6.1 Photosynthesis
- Syllabus
- 0610–2026–2027
- Topic
- 6.1
- Level
- —
Learning objectives
6.1.1Photosynthesis as the process by which• Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light6.1.2Word equation for photosynthesis• State the word equation for photosynthesis as: carbon dioxide + water → glucose + oxygen in the presence of light and chlorophyll6.1.3Chlorophyll is a green pigment that is• State that chlorophyll is a green pigment that is found in chloroplasts6.1.4Chlorophyll transfers energy• State that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates6.1.10Photosynthesis equation• State the balanced chemical equation for photosynthesis as: 6CO 2 + 6H2O → C6H12O6 + 6O26.1.5Subsequent use and storage• Outline the subsequent use and storage of the carbohydrates made in photosynthesis, limited to: (a) starch as an energy store (b) cellulose to build cell walls (c) glucose used in respiration to provide energy (d) sucrose for transport in the phloem (e) nectar to attract insects for pollination6.1.6Importance of: (a) nitrate ions• Explain the importance of: (a) nitrate ions for making amino acids (b) magnesium ions for making chlorophyll6.1.7Need for chlorophyll, light and carbon• Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls6.1.8Effects of varying light intensity• Investigate and describe the effects of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis6.1.9Effect of light and dark conditions on• Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution6.1.11Explain the limiting factors• Identify and explain the limiting factors of photosynthesis in different environmental conditions
Explain photosynthesis as chemical synthesis
Photosynthesis is the process by which plants synthesise carbohydrates from carbon dioxide and water, using energy from light.
| Part of the definition | Meaning |
|---|---|
| synthesise | build a larger carbohydrate molecule |
| raw materials | carbon dioxide and water |
| energy source | light |
| first named carbohydrate product | glucose |
Matter comes from the raw materials; light supplies the energy needed to rearrange that matter into carbohydrate. Oxygen is released as a product.
Plants do not obtain their food from soil. Mineral ions and water enter through roots, but carbohydrate is synthesised by photosynthesis.
Write the word equation for photosynthesis
carbon dioxide + water → glucose + oxygen
| Item | Position |
|---|---|
| carbon dioxide and water | reactants on the left |
| glucose and oxygen | products on the right |
| light and chlorophyll | conditions for the process, written above/beside the arrow rather than as reactants |
The arrow means 'are converted into'. The equation conserves the kinds of atoms even though the word equation does not show coefficients.
Do not put light or chlorophyll on the reactant side: light supplies energy and chlorophyll transfers it; neither is a raw material consumed to make glucose.
Locate chlorophyll in chloroplasts
Chlorophyll is a green pigment found in chloroplasts.
| Level | Correct term |
|---|---|
| organism | green plant |
| organ | leaf |
| cell structure | chloroplast |
| pigment inside it | chlorophyll |
In a variegated leaf, green regions contain chlorophyll; white regions do not. This difference can be used when testing whether chlorophyll is needed for starch production.
A chloroplast is the cell structure; chlorophyll is the pigment inside it. They are related but not interchangeable terms.
Track energy transfer by chlorophyll
Chlorophyll transfers energy from light into energy in chemicals for the synthesis of carbohydrates.
| Stage | Energy description |
|---|---|
| before photosynthesis | light energy reaches chlorophyll |
| during photosynthesis | chlorophyll transfers the energy |
| after synthesis | energy is stored chemically in carbohydrate molecules |
This energy transfer explains why light is required even though light is not a material reactant in the word equation.
Chlorophyll does not make energy and is not converted into glucose. It enables a transfer from light energy to chemical energy.
Write the balanced photosynthesis equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
| Element | Left side | Right side |
|---|---|---|
| carbon | 6 | 6 |
| hydrogen | 12 | 12 |
| oxygen | 18 | 18 |
Check balance by multiplying every subscript by its coefficient, then total each element on both sides. Light and chlorophyll remain conditions for the reaction.
Change coefficients to balance an equation; never change the subscripts in CO₂, H₂O, C₆H₁₂O₆ or O₂ because that would name different substances.
Trace the uses and storage of carbohydrates
Carbohydrates made in photosynthesis can be stored, built into structures, respired, transported or used in nectar.
| Product or conversion | Use |
|---|---|
| starch | insoluble energy store |
| cellulose | builds cell walls |
| glucose | used in respiration to provide energy |
| sucrose | transported in the phloem |
| nectar | attracts insects for pollination |
The form suits the job: insoluble starch remains stored, sucrose is a transport sugar, and cellulose becomes strong wall material.
Do not describe starch as the phloem transport sugar or cellulose as an energy store; these named fates are distinct in the syllabus.
Explain why plants need nitrate and magnesium ions
Plants need nitrate ions to make amino acids and magnesium ions to make chlorophyll.
| Mineral ion | Used to make | Consequence of shortage |
|---|---|---|
| nitrate | amino acids, then proteins | reduced protein synthesis and stunted growth |
| magnesium | chlorophyll | less chlorophyll, yellow leaves and reduced photosynthesis |
Both ions are absorbed from the soil through root hairs. Their roles differ: nitrate supplies nitrogen for amino acids, while magnesium is part of chlorophyll production.
Nitrate is not used directly to make glucose, and magnesium is not absorbed through stomata. Avoid swapping the two ions' functions.
Test the requirements for photosynthesis
Investigate whether chlorophyll, light or carbon dioxide is needed by changing one requirement and testing whether starch is produced.
| Requirement | Experimental comparison | Appropriate control |
|---|---|---|
| light | uncovered leaf area versus area covered on both sides | same leaf, same time and conditions |
| chlorophyll | green versus white region of a variegated leaf | same leaf exposed to light |
| carbon dioxide | leaf with CO₂ absorber versus identical setup with CO₂ available | same plant type, light, temperature and time |
Destarch the plant in darkness first, apply the treatment, expose it to light, then test the leaf with iodine. Blue-black shows starch; yellow-brown shows no starch.
Only the region or setup with the requirement should make starch. The control shows that the result is caused by the missing factor rather than another difference.
Boiling in ethanol removes chlorophyll so the iodine colour can be seen; it is not the step that destarches the plant. A fair test changes one factor.
Investigate factors affecting photosynthesis rate
Light intensity, carbon dioxide concentration and temperature can each change the rate of photosynthesis.
| Independent variable | Practical change | Possible rate measure | Key controls |
|---|---|---|---|
| light intensity | change lamp distance or measured light level | oxygen bubbles/volume per time | temperature and CO₂ concentration |
| CO₂ concentration | change hydrogencarbonate concentration | oxygen volume per time | light and temperature |
| temperature | use water baths | oxygen volume per time | light and CO₂ concentration |
Use several values, allow equilibration, measure for equal time intervals, repeat and calculate means. Bubble count is less reliable than gas volume because bubbles vary in size.
Rate usually rises as a factor increases, then levels off when another factor limits it. At excessive temperatures the rate can fall because photosynthesis is enzyme-controlled.
Lamp distance is not light intensity: increasing distance decreases intensity. State which variable is changed, measured and controlled.
Interpret hydrogencarbonate indicator experiments
Hydrogencarbonate indicator reveals changes in carbon dioxide concentration caused by gas exchange in sealed aquatic-plant setups.
| Net process | CO₂ change | Indicator relative to starting orange/red |
|---|---|---|
| photosynthesis faster than respiration | decreases | purple |
| photosynthesis equals respiration | no net change | remains orange/red |
| respiration faster than photosynthesis | increases | yellow |
In bright light, photosynthesis may remove CO₂ faster than respiration releases it. In darkness, photosynthesis stops but respiration continues, so CO₂ increases.
Compare equal plant amounts in equal indicator volumes for the same time and temperature; include a tube without an organism to show that any colour change is biological.
The indicator measures CO₂, not oxygen. Plants respire in both light and darkness; light changes whether photosynthesis also occurs.
Identify and explain limiting factors
A limiting factor is the environmental factor in shortest effective supply that restricts the rate of photosynthesis.
| Graph region | Likely limiting-factor reasoning |
|---|---|
| rising with light | light intensity is limiting because more light increases rate |
| plateau despite more light | light is no longer limiting; CO₂ concentration or temperature may limit |
| higher plateau after raising CO₂ | CO₂ limited the lower curve |
| different plateaus at different temperatures | temperature changes the maximum rate until another factor limits |
Identify the factor by asking which change still increases the rate under those exact conditions. A factor can stop being limiting as conditions change, so the limiting factor may shift along one graph.
Glasshouses can increase light, temperature or CO₂ to remove a limitation, but growers must compare the extra crop yield with heating, lighting and CO₂ costs.
A plateau does not mean photosynthesis has stopped. It means increasing the plotted factor alone cannot increase the rate because another factor is limiting.