Photosynthesis: The Chemistry

Every breath you take and almost every calorie you eat depend on one chemical process. Photosynthesis uses the energy of sunlight to turn carbon dioxide and water, two of the most stable, low-energy molecules around, into sugar and oxygen. It’s the reverse of burning, powered by light. Plants, algae and cyanobacteria carry it out on such a scale that they have changed the composition of the entire atmosphere.

The overall equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

Carbon dioxide and water are converted into glucose and oxygen. This is the reverse of the overall equation for aerobic respiration, and the energy involved is the same, with the opposite sign:

ΔG ≈ +2,870 kJ per mole of glucose

That positive value means the reaction can’t happen on its own. It must be driven by an outside energy source: light. Photosynthesis converts light energy into chemical energy stored in the bonds of sugar.

A redox reaction running uphill

Photosynthesis is a redox reaction (see oxidation and reduction):

• Carbon is reduced: its oxidation state falls from +4 in CO₂ to 0 (on average) in glucose.
• Oxygen is oxidised: its oxidation state rises from −2 in water to 0 in O₂.

In respiration, electrons flow “downhill” from glucose to oxygen and release energy. In photosynthesis, electrons are pushed “uphill” from water to carbon dioxide, and light supplies the energy to do the pushing.

Where does the oxygen come from?

It’s natural to guess that the O₂ given off comes from carbon dioxide, since CO₂ contains two oxygen atoms. It doesn’t. In 1941, Samuel Ruben and Martin Kamen supplied algae with water labelled with the heavy oxygen isotope oxygen-18. The oxygen gas produced contained the heavy isotope, showing it came from water, not CO₂.

A more accurate equation shows this:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Twelve water molecules are split to release six O₂; six new water molecules are formed in the process of building sugar. Isotopes are a powerful tool for tracing atoms through reactions (see mass spectrometry and isotopes).

Where it happens: the chloroplast

In plants and algae, photosynthesis takes place in chloroplasts, organelles found mainly in leaf cells. Each chloroplast has:

• an outer and inner membrane;
• thylakoids: flattened membrane sacs, often stacked into piles called grana, containing the light-absorbing pigments and the electron transport machinery;
• the stroma: the fluid around the thylakoids, containing the enzymes that build sugar.

Cyanobacteria, which invented oxygen-producing photosynthesis billions of years ago, carry out the same chemistry in membranes inside their cells. Chloroplasts are thought to be descended from ancient cyanobacteria that were engulfed by other cells.

The two stages

Stage 1: the light-dependent reactions (thylakoid membranes)

Light energy is captured and used to make two energy-rich molecules:

• ATP, the energy currency (see ATP);
• NADPH, a carrier of high-energy electrons (similar to NADH).

In the process, water is split, releasing oxygen as a by-product. The key players are pigment–protein complexes called photosystems, containing chlorophyll (see chlorophyll: why plants are green). The details are in the light-dependent reactions.

Stage 2: the Calvin cycle (stroma)

The ATP and NADPH are used to fix carbon dioxide: attach it to an organic molecule and reduce it to sugar. The key enzyme, RuBisCO, is probably the most abundant protein on Earth. This stage doesn’t need light directly, although it stops soon after dark because it runs out of ATP and NADPH. See the Calvin cycle.

The two stages depend on each other: the light reactions supply ATP and NADPH; the Calvin cycle returns ADP, phosphate and NADP⁺ to be recharged.

What limits the rate of photosynthesis?

The rate of photosynthesis depends on several factors, and at any moment, the one in shortest supply limits the overall rate:

• Light intensity: more light means faster light reactions, up to a point.
• Carbon dioxide concentration: at about 0.04% of air, CO₂ often limits photosynthesis on bright days. Commercial greenhouses sometimes raise CO₂ levels to boost growth.
• Temperature: the enzymes of the Calvin cycle work faster when warmer, up to an optimum, then slow down and eventually denature (see how temperature and pH affect enzymes).
• Water supply: plants short of water close the pores in their leaves (stomata) to reduce water loss, which also cuts off their CO₂ supply.

These are classic school investigations, often done by counting oxygen bubbles from pondweed at different light intensities.

What happens to the glucose?

Glucose made in photosynthesis is:

• used in respiration to supply the plant’s own energy needs;
• converted to starch for storage (see starch vs glycogen vs cellulose);
• converted to sucrose to be transported around the plant;
• used to make cellulose for cell walls;
• combined with nitrogen and other elements to make amino acids, proteins, fats and nucleic acids.

The leaf starch test, in which a leaf kept in light turns blue-black with iodine while one kept in the dark doesn’t, is direct evidence that photosynthesis produces carbohydrate (see the iodine test for starch).

Why photosynthesis matters

• Food: directly or indirectly, it supplies almost all the energy in food chains.
• Oxygen: it produced the oxygen in Earth’s atmosphere, starting with the “Great Oxidation Event” about 2.4 billion years ago, and it keeps replenishing it.
• Carbon cycle: plants and phytoplankton remove large amounts of CO₂ from the air every year, partly offsetting emissions from fossil fuels.
• Fossil fuels: coal, oil and natural gas are the remains of ancient photosynthetic organisms, so burning them releases stored solar energy captured millions of years ago.
• Efficiency: under ideal conditions, photosynthesis converts only a few percent of sunlight into chemical energy, and typical crops manage around 1%. Scientists are working on improving crop efficiency and on “artificial photosynthesis” to make fuels from sunlight, water and CO₂.

Common misconceptions

• “Plants get their mass from the soil.” Most of a plant’s dry mass comes from carbon dioxide from the air, plus water. Soil supplies only small amounts of minerals.
• “Plants photosynthesise instead of respiring.” Plants respire all the time, day and night. In daylight, photosynthesis usually runs faster than respiration.
• “The oxygen comes from CO₂.” It comes from water.
• “The Calvin cycle is a ‘dark reaction’ that happens at night.” It happens in the light, because it depends on ATP and NADPH from the light reactions.

Key takeaways

• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light, with ΔG ≈ +2,870 kJ mol⁻¹.
• It’s a redox process: CO₂ is reduced to sugar and water is oxidised to O₂. Isotope labelling showed the oxygen comes from water.
• It happens in chloroplasts: the light-dependent reactions in the thylakoids make ATP and NADPH; the Calvin cycle in the stroma uses them to fix CO₂.
• The rate is limited by light, CO₂, temperature and water.
• Photosynthesis feeds food chains, produces atmospheric oxygen and drives the carbon cycle. Compare it with respiration in photosynthesis vs respiration.