LECTURE 2: CLASS 10 : LIFE PROCESSES : PHOTOSYNTHESIS - I

CLASS X   |    SCIENCE    |    LIFE PROCESSES

      Notes prepared by Subhankar Karmakar

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SCIENCE: CLASS X: CBSE (CLASS NOTES)

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• NUTRITION IN PLANTS
• The process by which green plants make their own food like glucose from carbon di oxide and water by using sunlight energy in the presence of chlorophyll is called photosynthesis.
• Chlorophyll is present in the green coloured bodies called chloroplasts inside the plant cells. 
• The leaves of a plant are green because they contain tiny green coloured organelles called Chloroplasts. 
• The process of photosynthesis
• The chemical reaction of the photosynthesis

6CO₂ + 6H₂O + Light energy (in the presence of chlorophyll) ⟹ C₆H₁₂O₆ + 6O₂

• PHOTOSYNTHESIS

Plants prepare their food (glucose) in the green leaves of the plant by combining carbon-di-oxide and water in the presence of sunlight and chlorophyll. The process is known as Photosynthesis.

• CHLOROPHYLL:

Chlorophyll is a green pigment found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. It is needed for the process of photosynthesis.

• CARBON-DI-OXIDE

The plants derive carbon-di-oxide from the air by the plant leaves. Carbon-di-oxide enters the leaves through the small pores in them called stomata. 

• WATER

Water required for food preparation is taken from the soil through the roots. Water is transported to the leaves from the soil through the roots and stem. 

• SUNLIGHT

The sunlight provides the energy required to carry out the chemical reactions involved in the preparation of the food. The energy in the sunlight is absorbed with the help of chlorophyll. 

• Oxygen gas is produced as a by-product during the preparation of food by photosynthesis. This oxygen gas goes into the air. 
• The food prepared by the green leaves of a plant is in the form of a simple sugar called glucose. Glucose thus produced is sent to the different parts of the plant.
• The extra glucose is changed into another food called starch. This starch is stored in the leaves of the plant. Glucose and starch are called carbohydrates. 
• Green plants convert sunlight energy into chemical energy by making carbohydrates. 

• THE PHOTOSYNTHESIS PROCESS
• 1. Absorption of sunlight energy
• 2. Conversion of light energy into chemical energy and splitting of water into hydrogen and oxygen by light energy.
• 3. Reduction of carbon dioxide by hydrogen to form carbohydrate like glucose by utilising the chemical energy obtained by the transformation of light energy. 

• Conditions necessary for photosynthesis by green plants as well as autotrophic nutrition
• 1. Sunlight
• 2. Chlorophyll
• 3. Carbon dioxide
• 4. Water

A. Describe an experiment to show that Sunlight is necessary for photosynthesis

We know that green leaves make starch as food. As starch gives a black-blue colour with iodine solution. Plants store starch in their leaves. The green leaves of a plant are destarched by keeping this plant in a completely dark place in a room for atleast three days. 

• 1. We take a potted plant having Green leaves and place it in a completely dark place for about 3 days to destarch its leaves.
• 2. We take a thin strip of aluminium foil and wrap it in the centre of one leaf on both the sides. The covered part will not receive sunlight.
• 3. We keep this potted plant in bright sunshine for 3 to 4 days. 
• 4. We pluck the partially covered leaf from the plant and remove its Aluminium foil then immerse this leaf in boiling water for a few minutes. This will break down the cell membranes of leaf cells and make the leaf more permeable to iodine solution.
• 5. We put the plucked leaf in a beaker containing some alcohol. Place the beaker containing alcohol and leaf in a water bath and starts to heat it till the leaf becomes colourless. The boiling water will remove the chlorophyll from the green leaf. 
• 6. We remove the colourless leaf from alcohol, wash it in hot water and keep it in a petridish. We drop iodine solution on the leaf. 
• 7. The part which was wrapped in aluminium foil will not turn blue-black but rest of the parts of the leaf will become blue-black. 
• 8. Since the wrapped part didn't get sunlight, it did not produce starch. We can now conclude that 
• (i) sunlight is necessary for the process of photosynthesis.
• (ii) leaves make starch as food by photosynthesis.

• Variegated leaves:

The leaves which are partly green and partly white are called variegated leaves. The white part of the leaves doesn't contain chlorophyll. 

B. Describe an experiment to show that Chlorophyll is necessary for photosynthesis.

• 1. We take a potted plant having variegated leaves like a croton plant. The white part of the leaves doesn't contain chlorophyll whereas the green part contains chlorophyll. 
• 2. We place the plant in a completely dark place for about three days. 
• 3. We take out the plant and keep it under bright sun for 3 to 4 days 
• 4. We pluck the variegated leaf from the plant , boil it in water for few minutes, and then remove its green colour  chlorophyll by boiling in alcohol. The green part of the leaf  get decolourised.
• 5. We wash the decolourised leaf with hot water to soften and remove left-over Chlorophyll. 
• 6. We now pour iodine solution over the colourless leaf and observe the change in colour of the leaf. 
• 7. The inner part of leaf which was originally green turns blue black on adding iodine. 
• Chlorophyll is necessary for the process of photosynthesis to take place. 

C. Describe an experiment to show that Carbon Dioxide is necessary for photosynthesis.

• 1. We take a potted plant having narrow leaves and place it in a completely dark place for about three days to destarch its leaves. 
• 2. Take a glass bottle having a wide mouth and put some potassium hydroxide solution in it. 
• 3. Take a rubber cork which fits tightly into the mouth of the glass bottle and cut it into two halves. 
• 4. We put the destarched leaves in such a way, that upper half of the leaf should remain outside the glass bottle and only the lower half of the leaf should be inside the glass bottle.
• 5. The potted plant is kept in sunlight for 3 to 4 days. Upper half gets the carbon dioxide from air but lower half didn't get carbon dioxide as potassium hydroxide absorbed the carbon dioxide of the inside jar. 
• 6. We pluck the leaf and boiled it in alcohol then wash it with water.
• 7. We pour iodine solution over the colourless leaf. We observe that lower part of the leaf doesn't turned blue-black but the upper part became blue-black. 
• The photosynthesis to make starch in the leaf does not take place without carbon dioxide. 

Raw Materials for photosynthesis

The preparation of carbohydrates by plants by the process of photosynthesis requires two materials a. Carbon dioxide and b. Water

HOW THE PLANTS OBTAIN CARBON DIOXIDE

• STOMATA

There are large number of tiny pores called stomata on the surface of the leaves of plants. and green stem. The green plants take carbon dioxide from air for photosynthesis. The carbon dioxide gas enters the leaves of the plant through the stomata present on their surface. 

• STOMATAL PORES AND GUARD CELLS

STOMATAL PORES & GUARD CELL

• Each stomatal pore is surrounded by a pair of guard cells. The opening and closing of stomatal pore is controlled by the guard cells. 
• • When water flows into the guard cells, they swell, become curved and cause the pore to open. 
• • When guard cells lose water, they shrink, become straight and close the stomatal pore. 
• Plant losses water through the open stomatal pores hence when carbon dioxide is not needed by the plants, these pores are closed. 
• Oxygen gases produced during photosynthesis also goes out through the stomatal pores. 
• In most of the broad-leaved plants, stomata occur only on the lower surface of the leaf but in narrow-leaved plants, stomata are equally distributed on the both sides of the leaf. 
• Aquatic plants use the carbon dioxide gas dissolved in water for carrying out photosynthesis. 

HOW THE PLANTS OBTAIN WATER FOR PHOTOSYNTHESIS

• The water required by the plants for photosynthesis is absorbed by the roots of the plants from the soil through the process of osmosis. The water is absorbed by the roots of the plants is transported upward through the xylem vessels to the leaves. 
• Other nutrients like nitrogen, phosphorus, iron and magnesium etc. by the plants for its growth are taken by the plants from the soil through the roots of the plants. 

SITE OF PHOTOSYNTHESIS: CHLOROPLASTS

• Chloroplasts are the disc-like cell organelles of the photosynthetic cells of green plants which contain chlorophyll. At Chloroplasts photosynthesis take place. 
• The middle layer of the leaves are palisade layer and spongy layer and they contain photosynthetic cells which are called mesophyll cells. 
• Carbon dioxide enters through stomata and diffuses into the mesophyll cells and reaches the Chloroplasts. 
• Water is carried to the leaf by xylem and passes into the mesophyll cells by diffusion and reaches the Chloroplasts. 
• To reduce the water loss, there is a thin waxy protective layer called cuticle above and below a leaf.