Learning objectives
- Write the overall equation for photosynthesis and name its reactants and products.
- Describe how the structure of a leaf and of a chloroplast suits them to photosynthesis.
- Explain the two stages, the light-dependent reactions and the Calvin cycle, and how they are linked by ATP and NADPH.
- State that the oxygen released comes from water in the light-dependent reactions, while CO₂ is fixed in the Calvin cycle.
- Explain how light intensity, CO₂ concentration, temperature and water supply limit the rate, including light saturation.
What is photosynthesis?
Photosynthesis is the process by which plants, algae and some bacteria use light energy to make sugar from carbon dioxide and water, releasing oxygen. It converts light energy into chemical energy, stored in the bonds of glucose. The plant uses that glucose for respiration, turns it into sucrose to transport, starch to store and cellulose to build cell walls. Animals depend on both the food and the oxygen it produces.
Photosynthesis happens in two linked stages:
- The light-dependent reactions, in the thylakoid membranes, capture light energy, split water (releasing O₂) and make ATP and NADPH.
- The Calvin cycle (the light-independent reactions), in the stroma, uses that ATP and NADPH to fix CO₂ into sugar.
The overall equation
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Six molecules of carbon dioxide and six of water, using light energy absorbed by chlorophyll, make one molecule of glucose and six of oxygen. This is a summary of many steps: in reality the plant first makes a 3-carbon sugar (G3P), and the water split in the light-dependent reactions and the water formed in the Calvin cycle are counted together.
Inside a leaf
- Cuticle: a waxy, transparent layer that reduces water loss.
- Upper epidermis: transparent protective cells that let light through.
- Palisade mesophyll: tall cells packed with chloroplasts near the light; most photosynthesis happens here.
- Spongy mesophyll: loosely packed cells with air spaces that let CO₂ reach every cell and O₂ diffuse out.
- Stomata: pores in the lower epidermis, opened and closed by guard cells, where CO₂ enters and O₂ and water vapour leave.
- Vascular bundles (veins): xylem brings water in; phloem carries sugar away.
Inside a chloroplast
A chloroplast has a double membrane (envelope). Inside are flattened sacs calledthylakoids, stacked into grana and joined by stroma lamellae. The thylakoid membranes hold chlorophyll and the proteins of the light-dependent reactions; the space inside each thylakoid is the lumen. Around the thylakoids is the fluid stroma, which contains the enzymes of the Calvin cycle, as well as the chloroplast’s own DNA and ribosomes, and starch grains.
Stage 1: the light-dependent reactions
- Photosystem II absorbs light, which excites electrons in its chlorophyll.
- To replace them, PSII splits water (photolysis): 2H₂O → O₂ + 4H⁺ + 4e⁻. This is where the oxygen comes from.
- The electrons pass along an electron transport chain (plastoquinone, the cytochrome b₆f complex, plastocyanin), which pumps H⁺ into the lumen.
- Photosystem I absorbs light and re-energises the electrons.
- The electrons reduce NADP⁺ to NADPH.
- H⁺ flows back out of the lumen through ATP synthase, which makes ATP from ADP and phosphate.
Inputs: light, water, ADP, phosphate and NADP⁺. Outputs: O₂, ATP and NADPH. No CO₂ is used in this stage.
Stage 2: the Calvin cycle
- Carbon fixation: the enzyme RuBisCO joins CO₂ to RuBP (5 carbons); the product splits into two 3-PGA (3 carbons each).
- Reduction: ATP and NADPH from stage 1 convert 3-PGA into the 3-carbon sugar G3P.
- Release: for every 3 CO₂ fixed, one G3P leaves the cycle; two G3P make one glucose.
- Regeneration: the other G3P are rearranged, using more ATP, to remake RuBP.
Fixing 3 CO₂ uses 9 ATP and 6 NADPH. The Calvin cycle does not use light directly, but it stops in the dark within seconds because it runs out of ATP and NADPH. No O₂ is released in this stage.
Roles of the key molecules
| Molecule | Role |
|---|---|
| Chlorophyll | Green pigment in the thylakoid membranes that absorbs mainly red and blue light and passes the energy to electrons in the photosystems. |
| Water (H₂O) | Split in the light-dependent reactions to supply electrons and H⁺; its oxygen is released as O₂. |
| ATP | Energy carrier made by ATP synthase; powers the reduction and regeneration steps of the Calvin cycle. |
| NADPH | Carrier of high-energy electrons (reducing power) made at photosystem I; reduces 3-PGA to G3P. |
| Carbon dioxide (CO₂) | Source of the carbon in sugar; fixed by RuBisCO in the Calvin cycle. |
| Glucose (C₆H₁₂O₆) | Stores the captured energy; used for respiration and to make sucrose, starch and cellulose. |
| Oxygen (O₂) | By-product of splitting water; diffuses out through the stomata. |
Limiting factors and saturation
The rate of photosynthesis is held back by whichever factor is in shortest supply: the limiting factor. Increasing a factor that is not limiting barely changes the rate.
- Light intensity: in dim light the rate rises almost in proportion to light. But it does not rise forever: as light increases the curve bends over and reaches a plateau (light saturation), because the Calvin cycle cannot use the extra ATP and NADPH any faster. Very intense light can even damage the photosystems.
- Carbon dioxide: in bright light, the CO₂ available to RuBisCO is often what limits the rate. More CO₂ raises the plateau, which is why some greenhouses add CO₂.
- Temperature: the enzymes work slowly in the cold and fastest near an optimum (around 25–30 °C for many plants); above about 40 °C enzymes and membranes are damaged and the rate collapses.
- Water: only a little water is used in the reactions, but a plant short of water closes its stomata, which cuts off CO₂. Severe drought also damages the cells.
In the rate view, compare the three classic cases. In light-limited conditions, doubling the light raises the rate a lot while extra CO₂ does little. In CO₂-limited conditions, the reverse is true. Intemperature-limited conditions, warming towards the optimum helps most.
About this model
This simulator is a simplified representation of photosynthesis, not a complete biochemical model. The rate is calculated from two simple “ceilings”: a light-limited rate that saturates in bright light and a CO₂-limited rate that depends on the CO₂ inside the leaf; the lower of the two sets the rate. Temperature scales both, and water acts mainly by closing the stomata. The model leaves out photorespiration, the leaf’s changing pigments, light damage, the detailed chemistry of each step and differences between plant types (for example C₄ and CAM plants). Numbers are realistic in size but should be read as approximate. Each O₂ in the model corresponds to one CO₂ fixed, and the net O₂ release subtracts the leaf’s own respiration, which continues in the dark.
Key terms
- Chloroplast
- The organelle in which photosynthesis takes place.
- Thylakoid
- A flattened membrane sac in the chloroplast; site of the light-dependent reactions.
- Granum
- A stack of thylakoids (plural grana).
- Stroma
- The fluid around the thylakoids; site of the Calvin cycle.
- Photolysis
- The splitting of water by light-driven photosystem II, releasing O₂.
- Carbon fixation
- Joining CO₂ to an organic molecule (RuBP), catalysed by RuBisCO.
- Limiting factor
- The factor in shortest supply, which holds back the rate.
- Light saturation
- The point beyond which more light no longer increases the rate.
Why it matters
- Food: nearly all the food we eat comes, directly or indirectly, from photosynthesis.
- Oxygen: photosynthesis by plants and algae keeps oxygen in the atmosphere.
- Agriculture: greenhouse growers control light, CO₂ and temperature to remove limiting factors and boost yield.
- Climate: photosynthesis removes CO₂ from the air and stores the carbon in plants and soils.
- Energy research: scientists study photosynthesis to design artificial systems that turn sunlight into fuel.
Interesting facts
- In 1779 Jan Ingenhousz showed that plants only “restore” air in the light, and only in their green parts.
- In 1941 experiments with a heavy isotope of oxygen showed that the O₂ released comes from water, not from CO₂.
- Melvin Calvin traced the path of carbon with radioactive carbon-14 and won the 1961 Nobel Prize in Chemistry.
- RuBisCO is thought to be the most abundant protein on Earth.
Common misconceptions
- “The oxygen comes from carbon dioxide.” It comes from water, split in the light-dependent reactions.
- “The Calvin cycle happens in the dark.” It does not use light directly, but it needs ATP and NADPH, so it runs in the light.
- “More light always means more photosynthesis.” The rate saturates; then another factor limits it.
- “Plants photosynthesise instead of respiring.” Plants respire all the time; in the light, photosynthesis is usually faster, so they release O₂ overall.
- “Plants get their mass from the soil.” Most of a plant’s dry mass is carbon fixed from CO₂ in the air.
Key takeaways
- 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
- The light-dependent reactions (thylakoids) split water, release O₂ and make ATP and NADPH.
- The Calvin cycle (stroma) uses ATP and NADPH to fix CO₂ into sugar.
- Leaves are built to collect light, let CO₂ in, bring water in and carry sugar out.
- The rate is set by the limiting factor; light saturates, so more light does not help forever.