BCM - Biological membranes

Main function of biological membranes

• Compartmentation - plasma membrane, organelles (e.g. ER and Golgi apparatus; not all organelles have membranes)

• Regulation of transport - uptake of nutrients and excretion of waste (channels and transporters, endocytosis), vesicular transport within and out of the cells (secretory pathway)

• Energy conversion - mitochondria (outer and inner membrane, H+ gradient across inner membrane), chloroplasts (outer and inner membrane, H+ gradient across thylakoid membrane)

• Detection and transmission signals - cell surface receptors (e.g. hormones and neurotransmitters), nerve impulses (depend upon asymmetric ion distribution across the membrane)

Mitochondral ATP synthase

• energy derived from breakdown of food is used to pump protons from the matrix into the inter membrane space, generating an electrochemical gradient

• protons flowing from the inter membrane space back into the matrix drive rotation of the C-ring and axle of the ATP synthase

• the stator holds the 3 catalytic subunits in place - the rotating axle changes the structure of the catalytic subunits, leading to the ATP formation

Plasma membrane of an animal cell

Lipids that form biological membranes are amphipathic molecules

• Amphiphilic moelcules are also referred to as amphipathic molecules - the two terms are synonymous depending on -philic or -pathic

A typical detergent (only one tail)

• Sodium dodecyl sulphate (SDS) is a common ingredient of shower gels and shampoos (often labelled sodium lauryl sulphate) 

• in biochemical research, SDS is used to denature proteins for gel electrophoresis 

Hydrophilic and hydrophobic molecules interact differently with water

Amphipathic molecules in water form micelles and bilayers

Liposomes

Electron microscopy - freeze fracture reveals embedded proteins

Bilayer dynamics

Lateral diffusion:

• 2D fluid - diffusion speed ~2 micrometers/s

Transverse diffusion:

• large energy barrier because the head group must cross the hydrophobic interior of the bilayer 

• catalysed by 2 classes of enzymes - lipid scramblases (bidirectional and ATP-independent), lipid flippases (unidirectional and ATP-dependent)

Loss of fluidity below the transition temperature

• below the transition the bilayer is no longer liquid but adopt a more ordered gel phase - a type of solid state

• the transition temperature depends on the chemical structure of the lipids

Experimental evidence for lipid bilayer fluidity

• a fluorescent molecule is added to the plasma membrane

• an intense laser beam is used to ‘bleach’ a membrane spot - destroys the fluorphore

• the recovery of fluorescence in the bleached spot is due to diffusion of intact fluorescent molecules into the area - Fluorescence Recovery After Photobleaching (FRAP)

Experimental evidence for rapid lateral diffusion of proteins

• in this famous experiment, the fusion of mouse and human cells was induced by Sendai virus

• the mouse and human proteins were distinguished by labelling them with different fluorescent antibodies

Lipid rafts

• lipid rafts are micro domains within the lipid bilayer that have a characteristic glycolipid and protein composition, they are believed to float freely in the bilayer, like a raft on water 

• subject of lipid rafts is still hotly debated - everyone agrees the plasma membrane is not uniform though 

Hydropathy plot helps identify transmembrane helices

• because transmembrane helices typically contain only apolar amino acid residues, they can be predicted from the proteins sequence

• a hydropathy plot of the rhodopsin amino acid sequence identifies the 7 membrane helices

Permeability of lipid bilayer