Lesson 1 of 11 · 7 min

Tissues, organs and organ systems

NCERT §7.1

Arjun's diary opens with a question from Ms Rao: an amoeba lives as one cell, so why does a frog need millions of them, and how do they avoid getting in each other's way?

The story this chapter follows: Arjun's monsoon pond

Arjun lives beside the paddy fields at the edge of his village, and every monsoon the pond behind his house fills with croaking bullfrogs. This year he keeps a frog diary through the seasons, and his biology teacher, Ms Rao, helps him name every structure he reads about, from skin and limbs to heart, kidneys and brain.

The lesson in notes

In short

In a unicellular organism one cell does everything: digestion, respiration and reproduction. In a multicellular animal the same jobs are shared out among organised groups of cells.

Even a simple animal like Hydra has several types of cells, with the number of each type running into thousands; the human body has billions of cells.

A tissue is a group of similar cells, together with the intercellular substance around them, that carries out a particular function.

All complex animals are built from only four basic types of tissue: epithelial, connective, muscular and neural.

Tissues arranged in a definite proportion and pattern make an organ such as the stomach, lung, heart or kidney. Each organ is made of one or more types of tissue; the heart contains all four.

When two or more organs perform a common function through physical and/or chemical interaction, they form an organ system, such as the digestive or respiratory system.

Cells, tissues, organs and organ systems divide the work among themselves, which is division of labour, and this keeps the body as a whole alive. The organisation makes the activities of millions of cells more efficient and better coordinated.

The complexity of organs and organ systems shows a discernible trend, called the evolutionary trend (studied in Class XII).

Morphology is the study of form or externally visible features; in animals it means the external appearance of organs or body parts. Anatomy is conventionally the study of the morphology of internal organs in animals. The frog is studied as the representative vertebrate.