Photosynthesis

4. Bioenergetics

Photosynthesis is the reaction that builds glucose out of carbon dioxide and water, using energy transferred by light. It happens in chloroplasts, and it is endothermic - energy goes in rather than coming out.

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167 questions3 subtopicsFoundation and Higher1 required practical

What photosynthesis covers

Every subtopic below has its own questions, mark schemes and notes.

4.4.1.1The photosynthesis reaction

4.4.1.2Rate of photosynthesis

Anything a plant is short of will slow photosynthesis down. Light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll can each become the limiting factor - the one thing holding the rate back.

4.4.1.3Uses of glucose

The glucose a plant makes does not just sit there. It is respired, converted into storage compounds, built into cellulose for cell walls, or combined with nitrate ions to make amino acids.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Photosynthesis — The reaction in which plants and algae use energy transferred by light to make glucose and oxygen from carbon dioxide and water.

Chloroplast — The subcellular structure where photosynthesis happens.

Chlorophyll — The green pigment inside chloroplasts that absorbs light.

Endothermic — Describes a reaction that takes in energy from the surroundings.

Limiting factor — The factor that is in shortest supply and so is stopping the rate of photosynthesis from increasing any further.

Rate — How much of something happens in a given time.

Independent variable — The variable that is deliberately changed in an investigation.

Dependent variable — The variable that is measured in an investigation.

Inverse square law — The rule that light intensity is proportional to one divided by the square of the distance from the source.

Cellulose — The strong carbohydrate that plant cell walls are made of.

Starch — An insoluble carbohydrate that plants store glucose as.

Nitrate ions — Ions absorbed from the soil that a plant needs in order to turn glucose into amino acids.

Insoluble — Does not dissolve.

Where marks get lost

The mistakes that come up again and again in this topic.

Do not write light or sunlight as one of the reactants in the word equation - it belongs above the arrow.
Chlorophyll and chloroplast are different words: the chloroplast is the structure, the chlorophyll is the pigment inside it.
Photosynthesis is endothermic. Saying it is exothermic because the plant gains energy is a common and costly slip.
Write the formulae carefully - C6H12O6 with subscripts, and 6O2 rather than 12O.
When a graph levels off, say which factor has become limiting instead - do not just say the rate stops increasing.
Only the temperature graph falls again. Light and carbon dioxide graphs flatten out and stay flat.
Above the optimum the enzymes are damaged, so say denatured rather than killed - enzymes are not alive.
Doubling the distance from a lamp divides the light intensity by four, not by two.
In the practical, name a specific control variable such as the temperature of the water, rather than just saying everything else.
Cellulose strengthens the cell wall - it is not the same as starch, which is for storage.
Say that respiration transfers energy from glucose, rather than that it makes or produces energy.
If asked why glucose is stored as starch, the mark is for insoluble and for the osmosis consequence, not just for saying starch is better.
Remember where the nitrogen for amino acids comes from: nitrate ions taken up from the soil, not from the air.

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