Chloroplast structure and function

· Photosynthesis transfers light energy into chemical energy in organic molecules.
· In eukaryotic plants, photosynthesis occurs in chloroplasts.
· Chloroplast envelope = double membrane surrounding the chloroplast.
· Thylakoids = flattened membrane sacs; contain chloroplast pigments, electron carriers and ATP synthase.
· Grana = stacks of thylakoids; increase surface area for light absorption.
· Thylakoid membrane = site of the light-dependent stage.
· Thylakoid space = proton reservoir used to create a proton gradient.
· Stroma = fluid matrix containing enzymes for the Calvin cycle / light-independent stage.
· In diagrams and electron micrographs, link structure to function: large membrane surface area, compartmentalisation, and enzyme-rich stroma.

This diagram shows how chloroplast structure supports photosynthesis. The thylakoid membranes provide the site for the light-dependent stage, while the stroma contains enzymes for the Calvin cycle. The stacked grana increase surface area for light absorption. Source

Overall link between the two stages

· Light-dependent stage produces ATP and reduced NADP.
· Light-independent stage / Calvin cycle uses ATP and reduced NADP to make complex organic molecules.
· Energy transfer pathway: light energy → ATP + reduced NADP → chemical energy in organic molecules.
· ATP supplies energy for reactions in the Calvin cycle.
· Reduced NADP supplies hydrogen/electrons for reducing GP to TP.

Chloroplast pigments and light absorption

· Chlorophyll a, chlorophyll b, carotene and xanthophyll are chloroplast pigments in thylakoids.
· Pigments absorb different wavelengths of light, increasing the range of light usable for photosynthesis.
· Chlorophyll a is the main reaction centre pigment in photosystems.
· Accessory pigments such as chlorophyll b, carotene and xanthophyll absorb light and transfer energy to chlorophyll a.
· Pigments absorb mainly red and blue-violet light; less green light is absorbed, so leaves often appear green.
· Absorption spectrum = graph showing wavelengths of light absorbed by a pigment.
· Action spectrum = graph showing rate of photosynthesis at different wavelengths.
· Exam skill: compare peaks in absorption spectra with peaks in the action spectrum to show which pigments drive photosynthesis most effectively.

This graph shows that chlorophyll absorbs light strongly in the blue-violet and red regions. These wavelengths are most useful for photosynthesis because they provide energy for photoactivation of chlorophyll. Low absorption of green light explains why many leaves appear green. Source

Chromatography of chloroplast pigments

· Chromatography separates chloroplast pigments because pigments have different solubilities in the solvent and different attractions to the paper/stationary phase.
· Pigment spots move different distances up the chromatogram.
· Rf value is used to help identify pigments.
· Formula: Rf = distance moved by pigment ÷ distance moved by solvent front.
· Compare calculated Rf values with known values to identify chlorophyll a, chlorophyll b, carotene and xanthophyll.
· Exam tip: measure from the baseline to the centre of each pigment spot and to the solvent front.

Light-dependent stage: key locations and events

· Occurs on the thylakoid membranes.
· Uses light energy absorbed by pigments in photosystems.
· Photoactivation of chlorophyll = light energy excites electrons in chlorophyll to a higher energy level.
· Excited electrons pass along an electron transport chain.
· Energy released from electrons is used to pump protons across the thylakoid membrane into the thylakoid space.
· A proton gradient forms across the thylakoid membrane.
· Protons diffuse back into the stroma through ATP synthase by facilitated diffusion.
· This drives photophosphorylation, producing ATP from ADP and inorganic phosphate.
· Details of individual electron carriers and ATP synthase structure are not required.

This image shows how light energy excites electrons in photosystems. Electron movement through the thylakoid membrane helps create a proton gradient, which drives ATP synthesis. It also shows how reduced NADP is produced in the non-cyclic pathway. Source

Cyclic photophosphorylation

· Occurs during the light-dependent stage.
· Involves photosystem I / PSI only.
· Photoactivation of chlorophyll releases excited electrons from PSI.
· Electrons pass along an electron transport chain and return to PSI.
· Electron energy is used to make ATP by chemiosmosis.
· Produces ATP only.
· Does not produce reduced NADP.
· Does not produce oxygen, because photolysis of water is not involved.

Non-cyclic photophosphorylation

· Occurs during the light-dependent stage.
· Involves photosystem II / PSII and photosystem I / PSI.
· Photoactivation of chlorophyll occurs in both photosystems.
· Electrons lost from PSII are replaced by electrons from photolysis of water.
· The oxygen-evolving complex catalyses photolysis of water.
· Photolysis: water is split into protons, electrons and oxygen.
· Oxygen is released as a waste product.
· Electrons pass through an electron transport chain, releasing energy for proton pumping.
· Electrons from PSI reduce NADP to reduced NADP.
· Produces ATP, reduced NADP and oxygen.

Chemiosmosis in the thylakoid membrane

· Chemiosmosis = ATP synthesis using energy from a proton gradient across a membrane.
· Electron energy pumps protons from the stroma into the thylakoid space.
· High proton concentration builds up inside the thylakoid space.
· Protons move back into the stroma through ATP synthase.
· This proton movement provides energy for ATP synthesis.
· Exam wording: protons return by facilitated diffusion through ATP synthase, providing energy for ATP production.

Calvin cycle: light-independent stage

· Occurs in the stroma.
· Does not use light directly, but depends on ATP and reduced NADP from the light-dependent stage.
· Main purpose: convert carbon dioxide into organic molecules.
· The three main stages are carbon fixation, reduction and regeneration of RuBP.

Calvin cycle stage 1: carbon fixation

· RuBP = ribulose bisphosphate, a 5C compound.
· Carbon dioxide combines with RuBP.
· Reaction is catalysed by rubisco.
· The 6C intermediate formed is unstable.
· It immediately splits into two molecules of GP / glycerate 3-phosphate, each a 3C compound.

Calvin cycle stage 2: reduction of GP to TP

· GP is reduced to TP / triose phosphate.
· This uses reduced NADP as a source of hydrogen/electrons.
· This also uses ATP as an energy source.
· TP is a key product of the Calvin cycle.
· Some TP leaves the cycle to make other organic molecules.

Calvin cycle stage 3: regeneration of RuBP

· Most TP is used to regenerate RuBP.
· Regeneration of RuBP requires ATP.
· RuBP must be regenerated so the Calvin cycle can continue fixing carbon dioxide.
· Without RuBP regeneration, carbon fixation stops.

This diagram summarises the three main stages of the Calvin cycle. Carbon dioxide is fixed by rubisco, GP is reduced to TP, and RuBP is regenerated using ATP. It also shows where ATP and reduced NADP are used. Source

Uses of Calvin cycle intermediates

· GP can be used to produce some amino acids.
· TP can be used to produce carbohydrates, lipids and amino acids.
· Carbohydrates made from TP include sugars that can be converted into larger storage or structural molecules.
· Key exam phrase: Calvin cycle intermediates are used to produce complex organic molecules.

Common exam comparisons

· Cyclic photophosphorylation: PSI only; ATP only; electrons return to PSI; no photolysis; no oxygen; no reduced NADP.
· Non-cyclic photophosphorylation: PSI and PSII; ATP and reduced NADP made; photolysis of water; oxygen released; electrons do not return to the original chlorophyll.
· Light-dependent stage: thylakoid membranes; uses light; produces ATP and reduced NADP.
· Light-independent stage: stroma; uses ATP and reduced NADP; fixes carbon dioxide; produces organic molecules.
· Absorption spectrum shows light absorbed by pigments; action spectrum shows photosynthetic rate at each wavelength.