From tissue to organ and organ system

From a cell to a body

A house is built from bricks. The bricks make a wall, the walls make a room, and the rooms make a house. An animal is built in a similar way.

1. Cells are the units of life.
2. Tissues are groups of similar cells with one job.
3. Organs are made of two or more tissues that are arranged to do a particular job.
4. Organ systems are groups of organs that work together.
5. The organism is the whole animal.

For example, a muscle fibre is a cell. Cardiac muscle is a tissue. The heart is an organ. The circulatory system is an organ system. The person is the organism.

An organ has more than one tissue

The stomach is a good example. Its wall has all four basic tissues.

• Epithelial tissue lines it and makes mucus and juice.
• Connective tissue holds the layers together and carries blood vessels.
• Muscular tissue (smooth muscle) churns the food.
• Neural tissue controls the muscle and the glands.

Take any tissue away and the organ cannot work. This is the point of an organ: the tissues together do what none can do alone.

The wall of a hollow organ such as the small intestine has four layers.

The stomach has a third, oblique layer of muscle. NCERT does not give this detail, but the idea of the layers helps you to remember how tissues join.

Organ systems

Organs that work together form an organ system. The human body is usually described as having eleven organ systems.

Some books count in another way, and a few join the lymphatic system with the circulatory system. The number depends on the book. The idea is the same.

In animals the level of organisation differs. A sponge has cells and no true tissues. Hydra has tissues but no organs. A cockroach has organ systems, and so has a frog or a person. This is why the cockroach is a good study in the next course.

Four tissues compared

• Epithelial: tightly packed cells with very little matrix. It is found on surfaces, in linings and in glands. It covers, lines and secretes.
• Connective: few or many cells in much matrix. It is found everywhere. It links, supports and carries.
• Muscular: long fibres that contract. It is found in bones, organ walls and the heart. It makes movement.
• Neural: neurons and neuroglia. It is found in the brain, the cord and the nerves. It carries messages.

Try it in Python

The program lists the levels of organisation, with an example of each.

Practise with new numbers

Common mistakes

• Calling an organ a tissue. An organ has two or more tissues.
• Saying that the stomach is made of muscle only. It has all four tissues.
• Mixing up the order of the levels. Cell, tissue, organ, organ system, organism.
• Thinking that every animal has organ systems. Sponges have none, and Hydra has tissues but no organs.
• Counting one organ as two systems. An organ may serve two systems, as the pancreas serves digestion and the endocrine system.

Remember these

• Levels: cell, tissue, organ, organ system, organism.
• An organ has two or more tissues. The stomach has all four.
• The human body has eleven organ systems in the usual count.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Write the levels of organisation of an animal in order, from the smallest.

2. What is an organ?

3. Name the four tissues in the wall of the stomach, with a job for each.

4. Name the four layers in the wall of a hollow organ, from the inside.

5. A wall is 2 mm thick, and the muscle layer is 0.5 mm. What per cent of the wall is the muscle layer?

References

• OpenStax Biology 2e, chapter 33, The Animal Body: Basic Form and Function, sections 33.1 and 33.3 (organ systems).
• NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals.