How does photosynthesis work?

How does photosynthesis work? In short: a plant uses light energy to turn carbon dioxide and water into sugar, and releases oxygen as a by-product. It happens in two linked stages inside the chloroplasts of leaf cells. In the first, light is captured and stored as chemical energy. In the second, that energy is spent building sugar from carbon dioxide in the air. Here is each step, in order, with the numbers worked through.

The photosynthesis equation

Everything below adds up to one balanced equation:

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Count the atoms to check it. On the left: 6 carbon, 12 hydrogen (from 6 water molecules) and 18 oxygen (12 from the carbon dioxide plus 6 from the water). On the right: glucose has 6 carbon, 12 hydrogen and 6 oxygen, and the six O₂ molecules supply the other 12 oxygen atoms. Both sides come to 6 C, 12 H and 18 O.

Here is a detail that surprises people. The oxygen the plant releases comes from the water, not from the carbon dioxide. Scientists showed this by tracking a heavy form of oxygen through the reaction.

Where photosynthesis happens

Leaf cells contain chloroplasts, tiny compartments with two key regions:

• Thylakoids: stacked, flattened membranes that hold the green pigment chlorophyll. This is where the light-dependent reactions happen.
• Stroma: the fluid surrounding the thylakoids. This is where sugar is built.

Chlorophyll absorbs mostly red and blue light and reflects green, which is why leaves look green.

Stage 1: the light-dependent reactions

When light strikes chlorophyll in the thylakoid membranes, it energises electrons. The plant uses that energy to split water molecules. Splitting water releases three things: oxygen gas (which leaves through tiny pores called stomata), hydrogen ions, and electrons.

The energised electrons pass along a chain of proteins in the membrane. Along the way their energy is used to make two molecules that work as rechargeable batteries:

• ATP, the cell's general-purpose energy currency.
• NADPH, which carries electrons and hydrogen ready to be used in building sugar.

Nothing sugary has been made yet. Stage 1 only converts light into chemical energy and leaves oxygen as waste.

Stage 2: the Calvin cycle

The second stage happens in the stroma and does not need light directly, but it does need the ATP and NADPH from stage 1. It is a cycle:

1. Capture. An enzyme called rubisco attaches a CO₂ molecule to a five-carbon sugar called RuBP.
2. Reduce. The result is converted, using ATP and NADPH, into a three-carbon sugar called G3P.
3. Regenerate. Most of the G3P is rearranged, using more ATP, back into RuBP so the cycle can run again.

Here is the arithmetic. Each turn takes in one CO₂, so three turns bring in three carbons and produce six G3P. Five of those are recycled, and one is the net gain. To make a six-carbon glucose, the plant needs two G3P, and that takes six turns of the cycle.

Why photosynthesis matters

Photosynthesis is the entry point for nearly all the energy in food chains. The sugar feeds the plant itself, is stored as starch, or is built into cellulose for cell walls. Animals get that energy by eating plants, or by eating animals that ate plants. The oxygen released over billions of years is the oxygen we breathe.

A common exam slip is to say plants "breathe in CO₂ and breathe out oxygen" and stop there. Plants also respire, burning sugar for energy just as animals do. Photosynthesis simply makes more than respiration uses while the light is on.

Quick recap

• Photosynthesis turns CO₂, water and light into glucose and oxygen: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂.
• It takes place in chloroplasts: light reactions in the thylakoids, the Calvin cycle in the stroma.
• The light reactions split water, release O₂, and make ATP and NADPH.
• The Calvin cycle uses ATP and NADPH to fix CO₂ into sugar, with rubisco as the key enzyme; six turns make one glucose.
• The released oxygen comes from water, not from CO₂.

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