The surrounding cell walls and membranes can control the materials that pass in, out and through living cells. Four processes that can be related to this movement of materials are diffusion, osmosis, pinocytosis and phagocytosis.

To perform their functions, cells must maintain an internal steady state in the midst of an ever-changing environment. This consistency, or ‘homeostasis’, is maintained in part by the regulation of the movement of materials in and out of the cell. To achieve this control, a cell is bounded by a delicate membrane, which can distinguish between substances, slowing down or stopping the movement of some while allowing others to pass through freely. Because not all substances penetrate the cell membrane equally well, the membrane is known as being ‘differentially’, or ‘selectively permeable’.

The main purpose of the cell membrane is to regulate the movement of materials in and out of the cell. By doing so, the cell’s internal environment can di

Cell membranes consist mainly of phospholipids. These phospholipids have two parts; a polar phosphate "head" and two nonpolar fatty acid "tails." These molecules are arranged in a way that is known as a ‘bilayer’ (or double layer) so that the polar heads face the external and the internal environments of the cell, while the fatty acids form the inside of the cell membrane. The phospholipids are free to move around, often switching places with their neighbours. This movement allows the membrane to stretch and change shape.

Pinocytosis is a process that entails tiny pockets forming along the cell membrane, filling with liquid, and pinching off to form vacuoles within the cell.

Another form of diffusion is known as ‘facilitated diffusion’. This process uses a carrier, generally in the form of a protein molecule, which assists in the movement of the solute molecules. Many cells utilise this process for the uptake of certain sugars, amino acids, and vitamins. Some molecules also cross cell membranes by ‘active transport’, which requires a specialised carrier molecule, as well as a source of energy.

Phagocytosis is the process of ingestion of matter by cells known as phagocytes. Phagocytosis is displayed by single-celled life forms that engulf and ingest foreign matter such as bacteria, non-living materials, or other cells. In multiple-celled organisms, the process is relegated to specialised cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders such as infections.

· Adherence works reasonably well for whole bacteria or viruses, but less so for proteins or encapsulated bacteria. In order to deal more effectively with encapsulated bacteria, antibodies directed against the capsule enable the phagocytic cells to ingest the organisms.

The movement across cell membrane by way of osmosis is influenced by a number of different factors includin

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