lecture 8- membrane structure

What are the major functions of biological membranes?

Form specialized compartments through selective permeability, maintain unique environments, enable cell-cell recognition, serve as receptor sites for signaling, organize reaction sequences, compartmentalize redox reactions

How do membranes create specialized compartments?

By selective permeability that controls the kind, concentration, and movement of molecules across the membrane

What membrane properties contribute to unique cellular environments?

Differences in molecule concentration, pH, charge, and protein composition

What is asymmetric protein distribution?

Different proteins are located on the two sides of a membrane, allowing specialized functions

How do membranes participate in signaling?

Receptor proteins bind ligands and trigger intracellular responses

Why are membranes important for reaction sequences?

Products of one enzyme can be passed directly to the next enzyme in a pathway

What did the Davson-Danielli model propose?

A lipid bilayer coated on both sides by protein, forming a protein-lipid-protein sandwich

Why was the Davson-Danielli model incorrect?

Proteins are embedded within the membrane rather than only coating its surfaces

What does the Fluid Mosaic Model state?

Membranes are fluid phospholipid bilayers containing a mosaic of proteins that can move laterally

Who proposed the Fluid Mosaic Model?

Singer and Nicolson

What are the two major classes of membrane proteins in the Fluid Mosaic Model?

Integral proteins and peripheral proteins

What does amphipathic mean?

Having both hydrophilic and hydrophobic regions

Why are phospholipids amphipathic?

They contain a hydrophilic phosphate head and hydrophobic fatty acid tails

Why do phospholipids spontaneously form bilayers in water?

Hydrophobic tails avoid water while hydrophilic heads interact with water

What forms the hydrophobic core of a membrane?

The fatty acid tails of phospholipids

Why do membranes spontaneously reseal after damage?

Exposed hydrophobic regions are energetically unfavorable

Why do membranes never have free edges?

Exposed hydrophobic tails are unstable in water

What is the most abundant membrane lipid?

Phospholipid

What groups can be attached to phospholipid phosphate groups?

Choline, ethanolamine, serine, and inositol

How do additional phosphate head groups affect phospholipids?

They increase hydrophilicity

What are sphingolipids derived from?

Ceramides

What is a ceramide?

An amino alcohol attached to fatty acid chains

What percentage of the animal plasma membrane can be cholesterol?

Up to about 50%

How is cholesterol oriented in the membrane?

Its hydroxyl group faces the membrane surface while the steroid rings interact with fatty acid tails

What effect does cholesterol have on membrane permeability?

It decreases permeability

What effect does cholesterol have on membrane fluidity?

Generally decreases fluidity by filling spaces between phospholipids

What kinds of movement can membrane lipids undergo?

Lateral diffusion, rotation, flexing, and rare flip-flop movement

Which lipid movement occurs rapidly?

Lateral movement within the same leaflet

Which lipid movement occurs extremely slowly?

Flip-flop between membrane leaflets

What determines membrane fluidity?

Lipid composition, fatty acid saturation, chain length, cholesterol content, and temperature

How do unsaturated fatty acids affect membrane fluidity?

They increase fluidity

Why do unsaturated fatty acids increase fluidity?

Cis double bonds create bends that prevent tight packing

How do saturated fatty acids affect membrane fluidity?

They decrease fluidity

How does fatty acid chain length affect fluidity?

Shorter chains increase fluidity while longer chains decrease fluidity

How does cholesterol affect membrane fluidity?

It restricts phospholipid movement and decreases permeability

How does temperature affect membrane fluidity?

Lower temperature decreases fluidity while higher temperature increases fluidity

Why is membrane fluidity important?

Enables protein diffusion, receptor clustering, membrane growth, signaling, and membrane fusion

How do cells adapt membranes to cold temperatures?

Increase unsaturated fatty acids and shorten fatty acid chains

How do cells increase membrane fluidity in cold environments?

Desaturate fatty acids and produce shorter fatty acid chains

How do cells adapt membranes to high temperatures?

Increase saturated fatty acids and longer fatty acid chains

Why must cells regulate membrane fluidity?

Membranes must remain functional despite temperature changes

What is membrane asymmetry?

The two sides of a membrane have different lipid and protein compositions

Where are phospholipids synthesized?

On the cytoplasmic side of the smooth ER

What enzyme moves lipids between membrane leaflets?

Flippase

What is the function of flippases?

Transfer specific lipids from one leaflet to the other

Why are glycolipids found on the extracellular side of the plasma membrane?

Sugars are added in the Golgi lumen which becomes the extracellular surface after vesicle fusion

Why does the outside layer of a membrane never become the inside layer during vesicle transport?

Membrane orientation is preserved during budding and fusion

What does FRAP stand for?

Fluorescence Recovery After Photobleaching

What is the purpose of FRAP?

Measure membrane protein mobility and diffusion

How is FRAP performed?

Fluorescently label proteins, bleach an area with a laser, and monitor fluorescence recovery

What does rapid fluorescence recovery indicate?

High protein mobility within the membrane

What does little or no recovery indicate?

Proteins are immobilized or diffuse slowly

What is an integral membrane protein?

A protein embedded within the lipid bilayer

What is a transmembrane protein?

An integral protein that spans the entire membrane

What is the most common transmembrane structure?

Alpha helix

What membrane proteins commonly form channels?

Transmembrane proteins

What is a beta barrel?

A cylindrical membrane-spanning structure composed of beta sheets

What is a porin?

A beta-barrel protein that forms aqueous channels

What is a peripheral membrane protein?

A protein loosely attached to the membrane surface through ionic or hydrogen-bond interactions

What is a lipid-linked protein?

A protein covalently attached to a membrane lipid anchor

What is a monolayer-associated protein?

A protein embedded in only one leaflet of the bilayer

How can membrane proteins move?

By lateral diffusion within the membrane plane

How are integral membrane proteins experimentally removed from membranes?

By detergents that disrupt the lipid bilayer

How are peripheral membrane proteins removed?

High salt or alkaline pH disrupts surface interactions

Why do integral proteins require detergents for extraction?

They interact directly with the hydrophobic membrane interior

Why can detergents dissolve membrane proteins?

They are amphipathic and surround hydrophobic membrane regions

What is a micelle?

A spherical structure formed by detergent molecules with hydrophobic interiors

What is SDS?

Sodium dodecyl sulfate, an ionic detergent

What effect does SDS have on proteins?

Denatures proteins and gives them a uniform negative charge

What is Triton X-100?

A nonionic detergent that generally does not denature proteins

What is the difference between ionic and nonionic detergents?

Ionic detergents denature proteins while nonionic detergents usually preserve structure

What is the purpose of protease protection experiments?

Determine which protein domains face the cytoplasm or extracellular space

Why does protease protection work?

Proteases cannot cross intact membranes

What happens if intact cells are treated with protease?

Extracellular domains are digested

What happens if detergent is added before protease treatment?

All protein regions become accessible and are digested

What happens if membranes are permeabilized before protease treatment?

Cytoplasmic domains become accessible

What does SDS-PAGE stand for?

Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis

What is the purpose of SDS-PAGE?

Separate proteins based on size

Why does SDS allow size-based separation?

It gives proteins a nearly identical charge-to-mass ratio

How do large proteins migrate in SDS-PAGE compared to small proteins?

Large proteins move more slowly

Why is a reducing agent often added in SDS-PAGE?

To break disulfide bonds

Where are membrane carbohydrates located?

On the noncytoplasmic extracellular surface

What is a glycoprotein?

A protein with attached oligosaccharides

What is a glycolipid?

A lipid with attached oligosaccharides

What is a proteoglycan?

A protein with long unbranched polysaccharide chains attached

What amino acid receives N-linked glycosylation?

Asparagine

Where does N-linked glycosylation begin?

Endoplasmic reticulum

What amino acids receive O-linked glycosylation?

Serine and threonine

Where does O-linked glycosylation occur?

Golgi apparatus

What sequence motif is required for N-linked glycosylation?

Asn-X-Ser or Asn-X-Thr

What is the most common post-translational modification?

Glycosylation

What determines ABO blood groups?

Specific carbohydrate structures attached to cell-surface molecules

What distinguishes Type A blood from Type O?

Addition of N-acetylgalactosamine

What distinguishes Type B blood from Type O?

Addition of galactose

What is Type O blood missing?

The final sugar added by A or B transferases

What are the major functions of membrane carbohydrates?

Hydration, protection, cell recognition, signaling, and adhesion

How do membrane carbohydrates create slimy surfaces?

They absorb water

How do membrane carbohydrates contribute to cell recognition?

Specific carbohydrate structures act as molecular identifiers

What are lectins?

Proteins that bind specific carbohydrate structures

How do lectins help fight infection?

They bind neutrophils and promote migration into infected tissues