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Cells of the epithelium (lining) of the small intestine If you looked at a piece of skin under a powerful microscope, you would see that it was made up of tiny "bricks", called cells. Every part, or tissue, of your body is made of these tiny cells—more than 100 million million of them. There are over 200 different cell types, each with a size and shape related to the job it does: for example, message-carrying nerve cells are long and thin, while red blood cells are doughnut-shaped, ideal for carrying oxygen.

Inside a cell

All cells have the same basic structure, including a thin outer covering, called a cell membrane. Inside, there are “little organs”, or organelles, that float in the jelly-like cytoplasm. At the centre of the cell is the nucleus. Other organelles include the mitochondria, ribosomes and the endoplasmic reticulum.
In this illustration the organelles inside a cell have been cut away so we can see inside them. A chromosome in close-up. This thread-like strand is a coil made up of proteins (shown here as balls) with an extremely long,...Read More >>A chromosome in close-up. This thread-like strand is a coil made up of proteins (shown here as balls) with an extremely long, thin molecule of DNA wrapped around them. The largest organelle, the nucleus is surrounded by a membrane. The nucleus is packed with chromosomes, tightly wound strands of a substance called DNA. Here are to be found your genes: tiny instruction manuals containing all the information necessary to create your body in a certain way—blue eyes or brown eyes, curly or straight hair, tall or short, and so on. The nucleus sends out chemical instructions to the cell through tiny pores in the nuclear membrane. The nucleolus, a round blob inside the nucleus, makes ribosomes.
Sausage-shaped mitochondria are the cell’s energy releasers. They break up substances such as sugar (glucose) to release energy for use in the cell. The endoplasmic reticulum, a network of membranes, is the cell’s factory, which processes and transports cell-building substances called proteins.
Under the control of the nucleus, the cell works like a little factory. Fuel, in the form of sugars from the blood, is brought in through the cell membrane. Using energy released by the mitochondria, the ribosomes manufacture proteins, the basic building blocks of the body itself.

The longest cells in the body are the neurons that run from the spinal cord in the lower back to the big toe, which are around 1.3 m (4.25 ft) in adults.

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