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The fluid mosaic model of cell membranes (Proteins (Extrinsic (On either…
The fluid mosaic model of cell membranes
Phospholipids
Important components of cell surface membranes and form the basis of membrane structure because:
The outer layer of phospholipids has its hydrophilic head pointing out, interacting with the water surrounding the cell.
The hydrophobic tails of the 2 phospholipid layers point towards each other, to the centre of the membrane.
The inner layer of phospholipids has its hydrophilic heads pointing in, towards the cell, and interacts with the water in the cytoplasm.
The phospholipid component of a membrane allows lipid-soluble molecules across, but not water-soluble molecules.
Can form bilayers, with 1 sheet of phospholipid molecules opposite another.
Proteins
Extrinsic
On either surface of the bilayer.
Provide structural support and form recognition sites, by identifying cells, and receptor sites for hormone attachment.
Intrinsic
Some are carriers, transporting water soluble substances across and others allow active transport of ions across, by forming channels.
Extended across both layers of the phospholipid bilayer.
Proteins are scattered throughout the phospholipid bilayer of the membrane. There are 2 ways in which they are embedded.
Permeability of the membrane
Water-soluble substances (glucose and polar molecules) cannot readily diffuse through the phospholipids and must pass through intrinsic protein molecules, which form water filled channels across the membrane. As a result, the cell surface membrane is selectively permeable to water and some solutes.
Lipid-soluble substances (oxygen and carbon dioxide) dissolve in the phospholipid an diffuse across the membrane. The phospholipid later is hydrophilic so lipid-soluble molecules move through the cell membrane more easily than water-soluble substances.
Properties
The individual phospholipid molecules can move within a layer relative to one another (fluid).
The proteins embedded in the bilayer vary in shape, size and pattern (mosaic)
contains
Glycoproteins and glycolipids are found in the outer layer of the membrane. The carbohydrate layer around the membrane is called the glycocalyx. Some molecules in the glycocalyx have roles as hormone receptors or in cell to cell recognition.
Cholesterol occurs in the membranes of animal cells, between the phospholipid molecules, making the membrane more rigid and stable.