You should already know what chloroplasts do!

Chloroplasts are found in mesophyll cells (in the leaves) of green plants. Guard cells also contain chloroplasts. They are not normally found in the cells of roots!

Their function is to produce glucose from carbon dioxide and water. To do this they need light energy. You should already know this. You should also know that they contain a green chemical called chlorophyll. However, you need to know a little more for your “A” Level studies.

Have you ever wondered why green plants are all slightly different colours ? Well there is more than one kind of chlorophyll, and there are other photosynthetic pigments. When you extract these pigments from leaves and separate them using chromatography you will see that they are all slightly different colours.

Photosysnthesis actually takes place as two different sets of reactions. The “light reaction” requires light energy. Chlorophyll is required to convert light energy into chemical energy. This set of reactions produces two chemicals: ATP and NAHPH. In the “dark reaction” carbon dioxide and water are converted into carbohydrate. The dark reaction needs chemical energy which is supplied by ATP and NADPH.


  1. have a double membrane the inner membrane the outer membrane
  2. have their own DNA this carries the information to make the enzymes
  3. have their own ribosomes which are more like the ribosomes of prokaryotes than eukaryotes
  4. these are used to synthesise proteins (the enzymes) from amino-acids
  5. make their own enzymes
  6. are required for photosynthesis
  7. require carbon dioxide and water
  8. produce glucose
  9. contain chlorophyll this green chemical “traps” sunlight energy

N.B. Plant cells contain chloroplasts and mitochondria.

Click “Next”repeatedly to see the labels: You can use this for your Advanced Level studies to help you memorisethe parts of a chloroplast.

You already know from your GCSE biology that chloroplasts use sunlight energy to convert carbon dioxide and water into glucose and oxygen, but for your “A” level studies you need to know more detail than this. You have to be able to explain how structure and function are related, i.e. what each part of the chloroplast does.

The process of photosynthesis consists of two sets of chemical reactions called the light reaction and the dark reaction. The light reaction takes place in the grana. The dark reaction takes place in the stroma.

In the light reaction, electrons in chlorophyll molecules are excited. In other words the energy photons is used to activate electrons. The energy in excited electrons is used to split water molecules: this is photolysis. Oxygen is produced as a waste product and hydrogen is used to reduce NADP+ to form NADPH. The process involves non-cyclic electron transfer producing both NADPH and ATP and cyclic electron transfer producing just ATP.

The dark reaction requires energy from both NADPH and ATP. In this part of photosynthesis carbon dioxide is fixed. Energy from NADPH and ATP is used to convert glucose (a six carbon molecule) into ribulose diphosphate (a five carbon sugar molecule). Ribulose diphosphate can combine with carbon dioxide eventually forming glucose.

Chloroplasts contain both DNA and ribosomes. These are required to synthesise proteins. Remember that all these chemical reactions taking place in chloroplasts require enzymes.

My diagram shows some small starch granules. When a plant has been in the sun for some hours, the starch granules can get quite big as the glucose produced in photosynthesis is converted into starch.