membranes

MEMBRANE STRUCTURE

Content Statements:

B2.1.1 Lipid bilayers as the basis of cell membranes

B2.1.2 Lipid bilayers as barriers

B2.1.4 Integral and peripheral proteins in membranes

B2.1.9 Structure and func>on of glycoproteins and glycolipids

B2.1.10 Fluid mosaic model of membrane structure

B1.1.12 Forma>on of phospholipid bilayers as a consequence of hydrophobic and hydrophilic regions

CELL MEMBRANES

Cell (plasma) membranes enclose the contents of the cell, separa>ng intracellular components from the

extracellular environment. This allows for the precise control of internal condi>ons (i.e. homeostasis). Cell

membranes have two key proper>es that promote this homeosta>c regula>on:

• They are semi-permeable, in that some material cannot cross the membrane without assistance

• They are selective, in that membrane scan regulate the passage of certain material according to need

PHOSPHOLIPID BILAYER

Membranes consist of a phospholipid bilayer . Each phospholipid

consists of a polar phosphate head and two non-polar faOy acid

tails. The phosphate head is hydrophilic (water-loving), while the

faOy acid tails are hydrophobic (water-ha>ng). This makes the

phospholipid amphipathic (both hydrophilic and hydrophobic).

Phospholipids will spontaneously arrange into a bilayer, with the

hydrophilic phosphate heads facing out towards the surrounding

aqueous solu>ons (i.e. cytosolic and extracellular fluids), while

the hydrophobic faOy acids face inwards to avoid exposure to the

polar fluids. The bilayer is therefore held together by the weak

hydrophobic associaCons between the faOy acid tails, allowing for

membrane fluidity and flexibility (it can easily break and reform).

Hydrophilic head

AOracted to H2O

Hydrophobic tail

Repelled by H2O

Inters>>al Fluid (Extracellular)

Cytosolic Fluid (Intracellular)

MEMBRANE PROTEINS

Phospholipid bilayers are embedded with proteins, which may be permanently or temporarily aOached:

• Integral proteins are transmembrane (span the bilayer) and permanently attached to the membrane

• Peripheral proteins associate with one side of a membrane and are temporarily attached to the bilayer

Integral proteins cannot readily be dissociated from the membrane without disrup>ng the bilayer (such as

through the use of detergents). Examples of integral proteins include ion channels, carrier proteins and

protein pumps. Peripheral proteins can easily be dissociated from the membrane by using a polar solvent.

Examples include receptor complexes associated with signal transduc>on pathways (such as G proteins).PROTEIN FUNCTIONS

Membrane proteins serve a variety of key func>ons:

• Junctions: They can connect cells together to form tissues (tight junctions)

• Enzymes: Immobilising enzymes on membranes localises specific reactions

• Transport: Allows passage of material across the bilayer (channel proteins)

• Recognition: May function as markers for cell identification (e.g. antigens)

• Adhesion: Act as attachment points for cytoskeleton or extracellular matrix

• Transduction: Functions as receptors for signalling pathways (glycoproteins)

Hint: JET RAT

GLYCOSYLATION

Phospholipids and membrane proteins can have carbohydrate

chains aOached via the process of glycosylaCon. Glycosyla>on

of phospholipids result in glycolipids, whereas glycosyla>on of

membrane proteins produce glycoproteins. The carbohydrate

chains are located on the extracellular side of the membrane

and play important roles in cell adhesion and cell recogni>on.

• Adhesion: Surface carbohydrates can serve as attachment

points for cells (glycoproteins act as sperm binding sites)

• Recognition: Surface carbohydrates can also act as a point

of recognition between cells (ABO antigens are glycolipids)

Glycoproteins and glycolipids also play a role in maintaining the

structural integrity of the extracellular matrix. The carbohydrate

chains can link extracellular molecules together, helping to make

the matrix a cohesive network that provides external structure.

The glycocalyx is a sugar coat found

in ova that mediates sperm binding

FLUID-MOSAIC MODEL

The fluid-mosaic model of membrane structure describes two of the key quali>es of a plasma membrane:

• Fluid: Phospholipids can move position, making membranes amorphous (able to change size or shape)

• Mosaic: The bilayer is embedded with proteins and carbohydrates, resulting in a mosaic of components

The fluid-mosaic model was proposed by Singer and Nicolson in 1972 and is the currently accepted model.

integral

protein

glycoprotein

cholesterol

Phospholipid

phosphate

fa1y acids

peripheral protein