Photosynthesis

Photosynthesis is the process of preparing organic food (carbohydrate) by combining carbon dioxide and water, using solar energy with the help of chlorophyll pigments.

The sugar (glucose) produced in photosynthesis is stored in the form of starch in plants and it is the source of reserved internal energy.

The overall equation of photosynthesis is

$6C{O}_2+12H_{2}O\underset{Sunlight}{\overset{Chlorophyll}{\to }}{C}_6{H}_{12}{O}_6+6H_{2}O+6O_2$

First true and oxygenic photosynthesis started in cyanobacteria (blue green algae). Almost 90% of total photosynthesis is carried out by hydrophytes (mostly marine algae).

1.0Requirements of Photosynthesis

(1) Sunlight

(2) Photosynthetic Pigment

(3) Carbon Dioxide

(4) Water

Sunlight

Sun is a natural source of light for photosynthesis. Sunlight is necessary for photosynthesis.

Photosynthetic Pigments

These are chlorophylls which are responsible for absorbing light energy. Photosynthetic pigments are present in all the green parts of the plant. The plants reflect mainly the green colour of the white light falling on them due to the presence of chlorophyll, so plants look green to us.

Chlorophyll is necessary for photosynthesis. Only chlorophyll containing areas produce glucose which is a product of photosynthesis.

Carbon Dioxide

All the plants need carbon dioxide to form carbohydrates. The carbon dioxide is obtained by the plants from the atmospheric air. In the terrestrial plants, the CO₂ enters into the cells of leaves through tiny pores called stomata which are present on the surface of leaves.

Carbon dioxide is necessary for photosynthesis. In aquatic plants, CO₂ is obtained from the water which is available in the dissolved form. Such plants absorb CO₂ through diffusion all over their surface from the surrounding water.

Desert plants take up CO₂ at night and prepare an intermediate which is acted upon by the energy absorbed by the chlorophyll during the day and form glucose. In desert plants, stomata are opened in the night time to check excessive loss of water.

Note: KOH has the property to absorb CO₂.

Water

Water is always needed by the plants for its use during photosynthesis. It is absorbed by the roots of the plant from the soil through the process of osmosis.

Inside the chloroplasts of the leaves, the water molecules split into hydrogen and oxygen with the help of light energy of sun.

Some nutrients like nitrogen, phosphorus, iron, magnesium required by the plants are also transported to different parts of the plant along with the water.

Nitrogen is an essential element used in the synthesis of proteins and other compounds (chlorophyll, DNA and RNA). Nitrogen is taken up with water in the form of inorganic nitrates or nitrites from the soil.

2.0Stomata

These are tiny pores or microscopic apertures guarded by two kidney or bean shaped guard cells.

Functions of Stomata

(i) Massive amount of gaseous exchange takes place in the leaves through stomata. Exchange of gases also occurs across the surface of stem and roots.

(ii) Transpiration takes place through stomata. A large amount of water is lost in the form of water vapour through stomata is known as transpiration.

Guard Cells

These are kidney shaped cells which cover a single stoma (opening). They contain chloroplast also.

Function of Guard Cells

They regulate the opening and closing of the stoma and also perform photosynthesis.

Opening and closing of stomata

When the guard cells swell (turgid) due to the entry of water, the stomata open. But when the guard cells shrink (flaccid) due to the loss of water, the stomata get closed.

Generally, stomata are opened during daytime and closed during night time.

There are two theories about the opening and closing of stomata. 

(i) Sugar concentration theory

During the daytime, the cell-sap concentration becomes high due to accumulation of sugar. This leads to endosmosis and water is withdrawn inside guard cells from neighbouring cells. This makes guard cells turgid so that their thin outer walls get stretched out widening the stomatal pore and stomata open. The pressure developed in guard cells is turgor pressure.

During the night, there is no photosynthesis, carbon dioxide gets accumulated in guard cells. This carbon dioxide then combines with water to form carbonic acid which has a pH of 5.0. It promotes the conversion of sugar into starch which is insoluble in water. As a result, exosmosis takes place and guard cells become flaccid or lose turgidity. Thus, the slit-like stomatal pore gets narrowed down and closes. 

(ii) K+ ion concentration theory

According to this theory, opening and closing of stomata depends on the generation of potassium ion (K+) gradient.