Cell structure and function from "summary" of Biology by Neil A. Campbell
The cell is the basic structural and functional unit of all living organisms. It is the smallest unit that can carry out all the activities necessary for life. Cells vary greatly in size and shape, but they all have certain features in common. One of the most distinctive features of a cell is its outer boundary, the plasma membrane. This membrane separates the cell's interior from its surroundings and controls the passage of materials into and out of the cell. Inside the plasma membrane is the cytoplasm, a jelly-like substance that contains various structures responsible for the cell's functions. One of the most important structures in the cytoplasm is the nucleus. The nucleus contains the cell's genetic material, DNA, which carries the instructions for making proteins and other molecules essential for the cell's survival and growth. The nucleus is surrounded by a double membrane called the nuclear envelope, which controls the flow of materials in and out of the nucleus. Within the cytoplasm, there are also other structures called organelles that perform specific functions in the cell. For example, the endoplasmic reticulum is involved in protein synthesis and lipid metabolism, while the Golgi apparatus...
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