Biology IB HL - 1.3 Membrane Structure

Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule

Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains

Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic

Phospholipids spontaneously arrange into a bilayer

The hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively

The bilayer is held together by weak hydrophobic interactions between the tails

Hydrophilic / hydrophobic layers restrict the passage of many substances

Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility

This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)

Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)

Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane

The amino acids of a membrane protein are localised according to polarity

Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer

Polar (hydrophilic) amino acids are located internally and face aqueous solutions

Single helices / helical bundles

Beta barrels (common in channel proteins)

Junctions

Enzymes

Transport

Recognition

Anchorage

Transduction

Serve to connect and join two cells together

Fixing to membranes localises metabolic pathways

Responsible for facilitated diffusion and active transport

May function as markers for cellular identification

Attachment points for cytoskeleton and extracellular matrix

Function as receptors for peptide hormones

it functions to maintain integrity and mechanical stability

NO

It is absent in plant cells, as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose

Cholesterol is an amphipathic molecule (like phospholipids), meaning it has both hydrophilic and hydrophobic regions

Cholesterol’s hydroxyl (-OH) group is hydrophilic

aligns towards the phosphate heads of phospholipids

The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails

Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane

Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity

It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross

It functions to separate phospholipid tails and so prevent crystallisation of the membrane

It helps secure peripheral proteins by forming high-density lipid rafts capable of anchoring the protein

fluid-mosaic model

the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

phospholipids

cholesterol

proteins

The first model that attempted to describe the position of proteins within the bilayer was proposed by Hugh Davson and James Danielli in 1935

When viewed under a transmission electron microscope, membranes exhibit a characteristic 'trilaminar’ appearance

Trilaminar = 3 layers (two dark outer layers and a lighter inner region)

Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer

The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers

It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio

It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)

It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)

The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size

Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane

fluorescent antibody-tagging

| 2. freeze fracturing

Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place

Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively

When the two cells were fused, the markers became mixed throughout the membrane of the fused cell

This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)

Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane

These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure

In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972

called the fluid-mosaic model

According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers