PLANT NUTRITION

PLANT NUTRITION

One of the main characteristics of living things is their ability to feed. This is called nutrition. Both plants and animals feed. Whilesome organisms called autotrophs can manufacture their own food, other organisms called heterotrophs cannot. Therefore, most autotrophs can manufacture their own food through a process called photosynthesis.

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.

Photosynthesis can be represented by chemical equation as:

6CO2 + 6H2O      – Sunlight C6H12O6         +         6O2

(Carbon dioxide) (water) chlorophyll (glucose) (oxygen)

From the chemical equation above, 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 traped 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.  Photolysis of water is represented by the equation.

4H2O           light                      4H+                  +        4(OH-)

               Chloroplast         (Hydrogen ion)              (Hydroxyl ion) 

(OH) IS converted to water

4(OH)                            2H2O              +        O2

Hydrogen ion)              (Water) (Oxygen)

1. DARK REACTION

Dark reaction occurs at night or in the absence of light.  Together with energy provided by ATP, the reduced compound Nicotinamide Adenine Dinucleotide (NADP) then lead to the assimilation of Corbon dioxide. Through a series of steps, each controlled by a specific enzyme, a three carbon compound can be represented by a chemical equation as follows:

4H+ + CO2enzyme CH2O + H20 

(Hydrogen ion) (Carbon (iv) oxide) (Sugar) (water)

Materials and Conditions Necessary 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) oxides derived from the atmosphere and it diffuses into the intercellular spaces through the stomata of the leaves. From the intercellular spaces, carbon (iv) oxide, diffuses further into the mesophyll cells containing chloroplasts .

(2) Water and mineral Salts: Water and mineral salts are derived from the soil. They pass into the roots of plants through the roothairs by a process called osmosis. Water and dissolved mineral salts are conducted by yelm from the root through the stem and finally of the leaves.

(3) Sunlight: Sunlight is obtained from solar energy. The light from the sun is trapped by the chlorophyll of the leaves. The sunlight is used to split water into hydrogen ion and hydroxyl ions in a process called photolysis.

(4) Optimum temperature: Temperature is demoed partly from the solar energy and partly from chemical reactions within the leavesduring which heat is generated. Suitable temperature is important for enzymes to enable them function properly during photosynthesis.

(5) Chlorophyll: Chlorophyll is the green coloring pigment found in the palisade and spongy mesophyll of the leaves. Chlorophyll represents sites where food can be synthesized and it helps to trap solar energy and convert it to chemical energy. 

PRACTICAL 

Evidence of Photosynthesis in Plants

The 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 throughthe following, experiments:

Experiment 

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 decolorize 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 a few drops of iodine solution on the leaf. 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.

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