Cells, response and the meaning of life

The body is made of cells

Look at a thin slice of an onion under a microscope. You see small boxes that fit side by side. These are cells. An onion is made of cells. So are you, a mango tree and a mushroom.

• Some organisms have one cell: a bacterium, an Amoeba, yeast. They are unicellular.
• Others have many cells: a mango tree, a frog, a human. They are multicellular. In them, cells form tissues, and tissues form organs.

Cellular organisation of the body is a feature of all living things. A stone has no cells, and neither has a sand heap.

Sensing the surroundings

Touch a hot pan, and your hand pulls back before you think. Put a plant near a window, and it bends toward the light. A bacterium in a drop of water swims toward a grain of sugar. In all of these, the organism senses a change in its surroundings and responds.

The change that starts the response is a stimulus. It can be of three kinds:

• physical: light, temperature, touch, water, sound;
• chemical: a smell, a taste, a pollutant;
• biological: another organism, such as an enemy or a partner.

We sense our surroundings with sense organs. Plants respond to light, water, temperature, other organisms and pollutants. All organisms, from bacteria to the most complex animals, can sense and respond to the cues of the environment.

Photoperiod is the length of day and night. It affects reproduction in seasonal breeders, both plants and animals. A plant flowers in one season, and a bird lays eggs in another, because they sense the length of the day.

Awareness and self-awareness

All organisms are aware of their surroundings in this sense, and all of them deal with the chemicals that enter their bodies. This awareness is called consciousness. A human being is the only organism that is also aware of itself. This is self-consciousness.

Putting the features together

A dead dog still has cells, but it has no metabolism and does not respond, so it is not alive. A thermostat senses the temperature and switches a heater on and off. But it has no cells and no metabolism, so it is not alive either. We use the features together.

You should find that switching off growth and reproduction leaves the verdict "living", as for a mule. Switching off any one of the other three gives "not living".

Taking the features together: a living organism is a self-replicating, evolving and self-regulating interactive system that is capable of responding to external stimuli.

Try it in Python

This program records five features for six things, and it decides which are living by the rule: metabolism, cells and a response must all be present. Look at the mule and the salt crystal.

Practise with new numbers

Common mistakes

• Thinking plants do not respond. They bend toward light and respond to water, touch and temperature.
• Using one feature alone. A thermostat responds, and a dead leaf has cells, yet neither is alive.
• Mixing up the kinds of stimulus. Light is physical, a smell is chemical, an enemy is biological.
• Thinking only animals sense the day length. Photoperiod also decides when many plants flower.

Remember these

• Every living organism is made of one cell or many cells.
• Every living organism senses its surroundings and responds to physical, chemical or biological stimuli.
• Photoperiod, the length of the day, affects reproduction in seasonal breeders.
• Human beings are also self-conscious.
• Metabolism, cellular organisation and the response to the surroundings are found in all living things. We use the features together.

Check yourself

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

1. Name the three kinds of stimulus, with one example of each.

2. A dead leaf is made of cells. Why is it not alive?

3. A robot vacuum cleaner turns when it touches a wall. Is it living?

4. Which three features are found in every living organism?