Green plants are called autotrophs because they can prepare their own food from water and carbon dioxide using sunlight and chlorophyll. This process by which green plants make food is called photosynthesis. Photosynthesis mainly occurs in the leaves, which are often called the “food factories” of the plant.
The photosynthesis process is similar to a cooking process that requires raw materials, a kitchen, vessels, fuel, and a recipe. Raw materials are carbon dioxide from the air, water absorbed from the soil by the roots, and chlorophyll, the green pigment in leaves, which traps light energy. The chloroplast, containing the green pigment chlorophyll, is the vessel for the cooking process, and the leaves that contain this organelle are called the kitchen of a plant. The fuel or source of energy is sunlight. The food formed is a simple sugar called glucose, which may later be changed into starch and stored in different parts of the plant for later use.
Role of air (carbon dioxide) in photosynthesis:
Air contains many gases, one of which is carbon dioxide, which is essential for photosynthesis. Inside the leaf, carbon dioxide is used along with water and sunlight to make glucose. Carbon dioxide from the air enters the leaf through stomata, which are tiny pores present mostly on the lower surface of leaves.
Important!
Without carbon dioxide, the plant cannot complete the process of photosynthesis and cannot make starch.
Testing the role of carbon dioxide:
This activity helps to show that carbon dioxide is necessary for plants to prepare starch.
Materials needed:
A healthy potted green plant, a wide-mouthed bottle, caustic soda (sodium hydroxide) solution, split cork, iodine solution (for starch test).
Steps:
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Keep the potted green plant in a dark place for \(2\)–\(3\) days so that the plant becomes destarched (it uses up stored starch).
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Select one leaf from this plant for the experiment.
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Pour some caustic soda solution into the wide-mouthed bottle. Caustic soda absorbs carbon dioxide from the air inside the bottle.
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Carefully insert half of the selected leaf into the bottle through a split cork, leaving the other half of the leaf outside the bottle.
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Place the whole setup in sunlight for a few hours.
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After this period, remove the leaf carefully.
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Boil the leaf in water, then in alcohol (With the help of a teacher), and finally test the whole leaf with iodine solution to check for starch.
Observations:
The part of the leaf that was outside the bottle turns blue-black with iodine, showing the presence of starch. The part of the leaf that was inside the bottle does not turn blue-black, showing little or no starch.
Conclusion:
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The outer part of the leaf had access to carbon dioxide from the air, so it could perform photosynthesis and make starch. The inner part of the leaf did not get carbon dioxide because caustic soda absorbed it, so photosynthesis did not occur there, and no starch was produced.
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This activity proves that carbon dioxide from the air is essential for the preparation of starch by plants during photosynthesis, and it also confirms that the leaf, especially its green parts, is the main site of photosynthesis.
What happens during photosynthesis?
Inside the leaf cells, chloroplasts containing chlorophyll use light energy to bring about a chemical reaction between water and carbon dioxide. Water provides hydrogen. Carbon dioxide provides carbon and oxygen. Light energy is converted into chemical energy and stored in the bonds of glucose.
Word equation for photosynthesis:
Glucose is the food made by the plant. It is a simple sugar. Some glucose is used immediately for energy. Extra glucose is changed into starch and stored in leaves, stems, roots, and other parts. Oxygen formed in the process is released into the air through stomata. This oxygen is very important because animals and humans use it for respiration.
Release of oxygen during photosynthesis:
An activity that helps to show that oxygen is released during photosynthesis.
Materials needed:
A water plant (for example, Hydrilla), a beaker or a trough with water, two test tubes (\(A\) and \(B\)), a funnel, a source of light (sunlight or lamp), and a matchstick.
Steps:
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Fill the beaker with water and place the water plant under a funnel in the beaker.
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Fill two test tubes with water and invert one test tube (\(A\)) over the stem of the funnel placed in sunlight. In a similar way, arrange another test tube (\(B\)) over a second funnel kept in the dark.
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Leave both setups for some time.
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Observe the test tube in sunlight (Setup \(A\)). You will see small bubbles rising from the plant and collecting at the top of the inverted test tube.
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When enough gas is collected in test tube \(A\), close its mouth with your thumb and remove it from the water.
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Bring a glowing matchstick near the mouth of the test tube and then gently remove your thumb to let the gas come in contact with the flame.
Observations:
In Setup \(A\) (in sunlight), bubbles are seen, and the collected gas makes the glowing matchstick burn more brightly. In Setup \(B\) (in the dark), very few or no bubbles are seen, and gas collection is poor or absent.
Conclusion:
The gas collected in Setup \(A\) supports burning strongly, so it is rich in oxygen. This shows that oxygen is released from the water plant during photosynthesis. Since Setup \(A\) was in sunlight, it also shows that photosynthesis occurs in the presence of light.
Gas exchange in leaves – Role of stomata:
Leaves act as the main site where gases move in and out of the plant. Stomata are tiny pores found mainly on the lower surface of leaves. Each stoma (singular of stomata) is surrounded by two kidney-shaped guard cells. When guard cells swell with water, the stomatal pore opens; when they lose water, the pore closes. This movement helps the exchange of gases for both photosynthesis and respiration.
Functions of stomata:
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Allow carbon dioxide to enter the leaf for photosynthesis.
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Allow oxygen produced during photosynthesis to leave the leaf.
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Help in the loss of water vapour from leaves (transpiration). Thus, stomata help in the exchange of gases between the plant and the atmosphere.
Observing stomata under a microscope:
This is a demonstration activity done in the lab to show the presence of stomata.
Materials needed:
A leaf from plants such as money plant, onion, hibiscus, coleus, rhoeo, or grass, a beaker with water, a watch glass, forceps, a microscope slide and coverslip, a dropper, water and a drop of ink, compound microscope.
Steps
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Collect a fresh leaf and place it in a beaker of water.
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Carefully peel a very thin layer of the leaf from its lower surface using forceps.
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Place this peel in a watch glass containing water so that it does not dry.
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Take a clean microscope slide and put a drop of water on it.
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Using forceps, transfer the leaf peel onto the drop of water on the slide.
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Add a drop of ink or stain to make the cells clearer.
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Gently place a coverslip over the peel, avoiding air bubbles.
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Observe the slide under a microscope, starting with low power.
Observations:
Many small box-like cells are seen. Among them, small openings like tiny mouths are visible. Each opening has two special cells around it (guard cells). These openings are stomata. This activity shows that stomata are present on the leaf surface and are responsible for gas exchange.
Meet the scientist – Rustom Hormusji Dastur
Rustom Hormusji Dastur (1896–1961) was an Indian plant scientist. He worked as the head of the Botany Department at the Royal Institute of Science, Bombay (now Institute of Science, Mumbai). Dastur carried out detailed studies on photosynthesis. He investigated how factors like water, temperature, and the colour of light affect the rate of photosynthesis. His work helped in understanding how environmental conditions influence the food-producing ability of plants.
Factors essential for photosynthesis:
Photosynthesis depends on factors such as light intensity, carbon dioxide concentration, water availability, and temperature. From all the experiments, we can conclude that the following as the factors necessary for photosynthesis:
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Requirement
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Role in photosynthesis
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Sunlight
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Provides the energy needed to drive the photosynthesis reaction.
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Water
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A reactant used to form glucose; also a source of hydrogen, absorbed by roots from the soil and carried to leaves through xylem
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Chlorophyll
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A green pigment that traps light energy in chloroplasts.
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Carbon dioxide
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Raw material from the air through stomata and used as a raw material and used to build glucose molecules.
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Stomata
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Allow entry of carbon dioxide and exit of oxygen.
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Plants are autotrophs because they prepare their own food by photosynthesis using sunlight, water, carbon dioxide, and chlorophyll. The leaf is the primary site of photosynthesis, but all green parts containing chlorophyll can perform this process. In photosynthesis, carbon dioxide and water are converted into glucose; excess glucose is stored as starch. Oxygen is released as a byproduct, which supports the life of humans and other animals.