Overview: What Are Cells?
Most cells are too small to be seen without a microscope, so theories about them did not exist until after microscopes were in use. A scientist named Hooke called the small structures in a thin slice of cork “cells”, but it wasn’t until the 1800s that cell theory was really put together. According to cell theory, cells are the smallest, most basic unit of life.
Cell Theory
By the middle of the 1800’s, three parts of cell theory were in place. First, all organisms are composed of cells. Second, cells are the smallest, most basic unit of structure and function in organisms, whether they are one-celled amoeba, or many-celled whales. Third, all cells come from other cells. It is characteristic of life that it reproduces itself. Viruses and such forms may be an exception, but they do not function as life until they invade living cells and reproduce.
Size and Shape of Cells
The smallest of cells are a form of bacteria that can only be seen with an electron microscope, and are 0.1 micrometers or 1 ten-millionth of a meter long. The largest cell is an ostrich egg, which has a diameter of about 100 millimeters. Most cells are between 2 and 200 micrometers long, so that most of them can be seem with a regular light microscope. The shapes of cells vary according to their function. For example, the cells in the human body vary from the flat, thin cells of the skin and other membranes, to long, branched nerve fibers. Although different types of cells vary a great deal, they have three basic parts; the cell nucleus, the cell membrane and cell wall, and the cytoplasm.
The Cell Nucleus
The cell nucleus is the control center for most of the living cell’s activities. For example, proteins are built according to specialized instructions. The nucleus also contains chromosomes, made up of DNA, that contains the genetic information that is transmitted when each cell reproduces. Nucleoli within the cell nucleus contain DNA, RNA, and proteins. The nucleus is separated from the rest of the cell by the nuclear envelope, which has a double membrane.
Cell Membrane and Cell Wall
The cell membrane is the outer boundary of the cell, and it is composed of proteins and lipids. It gives the cell shape and regulates food that goes in and wastes that go out. Plants, algae, fungi, and some bacteria also have a more rigid cell wall in addition to the cell membrane. It allows water and dissolved substances to contact the membrane and also gives the cell its shape.
Cytoplasm
The cytoplasm is everything in the cell that is between the nuclear membrane and the outer cell membrane. Channels within the cytoplasm transport materials that are formed in the nucleus and other structures within the cell, such as proteins, energy produced within mitochondria, and nutrients.
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