Photosynthesis

The process by which green plants use energy from the sun to convert carbon dioxide and water into glucose in the presence of green-coloured pigment (chlorophyll) is called photosynthesis. Plants obtain carbon dioxide from the atmosphere through stomata and water is absorbed by the roots from the soil. The process of photosynthesis involves following steps:

1. Water taken up by the root system and carbon dioxide from the atmosphere reaches the leaves.
2. Light is absorbed by chlorophyll, which splits water (called photolysis) and oxygen is released. This reaction depends on sunlight for activation energy. It takes place in the thylakoids of the chloroplasts.
3. Carbon dioxide splits and this carbon is used to make sugars. This reaction does not require light energy. It occurs in stroma.
4. Sugars are formed and they combine to form complex sugars. Glucose and fructose are formed as a result of photosynthesis.

Photosynthesis can be represented by the following word equation:

Carbon dioxide + water $\xrightarrow[\text{chlorophyll}]{\text{sunlight}}$ Glucose + Oxygen

Small openings on the leaves present mostly on the lower surface of leaves and on green stems are called stomata. Each stoma has two bean-shaped cells called guard cells. The inner walls of the guard cells are thicker than the outer walls. When the guard cells are full of water, they swell and bulge outwards and the stomata opens. When the guard cells lose water, they lose their turgidity and the stomata closes. Normally stomata remains open during the day and closes at night. Stomata have following functions:

1. Exchange of gases (carbon dioxide and Oxygen)
2. Transpiration (loss of water from the plant surface)

Following are the three factors that affect photosynthesis:

1. Light — Light energy trapped by the chlorophyll is used to convert raw materials into sugar. The rate of photosynthesis also depends on the intensity of the light, higher the intensity of light higher is the rate of photosynthesis.
2. Chlorophyll — It is found in structures called chloroplasts. It enables the plant to trap light energy.
3. Temperature — The series of chemical reactions occurring during photosynthesis can take place only at a suitable temperature, the optimum being between 20°C to 40°C.

To test the presence of starch in green leaves, following steps are followed:

1. Pluck a leaf from a plant which has been exposed to sunlight for a few hours.
2. Put it in a test tube containing alcohol. Place the test tube in a beaker containing water and heat the water. As the alcohol begins to boil, the chlorophyll dissolves in it and the leaf becomes pale.
3. Remove the leaf from alcohol and wash it in warm water.
4. Lay the leaf in a dish and put a few drops of iodine on it.

We will observe that the colour of the leaf changes to blue-black. This shows that starch is present in the leaf.

To show that chlorophyll is necessary for photosynthesis, we will conduct following experiment:

1. Select a plant with variegated leaves. Chlorophyll is present in the green portion.
2. Put the plant in sunlight for a few hours.
3. Pluck any leaf and draw its rough outline indicating the green and non-green portions.
4. put the leaf in dilute iodine solution.

We observe that the portion of leaf with chlorophyll turns blue-black showing the presence starch. This shows that photosynthesis took places only in the green part of leaf. Thus, chlorophyll is necessary for photosynthesis.

The process of photosynthesis is of extreme importance as it is the source of food as well as oxygen for all animals directly or indirectly. Food is produced by the process of photosynthesis. Leaves capture light energy from the sun and use it to make food. Animals that feed on the plants get this energy. They pass it on further when they are eaten by other animals. The energy obtained from food is used by all organisms for various metabolic activities. All living organisms require oxygen. This oxygen is released by the plants during photosynthesis. In the absence of photosynthesis, the oxygen level in the atmosphere would decrease and life on earth would become impossible.

The living organism obtain energy from the food they eat. Animals get their food from plants and other animals. Plants on the other hand, make their own food by the process of photosynthesis.

Green plants make their own food by the process of photosynthesis. Plants convert carbon dioxide and water into glucose in presence of light and chlorophyll.

The glucose is converted into starch and stored in various storage organs within the plant.

When the guard cells are full of water, they swell and bulge outwards and the stomata opens.

Light is very important for photosynthesis. In the absence of light, plants cannot make their food. Light energy is required to convert raw materials of photosynthesis into glucose.

False

Corrected statement — Green plants make their own food by the process of photosynthesis.

True

False

Corrected statement — Oxygen is released as an end product during photosynthesis.

True

True

All organisms need energy to live and grow.

Animals get their food from plants and other animals.

The glucose produced during photosynthesis is transported from the leaves to the other parts of the plant.

The inner walls of the guard cells are thicker than the outer walls.

Chlorophyll absorbs light.

The rate of photosynthesis increases with an increase in the concentration of carbon dioxide.

1. The given picture shows Stomata.
2. They are found on the lower surface of leaves and on green stems.
3. Their functions are exchange of gases (carbon dioxide and oxygen) and transpiration.

1. It is used to show that light is necessary for photosynthesis.
2. Iodine solution is added to the leaf.
3. The part of the leaf covered with the black paper does not turn blue-black after adding iodine solution. This indicate that there is no starch formed in that area since no photosynthesis took place because that area of leaf was kept devoid of light. Hence, this activity proves that light is essential for photosynthesis.

No, we are not autotrophs. Autotrophs are the organisms that are capable to convert solar energy into food. We can cook food produced by plants but cannot produce it like autotrophs.

No, animals cannot make food from carbon dioxide, water and sunlight like plants do because they do not have chlorophyll and the mechanism to trap light energy and convert it into food. Hence, animals are not capable of performing photosynthesis to make their own food.

photosynthesis

Reason — The process by which plants make their own food is called photosynthesis

sun

Reason — Plants convert the energy of sun into chemical energy of food (glucose)

chlorophyll

Reason — The green pigment present in leaves is called chlorophyll which is present in chloroplasts

stomata

Reason — Stomata are small openings on the leaves present mostly on the lower surface of leaves and on green stems.

guard cells

Reason — Each stomata has two bean-shaped cells called guard cells. They regulate the opening and closing of stomata.

close

Reason — When the guard cells lose their turgidity, their inner walls come closer and the stomata closes.

starch

Reason — Iodine changes the colour of leaf to blue-black showing the presence of starch and thus shows the photosynthesis in that leaf.

30°C

Reason — The optimum temperature range for photosynthesis is 20°C to 40°C.