Energy for life
Every living thing needs energy, and almost all of it traces back to the Sun. Photosynthesis captures light energy and stores it in glucose; respiration releases that stored energy for the cell to use. Together they form a cycle that powers the living world and keeps the atmosphere's gases in balance.
Photosynthesis
Photosynthesis takes place in chloroplasts, using the green pigment chlorophyll to absorb light. Carbon dioxide and water combine, using light energy, to make glucose and oxygen. The glucose is used for respiration, stored as starch, or built into cellulose; the oxygen is released.
Its rate is limited by whichever factor is in shortest supply — light intensity, carbon dioxide concentration or temperature — the idea of a limiting factor.
Respiration
Respiration releases energy from glucose in every living cell. Aerobic respiration uses oxygen and releases a lot of energy (glucose + oxygen → carbon dioxide + water). When oxygen is short, cells switch to anaerobic respiration, releasing far less energy — in humans producing lactic acid (muscle fatigue), and in yeast producing ethanol and carbon dioxide (brewing and baking).
| Aerobic | Anaerobic | |
|---|---|---|
| Oxygen | used | not used |
| Energy released | a lot | a little |
| Products (humans) | CO₂ + water | lactic acid |
| Products (yeast) | CO₂ + water | ethanol + CO₂ |
Two halves of one cycle
Respiration is almost the reverse of photosynthesis: photosynthesis takes in CO₂ and water and gives out glucose and O₂, while respiration takes in glucose and O₂ and gives out CO₂ and water. Plants do both — photosynthesising in the light and respiring all the time.
Photosynthesis vs respiration
Photosynthesis STORES energy (builds glucose, needs light, only in cells with chlorophyll). Respiration RELEASES energy (breaks glucose, in every living cell, all the time).
A worked example
A pondweed gives off more oxygen bubbles as a lamp is moved closer, but above a certain brightness the rate stops rising. Explain.
- Oxygen bubbles show the rate of photosynthesis.Oxygen is a product of photosynthesis.
- Moving the lamp closer increases light intensity, so the rate rises.Light was the limiting factor.
- Above a certain brightness the rate levels off.Light is no longer limiting.
- Now another factor — CO₂ or temperature — limits the rate.The rate is set by whatever is in shortest supply.
More light speeds photosynthesis while light is limiting; the rate plateaus when another factor (CO₂ or temperature) becomes limiting.
The leaf — built for photosynthesis
The leaf is beautifully adapted to capture light and exchange gases. It is broad and flat for a large surface area, and thin so gases and light reach the inner cells quickly. Its upper cells are packed with chloroplasts to absorb light; stomata on the underside let carbon dioxide in and oxygen out; and a network of veins (xylem and phloem) brings water and carries away the sugars made.
Investigating photosynthesis
Several classic experiments prove what photosynthesis needs. To show a leaf has made starch, it is boiled (to kill it), decolourised in hot ethanol, and tested with iodine, which turns blue-black where starch is present. Covering part of a leaf with foil shows light is needed (the covered part makes no starch); a destarched plant in air without CO₂ shows carbon dioxide is needed; and a variegated leaf shows chlorophyll is needed (only the green parts make starch).
The starch test
Boil the leaf, decolourise it in hot ethanol (off a naked flame — ethanol is flammable), rinse, then add iodine. Blue-black means starch was made — evidence that photosynthesis happened there.