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Compiled by: Sumayyah

Be ready kids! You're about to start your tour of a world that is a lot more complex than the one you live in and yet is so small that you can't see it! Interesting, right?
It's the world inside your body, inside the body of all living things. It's the remarkable world of cells.
How big are cells? Take an idea of how small they are by the fact that the largest cells are about the size of the full stop at the end of this sentence. Most are very small, so small that we can't look at them with our naked eyes. We need very powerful lenses and microscopes to see them. 
Keep in mind that every inch of your body is made up of cells. From your heart muscles to a tiny piece of your skin. They're like those blocks which make up a building. They might be of different shapes in different places of your body, but they are there. 
Now let's dive into the tiny world of a cell. There are tiny rooms inside a cell which are known as Organelles. These organelles are very very small. You can imagine how small they are by the fact that they're inside cells which are already that tiny. These organelles work to provide for the needs of the cells, to bring in food supplies, get rid of waste, repair the cell and help it grow and reproduce. 
There is a membrane surrounding the cell, which is called the Plasma Membrane and it's like those guards sitting on your school gate, controlling who goes in and who goes out. They control all the substances which enter and leave the cell. It allows Oxygen and sugar molecules to pass through it. 
The cell is filled with a jelly which is known as the Cytoplasm. It fills up the cell and is mostly water. The other components of the cell just float around in the cytoplasm. 
There is a small nucleus inside the cell which controls all the activities of the cell, making sure the required substances are being manufactured. Then there are tiny sausage like structures in the cell which work for energy production and the energy produced is called ATP. This energy is used by the cell to carry out its functions. Then there are some sacs in your cell cytoplasm which create and pack large molecules used in other parts of the cell; these sacs are called Golgi Apparatus. There are some other organelles in the cell cytoplasm which are known as Ribosomes These are tiny factories which make proteins for the cell. There are small pouches also lying in the cytoplasm which help the cell kill the harmful bacteria or viruses that enter the cell. These pouches are called Lysosomes and they also help in digesting the food you've just eaten! Many similar cells combine to form a 'tissue'. These tissues then come together and form an organ. Your heart, your lungs and your kidneys are good examples of organs. Next, all of these organs are connected to each other and they form an organ system, which is US! 
Some cells, like bacteria, are made up of only one cell and so they're microscopic organisms. Our body has trillions of cells which work day and night to help us survive. You should also know that some cells have cell walls as well. But such cells are never present in animals. Only plant cells have cell walls and these are walls are very hard, made up of a substance called 'Cellulose'. They don't let the plant cell burst if it is overfilled with water. It's like an envelope around the plant cell. Human cells also don't have cell walls and so our body needs an environment which would not overfill our cells with water or else they'd all burst. Cells also keep reproducing and making copies of themselves by a process known as Mitosis. They keep dividing so that our body gets new cells to keep functioning. Once the old cells are old enough to function properly, they're sent to the liver where they are crushed and killed and then released as waste from our body.