Course 1.1Lesson 1 of 3

What makes something alive?

Biology is the science of life forms and living processes. Growth, an increase in mass and in the number of cells, is a feature of living things, but it is not a defining one. Living things grow from inside, and non-living things grow only by material that piles up on the outside. In a single-celled organism, growth and reproduction are one event.

about 30 min1 simulation2 practice questions1 Python program5 questions to check yourself

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Try the code and the simulations beside the lesson.

A stone, a seed and a dog

Look at three things on a table: a stone, a dry seed and a sleeping dog. You know at once that the dog is alive and the stone is not. But how do you know? Is it because the dog moves? A river moves too, and a plant on the windowsill hardly moves at all. Is it because the dog breathes? A dry seed does not seem to breathe, and it is still alive.

Biology is the science of life forms and of the processes that go on in them. So the first question of biology is the one you have just asked: what does it mean to be alive?

There is no single answer, because every feature that we try has a few exceptions. So we take the features one by one. We find what is true of all living things, and what is only usual. In this lesson we start with growth.

Growth: more mass and more cells

A living organism gets bigger. A puppy becomes a dog, and a seedling becomes a tree. This increase in mass, and in the number of cells that make the body, is called growth.

In a many-celled organism, growth happens because cells divide and the new cells grow to full size. The cells near the tip of a root divide, and the root gets longer.

In plants, cell division goes on in some tissues all through life. A tree gets taller and wider every year.
In animals, growth is seen only up to a certain age. An adult stops getting taller. But cells still divide in some tissues to replace lost cells, such as the cells of the skin.

Living things grow from inside

A heap of sand gets bigger when you add more sand. A crystal of salt grows in a glass of salty water. A mountain, a boulder and a snowball rolled over snow do the same. These things grow only by accumulation of material on the outside.

Two pictures. On the left, a living thing grows from inside: a seed becomes a seedling and then a young plant with leaves, and small cells that divide and grow are drawn beside the plant. On the right, a non-living thing grows by piling up: three heaps of sand, small, bigger and big, with arrows showing sand added from outside onto the biggest heap.

A living thing grows from inside. The cells divide in its body, and the new cells make the body bigger. Nobody has to pile anything on its surface. So the way of growing is different, even when the result looks the same.

Why growth is not the defining feature

Growth is common in living things, but it cannot be the defining feature of life, for two reasons.

Non-living things also increase in mass, as the sand heap shows.
Living things do not grow all the time. An adult human has stopped growing in height, and a dry seed on a shelf does not grow. Both are alive.

Growth in a single-celled organism

A bacterium, an Amoeba or a single-celled alga has one cell. When this cell grows and divides, there are two single-celled organisms, so the number of individuals has gone up. For such an organism, more cells and more individuals are the same thing. So growth and reproduction are one event in single-celled organisms.

Each division doubles the number of cells. After $n$ rounds of division there are $2^n$ cells.

You should find that halving the time for one division does far more than double the cells. In $4\ \text{h}$ , divisions every $40\ \text{min}$ give $64$ cells, but divisions every $20\ \text{min}$ give $4096$ .

Try it in Python

This program starts from one cell, divides it again and again, and counts the cells. It also finds how many bacteria come from one bacterium that divides every $20$ minutes.

Practise with new numbers

Common mistakes

Calling every increase in size growth of life. A snowball and a crystal get bigger from outside, with no cells dividing.
Making growth the defining feature. An adult human and a dry seed do not grow, and they are alive.
Adding 2 cells at each division. The number doubles: $2^n$ .
Thinking animals stop making cells as adults. Cells still divide to replace lost ones.