C1.3 Photosynthesis

Photosynthesis captures light energy and stores it as chemical energy in organic molecules.

Pigments absorb photons and excite electrons; electron transfer creates ATP and reduced NADP, which then drive carbon fixation. The process depends on light, pigments, membranes, water, carbon dioxide and suitable conditions.

Trace the energy conversion:

• photon absorbed by pigment
• electron transfer and proton gradient
• ATP and reduced NADP formed
• carbon compounds synthesized

A leaf exposed to light can use ATP and reduced NADP from the thylakoid reactions to build carbohydrate in the stroma.

Light provides energy but is not itself converted directly into glucose molecules.

The overall photosynthesis equation shows carbon dioxide and water converted to carbohydrate using light energy: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon dioxide supplies the carbon skeleton and is reduced using hydrogen derived from water. Oxygen is released when water is split by photolysis in the light-dependent reactions, not directly from CO₂. Glucose represents a carbohydrate product; the products can be used to make starch, sucrose and other organic compounds.

• Word equation: carbon dioxide + water → glucose + oxygen, using light energy.
• Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• O₂ comes from photolysis of H₂O.
• Plants, algae and cyanobacteria perform oxygenic photosynthesis.

Photosynthetic pigments can be separated and identified because they differ in solubility in the mobile solvent and attraction to the stationary phase.

Extract pigments, place a small concentrated spot on paper or a thin-layer plate, keep the spot above the solvent, allow the solvent front to rise, then mark the solvent front immediately and observe the separated bands.

If a pigment travels 4.2 cm while the solvent front travels 6.0 cm, Rf = 4.2 ÷ 6.0 = 0.70. Identify a pigment using both its colour and an Rf reference obtained with the same solvent and stationary phase.

Rf has no unit and normally lies between 0 and 1. It depends on the solvent and stationary phase, so an Rf from different conditions is not a secure identification; band distance alone is not pigment abundance.

An absorption spectrum shows the proportion of each wavelength absorbed by a pigment; an action spectrum shows the measured rate or effectiveness of photosynthesis at each wavelength.

Only photons with suitable energies are absorbed and excite electrons in pigment molecules. Chlorophylls and accessory pigments absorb different wavelength ranges, so their combined absorption helps explain the action spectrum.

Plot wavelength in nanometres, with the corresponding light colours, on the horizontal axis. For an action spectrum, calculate photosynthesis rate from oxygen production or carbon-dioxide consumption at each wavelength and plot rate on the vertical axis.

If oxygen production is greatest in blue and red light and lowest in green light, the action spectrum has blue and red peaks that broadly match pigment absorption; accessory pigments can make the curves differ.

Absorption and photosynthesis rate are different dependent variables. Do not label an action-spectrum y-axis as absorbance, and compare rates only when intensity and other limiting factors are controlled.

At any moment, the limiting factor is the condition in shortest effective supply relative to photosynthetic demand; increasing it raises rate until another factor becomes limiting.

State a testable hypothesis, vary one independent variable—carbon-dioxide concentration, light intensity or temperature—and measure photosynthesis rate as the dependent variable. Control the other two, plant material, time and measurement conditions; repeat measurements.

Vary light with lamp distance or a light meter, CO₂ with known hydrogencarbonate concentrations or gas control, and temperature with a thermostatically controlled water bath. Measure an initial oxygen-production or CO₂-consumption rate.

A rate rising with light intensity and then reaching a plateau supports the hypothesis that light was initially limiting; at the plateau, CO₂ concentration, temperature or biochemical capacity may limit instead.

A hypothesis is provisional and needs repeated testing. A plateau does not mean photosynthesis has stopped, and moving a lamp can also change temperature unless heat is controlled.

Carbon-dioxide enrichment experiments test how higher atmospheric CO₂ may alter photosynthesis and plant growth, but responses depend on other limiting factors.

Compare enriched and ambient-CO₂ treatments with replication. Record photosynthetic rate and longer-term growth or biomass while monitoring controlled variables such as light, temperature, water, nutrients and plant age.

A crop may show greater CO₂ uptake under enrichment when light and nutrients are sufficient, but little additional biomass under shade or nutrient limitation. This conditional response improves predictions of future growth.

Higher CO₂ does not guarantee a proportional or permanent rise in photosynthesis or yield. Greenhouse results cannot be transferred to natural ecosystems without considering enclosure effects and field interactions.