 °C and 25 °C overlap in dim light. Why? (Higher Tier Only)
At low light intensity, light is the limiting factor in both, so temperature makes no difference to the rate.
Once light is plentiful, why is the rate higher at 25 °C than at 15 °C? (Higher Tier Only)
Light is no longer limiting, so temperature now limits the rate.
The warmer plant has more kinetic energy and a faster rate.
How do you raise the rate once a line has levelled off? (Higher Tier Only)
Increase whichever factor has become limiting — raising the one already plotted will have no further effect.
Three curves level off at different heights as light increases. What does that show? (Higher Tier Only)
Each is limited by a different factor once light is plentiful.
The highest curve has the most favourable temperature and carbon dioxide concentration.
More than one factor can limit photosynthesis, but only the one in ＿＿＿＿＿＿＿＿ supply sets the rate. (Higher Tier Only)
More than one factor can limit photosynthesis, but only the one in shortest supply sets the rate.
How are light intensity and distance from a lamp related? (Higher Tier Only)
They are inversely proportional, following the inverse square law:
light intensity = 1 ÷ d²
A lamp is moved from 10 cm to 20 cm away. What happens? (Higher Tier Only)
The distance doubles, so the light intensity becomes four times smaller (2² = 4).
Calculate the light intensity 30 cm from a lamp. (Higher Tier Only)
light intensity = 1 ÷ d²
1 ÷ 30² = 0.001 a.u. (arbitrary units)
As distance from the light source increases, light intensity ＿＿＿＿＿＿＿＿. (Higher Tier Only)
As distance from the light source increases, light intensity decreases.
True or False?
Only one factor at a time can affect the rate. (Higher Tier Only)
False.
Several factors act together, but the rate is set by the one in shortest supply.
True or False?
Doubling the distance halves the light intensity. (Higher Tier Only)
False.
By the inverse square law it becomes four times smaller, not half.
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