Membrane Structure and Function: Phospholipids, Proteins, and Fluidity

Membrane structure

Phospholipid bilayer that acts as a selective barrier

Phospholipid

Amphipathic lipid with a hydrophilic head and hydrophobic tails

Amphipathic

Contains both hydrophilic and hydrophobic regions

Fluid mosaic model

Membrane is a fluid mixture of lipids, proteins, and carbohydrates that move within the bilayer

Most common phospholipid

Phosphatidylcholine

Membrane fluidity

The ease with which lipid molecules move within the bilayer

What increases membrane fluidity

Shorter fatty acid tails and more double bonds (unsaturated lipids)

What decreases membrane fluidity

Longer fatty acid tails and more saturated lipids

Effect of double bonds in fatty acids

Create kinks that prevent tight packing and increase fluidity

Temperature effect on membrane fluidity

Cold makes membranes rigid, heat makes membranes more fluid

How bacteria maintain membrane fluidity

Adjust hydrocarbon tail length and saturation

Cholesterol function in animal membranes

Regulates membrane fluidity and reduces permeability

Where new phospholipids are made

Cytosolic side of the ER membrane

Why scramblase is needed

Redistributes phospholipids between both layers of the ER membrane

Flip-flop movement

Phospholipid movement from one bilayer leaflet to the other

Lateral diffusion

Movement of phospholipids within the same layer of the membrane

Flippase function

Moves specific phospholipids between membrane layers in the Golgi

Membrane asymmetry

Two sides of membrane have different lipid and protein composition

Noncytosolic side

Faces Golgi lumen or extracellular space

Where membrane carbohydrates are located

Noncytosolic side of the membrane

Glycolipid

Lipid with a carbohydrate attached

Membrane protein main role

Perform most membrane functions such as transport and signaling

Integral membrane proteins

Proteins embedded in the membrane and removed only with detergents

Peripheral membrane proteins

Proteins loosely attached and removable by gentle extraction

Transmembrane proteins

Integral proteins that span the entire lipid bilayer

Alpha helix in membranes

Common structure used by proteins to cross membranes

Beta barrel

A cylindrical structure formed by beta sheets that creates membrane pores

Porins

Beta barrel proteins forming channels in membranes

Detergents

Disrupt lipid bilayers and solubilize membrane proteins

Cell cortex

Network of proteins under the plasma membrane that supports cell shape

Spectrin

Main protein component of the red blood cell cortex

Spectrin tetramer

Linked spectrin dimers that form part of the membrane support mesh

Actin role in cortex

Forms junction points connecting spectrin filaments

Purpose of cell cortex

Strengthens plasma membrane and maintains cell shape

Effect of spectrin mutation

Fragile spherical red blood cells and anemia

Diffusion barriers

Structures that restrict membrane protein movement

Example of diffusion barrier

Tight junctions

Glycocalyx

Carbohydrate coat on the outer surface of cells

Function of glycocalyx

Protection, lubrication, and cell recognition

Cell-cell recognition example

Immune cells recognize carbohydrates on infected cells

Why phospholipids form bilayers

Hydrophobic tails avoid water while hydrophilic heads interact with water