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Staying Alive – How Living Systems Live

Grade 8 Science Worksheets

The Basic Unit of Life – The Cell

All living things are made up of cells. All cells are “alive” – they live, breathe, reproduce, excrete and die. They take in food. They produce new cells through division. They get rid of waste. Often, they may get damaged and need to repair themselves. They also maintain a set of stable conditions for the body inside which they reside – conditions such as temperature, pressure, water content and so on. Where do they get the food from for all this?

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There are three main classes of food –

  1. Carbohydrates
  2. Proteins
  3. Fats

Plant cells use the energy of sunlight to produce rich sugars known as Carbohydrates through the process of Photosynthesis. Carbohydrates are a major source of energy for animals and plants. Animals get carbohydrates from eating plants and other animals. The digestive system of animals breaks down these carbohydrates into glucose, which is the primary fuel for all living cells. Grains and cereals, fruits and vegetables, seeds and nuts, are all important sources of carbohydrates. The basic elements in all carbohydrates are Carbon, Hydrogen and Oxygen.

Animals get Protein from the foods they eat. Plants build protein from nutrients in soil and air. Human body cells are largely made up of protein. Cells manufacture protein from special chemicals known as amino acids. Each protein has a specific function. Some are used to create new cells. Some, known as enzymes, help in the process of digestion. Some, known as antibodies, help ward off infection and disease. Some, known as hormones, maintain the conditions of the body. And so on.

Another class of food is Fat. Fat is essential in our diet as a high source of energy. It is an important structural constituent of cells. Fat helps in absorbing many vitamins essential for the body. Fat cells are found in animals and plants. Plants store fat in seeds. Most animals store fat under the surface of the skin in the form of adipose tissue, which is a source of insulation and warmth.

Homeostasis

Living organisms need to maintain a stable set of conditions in their bodies. This is referred to as Homeostasis. The living organism reacts to various external environmental factors and makes internal adjustments, known as reflexes. For example, on a hot day, the reflex is to allow the body to sweat more. Or while climbing a hill the reflex is to increase blood circulation to the muscles. All these and many more internal processes such as the nervous system, respiratory system, excretory system, and so on are maintained in this manner.

Check Point

  1. The following are functions of cells –
    1. Taking in food
    2. Creating new cells
    3. Maintaining Homeostasis
    4. All of the Above
  2. Enzymes, hormones, and antibodies all are forms of ______.
  3. The main function of adipose tissue is to store –
    1. Carbohydrates
    2. Fats
    3. Proteins
  4. Carbohydrates are broken down into ______ by the digestive system.
  5. Maintaining stable conditions in a body is a process known as ______.

Answer Key

  1. d) All of the above
  2. Protein
  3. Fats
  4. Glucose
  5. Homeostasis

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