Lecture 11 - Biological Membranes - Review

Membranes are

- 8nm thick

- dynamic (remodel constantly)

- fluid mosaic

- differ in composition across organelles

- asymmetrical (outer/inner layer differ in composition)

Fluid mosaic

A description of membrane structure, depicting a cellular membrane as a mosaic of diverse protein molecules embedded in a fluid bilayer made of phospholipid molecules.

Lipid composition of membranes _____

Vary (differ in lipid composition)

Long saturated FA allows for ______

Tighter packing

Unsaturated FA make membrane less ________ and more _______

dense (tight), fluid

A eukaryotic cell contains:

-60% phospholipids

- 10% sphingolipids

- 0.1-40% sterol lipids (20-50% in animals)

Lipid composition varies not only through plasma membranes but also among

Organelles

Phospholipids

PC- phosphatidylcholine

PE- Phosphatidylethanolamine

PI- phosphatidylinositol

PS- phosphatidylserine

PA- phosphatidic acid

Lipid composition varies as a function of physiological state during ______ in cell culture

growth

TAG lipids (triaglycerols)

Increase as culture reaches stationary phase and are important to sustain viability during starvation and and serve as precursors for fatty acids synthesis

The same phospholipid can vary in _______ and _________

Length, saturation

Cholesterol

essential component of biological membranes

- 90% of cholesterol in plasma membrane

- make 20-50% of lipids in plasma membrane

Cholesterol preferentially associated with

Saturated fatty acids

Amphiphatic molecule (cholesterol)

hydroxyl group aligns with the polar heads of phospholipids; four rings integrated within the fatty acid chains of phospholipids

Cholesterol functions as a...

Bidirectional regulator of membrane fluidity

At high temperatures....

Cholesterol stabilizes membranes by rising their melting point

At low temperatures, cholesterol...

Prevents the long fatty acid chains to cluster and make crystals that would make the membrane more rigid (below freezing point)

Eukaryotic genome for protein-coding gene coding for membrane proteins

25%

The ratio of proteins to lipids differ among...

different cells and cellular organelles

- mitochondria- 1:15

- plasma membrane- 1:25

- myelin- 1;70

Membrane and proteins mostly interact...

non-covalently

Integral proteins

-Span across the entire membrane

- can be deemed a transmembrane protein

- partially embedded in bilayer

- contain hydrophobic and hydrophilic regions

peripheral membrane proteins

-only on one side of the membrane (two sides of membrane differ in composition of peripheral proteins)

- lack surface hydrophobic groups

- have polar or charged regions that interact with exposed membrane proteins

- found on both sides of membrane

Peripheral proteins can be...

Anchored by glycolipid on ECM surface of membrane or attached to a membrane protein

Anchored membrane proteins.

Covalently bound to hydrophobic lipid components that allows them to tether in the phospholipid bilayer

Glycolipids, glycoproteins, and proteoglycans are only found...

On the outer side of membrane

Some membrane proteins move freely throughout the...

Cell membrane (think mouse and human heterokaryote cell)

Some membrane proteins are anchored at specific places and do not ______ move throughout the cell (attached to cytoskeleton)

freely

(Acetylcholine receptor- only at synaptic cleft side of membrane)

Glycolipids, glycoproteins, and proteoglycans all serve as

Recognition sites for other molecules

Glycolipid

A carbohydrate covalently bound to a lipid; recognition signal for interactions between cells

Glycoprotein

one or more short carbohydrate (oligosaccharides of 20 monosaccharides) chains covalently bound to a protein; cell recognition and adhesion

Proteoglycans

Contains numerous long polysaccharides; cell recognition and adhesion

Sponge reagregation

Cells adhere to other cell, eventually forming a colony of a new sponge

(Proteoglycans bind to eachother)

Homotypic binding

the same molecule sticks out of both cells and the exposed surfaces bind to each other

Heterotypic binding

the cells have different proteins that bind together

Cell junction

link animal cells together

Tight junctions

Proteins of tight junctions form a "quilted" seal, barring the movement of dissolved materials through the space between epithelial cells

Desmosomal adhesion

link adjacent cells tightly but permit materials to move around them in the intercellular space

Gap junctions

Let adjacent cells communicate

-connexins (6) = form connexon

- membrane lines channel