Bioenergetics · GCSE Biology
Photosynthesis
The flagship bioenergetics page: word and symbol equations, chloroplasts, limiting factors, uses of glucose and the pondweed practical.
Light energy in, glucose and oxygen out. The factory is the chloroplast. The rate is only as fast as the factor in shortest supply — light, carbon dioxide or temperature.
The important bits
What you need to know
- 1
Photosynthesis is an endothermic reaction: light energy is transferred to the chloroplasts and used to convert carbon dioxide and water into glucose. Oxygen is released.
- 2
Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light. Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- 3
Chlorophyll in chloroplasts absorbs light, mainly in the red and blue parts of the spectrum. Palisade mesophyll cells at the top of the leaf are packed with chloroplasts.
- 4
Glucose is used immediately in respiration, converted to starch for storage, used to make cellulose for cell walls, combined with nitrate ions to make amino acids, and used to make lipids.
- 5
Limiting factors are light intensity, carbon dioxide concentration and temperature. The factor in shortest supply caps the rate; the graph rises, then plateaus.
- 6
Temperature affects enzyme-controlled steps. Too low and collisions are rare; too high and enzymes denature, so the rate falls.
- 7
On Higher tier, light intensity follows an inverse square relationship: intensity ∝ 1/d², where d is the distance from the lamp.
- 8
Leaf adaptations support the process: broad shape, thin lamina, stomata for gas exchange, air spaces in the spongy mesophyll, and xylem bringing water.
Go deeper
The equation is the skeleton of every answer
If you cannot write both forms of the equation without hesitation, stop and learn them before anything else. Carbon dioxide enters through stomata. Water arrives in the xylem from the roots. Chlorophyll absorbs light and the chloroplast builds glucose. Oxygen is a by-product and leaves through stomata, which is why a pondweed practical counts bubbles as a proxy for rate. Photosynthesis is endothermic because energy is taken in from the surroundings — specifically from light — rather than released. That is the opposite of respiration. Students who mix the two equations lose a cluster of marks across both papers, so drill them as a pair: photosynthesis builds glucose; respiration breaks it down.
Go deeper
Limiting factors are a graph story
A limiting factor is the condition stopping the rate from rising further. Increase light intensity and the rate climbs until carbon dioxide or temperature takes over, then the line flattens. Increase carbon dioxide in a greenhouse and you see the same shape until light or temperature limits. Temperature is different: the line falls after the optimum because enzymes denature. In a six-mark answer, name the factor, describe the graph, and link it to collisions or to the chloroplast machinery. Growers raise carbon dioxide, use lamps and control temperature because extra glucose means extra biomass — provided no other factor is capping the process. Wasting energy on more light when carbon dioxide is limiting does not increase yield.
Go deeper
Glucose does not sit in the cell as sugar in a bowl
The plant’s next move is as important as the equation. Glucose is soluble and osmotically active, so storing it unchanged would draw in water. Starch is insoluble, so it is the storage form tested with iodine. Cellulose builds cell walls. Amino acids need nitrate ions from the soil; without nitrates the plant cannot make proteins and growth suffers. Lipids are made for membranes and for seeds. Respiration happens in every living cell, including palisade cells, which is why a leaf still needs oxygen at night. When a question asks “what happens to the glucose”, list at least three fates and attach a reason to each. “Used for energy” is too vague: write “used in respiration to release energy for cell processes”.
See the idea in action
A lamp is 10 cm from pondweed and 50 bubbles are counted in one minute. The lamp is moved to 20 cm. Distance has doubled, so on Higher tier light intensity falls to a quarter (1/2²). You predict fewer bubbles if light is limiting. If the count barely changes, light was not limiting — carbon dioxide or temperature was. In the method, use a ruler to set distance, a water bath to hold temperature, and sodium hydrogencarbonate if you want a controlled carbon dioxide supply. Count bubbles for a fixed time and repeat.
Exam technique
Turn knowledge into marks
Always state that photosynthesis occurs in chloroplasts and that chlorophyll absorbs light. Then give the equation. For limiting-factor graphs, say which factor is limiting on the rising section and which factor is likely to be limiting on the plateau. Inverse square calculations are Higher tier: intensity ∝ 1/d².
Common mistakes
Do not give these marks away
- 01
Writing the respiration equation instead of photosynthesis, or omitting that the reaction happens in chloroplasts.
- 02
Saying plants only photosynthesise and animals only respire — plant cells respire all the time.
- 03
Describing a plateau as “the reaction stops” rather than “another factor is now limiting”.
Where does photosynthesis take place in a plant cell?
AMitochondria
BNucleus
CChloroplasts
DRibosomes
Show the answer
Chloroplasts. Chloroplasts contain chlorophyll, which absorbs light energy used to convert carbon dioxide and water into glucose. Mitochondria are for aerobic respiration.
Quick questions
If this is the bit you searched
What is the word equation for photosynthesis?
Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
What is the symbol equation for photosynthesis?
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration and temperature. The factor in shortest supply determines the rate.
What is glucose used for after photosynthesis?
Respiration, stored as starch, used to make cellulose, combined with nitrates to make amino acids, and used to make lipids.