Photosynthesis

Learning Outcome
Students are able to explain :
1. the significance of photosynthesis
2. the requirements for and the process of photosynthesis
3. the effects of environmental factors on the rate of photosynthesis
4. the utilization of photosynthetic products
5. the structural adaptation of a leaf as a photosynthetic organ

What happens during photosynthesis?

Photosynthesis occurs in two stages, namely the light-dependent or light stage and the light-independent or dark stage.

In the light stage:

Light energy is absorbed by chlorophyll and then converted into chemical energy.

..........................chlorophyll

light energy--------------------------------> chemical energy

Light energy is also used to split water molecules into oxygen and hydrogen atoms. This is called PHOTOLYSIS of water.

In the dark stage:

The hydrogen produced in photolysis is used to reduce carbon dioxide to carbohydrates such as glucose. The energy required for this process to take place comes from the light stage. Since the formation of glucose from carbon dioxide does not requre light, it is called the dark stage.

In photosynthesis, light energy is transformed and stored in carbohydrate molecules. Glucose is the first stable form of carbohydrate formed in photosynthesis that the plant can use.

In the light stage, 12 molecules of water are split to yield 6 molecules of oxygen and 24 atoms of hydrogen. It has been found that all the oxygen gas released during photoynthesis comes from water. As the rate of photosynthesis increases, more oxygen is produced. The volume of oxygen produced thus indicates the rate of photosynthesis.

How is one molecule of glucose formed?

light stage

light energy----------------------------------------------------> chemical energy

............ photolysis

12 H2O-------------> 6 O2 + 24 H

............ of water

dark stage

24 H + 6CO2------------------------------------->C6H12O6 + 6H2O

In the dark stage, the hydrogen released during photosynthesis is used to reduce 6 molecules of carbon dioxide. As a result, 1 molecule of glucose is formed.

The equation for photosynthesis:

6CO2 + 12 H2O ---------------------------->C6H12O6 + 6O2 + 6H2O.

Task for students:

Based on present-day knowlegde, what do you thing is wrong with the equation given below?

6CO2 + 6H2O-------------------------> C6H12O6 + 6O2

In plants, photosynthesis takes place in chloroplasts and hence only chloroplast-containing cells can carry out photosynthesis. In light, chlorophyll, a green pigment in the chloroplast, absorbs light energy. The energy then drives the reactions of photosynthesis to produce carbohydrates.

To find out if photosynthesis has taken place or not, we can test for the presence of starch in a plant. However, starch in the leaf that is formed before the experiment will interfere eith the experimental results. Thus, the plants used in the experiments must first be put in the dark for at least 24 hours before any practical work work to remove all of the starch. This process of removing starch, without damaging the plants, is called destarching. Iodine solution is used to test for the presence of starch.

(adapted from:Lam Peng Kwan. GCE.'O'Level. Biology Matters. Marshall Cavendish Education).