1.3 Membrane Structure
Membrane proteins
Functions of membrane proteins
hormone binding site
immobilized enzyme with the active site on the outside
cell adhesion to form tight junctions between groups of cells in tissues and organs
cell-to-cell communication
channels for passive transport to allow hydrophilic particles across by facilitated diffusion
pumps for active transport which use ATP
Type of membrane protein
integral proteins
hydrophobic on at least part of their surface
many are transmembrane
embedded in the hydrocarbon chains in the centre of membrane
peripheral
hydrophilic on their surface
mostly are attached to surface of integral protein, this attachment is often reversible
some have hydrocarbon chain inserted into membrane
phospholipid bilayer
amphipathic, both hydrophilic (phosphate group) and hydrophobic (hydrocarbon chains)
Experiments
Davson and Danielli, membrane proteins are sandwiched between phospholipid layers, supported by electron microscope
Singer and Nicolson, membrane proteins occupy variety of positions, fluid mosaic model
Gorter and Grendel extracted phospholipids from plasma membrane, area of phopholipid is twice the area of cell
Cholesterol in membranes
Cholesterol
a type of lipid, grouped under steroid
one end of is a hydroxyl group
mostly made of hydrocarbon tails
Roles of cholesterol
disrupts the regular picking of hydrocarbon tails preventing them from crystallizing and behaving as a solid
reduces permeability to hydrophilic particles such as sodium ions and hydrogen ions
restricts molecular motion and fluidity of mebrane
help membrane to curve into concave shape, helps in the formation of vesicles during endocytosis