What Is a Cell?
By Jim V.
Cells all have different sizes, shapes, and jobs to do. Did you know that the yoke from an ostrich's egg is the biggest cell? Even the cells in a single organism may have different shapes, sizes, and jobs. Organisms like humans are made up of trillions of cells. There are also one celled organisms such as euglenas, amoebas, and bacteria.
All cells have some parts in
common. One part found in all cells is the cell membrane. The
cell membrane surrounds the cell, holds the other parts
of the cell in place, and protects the cell. Molecules can pass
in and out of the cell membrane. Inside the membrane, all cells,
except for bacterial cells, contain a nucleus and cytoplasm. The
nucleus is a dark structure located in the middle of the
cell. It controls the cell's activities, and acts like the cell's
brain. Inside the nucleus there is DNA
which contains genetic information. The cytoplasm is a
jelly-like substance inside the cell where most of the cell's
activities take place. It's made out of water and other chemicals.
All cell parts, except the nucleus, are located in the cytoplasm.
Like all living things, cells die. If you watched a clock for one minute, about three billion cells in your body die. In that same minute, about three billion cells divided so that all the cells that died were replaced. Dead skin cells flake off and dead cells from internal organs will pass through and out of the body with waste products. The length of time cells live can vary. For example, white blood cells only live for thirteen days, red blood cells live for about one hundred and twenty days, liver cells live about eighteen months, and nerve cells can live up to one hundred years. That is a very big difference!
Basically, each cell was made from an already existing cell.
New cells are made through a process called cell division or mitosis. One cell turns into two
cells and then two cells turn into four cells, etc. Even humans
started life with only one cell. Most of the cells in many celled
organisms use mitosis to reproduce.
The animation shows the basic steps in mitosis which are:
1) At the start of mitosis chromosomes are in the nucleus.
2) The chromosomes in the nucleus will then make a copy of themselves
3) Next the cell divides.
4) Last one set of chromosomes goes to the new cell and one set
remains in the parent cell.
One of the most important tools that scientists use to study cells is the microscope. An optical microscope can magnify a cell up to two thousand times. If an ant were to be magnified at two hundred thousand times, it would be more than two and one half miles (four kilometers) long. Electron microscopes can magnify a cell by one million times. But even with such large magnifications, the very detailed parts of some cells can't be seen. If you would like to see real photographs of cells viewed through the lens of an electron microscope, make sure you visit the Nanoworld Image Gallery (you'll need to register in order to view the gallery).
Scientists also put dyes in cells to make certain parts look bright under a microscope. Another way that scientists use to examine cells is to separate them into their parts by using a machine called a centrifuge. It spins substances at really high speeds. First scientists grind up cells in a tube, and then they place the tube in the machine. After whirling around in the centrifuge, the parts of the cell that weigh more sink, while the lighter parts stay on top. That makes it easier for the scientist to study the parts.