Photosynthesis¶

Part of 4.4 Bioenergetics.

Photosynthesis is how plants transfer energy from light into the chemical energy of glucose. The topic is easier to understand when you connect the equation to leaf structure and limiting factors.

Learning Objectives¶

Photosynthetic Reaction¶

Photosynthesis is an endothermic process — the plant takes in light energy and stores it as chemical energy.

• Photosynthesis takes place in chloroplasts and uses light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen.
• Word equation: carbon dioxide + water → glucose + oxygen
• Photosynthesis occurs in two main stages: first, light energy is used to split water into oxygen and hydrogen ions; second, the hydrogen ions combine with carbon dioxide to form glucose.
• Leaves are adapted for photosynthesis because they are broad, thin and packed with chloroplast-containing cells near the upper surface.
• Photosynthetic organisms are the primary producers of biomass on Earth. The glucose they make passes energy along food chains to all other organisms.

Respiration and Photosynthesis in Plants¶

Plants respire all the time — day and night — to obtain energy for their internal processes. Photosynthesis only occurs during daylight.

• During the day, photosynthesis runs faster than respiration, so plants take in more carbon dioxide than they release and produce a net surplus of oxygen.
• During the night, with no light available, only respiration occurs. Plants consume oxygen and release carbon dioxide.

Hydrogencarbonate Indicator Investigation¶

Hydrogencarbonate indicator is used to detect changes in carbon dioxide concentration. It starts orange; it turns yellow when CO₂ increases and purple when CO₂ decreases.

A typical experiment uses four sealed tubes containing the indicator: 1. A control tube with no plant — remains orange. 2. A tube with a leaf wrapped in foil (no light) — turns yellow, as respiration releases CO₂ with no photosynthesis to remove it. 3. A tube with a leaf wrapped in gauze (dim light) — little colour change, as respiration and limited photosynthesis roughly balance. 4. An uncovered tube with a leaf (full light) — turns purple, as rapid photosynthesis removes more CO₂ than respiration produces.

Rate of Photosynthesis¶

A limiting factor is whichever condition is in shortest supply at a given moment; increasing it will raise the rate until a different factor becomes limiting.

Light Intensity¶

• Greater light intensity → more light captured by chlorophyll → faster rate of photosynthesis.
• Above a certain intensity the rate levels off because the chloroplasts are already working at maximum capacity and light is no longer limiting.
• Inverse square law: light intensity is inversely proportional to the square of the distance from the light source (intensity ∝ 1/distance²). Doubling the distance quarters the light intensity; halving the distance quadruples it.

Carbon Dioxide Concentration¶

• Higher CO₂ drives the reaction forward, increasing the rate.
• At very high concentrations, the enzymes involved become saturated and the rate plateaus; CO₂ is no longer the limiting factor.

Temperature¶

• Higher temperature gives enzymes more kinetic energy, increasing the rate up to the optimum (typically around 25–30 °C for most plants).
• Above about 45 °C, enzymes denature — their active site changes shape permanently — and the rate falls sharply.

Amount of Chlorophyll¶

• More chlorophyll means more light can be captured, increasing the rate.
• Disease and infection can reduce the amount of functional chlorophyll and thus limit photosynthesis.

Practical Investigation: Pondweed¶

The effect of limiting factors on photosynthesis can be measured using Canadian pondweed submerged in water. The rate of oxygen production (measured by counting bubbles or observing displacement of a gas bubble in a capillary tube) is proportional to the rate of photosynthesis.

• Light intensity: move a lamp to different distances; apply the inverse square law to calculate intensity.
• Temperature: place the boiling tube in water baths of different temperatures.
• CO₂ concentration: dissolve different amounts of sodium hydrogencarbonate in the water (it releases CO₂).
• Always control variables not being tested, and repeat at least three times to calculate a mean.

Starch Test for Photosynthesis¶

Starch is the storage product of photosynthesis in leaves. The iodine test reveals whether photosynthesis occurred:

1. Dip the leaf in boiling water to stop reactions.
2. Place in warm ethanol to remove chlorophyll (the leaf turns pale/white).
3. Rinse with cold water.
4. Add iodine solution — blue-black = starch present (photosynthesis occurred); orange = no starch (no photosynthesis).

Greenhouses and Commercial Optimisation¶

Greenhouses allow growers to manipulate all major limiting factors to maximise yield year-round:

• Temperature: glass panels trap solar heat via the greenhouse effect. Heaters maintain warmth in winter. Blinds prevent overheating and enzyme denaturation in summer.
• CO₂ concentration: burning paraffin heaters raises CO₂ levels inside the greenhouse while also providing heat.
• Light intensity: artificial lighting supplements natural daylight, extending the photoperiod and allowing growth during winter months.

Monitoring and adjusting these conditions involves significant cost, so growers must balance efficiency against expenditure to make production economically viable.

Uses of Glucose from Photosynthesis¶

• Respiration — glucose is broken down to release energy for cell processes.
• Starch — insoluble storage form; built up when glucose supply exceeds demand.
• Cellulose — used to make cell walls.
• Lipids — formed for energy storage, especially in seeds.
• Amino acids — made when glucose combines with nitrate ions absorbed from the soil; used to build proteins.

Common Confusions¶

• Endothermic vs endergonic: Photosynthesis is described as endothermic at GCSE because the plant takes energy in from light. Do not confuse this with a reaction that simply releases heat.
• The inverse square law direction: doubling the distance reduces intensity to one quarter — not one half. A common error is to halve rather than quarter.
• Plants only photosynthesise, not respire: Plants always respire. In daylight they also photosynthesise; at night only respiration occurs. The net exchange depends on which process is faster.
• Limiting factor plateau: A graph line levelling off does not mean photosynthesis has stopped — it means that factor is no longer limiting. Another factor is now the bottleneck.

Key Terms¶

• Photosynthesis: the process by which plants use light energy to make glucose from carbon dioxide and water.
• Chlorophyll: the green pigment that absorbs light energy for photosynthesis.
• Limiting factor: a factor in shortest supply that restricts the rate of a process.
• Glucose: a simple sugar made during photosynthesis and used for respiration and growth.
• Starch: an insoluble storage carbohydrate made from glucose in plants.
• Endothermic reaction: a reaction that takes in energy from the surroundings; photosynthesis is endothermic because it absorbs light energy.
• Chloroplast: the organelle in plant cells where photosynthesis takes place; contains chlorophyll.
• Inverse square law: the rule that light intensity is inversely proportional to the square of the distance from the source (intensity ∝ 1/d²).
• Hydrogencarbonate indicator: a pH-sensitive indicator used to detect changes in CO₂ concentration; orange at normal levels, yellow when CO₂ increases, purple when CO₂ decreases.
• Optimum temperature: the temperature at which enzyme-controlled reactions, including photosynthesis, proceed at the fastest rate.

Connected Pages¶

• 4.4.2 Respiration
• GCSE Biology Key Terms Glossary
• Required Practical Reference