Photosynthesis

The big idea

A plant can't go out and find food, so it makes its own — out of air, water and sunlight. Photosynthesis is how it does it: the plant takes in carbon dioxide from the air and water from the soil, and uses the energy in sunlight to turn them into sugar. Oxygen is left over, and the plant lets it go. That's the oxygen you're breathing right now.

Think of a leaf as a tiny solar-powered kitchen. Sunlight is the power, water and carbon dioxide are the ingredients, and sugar is the meal.

What you'll learn

• What goes in and what comes out of photosynthesis
• The two stages, and where in the cell each one happens
• Why leaves are green
• What a "limiting factor" is — and how to read one on a graph

Where it happens: the chloroplast

Photosynthesis happens inside chloroplasts — small green structures packed into the cells of a leaf. Two parts of the chloroplast matter:

• Thylakoids are flat, disc-shaped sacs. Their membranes hold chlorophyll, the green pigment that catches light. Thylakoids are stacked like pancakes, and each stack is called a granum (plural: grana).
• The stroma is the fluid that fills the space around the stacks.

Keep this picture in your head: stage 1 happens in the thylakoids, stage 2 happens in the stroma.

Stage 1: Catching the light

The first stage needs light, so it's called the light-dependent reactions. It happens in the thylakoid membranes.

1. Chlorophyll absorbs light. The light's energy "excites" electrons in the chlorophyll — it bumps them up to a higher energy.
2. Water is split. To replace those electrons, the plant breaks water molecules apart. This is called photolysis ("splitting with light"). Splitting water releases oxygen — so the oxygen a plant gives off comes from water, not from carbon dioxide.
3. The energy is stored. The excited electrons' energy is packed into two carrier molecules: ATP (stores energy) and NADPH (carries high-energy electrons).

Think of stage 1 as a solar panel charging two batteries. No sugar has been made yet — the plant has only turned light energy into chemical energy it can use.

Stage 2: Building the sugar

The second stage is the Calvin cycle. It happens in the stroma, and it doesn't use light directly — it runs on the charged "batteries" (ATP and NADPH) from stage 1.

1. Grab carbon dioxide. CO₂ from the air is attached to a five-carbon molecule already waiting in the stroma, called RuBP. Turning CO₂ from the air into part of a living molecule is called carbon fixation.
2. Spend the energy. ATP and NADPH provide the energy and electrons to turn the result into G3P, a small three-carbon sugar.
3. Build and repeat. Some G3P leaves the cycle, and the plant joins G3P molecules together to make glucose and other carbohydrates. The rest of the G3P is used to rebuild RuBP, so the cycle can keep going.

The "empty batteries" (ADP and NADP⁺) go back to the thylakoids to be recharged. That's why the two stages depend on each other: stage 1 charges, stage 2 spends.

Putting it together

The whole process is summed up in one equation:

$6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}$

Read it left to right: six carbon dioxide plus six water, using light energy, make one glucose and six oxygen. It's a summary — the real process takes many small steps — but it's the version tests ask for.

Why leaves are green

Chlorophyll absorbs mostly blue-violet and red light and uses that energy. It absorbs very little green light — green is reflected, bouncing off the leaf — so green is the colour that reaches your eyes.

Limiting factors

Photosynthesis needs enough light, enough carbon dioxide and the right temperature (the enzymes in the Calvin cycle work best in a certain range). Whichever one is in shortest supply holds the whole process back. That one is the limiting factor.

Example a greenhouse has bright lamps and plenty of water, but the vents are shut and the CO₂ level drops. Turning up the lamps does nothing — the Calvin cycle is starved of CO₂. Pump in CO₂ and the rate climbs, until light or temperature becomes the new limit. On a graph, that's the point where the rising line levels off.

Check yourself

1. Where does the oxygen released by a plant come from? Answer From water, split in stage 1 — not from carbon dioxide.
2. Does the Calvin cycle need light? Answer Not directly. It needs the ATP and NADPH that stage 1 makes using light, so it stops soon after the lights go out.
3. A plant gets more light, but its rate of photosynthesis doesn't change. Why? Answer Something else — most likely carbon dioxide or temperature — is now the limiting factor.

Common mistakes

• "The oxygen comes from CO₂." It comes from water.
• "The Calvin cycle is the dark reaction, so it happens at night." It doesn't need light directly, but it runs during the day, while stage 1 keeps its batteries charged.
• Mixing up the places. Thylakoids = catching light (stage 1). Stroma = building sugar (stage 2).

Remember this

• In: carbon dioxide + water + light. Out: glucose + oxygen.
• Stage 1 (thylakoids) makes ATP, NADPH and oxygen. Stage 2 (stroma) uses ATP and NADPH to fix CO₂ into sugar.
• Oxygen comes from water. Leaves are green because chlorophyll reflects green light.