Each cell has the information to build an entire organism. Yet each cell has its own function. There are a few organisms that consist of just one cell. But for most organisms, individual cells cannot survive on their own, they cooperate with other cells. Each cell has the information to recreate itself. Cells come in all shapes and functions, but they all share three basic components: DNA region, cytoplasm region, and plasma membrane.
DNA region - is the brain region of the cell.
Cytoplasm region - is everything between the DNA region and the outer plasma membrane.
Plasma membrane - is a thin outer layer that defines a cell as a separate unit. It does not isolate the cell from its surroundings. Instead, it serves as a gate-keeper which determines what goes in and what comes out. Both the inner side and the outer side of the plasma membrane are bathed in fluid, which plays a vital role in the function of the membrane.
The membrane itself is more of a fluid than a rigid shell. Membranes are composed of two layers of molecules called phospholipids. Phospholipids have one head and two tails. Heads form two shells around the DNA region, with tails sandwiched between the shells. Interactions between the phospholipids are weak, so the individual phospholipids are free to move around, spin, drift, and flex its tails. This motion prevents molecules from packing together into a solid layer, allowing things to pass through the membrane.
The above description is a good starting point at describing a cell membrane. But of course, these membranes come in all sorts of compositions and arrangements.
In between the phospholipds are embedded various other molecules called proteins, which serve different functions:
Enzymes - they speed up cell processes.
Transport proteins - they bind molecules or ions on one side of the membrane, and release them on the other side.
Receptor proteins - they wait for specific substances on the outside of the cell that trigger changes in cell activities. For example, enzymes that crank up the machinery for cell division are switched on when molecules of hormone somatotropin dock at a specific type of receptors designed to recognize them. Recognition proteins - are like fingerprint scanners placed on the outside of the cell. They scan proteins on the surface of other cells, and identify them as "self" or "not-self". Cells identified as "self" are left alone, Cells identified as "non-self" are attacked and destroyed.
Adhesion proteins - these help identify other cells of the same type so they could stick to one another. This helps to keep cells of one organ next to each other.
Sources:
Starr, Cecie. Basic Concepts in Biology: From Biology: Concepts and Applications, 5th Edition. Pacific Grove, CA: Brooks/Cole-Thomson Learning, 2003. Print.
Image Credit: Creative Commons, by monstara