Photosynthesis

What you'll learn

Photosynthesis is the process by which plants and algae convert light energy into chemical energy stored in glucose. This topic examines the word and symbol equations, the factors affecting the rate of photosynthesis, how leaves are adapted for photosynthesis, and what plants do with the glucose they produce. Understanding photosynthesis is essential for explaining food chains, carbon cycling, and how organisms obtain energy.

Key terms and definitions

Photosynthesis — the process by which plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen

Chloroplast — the organelle in plant cells where photosynthesis occurs, containing the green pigment chlorophyll

Chlorophyll — the green pigment found in chloroplasts that absorbs light energy for photosynthesis

Limiting factor — a variable that, when in short supply, restricts the rate of photosynthesis (such as light intensity, carbon dioxide concentration or temperature)

Stomata — small pores (mainly on the underside of leaves) that allow gas exchange; carbon dioxide enters and oxygen exits through these openings

Guard cells — pairs of specialised cells surrounding each stoma that control its opening and closing

Glucose — the simple sugar produced during photosynthesis that plants use for respiration or convert into other substances

Endothermic reaction — a reaction that takes in energy from the surroundings; photosynthesis requires light energy to proceed

Core concepts

The photosynthesis equation

Photosynthesis can be expressed as both a word equation and a symbol equation. You must be able to write both correctly.

Word equation: carbon dioxide + water → glucose + oxygen

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Key points about the equation:

• Light energy is required (shown above the arrow)
• Chlorophyll is needed but is not used up (also shown above the arrow)
• The reaction is endothermic — energy is transferred from the environment to the chloroplasts by light
• For every 6 molecules of carbon dioxide used, 6 molecules of oxygen are produced
• Water is split to release oxygen; the hydrogen combines with carbon dioxide to form glucose

The carbon dioxide enters through stomata on the leaves. Water is absorbed by the roots and transported to the leaves via xylem vessels. Oxygen is released as a waste product through the stomata.

Structure and function of the leaf

Leaves are highly adapted for photosynthesis. Each structural feature maximises the efficiency of the process.

Upper epidermis:

• Transparent to allow light to pass through to photosynthetic cells below
• Covered by a waxy cuticle that reduces water loss by evaporation

Palisade mesophyll layer:

• Located near the top of the leaf to receive maximum light
• Cells are columnar (tall and narrow) and tightly packed
• Contains the highest concentration of chloroplasts
• Main site of photosynthesis

Spongy mesophyll layer:

• Contains cells with large air spaces between them
• Allows carbon dioxide to diffuse to photosynthesising cells
• Provides a large surface area for gas exchange
• Also contains chloroplasts but fewer than palisade cells

Lower epidermis:

• Contains most of the stomata for gas exchange
• Guard cells control stomatal opening
• Stomata close in darkness or when water is scarce to reduce water loss

Vascular bundles (veins):

• Xylem transports water and minerals to leaf cells
• Phloem transports glucose (as sucrose) away from the leaf to other parts of the plant
• Provides structural support to the leaf

The broad, flat shape of most leaves provides a large surface area to absorb light. The thin structure ensures carbon dioxide can diffuse quickly to all cells. The extensive network of veins ensures all cells receive water and can export glucose.

Limiting factors of photosynthesis

The rate of photosynthesis can be limited by several factors. A limiting factor is the variable that is in shortest supply at any given moment, preventing the rate from increasing further.

Light intensity:

• At low light levels, increasing light intensity increases the rate of photosynthesis
• Light provides the energy for the reaction
• At very high light intensities, light is no longer the limiting factor
• Represented by the inverse square law: light intensity ∝ 1/distance²

Carbon dioxide concentration:

• Atmospheric CO₂ is approximately 0.04%
• Increasing CO₂ concentration increases photosynthesis rate (up to a point)
• CO₂ is a raw material required for the reaction
• In enclosed spaces like greenhouses, CO₂ can become limiting

Temperature:

• Photosynthesis is controlled by enzymes
• As temperature increases, enzyme and substrate molecules move faster, increasing collision rate
• Optimum temperature for most plants is 25-35°C
• Above 45°C, enzymes denature and the rate drops to zero
• Temperature affects the rate but is not a raw material

Chlorophyll concentration:

• Plants with more chlorophyll can absorb more light energy
• Mineral deficiencies (such as magnesium or iron) reduce chlorophyll production
• Chlorophyll content can vary with disease or environmental stress

Graphs showing limiting factors:

When analysing graphs, identify which factor is limiting at different points:

• A steep upward slope shows that increasing the factor increases the rate
• A plateau (flat line) indicates that factor is no longer limiting; something else restricts the rate
• To increase the rate when the graph plateaus, a different variable must be changed

For example, if increasing light intensity no longer increases the rate, either temperature or CO₂ concentration has become the limiting factor.

Investigating the rate of photosynthesis

A common practical investigation uses aquatic plants such as Canadian pondweed (Elodea) to measure oxygen production.

Method:

• Place pondweed in a test tube or beaker of water
• Position a lamp at a measured distance from the plant
• Count the number of bubbles produced per minute, or collect oxygen in an inverted measuring cylinder
• Change one variable (distance of lamp, temperature, or CO₂ concentration by adding sodium hydrogencarbonate)
• Keep all other variables constant (controlled variables)

Key considerations:

• Allow time for the plant to adjust to new conditions before counting
• Repeat readings and calculate a mean to improve reliability
• Use the same piece of pondweed or select pieces of similar size
• Control temperature using a water bath or heat shield
• Increasing distance from lamp decreases light intensity according to the inverse square law

Limitations:

• Not all oxygen escapes as bubbles; some dissolves in water
• Bubble size varies
• The plant also respires, using some oxygen
• Light intensity is affected by distance and also by lamp brightness

Uses of glucose in plants

Plants do not use all the glucose produced immediately. Glucose is converted into other substances or stored for later use.

Respiration:

• Glucose is broken down to release energy for cell processes
• Occurs continuously in all living cells, day and night
• Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

Starch for storage:

• Glucose is insoluble and would affect water movement by osmosis if stored directly
• Converted to starch (a polymer of glucose), which is insoluble
• Stored in roots, stems, leaves and seeds
• Can be broken down to glucose when needed (e.g., at night or during germination)

Cellulose for cell walls:

• Another glucose polymer, with different bonds to starch
• Provides structural support and strength to plant cells
• Cannot be digested by most animals

Proteins for growth and enzymes:

• Glucose combines with nitrate ions (absorbed from soil by roots) to form amino acids
• Amino acids join together to form proteins
• Essential for growth, repair and enzyme production

Lipids (fats and oils) for storage:

• Seeds store lipids as energy reserves for germination
• More energy-dense than carbohydrates
• Found in nuts, seeds and some fruits (e.g., avocados, olives)

Sucrose for transport:

• Glucose is converted to sucrose (a disaccharide) for transport in phloem
• Transported from leaves to other parts of the plant where it is needed or stored

Commercial applications: greenhouses

Farmers use knowledge of limiting factors to increase crop yields in greenhouses.

Optimising conditions:

• Artificial lighting extends the hours of photosynthesis, especially in winter
• Heating maintains optimum temperature for enzymes, allowing year-round growth
• Adding CO₂ (from burning fuels or CO₂ generators) increases the rate when light and temperature are high
• Automated watering systems ensure water is never limiting

Economic considerations:

• Costs of heating, lighting and CO₂ must be balanced against increased crop value
• Farmers calculate the optimum investment for maximum profit
• Insulation reduces heating costs
• LED lights are more efficient than traditional bulbs

Environmental control:

• Thermostats and sensors maintain ideal conditions automatically
• Humidity control prevents excess water loss from plants
• Pest control may use biological methods or pesticides
• Ventilation prevents overheating and allows gas exchange

Worked examples

Example 1: A student investigates how light intensity affects the rate of photosynthesis in pondweed. She places a lamp at 10 cm from the pondweed and counts 40 bubbles per minute. When she moves the lamp to 20 cm, she counts 10 bubbles per minute.

(a) Name two variables the student should control in this investigation. [2 marks]

Answer:

• Temperature (of the water) [1]
• Carbon dioxide concentration / same piece of pondweed / volume of water [1]

(Any two suitable controlled variables; accept: type of pondweed, time period for counting, same lamp)

(b) Explain why the rate of photosynthesis decreased when the lamp was moved further away. [3 marks]

Answer:

• Light intensity decreased / less light reached the plant [1]
• Light provides energy for photosynthesis [1]
• So less glucose / fewer products can be made / rate decreases [1]

(Award marks for: identifying decreased light intensity, explaining light's role, stating the consequence on the reaction)

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Example 2: A student recorded the rate of photosynthesis at different temperatures. At 25°C the rate was 15 bubbles per minute. At 45°C the rate was 18 bubbles per minute. At 55°C the rate was 0 bubbles per minute.

Explain the results at 55°C. [3 marks]

Answer:

• At high temperatures, enzymes denature [1]
• The active site changes shape / loses its specific shape [1]
• So the substrate can no longer fit / bind to the enzyme / photosynthesis cannot occur [1]

(Accept: enzymes are damaged / destroyed; do not accept "enzymes die")

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Example 3: Explain why plants store glucose as starch rather than as glucose.** [2 marks]

Answer:

• Starch is insoluble (whereas glucose is soluble) [1]
• So it does not affect water movement by osmosis / does not affect water concentration in cells [1]

(Both points required for full marks; accept: does not dissolve, easier to store)

Common mistakes and how to avoid them

• Confusing respiration and photosynthesis — remember that photosynthesis makes glucose (in chloroplasts, requires light) while respiration breaks down glucose (in mitochondria, happens all the time). Plants do both processes.

• Writing incorrect equations — ensure you know both word and symbol equations precisely. Don't write "chlorophyll" as a reactant; it's needed but not used up. The symbol equation must be balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

• Misidentifying limiting factors on graphs — a plateau means that factor is no longer limiting. To increase the rate, you must increase a different factor. Read the graph axis labels carefully.

• Saying enzymes "die" when denatured — enzymes are proteins, not living things. Use "denatured" or "change shape" when describing the effect of high temperature or extreme pH.

• Forgetting that temperature affects enzyme activity, not just providing warmth — temperature increases the kinetic energy of molecules, leading to more frequent collisions between enzymes and substrates, increasing the rate up to the optimum.

• Confusing the functions of xylem and phloem — xylem transports water and minerals upwards from roots to leaves; phloem transports sugars (sucrose) from leaves to all parts of the plant in both directions.

Exam technique for "Photosynthesis"

• Command words matter: "State" requires a simple answer with no explanation (1 mark). "Describe" needs you to say what happens without explaining why (usually 2-3 marks). "Explain" requires reasons or mechanisms — use "because" or "so" to link points (typically 3+ marks).

• Use data from graphs and tables: Questions often provide experimental results. Quote figures from the data ("increased from 10 to 25 bubbles per minute") before explaining the pattern. This demonstrates analysis and typically earns you an additional mark.

• Link structure to function: When asked about leaf adaptations, always connect the structural feature to how it helps photosynthesis. For example: "Palisade cells contain many chloroplasts [structure] so they can absorb more light energy [function]."

• Show working in calculations: Even if the final answer is wrong, you can gain method marks. For light intensity calculations using the inverse square law, write the formula, substitute values, then calculate.

Quick revision summary

Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy absorbed by chlorophyll. The equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Leaves are adapted with palisade cells containing many chloroplasts, stomata for gas exchange, and veins for transport. The rate is limited by light intensity, CO₂ concentration, temperature and chlorophyll. Plants use glucose for respiration, or convert it to starch (storage), cellulose (cell walls), proteins (growth) or lipids (seeds). Farmers optimise greenhouse conditions to maximise photosynthesis and crop yield.