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.

A photosystem is a membrane-bound molecular array of chlorophyll and accessory pigments surrounding a special reaction-centre chlorophyll.

Different antenna pigments absorb different wavelengths and transfer excitation energy toward the reaction centre. The reaction-centre chlorophyll emits an excited electron to an electron acceptor, initiating electron flow.

Photosystems occur in chloroplast thylakoid membranes and photosynthetic membranes of cyanobacteria. The structured array broadens light capture and funnels energy efficiently to one reaction centre.

A wavelength weakly absorbed by the reaction-centre chlorophyll can still drive electron emission when an accessory pigment absorbs it and transfers the excitation energy through the array.

A single isolated chlorophyll or accessory-pigment molecule cannot perform any part of photosynthesis by itself; the organized pigment–protein membrane system is essential.

In photosystem II, light-driven photolysis splits water; later, photosystem I supplies excited electrons that reduce NADP on the stromal side of the thylakoid.

Photolysis replaces electrons lost by photosystem II. Its protons contribute to the thylakoid proton gradient and its oxygen is released as waste; oxygen generation profoundly changed Earth's living and geological systems.

At photosystem I, NADP accepts two excited electrons originating from photosystem I and one H⁺ from the stroma to form reduced NADP (NADPH), which carries reducing power to the Calvin cycle.

Use one consistent pair of names: NADP/reduced NADP or NADP⁺/NADPH. The released O₂ comes from water, not carbon dioxide, and photolysis does not directly make glucose.

The thylakoid is an integrated membrane system that positions photosystems, electron carriers and ATP synthase for the light-dependent reactions.

Electron transfer releases energy that pumps H⁺ from the stroma into the thylakoid lumen. H⁺ then moves down its electrochemical gradient through ATP synthase, coupling that flow to ADP + Pi → ATP in the stroma.

Photolysis occurs at photosystem II, ATP synthesis across the membrane, and NADP reduction at photosystem I on the stromal side. Non-cyclic electron flow begins with electrons from photosystem II and makes ATP plus reduced NADP; cyclic flow returns electrons from photosystem I and supports ATP production without reducing NADP.

The membrane separates the H⁺-rich lumen from the stroma, so electron-carrier proton pumping stores energy as a gradient and ATP synthase converts that gradient energy into chemical energy in ATP.

Electron carriers build the gradient; ATP synthase uses it. Chemiosmosis is proton movement through ATP synthase, not electron passage through the enzyme.

Rubisco catalyses carbon fixation in the chloroplast stroma by adding CO₂ to the five-carbon acceptor RuBP.

The unstable six-carbon product immediately divides into two molecules of glycerate 3-phosphate (GP). This is carbon's entry into the Calvin cycle.

Substrates: CO₂ and RuBP; enzyme: Rubisco; product: two GP molecules. Rubisco is the most abundant enzyme on Earth, and plants maintain high stromal concentrations because it works relatively slowly and is ineffective at low CO₂ concentrations.

Fixing three CO₂ molecules to three RuBP molecules produces six GP molecules before their reduction to triose phosphate.

Rubisco does not directly produce glucose or triose phosphate. It catalyses the fixation step that produces GP, which is modified in later Calvin-cycle reactions.

Glycerate 3-phosphate (GP) is converted to triose phosphate (TP) using ATP and reduced NADP; most TP is then recycled to regenerate RuBP using ATP.

ATP supplies energy and reduced NADP supplies hydrogen/electrons for GP reduction. Regenerating the five-carbon CO₂ acceptor RuBP allows carbon fixation to continue.

For every six TP produced, five TP molecules are rearranged using ATP to form three RuBP molecules. Thus five-sixths of TP is recycled when glucose is treated as the photosynthesis product; one-sixth is available for product synthesis.

Three fixed CO₂ yield six TP after reduction: five TP regenerate three RuBP, while the carbon from one TP is the net gain. Two such net TP gains can supply the six carbons needed for one glucose.

The individual regeneration reactions are not required. Do not imply that each turn releases glucose or that all TP leaves the cycle.

The Calvin cycle supplies carbon intermediates for carbohydrates, amino acids and other carbon compounds, while depending on ATP and reduced NADP from the light-dependent reactions.

All carbon in compounds made by a photosynthesizing organism is first fixed through the Calvin cycle. TP and other intermediates enter metabolic pathways; mineral nutrients supply elements such as nitrogen needed for amino acids and other products.

The light-dependent reactions supply ATP and reduced NADP; the Calvin cycle returns ADP, phosphate and NADP. Without light, ATP and reduced NADP production stops. Without CO₂, carbon fixation stops and the coupled recycling of electron acceptors is disrupted, eventually preventing photosystem II from functioning.

Some TP contributes to sucrose or starch, while carbon skeletons combined with mineral nitrogen can form amino acids. Most TP remains in the cycle to regenerate RuBP.

‘Light-independent’ means light is not absorbed directly by these reactions; it does not mean they can continue indefinitely without light-reaction products. Glucose is not formed in one Calvin-cycle step.