The cell: the unit of living things
Aprende
All living things are made of cells. A cell is the smallest part of a living thing that is alive: it gets nutrition, interacts with what is around it and reproduces. A stone has no cells, because it has never been alive.
Cells were discovered with the microscope. In 1665, Robert Hooke looked at a very thin slice of cork and saw tiny compartments, like those of a honeycomb. He called them cells. Later it was found that all living things are made of cells and that every cell comes from another cell.
Hooke and the cork Some living things have just one cell and are called unicellular: bacteria, yeasts and amoebas. Multicellular ones have many cells. A person has about 30 trillion, that is, 30 million million.
One cell or many Almost all cells are so small that they can only be seen with a microscope. They are measured in micrometres (µm): a micrometre is one thousandth of a millimetre. A red blood cell measures about 7 micrometres. A famous exception is an egg yolk: a single cell, so big that it can be seen with the naked eye.
Cells are not all the same: some are round, some are long and some have branches, and each type does its own job. But almost all of them have three parts. The cell membrane wraps round the cell and lets some substances through and not others. The cytoplasm is its soft inside, like jelly. The nucleus holds the instructions of the cell. Bacteria do not have a nucleus.
Three parts of an animal cell; the other parts will be seen in another topic
Worked examples
Ejemplo 1
Ellie is kneading bread with yeast. Julie says: “Yeast is a little creature so tiny that it has no cells.” Is she right?
- Yeast is a living thing, and all living things are made of cells.
- It is very small, but it has a cell.
- It is called unicellular: it has a single cell, which carries out all the vital functions.
She is not right: yeast is a living thing with a single cell.
¡Ojo! Many children think that a living thing so small has no cells.
Ejemplo 2
Vera looks at a yeast through the microscope; it measures 8 µm. How many yeasts, placed in a row, would cover 1 millimetre?
- A millimetre has 1000 micrometres.
- Each yeast takes up 8 micrometres.
- Divide: 1000 ÷ 8 = 125.
It would take 125 yeasts in a row.
¡Ojo! Many children get the units wrong and say 8 or 8000.
Ejemplo 3
Tony says: “The elephant is bigger than the mouse because its cells are bigger.” Is that true?
- The cells of the elephant and those of the mouse are about the same size.
- The elephant is bigger because it has many, many more cells.
- The same happens when a young animal grows: more cells are made.
That is not true: the elephant has more cells, not bigger cells.
¡Ojo! Many children think that the bigger the body, the bigger the cells.
Un reto para pensar
With an adult, crack open a raw hen’s egg onto a plate and look at the yolk: it is a single cell. Draw it and wash your hands with soap and water when you have finished. Then find three things at home that are made of cells and three that are not, and explain how you decided about each one.
Practica
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