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Cells, the body and growth

Picture 4. A human cheek cell. It is magnified about 10,000 times. It would be about 25 millionths of a metre across the cell. The nucleus is surrounded by cytoplasm.
(Image courtesy of The Ohio State University at Lima).

	Humans are multi cellular organisms. That means they are made up of billions of individual cells. These cells are not all the same. There are many different cell types that are specialised to perform the range of functions needed in a complex organism.
Different types of cells
	Nerve cells that carry electrical signals, muscle cells which generate force and pancreas cells making insulin are just a few examples. Amazingly, all of these cells have grown from just one single fertilised egg cell.

Structure of an animal cell
	All animal cells are based on the same structures. Explore the diagram to see what the different parts of a cell does.

Picture 5. Structure of an animal cell . Roll your cursor over the red dots to find out more.

How molecules move in and out of cells
	Substances cross the cell membrane in four ways: diffusion, osmosis, active transport and phagocytosis. Diffusion is the movement of molecules from a region of their high concentration to a region of their low concentration. Osmosis is the movement of water through a differentially-permeable membrane, from a high water potential to a low water potential. Active transport requires energy and moves molecules in the opposite direction to diffusion. i.e. from a region of low concentration into a region of their high concentration. Phagocytosis happens when the cell engulfs large particles. This is seen when white blood cells surround and "eat" bacteria.

Question 2

Different parts of the cell carry out different functions. Look at the functions described in the table below. From the drop down list, choose the part of the cell that carries out each function.

Function	Part of cell
Controls the substances that can get in and out of the cell. It is also involved in active transport.
Contains the cell's DNA.
Is involved in the generation of energy during aerobic respiration.
Where many of the chemical reactions of the cell takes place.

Membrane

important in controlling what enters and leaves the cell

Cytoplasm

where the hundreds of chemical reactions take place

Mitochondria

they supply energy to the cell

Food store

glycogen granules

Nucleus

important in cell division and control of other processes in the cell

Chromosome

Chromosomes are found inside the nucleus. They contain the genetic code needed to make new copies of the cell. Every human cell has 23 pairs of chromosomes.