A lipid that containing phosphate group in its molecule. From phospholipids, little bags can be formed in which chemicals can be isolated from the external environment. These bags are the membrane-bound compartments that we know as cells and organelles.
The basic structure of a membrane is a 7 nm thick phospholipid bilayer with protein molecules
spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the
layers. Hence the structure is described as a fl uid mosaic – the scattered protein molecules resemble pieces of a mosaic.
Phospholipid bilayers are a barrier to most water
soluble substances because the interior of the membrane is hydrophobic.
Structure Of Membrane
Cholesterol is needed for membrane
fluidity and stability.
Some proteins are transport proteins, transporting molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape. Some proteins act as enzymes – for example, in the cell surface membranes of microvilli in the gut. Glycolipids and glycoproteins form receptors – for example, for hormones or neurotransmitters.
They also form antigens, which are cell recognition markers.
Transport Across The Cell Surface Membrane
Th e cell surface membrane controls exchange between the cell and its environment. Some chemical reactions take place on membranes inside cell organelles, as in photosynthesis and respiration.
Diffusion is the net movement of molecules or ions from a region of their higher concentration to one of lower concentration. Oxygen, carbon dioxide and water cross membranes by diffusion through the phospholipid bilayer. Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins. This process is called facilitated diffusion.
Active transport – the sodium–potassium pump
 |
No comments:
Post a Comment