IB Biology HL C1.3 Photosynthesis Notes

Photosynthesis converts light energy into chemical energy in carbon compounds. Photoautotrophs use chlorophyll to capture light; in plants it is in chloroplast thylakoid membranes, while cyanobacteria use chlorophyll in internal membranes instead of chloroplasts.

The photosynthesis equation maps the sources of atoms and products. Carbon dioxide is reduced to carbohydrate using hydrogen from water. Glucose represents the main stored product, though many compounds are synthesized. Oxygen is released from photolysis of water, not directly from CO2; oxygenic photosynthesis occurs in plants, algae, and cyanobacteria.

Paper chromatography separates chlorophylls and accessory pigments because pigments differ in solubility and attraction to the paper. Use Rf as a ratio: distance moved by pigment divided by distance moved by solvent front. A larger Rf means the pigment moved further relative to the solvent.

Spectra questions test whether you know what the graph is measuring. Chlorophyll and accessory pigments absorb specific wavelengths; absorbed light excites electrons for light-dependent reactions. An absorption spectrum shows wavelengths absorbed by pigments, while an action spectrum shows wavelengths most effective for photosynthesis rate, often using oxygen production or CO2 uptake.

Limiting-factor experiments ask what is currently holding photosynthesis back. Light intensity, CO2 concentration, and temperature can limit photosynthesis. Investigations change one variable and estimate rate from O2 production or CO2 uptake per unit time while other variables are controlled. When a graph plateaus, the tested factor is no longer limiting.

CO2 enrichment tests whether increased CO2 raises photosynthesis or growth. Greenhouse experiments are controlled but less natural; FACE experiments enrich CO2 in open air and compare realistic crop and ecosystem responses. The teacher move is to ask: was CO2 really limiting, and were water, nutrients, temperature, and species differences controlled or considered?

HL starts by replacing the idea of one lonely chlorophyll molecule. Photosystems are pigment-protein arrays in thylakoid membranes. Antenna pigments pass energy to reaction-centre chlorophyll. Pigment arrays broaden wavelength absorption and funnel energy efficiently; a single chlorophyll molecule cannot sustain the full light reaction system.

Follow the electrons. Photosystem II uses light energy to split water. Photolysis supplies replacement electrons, protons, and oxygen. The electrons move through carriers, then Photosystem I re-excites electrons for NADP reduction. NADP accepts electrons and H+ to form reduced NADP/NADPH for the Calvin cycle.

Thylakoids are organized to turn electron flow into ATP. Electron transport pumps protons into the thylakoid space, and photolysis adds more protons there. Proton flow through ATP synthase produces ATP by photophosphorylation. Thylakoids organize photosystems, electron carriers, and ATP synthase; key outputs on the stromal side are ATP and reduced NADP.

The Calvin cycle begins with carbon fixation. Rubisco fixes CO2 to RuBP, forming glycerate 3-phosphate. Rubisco is abundant but slow and limited by low CO2, which is why CO2 availability matters in photosynthesis-rate questions.

After carbon fixation, glycerate 3-phosphate is reduced to triose phosphate. ATP supplies energy and reduced NADP supplies hydrogen. Most triose phosphate regenerates RuBP so the Calvin cycle continues, and ATP is required for regeneration of the CO2 acceptor.

Triose phosphate is converted into sugars, starch, lipids, and organic acids. Mineral nutrients allow synthesis of amino acids and other compounds. Light-dependent reactions supply ATP and reduced NADP for the Calvin cycle; the Calvin cycle returns ADP and NADP and depends on CO2 or hydrogen carbonate availability.

Photosynthesis converts light energy into chemical energy in carbon compounds. Carbon dioxide is reduced to carbohydrate using hydrogen from water, glucose is the main stored product, and released oxygen comes from photolysis of water. Pigment evidence is tested with chromatography and Rf, spectra questions separate absorption from action, and rate/evidence questions use limiting factors, controls, and CO2 enrichment context.

Use this for SL/core questions on photosynthesis definition, source of carbohydrate/oxygen, pigment separation, spectra, limiting factors, and CO2 enrichment evidence.

HL photosynthesis links light-dependent reactions to the Calvin cycle. Photosystems are pigment-protein arrays in thylakoid membranes; antenna pigments broaden absorption and pass energy to reaction-centre chlorophyll. PSII uses light to split water, supplying electrons, protons, and oxygen. Electron transport pumps protons into the thylakoid space, ATP synthase makes ATP by photophosphorylation, PSI re-excites electrons, and NADP is reduced. The Calvin cycle uses ATP and reduced NADP: Rubisco fixes CO2 to RuBP to form GP, GP is reduced to triose phosphate, most triose phosphate regenerates RuBP, and some becomes wider biomolecules.

Use this for HL structured questions asking for light-dependent reactions, photosystems, proton gradients, NADP reduction, Calvin-cycle steps, product synthesis, or stage interdependence.