Organisation of Life — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Biology — Chapter 18. From a single cell to a whole organism.

1. About this chapter

This chapter covers the levels of organisation, plant tissues (meristematic and permanent) and animal tissues (epithelial, muscular, connective, nervous).

2. Levels of organisation

• A living body is built up in stages: cell → tissue → organ → organ system → organism.
• A tissue is a group of similar cells that perform a common function.

3. Plant tissues

Plant tissues are of two kinds:

• Meristematic tissue — groups of immature, continuously dividing cells with a dense cytoplasm, a prominent nucleus and no vacuole; found at growing tips (apical), in the girth (lateral) and at nodes (intercalary). They are responsible for growth.
• Permanent tissue — cells that have stopped dividing:
  • Parenchyma — thin-walled living cells; store food and carry out photosynthesis.
  • Collenchyma — give flexibility and support.
  • Sclerenchyma — thick, dead cells that give mechanical strength.

4. Animal tissues

There are four types:

• Epithelial tissue — covers and protects body surfaces and lines organs (e.g. squamous epithelium lining the cheek and lungs).
• Muscular tissue — contracts to bring about movement (skeletal, smooth, cardiac).
• Connective tissue — binds and supports (blood, bone, cartilage, ligaments).
• Nervous tissue — conducts impulses; its unit is the neuron (nerve cell).

5. Worked examples

Example 1. Write the correct order of organisation. Cell → tissue → organ → organ system → organism.

Example 2. Which plant tissue is responsible for growth? Meristematic tissue.

Example 3. What is the functional unit of nervous tissue? The neuron (nerve cell).

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

1. The correct order of organisation is — (a) cell → tissue → organ → organ system → organism / (b) tissue → cell → organ. Ans: (a).
2. The dividing tissue in plants is — (a) meristematic / (b) parenchyma. Ans: (a) meristematic.
3. The tissue that conducts impulses is — (a) muscular / (b) nervous. Ans: (b) nervous.
4. Blood and bone are examples of ____ tissue — (a) epithelial / (b) connective. Ans: (b) connective.
5. Meristematic cells lack a — (a) nucleus / (b) vacuole. Ans: (b) vacuole.

II. Fill in the blanks 6. A group of similar cells performing a common function is called a tissue. 7. Parenchyma stores food in plants. 8. The unit of nervous tissue is the neuron.

III. Answer briefly 9. Name the four types of animal tissue. 10. Differentiate meristematic and permanent tissue.

7. Common mistakes

• Mistake: Reversing the order of organisation. Fix: It runs cell → tissue → organ → organ system → organism.
• Mistake: Saying meristematic cells have large vacuoles. Fix: Meristematic cells have dense cytoplasm and no vacuole.
• Mistake: Calling blood an epithelial tissue. Fix: Blood is a connective tissue.

8. Quick revision

• Biology Ch 18 · cells, tissues, organisation.
• Order: cell → tissue → organ → organ system → organism.
• Plant tissues: meristematic (dividing, growth) and permanent (parenchyma, collenchyma, sclerenchyma).
• Animal tissues: epithelial (cover), muscular (movement), connective (bind — blood, bone), nervous (conduct — neuron).