Membrane Structure and Transport

Animal Specific structures

Extracellular matrix and lysosomes

Plant Specific Structures

Cell wall (Creates shape and protects against mechanical stress)

Vacuoles (2 types, degradation and storage)

Chloroplast (Site of photosynthesis)

Cytoplasm vs Cytosol vs Lumen

Cytoplasm - Contents of the cell outside nucleus

Cytosol - Just aqueous part of cytoplasm (no organelles)

Lumen - Area inside organelles

Structure of Lipid in lipid bilayer

Hydrophilic head (polar) and hydrophobic tail

“amphipathic” - Both hydro philic + phobic parts

Phospholipid

Membrane lipid with phosphate group in polar head

Composes the bilayer (Spontaneously self-associate

Energetic favourability

Phospholipid bilayers are in a more favourable state when a sealed compartment is created (No area where hydrophobic tail is interacting with water)

Use of Liposomes (3)

Study lipid properties, membrane protein properties, drug delivery (They are artificial vesicles made of a phospholipid bilayer)

Phospholipid movement 3

Phospholipids can:

diffuse laterally, rotate, and flex

They rarely:

mofe from one leaflet to other

Leaflet

One half of the lipid bilayer (One side,inner or outer, so to speak)

Factors than can affect membrane fluidity (4) TSTC

Temperature - lower temp, less fluid

Composition:

saturation - cis-double bonds increase fluidity at lower temp since the packing is less tight

tail length - Shorter tails means more fluid (Less interaction)

Lipid composition - addition of cholesterol stiffens membrane

Sterols (3)

Up to 1-to-1 ratio of cholestoral and phospholipid

decrease mobility of tails

membrane becomes less permeable to polar molecules

(Does this because it makes packing tighter - fits between phospholipids)

Lipid movement between leaflets

Enzymes such as scramblase catalyzes random flip-flop of phospholipids from one leaflet to the other

occurs because phospholipids are synthesized in the cytosolic leaflet, so to preserve symmetric growth they need to be shared randomly

Membrane orientation during transfer

membranes retain their orientation during transfer between cell compartments - cytosolic side stays facing cytosol, and the lumen side stays facing lumen

Lipid distribution

lipids are asymmetrically distributed across the membrane - example is glycolipids which are located mainly in the plasma membrane and only on the noncytosolic side to act as protection for the cell

Flippase

Flippases catalyze the flip-flop of specific phospholipid to the correct leaflet

Some can bind cytoslic proteins

Glycolipid synthesis

Formed by adding sugar to lipid/protein on luminal face of golgi

Ways proteins are attached directly to bilayer

1. Inserted in lipid bilayer (Transmembrane/monolayer-associated)

2. Attached to a lipid which is inserted (Lipid linked)

Peripheral membrane protein attachment 3

1. Bound to other proteins

2. Bound to lipids

3. Bound by non-covalent interactions

Extraction of membrane proteins

If lipid is inserted - use detergent that destroys membrane

If lipid is peripheral - gentle extraction that keeps bilayer intact)

Transmembrane proteins are… (a)

amphipathic - have both polar and nonpolar domains

3 examples of membrane spanning domains

1. Single alpha helix -

2. multiple alpha helices - hydrophobic side chains with hydrophilic side chains in the middle that form a pore

3. Beta barrel - Rigid channel

4 examples of membrane protein functions

1. Transporter/channels

2. Anchors

3. Receptors

4. enzymes

Ways to identify membrane structures

1. x-ray crystallography for 3D structure

2. Hydrophobicity plots - determine hydrophobic or philic regions

Monolayer-associated membrane proteins

• Anchored on cytosolic of bilayer by an amphipathic alpha helix

Lipid-linked membrane proteins (2 versions)

• One has GPI anchor that is synthesized in the ER lumen and ends up on noncytosolic face

• Other also has lipid anchor, cytosilic enzymes add it and directs protein to cytosolic face

Lateral Diffusion

• No flip fop

• Fleuroescence Recovery after photobleaching (FRAP) - allows the tracking of proteins across the membrane

• Can be used to measure rate of lateral diffusion (Track how fast bleached patch is refluoresced)