Molecular Structures WIP

What is the fluid-mosaic membrane model

• Describes molecular organisation of lipids and proteins in membranes

• Membrane = phospholipid bilayer with embedded or associated proteins

• Both lipids and proteins can diffuse laterally within the bilayer plane

• Lipid-anchored proteins inserted into the bilayer

What does amphipathic mean, why is it important for bilayer formation

• Amphipathic = molecule has both a hydrophilic head and hydrophobic tail

• In water, heads orient outward to maximise hydrogen bonding with water (hydrophilic effect)

• Tails cluster inward to minimise contact with water (hydrophobic effect)

• The bilayer is the minimum-energy self-assembly structure for most membrane lipids

What are peripheral membrane proteins

• Water-soluble proteins that do not penetrate the lipid bilayer

• Bind to integral proteins or lipid head groups via salt bridges, electrostatic interactions, or hydrogen bonds

• Can be removed by mild washing with salt or acid solutions

• Some link membranes to cytoskeletal networks

• Cell signalling , Structure , Act as enzymes

What are integral membrane proteins

• Amphipathic proteins that are embedded within the hydrophobic bilayer interior

• Insoluble in water — require detergents or organic solvents for removal

• May span the bilayer fully (transmembrane) or be partly embedded

• Perform functions such as transport, signalling, and enzymatic processing

How do GPI-anchored proteins associate with the membranes

• Anchored to the extracellular face via a glycosylphosphatidylinositol (GPI) lipid anchor

• Initially synthesised as integral proteins before the transmembrane domain (specific segment of an integral protein that spans the lipid bilayer of a cell membrane) is enzymatically cleaved in the ER

• Arabinogalactan proteins (AGPs) are a key example in plants

• Can be released from the cell surface by phospholipase C

What are lipid rafts

• Membrane microdomains enriched in phytosterols and saturated lipids

• More ordered and less fluid than the surrounding bilayer

• Characteristically associated with GPI-anchored proteins

• Proposed roles in membrane sorting and transmembrane signalling

• Defined biochemically by resistance to certain detergents at low temperature