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