B4.2 How do plants make food?

B4.2 How do plants make food?

Photosynthesis can be written as


There are three main stages in photosynthesis:

❶ Light energy is absorbed by the green chlorophyll

❷ Energy used to bring about the reaction between carbon dioxide and water to produce glucose (a sugar)

❸ Oxygen produced as a waste product

Glucose is made up of carbon, hydrogen and oxygen atoms. Glucose made by the process of photosynthesis may be used in three ways:

❶ It can be converted into chemicals required for growth of plant cells such as cellulose

❷ It can be converted into starch, a storage molecule, that can be converted back to glucose when the plant requires it

❸ It can be broken down during the process of respiration, releasing energy stored in the glucose molecules

Plant cell:

  • Cell wall – provides support for cell
  • Cell membrane – allows gases and water to pass in and out of the cell freely while acting as a barrier to other, larger chemicals
  • Nucleus – contains DNA which carries the genetic code for making enzymes and other proteins used in the chemical reactions of photosynthesis
  • Vacuole – used by the cell to store waste materials and to regulate water levels
  • Cytoplasm – where the enzymes and other proteins are made
  • Mitochondria – where respiration occurs
  • Chloroplasts – contain chlorophyll and the enzymes for the reactions in photosynthesis



Plants need other chemicals in addition to glucose. The roots take up minerals from the soil in solution. Nitrogen, in the form of nitrates is absorbed and used by plant cells to make proteins.

Substances move through cells via the process of DIFFUSION. Diffusion is the overall movement of a substance from a region where it is in high concentration to an area where it is in lower concentration. Diffusion is a passive process as it does not need an energy input to happen.



OSMOSIS is a specific type of diffusion – it is the overall movement of water from a dilute to a more to a more concentrated solution through a partially permeable membrane. A partially permeable membrane allows water molecules through, but not solute molecules because they are too large.

The movement water into plant roots occur by osmosis.




ACTIVE TRANSPORT is the overall movement of a chemical substance across a cell membrane (from where the substance is in low concentration to where it is in higher concentration). This requires energy, which is provided by respiration. Active transport is used in the absorption of nitrates by plant roots.

There are several factors that can limit the rate the rate of photosynthesis:

  • Temperature – too low and photosynthesis stops until the temperature rises again. Too high and the enzymes stop working permanently
  • Carbon dioxide concentration – as carbon dioxide concentration increases, so does the rate of photosynthesis
  • Light intensity – light is needed for photosynthesis. The greater the availability of light the quicker photosynthesis will take place

To identify the effect of light on plants, biologists have to carry out fieldwork. This involves using a variety of techniques to measure the amount of available light and to see how this has affected the growth of plants.

  • A light meter can measure the amount of light that is hitting the leaf. The amount of light is measured in units of lux. Data-loggers can be fitted with a light meter and readings taken over a period of time.
  • A quadrat is a square shape, often divided up into smaller squares. The quadrat is placed randomly on sections of the area in question and the plants that fall inside the quadrat are vaunted
  • It is vital to use an identification key to ensure that plants are correctly identified. A key enables the rapid identification of plants and animals by asking questions such as ‘does the plant have parallel veins in its leaves?’

Sometimes it is preferable to measure the changes in plant life along a straight line – in this case a transect may be taken. A line is drawn and the quadrat is placed at set intervals along the line and the plants are counted. This gives a picture of the changes in plant life over the line of the transect.