Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 19

All human cells are surrounded by an outer membrane referred to as the plasma membrane. This has a multitude of functions including: holding the cell together as a discrete intact unit, regulating the movement of materials into and out of the cell and communication and recognition between cells.

Figure 1.4 Structure of the plasma membrane

The plasma membrane is predominantly composed of a phospholipid bilayer within which are located a variety of proteins (Figure 1.4). Since phospholipid is a fluid, with a similar consistency to vegetable oil, and the denser proteins are positioned throughout its structure, under a microscope it has a mosaic-like appearance, hence the plasma membrane is frequently referred to as having a fluid-mosaic structure. The phospholipid molecules originate from the smooth ER while the integral proteins initially are synthesised by ribosomes, and refined in the Golgi before being transferred to and inserted into the membrane.

The plasma membrane is not a static structure; phospholipid and protein molecules are continually being added and removed depending on the current needs of the cell. The phospholipid bilayer is often referred to as being self-forming. Each phospholipid molecule consists of a hydrophilic (water-loving) head portion and two hydrophobic (water-hating) tails. Since the intracellular compartment of the cell is full of the water-based cytosol and the outside of the cell is surrounded by watery interstitial fluid, the phospholipid molecules naturally form a bilayer as the hydrophobic tails orientate themselves away from the aqueous environments of both the intracellular and extracellular compartments (Figure 1.4).

There are many different types of protein molecules within the phospholipid bilayer, including channel proteins which span the entire width of the membrane and form pores through which materials can enter and leave (see below), and receptor proteins which form three-dimensional pockets into which chemical signals such as hormones can fit.