Photosynthesis: Chemistry and Chloroplast Function

Photosynthesis is the process by which chloroplasts use light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. In brief: light energy is converted into chemical energy stored in glucose. The reaction is endothermic, which means it takes in energy, and it takes place in two linked stages inside the chloroplast: the light-dependent reactions in the thylakoid membranes and the Calvin cycle in the stroma.

Photosynthesis is the source of nearly all the chemical energy in food webs and of the oxygen in the atmosphere. This guide gives the word and chemical equations, describes the structure of the chloroplast and the role of chlorophyll, explains how light is used, works through an atom-balance and energy example, and corrects common mistakes.

What are the equations for photosynthesis?

Word equation: carbon dioxide + water → glucose + oxygen (using light energy and chlorophyll).

Chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This summary hides many steps. The oxygen released comes from the splitting of water, not from carbon dioxide. This was shown by experiments with a heavy isotope of oxygen (¹⁸O) in the 1940s, which appeared in the oxygen gas only when the labeled atoms were in the water. Light energy is stored as chemical energy in the bonds of glucose. About 2 870 kJ is stored for each mole of glucose (180 g) made, the same amount that is released when it is fully respired.

What is the structure of a chloroplast?

Chloroplasts are organelles found in the cells of plants and algae, mostly in the palisade and spongy mesophyll of leaves. A typical leaf cell has 20 to 100 of them. They are lens-shaped, about 4 to 10 micrometers long. Their structure is closely related to function.

• Double membrane (envelope): encloses the organelle and controls what enters and leaves.
• Thylakoids: flattened membrane sacs that contain chlorophyll and the proteins of the light-dependent reactions. Their large surface area captures more light.
• Grana (singular: granum): stacks of thylakoids, which resemble piles of coins.
• Stroma: the fluid around the thylakoids. It contains the enzymes of the Calvin cycle, including rubisco, as well as DNA, ribosomes and starch grains.

Chloroplasts, like mitochondria, contain their own circular DNA and ribosomes, and divide by a process like binary fission. This is evidence for the endosymbiotic theory, according to which they descend from cyanobacteria.

What does chlorophyll do?

Chlorophyll is a green pigment that absorbs light energy. The main type, chlorophyll a, absorbs mostly blue-violet light (around 430 nanometers) and red light (around 660 nanometers) and reflects green light, which is why leaves look green. Chlorophyll b and carotenoids (yellow to orange pigments) absorb at other wavelengths and pass the energy to chlorophyll a. This broadens the range of light that can be used. In autumn, chlorophyll breaks down in many trees, revealing the carotenoids and giving yellow and orange colors.

Pigments are arranged in clusters called photosystems in the thylakoid membrane. An absorbed photon excites electrons in the chlorophyll, and the high-energy electrons are passed along a chain of carriers, which is where the energy is first captured.

What are the two stages of photosynthesis?

Light-dependent reactions (thylakoid membranes)

Light energy is used to split water molecules in a process called photolysis, which releases oxygen, hydrogen ions and electrons. The energy of the electrons is used to make ATP and to reduce NADP⁺ to NADPH, a carrier that holds hydrogen. Oxygen is released as a by-product. These reactions need light.

Light-independent reactions: the Calvin cycle (stroma)

The ATP and NADPH from the first stage are used to convert carbon dioxide into sugar. The enzyme rubisco attaches carbon dioxide to a five-carbon sugar (RuBP), and the resulting molecules are reduced to triose phosphate. Some triose phosphate is used to make glucose and other organic compounds, and the rest is used to regenerate RuBP. The Calvin cycle does not use light directly, but it stops in darkness because it depends on ATP and NADPH. Both stages are described in more detail in the advanced articles.

Photosynthesis and respiration compared

Worked example: balancing the equation and energy storage

Check the atom balance of the photosynthesis equation and calculate the mass of glucose that a plant makes from a given amount of carbon dioxide.

1. Carbon: left, 6 (6CO₂); right, 6 (C₆H₁₂O₆). Balanced.
2. Hydrogen: left, 6 × 2 = 12 (6H₂O); right, 12 (C₆H₁₂O₆). Balanced.
3. Oxygen: left, 6 × 2 + 6 × 1 = 18; right, 6 (glucose) + 6 × 2 (6O₂) = 18. Balanced.
4. Mass relationship: molar masses are CO₂ = 44 g/mol, glucose = 180 g/mol and O₂ = 32 g/mol. Six moles of CO₂ (264 g) give one mole of glucose (180 g) and six moles of oxygen (192 g).
5. Example: if a plant absorbs 22 g of CO₂, then moles = 22 ÷ 44 = 0.5 mol. Glucose = 0.5 ÷ 6 = 0.0833 mol, which is 0.0833 × 180 = 15 g. Oxygen = 0.5 mol, which is 0.5 × 32 = 16 g.

Mass is conserved: the reactants 22 g of CO₂ plus 9 g of water (0.5 mol of H₂O, using 18 g/mol) total 31 g, and the products 15 g of glucose plus 16 g of oxygen total 31 g. The plant’s increase in mass comes mostly from the carbon dioxide taken from the air and the water, not from the soil.

How was photosynthesis explained?

Jan van Helmont showed in the 1600s that a growing willow tree gained mass without any significant loss of soil. Joseph Priestley (1771) and Jan Ingenhousz (1779) showed that plants restore air and that light is needed. In the 1930s Cornelis van Niel proposed that the oxygen comes from water, and in the 1950s Melvin Calvin and colleagues used radioactive carbon-14 to trace the path of carbon in the Calvin cycle, for which Calvin received the Nobel Prize in Chemistry in 1961.

Which mistakes are common?

Mistake 1: Saying that oxygen comes from carbon dioxide

It comes from water, which is split in the light-dependent reactions.

Mistake 2: Writing that light is a reactant

Light provides energy. It is shown above the arrow, not as a substance.

Mistake 3: Calling the Calvin cycle the “dark reaction”

It does not need light directly, but it depends on products of the light-dependent reactions and in most plants runs mainly in light.

Mistake 4: Thinking that photosynthesis happens in all plant cells

Only cells with chloroplasts photosynthesize. Root cells, for example, do not.