Where photosynthesis happens

1. Photosynthesis is the process in which light energy is used to build glucose from carbon dioxide and water, with oxygen released as a by-product.
2. It is carried out by plants, by algae and by some bacteria, and it is the only process on Earth that brings a large amount of new chemical energy into living things.
3. In a plant it takes place inside the chloroplasts, so only cells that contain chloroplasts can photosynthesise.
4. Chloroplasts are most tightly packed into the palisade mesophyll cells just below the upper surface of a leaf, where the most light arrives.
5. Inside each chloroplast the green pigment chlorophyll is spread over stacks of flattened membranes, which spreads it out so that as much light as possible is absorbed.
6. Chlorophyll absorbs light most strongly at the red and blue ends of the visible spectrum.
7. It reflects green light rather than absorbing it, and that reflected light is what reaches your eye, which is why leaves look green.
8. Chlorophyll is a pigment and not an enzyme, so its job is to absorb light energy rather than to speed a reaction up.
9. A cell with no chlorophyll cannot photosynthesise at all, which is why the white parts of a variegated leaf make no glucose.

Reactants, products and the equations

1. The reactants of photosynthesis are carbon dioxide and water, and the products are glucose and oxygen.
2. Carbon dioxide diffuses into the leaf from the air through the stomata, which are the pores on the underside of the leaf.
3. It then diffuses through the air spaces of the spongy mesophyll and into the mesophyll cells that hold the chloroplasts.
4. Water is absorbed from the soil by the root hair cells and carried up to the leaf in the xylem.
5. Oxygen diffuses out through the same stomata, and any that the plant does not use for its own respiration is released into the air.
6. The word equation is carbon dioxide + water $\to$ glucose + oxygen.
7. Light and chlorophyll are written above the arrow rather than on the left, because they make the reaction happen without being used up in it.
8. The balanced symbol equation is $6C{O}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$.
9. Check the balancing by counting atoms: each side has $6$ carbon, $12$ hydrogen and $18$ oxygen atoms.
10. The eighteen oxygen atoms on the right are split between the $6$ in the glucose and the $12$ in the six molecules of oxygen gas.
11. The glucose made is used in several ways: some is respired straight away, some is converted to starch for storage, some to cellulose for new cell walls, and some to lipids for seeds.
12. Glucose is also combined with nitrate ions taken up by the roots to build the amino acids the plant needs for its proteins.

The two stages of the reaction

1. Photosynthesis is not a single step; it happens in two stages, and only the first of them needs light directly.
2. In the first stage, light energy absorbed by chlorophyll is used to split water molecules apart.
3. Splitting the water releases hydrogen, which is held ready for the second stage, and oxygen, which is the by-product that leaves the leaf.
4. Splitting water needs a large input of energy, and that is exactly what the absorbed light provides.
5. In the second stage, the hydrogen is joined with carbon dioxide to build glucose.
6. The second stage does not use light itself, so it depends on the first stage to keep supplying it with hydrogen.
7. This is why the whole process stops in the dark: with no light there is no hydrogen, so no glucose can be assembled.
8. It also explains why the oxygen released comes from the water rather than from the carbon dioxide.

Photosynthesis as an endothermic reaction

1. An endothermic reaction is one that takes energy in from its surroundings, so energy has to be supplied before it will happen.
2. Photosynthesis takes in light energy, which means it cannot run at all unless light is being absorbed.
3. The bonds in glucose and oxygen hold more energy than the bonds in carbon dioxide and water, so energy has to be put in to make the change.
4. The light energy absorbed is therefore transferred into chemical energy stored in the glucose, where it stays until something respires it.
5. Respiration is the opposite, an exothermic reaction that releases the energy the glucose is holding, so the two processes run in opposite directions.
6. A plant carries out both at once, photosynthesising only in the light but respiring day and night.

Producers, biomass and the rest of life

1. A producer is an organism that makes its own food by photosynthesis, and plants, algae and photosynthetic bacteria are all producers.
2. Producers sit at the start of almost every food chain, because they are the only organisms that do not have to eat something else.
3. The glucose they make is used to build new plant material, and the mass of that material is called biomass.
4. Biomass is measured as dry mass, meaning the mass left after all the water has been driven off, because the amount of water in a living thing varies from hour to hour.
5. Only about one or two per cent of the light energy falling on a plant ends up stored in its biomass, because much of the light misses the chloroplasts, passes straight through or is the wrong wavelength.
6. Small as that fraction is, it is the energy that supports almost all life on Earth.
7. When a herbivore eats a plant, the biomass and the energy stored in it pass along the food chain to the next organism.
8. Photosynthetic organisms in the oceans, particularly the microscopic algae of the plankton, produce a large share of the world's biomass and oxygen.
9. Fossil fuels are the remains of organisms that photosynthesised long ago, so even the energy in coal and oil arrived through this one reaction.

Investigating photosynthesis in the laboratory

1. Glucose is difficult to detect in a leaf, because it is used and moved almost as soon as it is made.
2. A plant converts surplus glucose into starch, which stays put in the leaf, so testing a leaf for starch is the practical way of finding out whether it has photosynthesised.
3. Iodine solution is the reagent used, and it turns from orange-brown to blue-black where starch is present.
4. Before any experiment the plant has to be destarched by leaving it in a dark cupboard for one or two days.
5. Destarching works because the plant keeps respiring in the dark and uses up or moves away the starch already stored in its leaves.
6. Any starch found after the experiment must then have been made during the experiment itself, which is what makes the result valid.
7. Once the starch test itself is reliable, you can find out which conditions a leaf actually needs by depriving one leaf of one condition at a time.