13.1 Photosynthesis as Energy Transfer Process
- Syllabus
- 9700–2028–2029
- Topic
- 13.1
- Level
- A2
Learning objectives
13.1.1Chloroplast structure-function relationship• Chloroplast structure-function relationship13.1.2ATP and reduced NADP in Calvin cycle• ATP and reduced NADP from light-dependent stage used in Calvin cycle for complex organic molecules13.1.3Sites within chloroplast• Sites within chloroplast:- Thylakoids (in grana stacks): light-dependent stage- Stroma: light-independent stage (Calvin cycle)13.1.4Chloroplast pigments in light absorption• Chloroplast pigments in light absorption: chlorophyll a, chlorophyll b, carotene, xanthophyll13.1.5Absorption and action spectra• Interpret absorption spectra and action spectra13.1.6Pigment chromatography• Chromatography to separate and identify pigments (Rf values)13.1.7Light-dependent stage• Light-dependent stage: cyclic and non-cyclic photophosphorylation13.1.8Cyclic photophosphorylation• Cyclic photophosphorylation:- Only PSI involved- Photoactivation of chlorophyll- ATP synthesised13.1.9Non-cyclic photophosphorylation• Non-cyclic photophosphorylation:- Both PSI and PSII involved- Photoactivation of chlorophyll- Oxygen-evolving complex catalyses photolysis of water- ATP and reduced NADP synthesised13.1.10During photophosphorylation• During photophosphorylation:- Electrons release energy through electron transport chain- Energy transfers protons across thylakoid membrane- Protons return to stroma via ATP synthase by facilitated diffusion, providing energy for ATP synthesis13.1.11Calvin cycle three main stages• Calvin cycle three main stages:- Rubisco catalyses CO2 fixation with RuBP (5C) to yield two GP molecules (3C)- GP reduced to TP using reduced NADP and ATP- RuBP regenerated from TP using ATP13.1.12Calvin cycle intermediates• Calvin cycle intermediates produce other molecules:- GP: some amino acids- TP: carbohydrates, lipids, amino acids
Chloroplast structure separates light capture from carbon fixation
A chloroplast is compartmentalised so light energy is captured at an internal membrane system while carbon-fixation reactions occur in the surrounding stroma. Its structure therefore supports the transfer of light energy into chemical energy.
- Envelope: a double phospholipid-bilayer envelope surrounds the chloroplast and separates its internal reaction spaces from the cytoplasm.
- Stroma: the fluid-filled stroma surrounds the internal membranes and is the compartment for the light-independent reactions; carbon dioxide, enzymes and products are dissolved there.
- Thylakoid membranes: flattened fluid-filled thylakoids form an internal membrane system. The membranes contain photosynthetic pigments, electron carriers and enzymes for light-dependent reactions.
- Grana and connections: stacks of thylakoids form grana, linked by stroma lamellae. The extensive membrane area provides many sites for light capture and electron-transfer reactions.
- Functional boundary: pigments are located in the thylakoid membrane system, whereas the stroma provides the separate reaction environment for carbon fixation. This is a location/function boundary, not a complete description of every photosynthetic step.
Do not treat the chloroplast as one undivided space, place pigments in the stroma, or assume the double envelope itself is the light-capturing surface. Detailed pigment spectra, chromatography, photophosphorylation pathways and Calvin-cycle carbon accounting belong to neighbouring cards.
Light-dependent reactions supply ATP and reduced NADP to the Calvin cycle
Photosynthesis links two compartmentalised stages: light-dependent reactions in the thylakoid system transfer light energy into ATP and reduced NADP, and the Calvin cycle in the stroma uses these products to build complex organic molecules from carbon dioxide.
- Light-dependent stage — thylakoids: light energy drives reactions that produce ATP by photophosphorylation and reduced NADP using hydrogen ions and electrons from water. These products carry usable energy and reducing power to the stroma.
- Transfer between stages: ATP and reduced NADP are supplied from the light-dependent stage to the light-independent stage; they are the connection between light capture and carbon fixation.
- Light-independent stage — stroma: the Calvin cycle uses the energy from ATP and hydrogen from reduced NADP to make complex organic molecules, including carbohydrates such as starch, sucrose and cellulose.
- Dependency boundary: “light-independent” means the Calvin-cycle reactions do not directly require light, not that they are independent of the light-dependent stage. Without a continuing supply of ATP and reduced NADP, they cannot continue indefinitely in darkness.
Do not treat the two stages as unrelated or say that the Calvin cycle makes carbohydrate without ATP and reduced NADP. Detailed pigment spectra, chromatography, individual photosystems, full photophosphorylation mechanisms and carbon-accounting steps belong to neighbouring cards.
Thylakoid membranes and stroma divide photosynthetic functions
A chloroplast separates photosynthetic work between an internal thylakoid membrane system and the surrounding stroma: thylakoids provide the organised membrane surface for light-dependent reactions, while the stroma provides the fluid reaction space for the Calvin cycle.
- Thylakoid system: flattened, fluid-filled thylakoids contain pigments, enzymes and electron carriers needed for the light-dependent stage. Pigments are arranged in photosystems within the thylakoid membranes, so light energy is captured at this membrane surface.
- Grana and stroma lamellae: stacks of thylakoids form grana, with connecting membrane regions between stacks. The membrane arrangement gives a large surface area and many organised sites for light-dependent reactions.
- Thylakoid spaces: the internal spaces of the thylakoids are part of the light-dependent system; keeping them enclosed allows the membrane system to separate reaction spaces.
- Stroma: the fluid surrounding the thylakoids contains dissolved carbon dioxide, sugars, enzymes and other molecules. It is the site of the light-independent stage, including the Calvin cycle.
- Functional boundary: thylakoid membranes capture light and transfer energy, whereas the stroma provides the separate environment for carbon fixation. The ATP and reduced NADP made by the light-dependent stage connect these regions without making them the same reaction space.
Do not place photosynthetic pigments in the stroma or treat grana as the site of the Calvin cycle. This card establishes structure and location; detailed pigment spectra, chromatography and individual photochemical steps belong to neighbouring cards.
Chloroplast pigments broaden the light available for photosynthesis
Chloroplasts contain several photosynthetic pigments in the thylakoid membranes. Because different pigments absorb different wavelengths, the pigment mixture broadens the light that can be captured and transferred into photosynthetic reactions.
- Chlorophylls: chlorophyll a and chlorophyll b are located in the thylakoid membrane system. They absorb mainly in the blue-violet and red regions, while reflected green light contributes to the green appearance of plants.
- Carotenoids: β-carotene and xanthophyll are accessory pigments in the thylakoid system. Their different absorption range, mainly including blue-violet light, adds to the wavelengths available for light capture.
- Energy transfer: pigment molecules are organised in photosystems. Absorbed light excites pigment electrons, and the captured energy is transferred through the photosystem towards the reaction centre for the light-dependent stage.
- Functional consequence: a mixture of pigments absorbs a broader range of usable wavelengths than one pigment alone, helping the chloroplast capture light energy under varied illumination. This card establishes the pigment-to-light-capture link; it does not interpret absorption/action spectra or separate pigments by chromatography.
Do not confuse the colour a pigment reflects with the wavelengths it absorbs, place pigments in the stroma, or treat accessory pigments as a replacement for the reaction centre. Detailed spectrum graphs, chromatography and the full light-dependent pathway belong to neighbouring cards.
Absorption spectra and action spectra answer different questions
An absorption spectrum shows how strongly a particular photosynthetic pigment absorbs different wavelengths of light. An action spectrum shows the rate of photosynthesis at different wavelengths, so comparing them links pigment absorption to photosynthetic activity.
- Read an absorption spectrum: identify the axes, then locate peaks where the named pigment absorbs strongly and troughs where absorption is low. The graph describes light absorbed by that pigment, not the rate of photosynthesis.
- Read an action spectrum: use the wavelength axis and photosynthesis-rate axis to locate wavelengths where photosynthesis is fastest or slowest. High action-spectrum values indicate effective photosynthetic activity under those wavelengths.
- Compare the graphs: the combined absorption of the chloroplast’s pigments should broadly match the action spectrum. Peaks in the blue-violet and red regions and a trough in the green-yellow region show the relationship supported by the pigment mixture.
- Interpret the link: wavelengths absorbed by chlorophylls and carotenoids can provide energy for photosynthesis, so strong cumulative absorption can correspond to a higher photosynthesis rate. Keep the graph boundary: an absorption spectrum is not itself a photosynthesis-rate measurement.
Do not read an absorption spectrum as if it directly measured photosynthesis, infer that a pigment’s visible colour is its main absorbed wavelength, or add chromatography and full light-reaction detail to this graph-reading objective.
Chromatography separates pigments so their movement can be compared
Chromatography separates a leaf-pigment mixture because its components move through a stationary phase at different speeds. The separated bands provide evidence for different pigments and their relative movement under the chosen solvent conditions.
- Extract and apply: dissolve pigments from leaf material in a suitable solvent, then apply a concentrated sample to the stationary phase. Keep the starting spot small so separated bands can be distinguished.
- Develop the chromatogram: allow the solvent mobile phase to travel through the stationary material. Different pigment components move at different rates because their interactions with the two phases differ.
- Observe separation: record the number, colour and relative positions of the bands. Separate bands are evidence that the extract contained more than one pigment; colour supports description but is not, by itself, a complete identification.
- Compare movement: measure from the origin to a pigment band and from the origin to the solvent front. Where required, calculate the retardation factor: Rf = distance travelled by component ÷ distance travelled by solvent front.
- Interpret cautiously: compare bands or Rf values with a reference run made using the same solvent and conditions. A larger Rf means farther movement in that setup; it does not provide a universal value independent of solvent.
Use the same solvent, stationary material, sample preparation and development conditions when comparing extracts. Repeat or run a reference where possible, and report that Rf values and band positions depend on the solvent and experimental conditions. Chromatography separates pigments; it does not measure their absorption spectrum or demonstrate the full light-dependent pathway.
Do not invent exact Rf values, treat a band colour alone as conclusive identification, or confuse chromatographic movement with wavelength absorption or photosynthesis rate.
Light-dependent reactions use cyclic and non-cyclic photophosphorylation
Both cyclic and non-cyclic photophosphorylation occur in the light-dependent stage on thylakoid membranes. In both, photoactivated electrons help generate ATP.
| Pathway | Photosystems and electron destination | Main products |
|---|---|---|
| Cyclic | PSI only; the excited electron returns to PSI | ATP only |
| Non-cyclic | PSII and PSI; electrons from water pass ultimately to NADP | ATP, reduced NADP and oxygen |
This card identifies the two pathways and their defining contrast. The next cards explain their detailed electron routes; cyclic flow does not produce reduced NADP or oxygen.
Cyclic photophosphorylation makes ATP without producing reduced NADP or oxygen
Cyclic photophosphorylation is a light-dependent process in the thylakoid membrane in which excited electrons from photosystem I pass through carriers and return to photosystem I. Their energy builds a proton gradient, which drives ATP synthesis, but the cycle does not produce reduced NADP or oxygen.
- Excite: light is absorbed by photosystem I and raises the energy of an electron in its primary pigment.
- Cycle the electron: the excited electron is accepted and passed along an electron-carrier chain before returning to photosystem I. The electron is therefore recycled rather than replaced by photolysis of water.
- Build the gradient: as the electron moves through the carriers, released energy powers proton pumping from the stroma into the thylakoid lumen.
- Make ATP: protons move back down their gradient through ATP synthase by chemiosmosis, driving ADP + Pi → ATP. The ATP can then supply the light-independent stage.
- Product boundary: cyclic flow uses photosystem I only and makes ATP, but it does not split water; therefore it produces neither oxygen from photolysis nor reduced NADP.
- Contrast: non-cyclic photophosphorylation uses photosystems II and I, replaces electrons from water and transfers electrons onward to reduce NADP, so it produces ATP together with reduced NADP and oxygen.
Do not describe cyclic photophosphorylation as carbon fixation or as a source of reduced NADP. The defining loop is photosystem I → electron carriers → photosystem I; detailed Calvin-cycle reactions belong to a separate card.
Non-cyclic photophosphorylation uses two photosystems and replaces electrons from water
Non-cyclic photophosphorylation is the light-dependent electron flow through photosystems II and I in the thylakoid membrane. Water replaces electrons lost from photosystem II, the flow builds a proton gradient for ATP synthesis, and electrons from photosystem I reduce NADP to reduced NADP; photolysis also releases oxygen.
- Photosystem II starts the flow: light excites an electron in the primary pigment of photosystem II. The electron enters an electron-carrier chain and is passed towards photosystem I.
- Replace the lost electron: water is split in photolysis, producing electrons, H+ and oxygen. The electrons replace those removed from photosystem II; oxygen is released as a product of water splitting.
- Make ATP: energy released as electrons move through the carriers powers proton movement from the stroma into the thylakoid lumen. Protons return through ATP synthase by chemiosmosis, driving ADP + Pi → ATP.
- Photosystem I and reduced NADP: light also excites an electron in photosystem I. Electrons from its carrier chain combine with H+ and NADP to form reduced NADP, which passes to the light-independent stage.
- Boundary with cyclic flow: non-cyclic flow uses both photosystems and sends electrons onward to NADP, producing ATP, reduced NADP and oxygen. It is not an electron loop returning to photosystem I.
Do not say that oxygen comes from carbon dioxide or that non-cyclic electrons return to their original photosystem. This card explains light-dependent electron flow and products; Calvin-cycle carbon fixation and limiting-factor investigations belong elsewhere.
Photophosphorylation depends on electron flow, proton pumping and ATP synthase
Photophosphorylation is ATP formation in the light-dependent stage. In the thylakoid membrane, light-excited electrons pass through carriers whose released energy pumps protons into the thylakoid lumen; protons then return through ATP synthase and drive ADP + Pi → ATP.
- Capture and transfer: photosynthetic pigments absorb light and transfer energy to electrons in a photosystem. The excited electrons enter an electron-carrier chain in the thylakoid membrane.
- Pump protons: as electrons move through the carriers, they release energy. That energy powers proton transport from the stroma into the thylakoid lumen, establishing a high H+ concentration in the lumen and a lower concentration in the stroma.
- Chemiosmosis: H+ moves back down its concentration gradient through transmembrane ATP synthase. The enzyme couples this proton flow to phosphorylation of ADP, forming ATP.
- Product connection: ATP produced in the light-dependent stage is supplied to the light-independent reactions. In non-cyclic flow, electron transfer also contributes to reduced NADP formation; cyclic flow returns electrons to photosystem I and does not produce reduced NADP.
- Interpretation boundary: light supplies the initial energy, electron carriers build the gradient, and ATP synthase converts the gradient into ATP. These are linked steps, not separate sources of ATP.
Do not reverse proton directions: pumping is from stroma to lumen, while chemiosmotic return is from lumen to stroma through ATP synthase. Do not confuse photophosphorylation with carbon fixation, or treat reduced NADP as an ATP product of cyclic flow.
The Calvin cycle fixes, reduces and regenerates carbon compounds
In the chloroplast stroma, the Calvin cycle fixes carbon dioxide, reduces GP to TP and regenerates the carbon-dioxide acceptor RuBP using ATP and reduced NADP from the light-dependent stage.
- Fixation: rubisco catalyses combination of CO2 (1C) with RuBP (5C). The unstable 6C product immediately forms two GP molecules (3C each).
- Reduction: ATP supplies energy and reduced NADP supplies hydrogen/electrons to convert GP into TP (3C).
- Regeneration: some TP leaves for biosynthesis; the remaining TP is rearranged using ATP to regenerate RuBP (5C), allowing further CO2 fixation.
Rubisco fixes CO2 but does not make carbohydrate in one step. The immediate stable products are two 3C GP molecules, and RuBP must be regenerated for the cycle to continue.
GP and TP supply carbon skeletons for specified organic products
Calvin-cycle intermediates are not only recycled: some provide carbon skeletons for other organic molecules made by the plant.
| Calvin-cycle intermediate | Syllabus-limited products |
|---|---|
| GP (glycerate 3-phosphate) | Some amino acids |
| TP (triose phosphate) | Carbohydrates, lipids and amino acids |
TP molecules can combine and be rearranged to form carbohydrate precursors; carbon skeletons from TP can also contribute to lipid and amino-acid synthesis. GP provides carbon skeletons for some amino acids. These biosynthetic withdrawals must be balanced by continued carbon fixation and RuBP regeneration.
GP is a Calvin-cycle intermediate, not itself classified here as a carbohydrate. Do not restrict TP only to starch/sucrose or omit its lipid and amino-acid uses; the required product list is deliberately limited to the table.