Photosynthesis — the four marked elements
Cambridge credits four things: the reactants carbon dioxide and water; the energy source, light; the role of chlorophyll, the green pigment that absorbs the light energy and transfers it to drive the reactions; and the products, glucose and oxygen. Word equation: carbon dioxide + water → glucose + oxygen, with "light energy / chlorophyll" written over the arrow. (Extended) The balanced symbol equation is — balance C first, then H, then O (18 each side).
Glucose has five fates in a plant
Respiration breaks it down to release energy for every cell activity. Cellulose, built by condensation, forms the cell walls. Starch, also by condensation, is the store held in leaves and other organs — insoluble, so it does not alter the cell's water potential, and quickly hydrolysed back to glucose. Amino acids, and then proteins, need glucose plus nitrate ions from the soil. Fats and oils store energy densely, especially in seeds. A 4-mark "uses" question wants at least three distinct fates.
Limiting factors — light, CO₂, temperature
A limiting factor is the factor in shortest supply: the one whose increase would raise the rate of photosynthesis. The 0610 syllabus recognises three — light intensity, CO₂ concentration and temperature (the rate climbs to the enzyme optimum, then falls as enzymes denature). Reading a rate graph: a steeply rising section means the factor on the x-axis is limiting; a plateau means it is no longer limiting and a different factor has taken over.
Drawn from real examiner reports.
Chloroplast ≠ chlorophyll
The chloroplast is the organelle in which photosynthesis takes place; chlorophyll is the green pigment inside it that absorbs the light energy. Asked which cell structure is needed for photosynthesis, the answer is chloroplast — "chlorophyll" scores nothing. Chlorophyll is to the chloroplast what haemoglobin is to a red blood cell: the working molecule, not the structure.
Flagged s22 P33 Q2b ii
A plateau is never chlorophyll saturation
When a rate curve levels off, write that light intensity is no longer limiting and that a different factor — CO₂ concentration or temperature — has become the new limiting factor. "The chlorophyll cannot absorb any more light" and "the plant has enough light now" earn no credit. Examiners also found candidates failing to name light intensity as limiting on the rising part of the same curve.
Flagged s23 P23 Q12 · s22 P42 Q1d
Purple indicator means CO₂ has fallen
Hydrogencarbonate indicator is orange/red at atmospheric CO₂. A plant photosynthesising in light removes CO₂, so the solution becomes less acidic and turns purple. Yellow means CO₂ has increased — respiration in the dark. Candidates routinely reverse the pair and give yellow for the lit tube, losing both marks in one stroke.
Flagged s23 P11/21 Q11–12 · w22 P21/22 Q11
White leaf areas make no starch
Iodine turns the green areas of a variegated leaf blue-black and leaves the white areas amber. White areas hold no chlorophyll, so no light energy is absorbed, no photosynthesis occurs and no starch forms. The same reasoning covers the CO₂ test: the leaf sealed with sodium hydroxide has its CO₂ absorbed, so it makes no starch — not "a little".
Flagged s23 P23 Q11
The product is glucose, not starch
The immediate organic product of photosynthesis is glucose. Writing "carbon dioxide + water → starch + oxygen" loses the mark, because starch, cellulose, amino acids and oils are all made from glucose afterwards. "Food" is no better — it is not precise enough to earn the product mark. Name glucose every time.
Plants store starch, never glycogen
Asked to name the large carbohydrate a plant makes or stores, candidates offer glycogen — that is the animal and fungal store, not a plant one. Plants store starch and build cellulose. A second trap in the same family: plants translocate sucrose in the phloem, not glucose and certainly not starch.
Flagged w22 P31 Q6c i
Don't hand back the respiration equation
Photosynthesis takes in carbon dioxide and water and gives out glucose and oxygen; aerobic respiration runs exactly the other way. Candidates write one equation when the other was asked for, and the two are not interchangeable. Quick check before you commit: if oxygen sits on the left, you have written respiration.
Flagged s23 P11 Q24 · w22 P32 Q6b ii · s23 P32 Q5b
Define it in four elements
Reactants (carbon dioxide and water), energy source (light), role of chlorophyll (it absorbs and transfers that light energy) and products (glucose, with oxygen as a by-product). "Plants use sunlight and water to make food" hits none of the four cleanly and scores almost nothing.
Investigations: IV → CVs → DV
Name what you change (lamp distance for light intensity; sodium hydrogencarbonate concentration for CO₂), at least two things you keep the same (temperature, plant species, volume of water, colour of light), and what you measure — bubbles per minute or cm³ of oxygen per minute.
Always name the limiting factor
"Another factor is limiting" earns no mark. Say which one: CO₂ concentration or temperature. On the rising part of the curve, state that light intensity is limiting and that increasing it increases the rate because more light energy is absorbed by chlorophyll.
Destarch first, then decolourise
Keep the plant in the dark for 24–48 hours so that any starch found afterwards was definitely made during the experiment. Then boil the leaf in water, boil it in ethanol over a water bath to remove the chlorophyll, rinse in warm water and add iodine — skip the ethanol and the green masks the blue-black.
Photosynthesis is the process by which plants manufacture glucose using carbon dioxide () and water (), with light energy absorbed by chlorophyll.
Word equation:
(Extended) Balanced symbol equation:
Chlorophyll is the green pigment located inside chloroplasts (the organelles in which photosynthesis occurs). Chlorophyll absorbs light energy and transfers it to drive the reactions of photosynthesis. No chlorophyll = no photosynthesis.
What is the difference between chlorophyll and a chloroplast? Which is the cell ORGANELLE and which is the PIGMENT?
(a) Define photosynthesis. [3]
(b) State the word equation for photosynthesis. [1]
