CBSE Class 10 Science Life Processes Assignment Set 05

NUTRITION IN PLANTS

All living organisms need energy to grow, synthesise other molecules and perform basic processes. This energy is obtained from food.

The process by which living organisms procure food as a source of energy is called nutrition.

There are two modes of nutrition- AUTOTROPHIC AND HETEROTROPHIC.

Autotrophic nutrition

The mode of nutrition in which the organisms take in raw materials like carbon dioxide and water from outside environment and convert them into carbohydrates in the presence of sunlight and chlorophyll is known as autotrophic nutrition. The organisms that are able to carry out this process are autotrophs eg. Green plants and cyanobacteria.

The carbohydrates are utilized by plants to obtain energy. The carbohydrates that are not utilized immediately are stored in the form of starch in different parts of the plant body like underground stems, roots seeds etc as reserve food. It can be used by the plant as and when required. Similarly, animals store some carbohydrates in the liver in the form of glycogen.

Photosynthesis

It is the process during which green plants manufacture organic food in the form of carbohydrate from carbon dioxide and water in the presence of chlorophyll and sunlight. Oxygen gas is given out.

\[ \underset{\text{Carbon dioxide}}{6\text{CO}_2} + \underset{\text{Water}}{12\text{H}_2\text{O}} \xrightarrow[\text{Chlorophyll}]{\text{Light}} \underset{\text{Glucose}}{\text{C}_6\text{H}_{12}\text{O}_6} + \underset{\text{Water}}{6\text{H}_2\text{O}} + \underset{\text{Oxygen}}{6\text{O}_2} \]

Raw materials required for photosynthesis

The two raw materials required for photosynthesis are carbon dioxide and water.

Carbon dioxide – Plants obtain carbon dioxide from the atmosphere through stomata (leaves), lenticels (stem) and root surface as well. Stomata are tiny pores present on the surface of the leaves for gaseous exchange. Stomatal pore is bound by two bean shaped guard cells. Guard cells contain chloroplasts and they regulate the opening and closing of stomatal pore. At night when there is no photosynthesis, the pores close as guard cells lose water and shrink ie become flaccid.. This prevents loss of water from the pores. During the day , guard cells swell when water flows into them, become turgid causing the pore to open.

Water – It is absorbed from the soil with the help of roots. Tiny root hair on the roots increase the surface area for absorption. Along with water, minerals rich in nitrogen, phosphorus, iron and magnesium are also absorbed from the soil for body building eg. Nitrogen is essential for making proteins. It is absorbed in the form of nitrates and nitrites.

Conditions essential for photosynthesis

Chlorophyll – it is the green pigment present in chloroplasts. It traps light energy from the sun.

Sunlight – Sun is the main source of light. The visible component of light is absorbed by chlorophyll. Rate of photosynthesis is maximum in red and blue component of white light and no photosynthesis occurs in green light . Only 1% of sunlight is required for photosynthesis.

Site of photosynthesis

The actual sites of photosynthesis are the chloroplasts which are present mainly in the cells of the leaf. The cross section of leaf under the microscope shows that it is bound by upper and lower epidermis. Lower epidermis has stomata for exchange of gases. In between these layers the tissue present is mesophyll tissue which has two parts, the upper part made up of generally compact cells is palisade and the lower part made up of cells having air spaces is spongy parenchyma. Cells of mesophyll tissue have chloroplasts that contain chlorophyll.

Mechanism

The process of photosynthesis involves the following events-

• i) Absorption of light energy or radiant energy by the photosynthetic pigment chlorophyll and its activation.
• ii) Conversion of light energy into chemical energy and splitting of water molecules into hydrogen and oxygen in the presence of light. This is called photolysis of water.
• iii) Hydrogen formed in the above step helps in reduction of carbon dioxide to carbohydrates ( glucose) and oxygen is released as a waste through stomata.

ACTIVITIES

To show that chlorophyll is essential for photosynthesis.

Method

• 1. Take a potted plant with variegated leaves i.e. leaves having green and non- green areas eg. moneyplant, croton, coleus.
• 2. Select a leaf and mark on it the green areas and trace them on a sheet of paper.
• 3. Destarch the potted plant by keeping it in a dark room for 2-3 days. Keep the plant back in sunlight.
• 4. Now perform the starch test, the steps are –
  • a) Dip the leaf in boiling water for a few minutes to make the tissue soft and loose.
  • b) Boil the leaf in alcohol in a water bath to remove chlorophyll.
  • c) Take out the leaf from alcohol and wash it.
  • d) Dip the leaf in dilute solution of iodine in a petridish.
• 5. Observe the colour of the leaf and compare it with the tracing done on a sheet of paper.

Observation

The part of the leaf marked green turns blue black with iodine with starch test.

To show that CO2 is required during photosynthesis.

• 1. Take two healthy potted plants of the same type and nearly the same size.
• 2. Destarch them by keeping them in the dark room for 2-3 days.
• 3. Place them on separate plates in the sun and place a dish containing KOH near one of the pots. (KOH absorbs CO2)
• 4. Cover the pots with separate bell jars. Seal their bottoms by using Vaseline so as to make the setup air tight.
• 5. Keep the plants in sunlight for 4-5 hours.
• 6. Pluck one leaf from each plant and test for the presence of starch (as given in the above activity).
• 7. The leaf taken from the plant kept with KOH does not turn blue black as KOH absorbed CO2 and photosynthesis did not take place.
• 8. The other leaf turns blue black with iodine.

To show that sunlight is essential for photosynthesis.

Method

• a) Take a healthy potted plant.
• b) Keep it in dark for 3 days to destarch it.
• c) Select a leaf and attach strips of black paper on a part of upper and lower surface or ganong’s potometer.
• d) Put it back in sunlight for 6-8 hours.
• e) Pluck the leaf and perform the starch test as mentioned earlier.
• f) Record your observations.

Observation

The part of the leaf covered with black paper does not show blue black colour with iodine solution while the uncovered part received sunlight, so could photosynthesize and therefore turned blue black with iodine solution.

Heterotrophic nutrition

The mode of nutrition in which organisms obtain their nutrients from other living organisms. Heterotrophs directly or indirectly depend on autotrophs. eg. many bacteria, fungi, non-green plants and all animals. This form of nutrition depends on availability of food material as well as the method of obtaining it. The organisms show special adaptations to procure food in different ways. This mode of nutrition is of the following types –

a) Saprophytic Nutrition

In this mode, the organisms live on dead and decaying organic matter and break down the complex food material into simple absorbable form outside their body and then absorb them therefore it is absorptive mode of taking in nutrients . To do this, they secrete certain enzymes outside their body eg many bacteria, fungi like bread mould, yeast and mushrooms. These organisms are called saprophytes. This type of digestion which occurs outside the body is known as extra- cellular digestion.

b) Parasitic Nutrition

In this mode the organisms derive their nutrition from plants and animals without killing them but they do cause harm to them. The organism that obtains food is called ‘parasite’ and the one from which food is absorbed is called the ‘host’.

Animal parasites are of two types.

• Ectoparasites – feed on the surface of the host eg ticks, lice, leeches.
• Endoparasites – take shelter and nutrition from within the body of the host.

Parasitic plants – Cuscuta, mistletoe, orchids.

c) Holozoic Nutrition

In this mode of nutrition complex organic matter in the form of solid food is ingested, digested, absorbed and utilized in the body. eg all animals like Amoeba , frog, human beings etc