6. Life Processes | Class 10 CBSE | Web Notes | Part 2 | Nutrition



It is the process of transfer of energy source (food) from outside the body to the inside.

It is needed for body growth, development and synthesis of protein and other substances.

Even we are inactive, energy is needed to maintain a state of order in the body.

General requirement for energy & materials is common in all organisms, but it is fulfilled in different ways.

Some organisms use simple food material obtained from inorganic sources such as CO2 and water. They are called autotrophs. E.g. green plants and some bacteria.

Some organisms directly or indirectly depend on autotrophs for nutrition. They are called heterotrophs. E.g. animals and fungi. They utilise complex substances which are broken down into simpler ones with the help of biocatalysts called enzymes.

Autotrophic Nutrition

It is a process where an organism prepares its own food from simple inorganic materials.

Photosynthesis: It is the process by which CO2 & water is converted into carbohydrates in presence of sunlight & chlorophyll. It occurs in autotrophs like green plants.

Carbohydrates provide energy to the plant.

The carbohydrates which are not used immediately are stored as starch. It serves as the internal energy reserve.

In our body, some of the energy derived from food is stored as glycogen.

A cross-section of a leaf under the microscope shows that some cells contain green dots. They are cell organelles called chloroplasts. It contains chlorophyll.

Events in photosynthesis:

  1. Absorption of light energy by chlorophyll.
  2. Conversion of light energy to chemical energy and splitting of water molecules into hydrogen & oxygen.
  3. Reduction of CO2 to carbohydrates.

These steps need not occur in sequence immediately. E.g. desert plants take up CO2 at night and prepare an intermediate. It is acted upon by the energy absorbed by chlorophyll during the day.

Experiment to show that chlorophyll is essential for photosynthesis:

  • Take a potted plant with variegated leaves (e.g. money plant or crotons).
  • Keep it in a dark room for 3 days to use up all the starch (destarch).
  • Keep the plant in sunlight for about 6 hours.
  • Pluck a leaf and mark the green areas in it and trace them on a paper.
  • Dip the leaf in boiling water for a few minutes.
  • Then immerse it in a beaker containing alcohol. Place the beaker in a water-bath and heat to boil the alcohol.
  • The leaf becomes colourless. Chlorophyll is dissolved in alcohol and the alcohol turns green.
  • Dip the leaf in a dilute iodine solution for a few minutes.
  • The green areas of leaf turn dark blue. It indicates the presence of starch. Colourless part of leaf shows no formation of starch.
Gaseous exchange occurs in the leaves through stomata for photosynthesis. It also occurs across the surface of stems, roots and leaves.

During this, much water is also lost through stomata. So the plant closes stomata when it does not need CO2

The opening and closing of stomatal pore is regulated by guard cells. When water flows into guard cells, they swell and the pore opens. If the guard cells shrink, the pore closes.

Experiment to show that CO2 is essential for photosynthesis:

  • Take 2 healthy potted plants having nearly same size. 
  • Keep them in a dark room for 3 days.
  • Now place each plant on separate glass plates. In one, place a watch-glass containing potassium hydroxide (KOH). KOH is used to absorb CO2.
  • Cover both plants with separate bell-jars.
  • Using Vaseline, seal the bottom of the jars to the glass plates to make it air-tight.
  • Keep the plants in sunlight for about two hours.
  • Test the leaves from both plants using iodine.
  • Leaf of plant kept without KOH turn blue. It indicates the presence of starch. Plant with KOH does not turn blue.

Experiment to show that sunlight is essential for photosynthesis:

  • Keep a plant in dark room for 3 days to destarch leaves.
  • Cover a leaf partially with a black paper.
  • Expose the plant to sunlight for 3-4 hours.
  • Remove chlorophyll from the leaf and perform a starch test with iodine.
  • Covered leaf part shows brown colour. Exposed leaf turns dark blue due to the presence of starch.
Autotrophs also need other raw materials such as water and minerals like N, P, Fe and Mg. They are taken up from the soil. Nitrogen is an essential element for protein synthesis. It is absorbed as inorganic nitrates or nitrites. Or it is taken up as organic compounds prepared by bacteria from atmospheric nitrogen.

Heterotrophic Nutrition

It is the nutrition in which an organism depends on other living organisms for food.

Organisms take and use food in various ways. E.g.
  • Some organisms break down food outside the body and absorb it. E.g. fungi like bread moulds, yeast and mushrooms.
  • Some take in food and break down inside the body.
  • Some organisms take food from organisms without killing them (parasitism). E.g., Cuscuta (amar-bel), ticks, lice, leeches and tape-worms.

How do organisms obtain their Nutrition?

Unicellular organisms take food by their entire surface.

As the complexity increases, specialised parts are formed to perform different functions.

E.g., Amoeba captures food using temporary finger-like extensions (Pseudopodia) of the cell surface. They fuse over the food particle forming a food-vacuole. In this, food breaks down into simpler ones and diffuse into cytoplasm. The undigested material is moved to cell surface and thrown out.

Paramoecium takes food in at a specific spot by the movement of cilia (hair-like structure covering the cell).

Nutrition in Human Beings

Human alimentary canal is a long tube extending from the mouth to the anus.

Food digestion:

It is the breakdown of complex food with the help of enzymes into smaller absorbable molecules.

In mouth, food is chewed with the help of teeth and muscular tongue and it is mixed with saliva secreted by the salivary glands.

Teeth are used to crush the food into small particles.

Saliva contains salivary amylase enzyme that breaks down starch to simple sugar.

Experiment to show the action of saliva:
  • Take 1 mL starch solution (1%) in test tubes A & B.
  • Add 1 mL saliva to test tube A and leave both test tubes undisturbed for 20-30 minutes. Now add a few drops of dilute iodine solution to the test tubes.
  • In test tube B, blue colour develops. It indicates presence of starch. In test-tube A, no colour change occurs because starch is digested by salivary amylase.
The lining of the alimentary canal is soft for smooth passage of food. The muscles in the lining, contract rhythmically to push the food forward. It is called peristalsis. It helps to process the food properly in each part.

From the mouth, food is moved through oesophagus (food-pipe) and reaches stomach.

Stomach is a large organ that expands when food enters it. Its muscular walls help in mixing the food thoroughly with digestive juices.

Stomach wall contains gastric glands. They release hydrochloric acid, pepsin (a protein digesting enzyme) and mucus.

HCl creates an acidic medium which facilitates the action of pepsin. It also helps to destroy harmful bacteria.

The mucus protects the inner lining of the stomach from the action of the acid.

Sometimes HCl acts on the wall causing a burning sensation. This is called acidity.

A sphincter muscle regulates the exit of food in small amounts from stomach into small intestine.

Small intestine is the longest and highly coiled part of the alimentary canal.

In animals, the length of the small intestine differs based on the food they eat. E.g.
  • Herbivores have a longer small intestine for cellulose digestion.
  • Meat is easier to digest. So carnivores have a shorter small intestine.
Small intestine is the site of the complete digestion of carbohydrates, proteins & fats. It receives the secretions of the liver & pancreas.

Liver secretes bile juice. It
  • Emulsifies fat. It is the breakdown of large fat globules into the smaller globules with the help of bile salts. It increases the efficiency of enzyme action. This is similar to emulsifying action of soaps on dirt.
  • Makes the acidic food (from stomach) alkaline.
Pancreas secretes pancreatic juice. It contains enzymes like trypsin (to digest proteins) and lipase (to digest emulsified fats).

Glands in the walls of small intestine secrete intestinal juice which contains enzymes. They finally convert
  • Proteins → amino acids.
  • Complex carbohydrates → glucose.
  • Fats → fatty acids & glycerol.
Digested food is taken up by the walls of the intestine.

Inner lining of the small intestine has many finger-like projections called villi. They increase the surface area for absorption. Villi contain blood vessels that take the absorbed food to all cells. In cells, it is utilised to obtain energy, build up new tissues and repair old tissues.

The unabsorbed food is moved to the large intestine where its wall absorbs more water from this material.

Remaining waste material is removed from the body via anus. This removal is regulated by anal sphincter.

Dental caries (Tooth decay):

It is the gradual softening & demineralisation of enamel & dentine due to the production of acids by bacteria. They convert sugary foods to acids.

Bacteria invade the pulp causing inflammation & infection.

Masses of bacterial cells together with food particles stick to the teeth to form dental plaque. It prevents saliva reaching the tooth surface to neutralise the acid.

Brushing the teeth after eating can remove plaque.

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