Photosynthesis — definition and word equation
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light. Word equation: carbon dioxide + water → glucose + oxygen, with light and chlorophyll written on the arrow — reactants, products and both conditions are separately markable. Chlorophyll is the green pigment that absorbs the light; the chloroplast is the organelle containing it, and is where photosynthesis happens.
(Extended) Balanced symbol equation
(Extended) 6CO2 + 6H2O → C6H12O6 + 6O2, in light with chlorophyll. Check it balances: 6 carbon each side; 18 oxygen each side (6×2 + 6×1 on the left, 6 + 6×2 on the right); 12 hydrogen each side (6×2 on the left, 12 in glucose on the right). Chlorophyll transfers energy from light into energy in chemicals for the synthesis of carbohydrates — it converts one form of energy into another, it never creates energy.
(Extended) Products and mineral ions
(Extended) Glucose made in photosynthesis is stored as starch (insoluble energy store), built into cellulose for cell walls, used directly in respiration, converted to sucrose for transport in the phloem, or made into nectar to attract pollinating insects. Two soil ions work alongside it: nitrate ions build amino acids (both contain nitrogen), magnesium ions build chlorophyll (a magnesium atom sits at chlorophyll's centre).
Drawn from real examiner reports.
Chlorophyll absorbs, chloroplast houses
Substance and structure are different marks. "Which substance absorbs light?" → chlorophyll, the green pigment. "Which structure carries out photosynthesis?" → the chloroplast, the organelle containing chlorophyll. Answering "chloroplast" to the substance question scores nothing, and the same swap the other way costs the structure mark.
Flagged Jun 2023 P42 Q4b
Reversing the equation gives respiration
The reactants are carbon dioxide and water; the products are glucose and oxygen. Writing "glucose + oxygen → carbon dioxide + water" is the equation for respiration, not photosynthesis. Candidates swap the two sides, or reverse the arrow, and lose the whole equation. Photosynthesis stores energy; respiration releases it.
Flagged Nov 2023 P32 Q4di
Nitrate ions ≠ magnesium ions
(Extended) Nitrate ions make amino acids, and so proteins — not chlorophyll. Magnesium ions make chlorophyll — not amino acids. The two are routinely reversed. The symptoms follow from the roles: too little magnesium → yellow leaves (chlorosis), no green pigment being made; too little nitrate → poor, stunted growth.
Flagged Jun 2022 P32 Q10ci · Nov 2022 P31 Q7d · Nov 2023 P13 Q5
"More collisions" is not an explanation
When a rate graph becomes constant, the explanation must be about photosynthesis, not generic reaction kinetics — vague "more collisions happen" earns nothing. Say instead that light intensity is no longer the factor limiting the rate, because another factor (carbon dioxide concentration or temperature) is now in shortest supply relative to what the plant could use.
Flagged Jun 2022 P43 Q4bi · Jun 2023 P41 Q4dii
Variegated leaf: light alone is not enough
In a destarched variegated leaf, iodine turns blue-black only where chlorophyll AND light were both present. A white area given full light makes no starch, because it cannot absorb the light energy; a green area covered by foil makes none either, because no light reached the chlorophyll. Both halves of the control matter.
Flagged Jun 2023 P12 Q5
Testing for starch without destarching
Before any starch test the plant must sit in the dark for 24–48 hours to remove the starch already there. Skip it and a blue-black iodine result proves nothing, since that starch may predate the experiment. The same logic demands a control — a foil-covered area, or a plant deprived of carbon dioxide by soda lime — to compare against.
Chloroplasts, not "in the leaves"
Asked WHERE photosynthesis happens, the mark is for chloroplasts. "In the leaves" is too imprecise — the leaf is the organ, and not every cell in it photosynthesises. "In chlorophyll" is wrong outright: chlorophyll is a pigment, not a place. Name the chloroplast, then add that it contains chlorophyll if the tariff allows.
Flagged Nov 2022 P31 Q7b · Jun 2023 P32 Q4dii
The limiting-factor chain
Identify the factor on the x-axis. While the line is still rising, that factor is the limiting one. Once it plateaus, that factor is no longer limiting and another has taken over — name it (carbon dioxide concentration or temperature, whichever was held constant).
Describe the graph, then explain it
"Describe" wants the shape only — the rate rises, then levels off at a constant value. "Explain" wants the limiting-factor chain, and must name photosynthesis specifically rather than "the reaction". Answering the wrong one of the two scores zero on a part you understood.
Temperature graphs fall, not plateau
Light-intensity and carbon-dioxide graphs level off. A temperature graph rises to an optimum and then falls, because the enzymes controlling photosynthesis begin to denature. Writing "levels off" for temperature describes the wrong curve and loses the denaturation mark.
Hydrogencarbonate indicator colours
(Extended) Decide which process is winning, then read the colour off. Light: photosynthesis removes carbon dioxide faster than respiration releases it, so red → purple/pink. Dark: only respiration runs, so red → yellow. Never claim the plant stops respiring in the light.
Cambridge 0654 spec reference: Section B6 "Plant nutrition", sub-topic B6.1 (Core + Extended). This leaf covers the definition and word equation for photosynthesis, chlorophyll/chloroplasts, investigating the need for chlorophyll/light/carbon dioxide, and (Extended) the balanced symbol equation, chlorophyll's energy-transfer role, storage/use of products, the importance of nitrate and magnesium ions, the effects of light intensity/carbon dioxide concentration/temperature on rate, and gas exchange in an aquatic plant shown with hydrogencarbonate indicator.
Out of scope for B6.1 (covered elsewhere): leaf structure and diagram labelling (chloroplasts, cuticle, guard cells, stomata, epidermis, mesophyll, vascular bundles) -- that is B6.2 "Leaf structure". Xylem/phloem transport mechanism detail belongs to B8 "Transport in plants". Detailed enzyme active-site/denaturation mechanism belongs to B5 "Enzymes" (named here only briefly, as one reason the rate falls at very high temperature).
| Term | Mark-scheme-precise definition |
|---|---|
| Photosynthesis | The process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light |
| Chlorophyll | A green pigment found in chloroplasts, that absorbs light energy |
| Chloroplast | The structure (organelle) inside plant cells that contains chlorophyll and is where photosynthesis takes place |
| (Extended) Limiting factor | The factor which, at a given time, is in the shortest supply relative to what the plant could otherwise use, and so determines the actual rate of photosynthesis at that time |
Word equation (Core):
(Extended) Balanced symbol equation:
(Extended) Chlorophyll's role: chlorophyll transfers energy from light into energy in chemicals (chemical energy), for the synthesis of carbohydrates -- it is a converter of energy form, not a creator of energy.
Define photosynthesis.
(a) Define photosynthesis. (2 marks)
(b) State the word equation for photosynthesis, including the conditions needed. (3 marks)
(c) Name the structure in a plant cell where photosynthesis takes place, and the substance inside it that absorbs light. (2 marks)
