Plants make their own food, and almost all life on Earth depends on this ability. In plant nutrition you will learn how leaves capture light energy and turn carbon dioxide and water into glucose – the process of photosynthesis – and what controls how fast it happens.
The photosynthesis equation
Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light.
- Word equation: carbon dioxide + water → glucose + oxygen.
- Symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Light energy is absorbed by the green pigment chlorophyll inside chloroplasts.
- The glucose made can be used for respiration, stored as starch, or converted into cellulose, proteins and fats.
Limiting factors
A limiting factor is the factor in shortest supply that holds back the rate of photosynthesis.
- The three main limiting factors are light intensity, carbon dioxide concentration and temperature.
- If you increase a limiting factor, the rate rises until another factor becomes limiting and the graph levels off.
- Temperature works through enzymes, so too high a temperature denatures them and the rate falls.
- Growers in greenhouses raise light, carbon dioxide and warmth together to increase crop yields.
Leaf structure
The leaf is adapted to be an efficient organ for photosynthesis.
- A broad, flat lamina gives a large surface area to absorb light.
- The palisade mesophyll near the top is packed with chloroplasts to capture light.
- Stomata (mainly on the lower surface) let carbon dioxide diffuse in; guard cells open and close them.
- A network of veins (xylem and phloem) brings water and carries away sugars; air spaces in the spongy mesophyll allow gases to move.
Mineral needs and testing leaves
Plants also need minerals from the soil and can be tested for starch to show photosynthesis has occurred.
- Nitrate ions are needed to make amino acids and proteins; without them growth is stunted and leaves yellow.
- Magnesium ions are needed to make chlorophyll; a shortage causes yellowing between veins (chlorosis).
- The starch test: destarch the plant, then boil a leaf in water, then in ethanol to remove chlorophyll, rinse, and add iodine – a blue-black colour shows starch.
- This test is used to prove that light, chlorophyll and carbon dioxide are each needed for photosynthesis.