Table of Content
- Definition of Photosynthesis
- Materials And Conditions Necessary For Photosynthesis
- Evidence Of Photosynthesis In Plants
- Importance Of Photosynthesis
Definition Of Photosynthesis
Photosynthesis is defined as the process by which green plants manufacture their food (organic compounds) making use of carbon dioxide and water in the presence of sunlight.
Water combines with carbon dioxide in the presence of sunlight within the chlorophyll of leaves to manufacture food (glucose) and oxygen is liberated as a by-product. Photosynthesis occurs in all green parts of plants.
Mechanism of Photosynthesis
Photosynthesis is an endothermic reaction and it consists of a series of complex reactions. These reactions during photosynthesis is grouped into two stages called Light reaction and Dark reaction.
1. Light Reaction Stage
Light reaction stage occurs during the day or in the presence of sunlight. The light energy or solar energy is captured by the chlorophyll and electrons are excited. The energy so trapped is used to split water into hydrogen ion (H+) and hydroxyl ion (OH-). This splitting of water into hydrogen ion (H+) and hydroxyl ion (OH-) is called photolysis of water.
2. Dark Reaction Stage
Dark reaction occurs at night or in the absence of light. Together with the energy provided by ATP, the reduced compound Nicotinamide Adenine Dinucleotide (NADP) then lead to the assimilation of carbon dioxide. Through a series of steps controlled by a specific enzyme, a three carbon compound (CH20) or sugar is formed.
CH20 is the carbon structure from which simple sugar, fat and oil, protein etc are formed during the dark reaction.
Materials And Conditions Needed For Photosynthesis
For photosynthesis to take place, certain materials or conditions must be available. These are carbon (iv) oxide, water, chlorophyll, sunlight, mineral salts, suitable temperature and enzymes.
While carbon (iv) oxide, water, sunlight, mineral salts and suitable temperature are external conditions or factors, chlorophyll and enzymes are internal conditions or factors necessary for photosynthesis to take place.
1. Carbon (iv) oxide: Carbon (iv) oxide is derived from the atmosphere and it diffuses into the intercellular spaces through the stomata of leaves.
2. Water and mineral salts: Water and mineral salts are derived from the soil. They pass into the roots of plants through the root hairs by a process called osmosis. Water and dissolved mineral salts are conducted by xylem from the roots through the stem and finally to the mesophyll cells containing chlorophyll of the leaves.
3. Sunlight: Sunlight is obtained from solar energy. The light from the sun is trapped by the chlorophyll of the leaves.
4. Optimum temperature: Temperature is derived partly from the solar energy and partly from chemical reactions within the leaves during which heat is generated. Suitable temperature is important for enzymes to enable them function properly during photosynthesis.
5. Chlorophyll: Chlorophyll is the green colouring pigment found in the pallisade and spongy mesophyll of the leaves. The chlorophyll represents sites where food can be synthesized and it helps to trap solar energy and convert it to chemical energy.
Evidence of Photosynthesis in Plants
1. Formation of sugar and starch: The main product formed during photosynthesis is simple sugar. The simple sugar formed is partly used by the plant and excess of it is converted to starch immediately for storage. The starch is then transported to other parts of the plant through the phloem vessels for storage. This process is called translocation.
The best way to show the evidence of photosynthesis in plant is to test the leaf for starch. The test for starch can be carried out through this experiment below:
Aim: To test for the presence of starch in the leaf.
Materials Required: Fresh green leaves from outdoor plants, beakers, burner, boiling tubes, dropping tube, white tile and iodine solution.
Method: First boil the leaf in water for 4-6 minutes so as to kill the cells, inactivate the enzymes and burst starch grains present.
Then dip the leaf into a test tube containing 70% alcohol to decolourize the leaf. After that, the decolorized leaf is dipped into a beaker containing hot water to soften it. Finally, place the leaf in a white tile and pour few drops of iodine solution on the lead. In the control experiment, a leaf from a plant kept in the dark cupboard is plucked and tested for starch.
Observation: It is seen that the leaf that was plucked from the potted plant outside turned blue-black with iodine solution while the other leaf (control experiment) remained colourless.
Conclusion: Since the leaves in the real experiment has turned blue-black with iodine solution, it shows that photosynthesis has taken place or starch is formed by the leaf.
Importance of Photosynthesis
Photosynthesis is very important both to plants and animals for the following reasons:
1. Production of Food: Photosynthesis provides food for both plants and animals. All green plants are able to manufacture their food through the process of photosynthesis while animals depend directly or indirectly on these green plants for their food.
2. Purification of the Atmosphere: Waste product like carbon (iv) oxide released during respiration by both plants and animals is removed from the atmosphere by plants for use during photosynthesis.
3. <>Release of Oxygen to the Environment: Oxygen needed for respiration by plants and animals is released into the environment during photosynthesis.
4. It serves as building blocks for other substances: Photosynthesis provides the building block or carbon skeleton on which other food substances such as proteins, fats, oil etc are built.