Membrane Strcuture

Description of the cell membrane

The film of lipid and protein molecules held together by noncovalent interactions

What does the cell membrane serve as

Relatively imperameable barrier to water soluble molecules

What is the breakdown for the structure of cell membranes

50% of animal cell membranes are lipid

Most of the rest is protein

Amphiphilic

Hydrophilic polar and hydrophobic nonpolar ends

Most abundant membrane lipids

Phospholipids with two hydrophobic hydrocarbon tails (fatty tails)

Main phospholipids

Phosphoglycerides

Backbone for common phospholipids

3-Carbon glycerol

Most common phospholipids

• phosphatidylethanolamine

• Phosphatidylserine

• Phosphatidylcholine

Most common sphingolipid

Sphingomyelin

Difference from phospholipids and sphingolipid

Built from sphingosine, not glycerol

What is also present in the belayer

Cholesterol and glycolipids

What does cholesterol serve for

To increase impermeability of the lipid bilayer and prevent hydrocarbon tails from crystallizing

Orientation of cholesterol

Hydroxyl polar head close to polar heads of phospholipids

What does it mean that lipid molecules in membranes are disoriented

Spacing between them is irregular and flexible

Why is fluidity of membrane important

Must be maintained for transport and enzymes to work

Why does the lipid bilayer have bent tails

To make chains harder to pack together and bilayer more difficult to freeze, maintain functionality at lower temps

What is the controversy about lipid rafts

Most likely formed transiently only when needed for protein transport or signal conversion

Where are excess cellular lipids stored

Lipid droplets

What surrounds lipid droplets

Phospholipids monolayer

What do lipid droplets contain

Neutral lipids like triglycerides and cholesterol esters

Where are droplets formed

In ER membrane

What are adipocytes

One big fat droplet

What does asymmetrical lipid composition of monolayers cause

Makes charges between the two halves of the bilayer different

Why is asymmetry of lipid bilayer important

For converting Extracellular signals into intracellular ones; also marking dying cells

What are glycolipids

Sugar-containing lipid molecules

Where are glycolipids found

Exclusively in noncytosolic side of lipid bilayer

Where are sugars added to lipid molecules

In lumen of Golgi apparatus

Function of glycolipids

Help cell interact with its surrounding, protect from harsh surroundings, affect electrical field of membrane and ion concentration

Gangliosides

Complex glycolipids found mostly in nerve cells

Three kinds of membrane proteins

• trans membrane

• Entirely cytosolic but associated to the cytosol side of bilayer

• Entirely on cell curface but attached to lipid bilayer

GPI anchor

Special membrane proteins that are actually Extracellular

Where are GPI molecules made

In ER of cell as single-pass initially

When is the GPI anchor added

After the transmembrane segment is cleaved

Peripheral membrane proteins

Bound via some noncovalent interaction with other membrane proteins

Transmembrane proteins

Held in the hydrophobic core of the lipid bilayer and cannot be released easily

How do transmembrane proteins work

Function on both sides of the bilayer or transport molecules across it

What do the membrane-spanning domains of transmembrane proteins contact

Hydrophobic part of lipid bilayer

Why do transmembrane proteins turn into alpha-helix as it cross bilayer

Every peptide bond is polar and no water is present in bilayer, aa form hydrogen bonds with each other

Single-pass membrane proteins

Cross lipid bilayer only once

Multi-pass membrane protein

Cross lipid bilayer multiple times

What can multi-pass proteins form

Beta sheets which form into a barrel which is a protein to satisfy hydrogen bonding requirements

What are multi-pass proteins also called

Porin proteins

Where do porin proteins most often occur

In bacterial membranes and mitochondrial membranes

What are most multi-pass proteins in eukaryotic cells constructed from

Alpha helices that can move, transport, transduce

What can beta barrels also form into

Channels

What can some beta barrels have

Amino acid loops that project into the center, filling the hole, may function as receptors or enzymes

What can multiple transmembrane helices arrange to create

Channels through the membrane

What do hydropathy plots do

Illustrate the number of amino acids of a transmembrane protein located within a cell membranes lipid bilayer and how many membrane-spanning domains there are

Where are sugars added to proteins destined for some membrane

In the ER and Golgi

Where are sugars always present

On non cytosolic side of membrane

Where can you find carbs

On the surface of all eukaryotic cells covalently bound to membrane proteins and lipids

Lectins

Carbohydrate-binding proteins

What are lectins used for

Study carb coating and mediate cell-cell adhesion

Detergents

Small amphiphilic molecules of various structure

How do detergents and membrane proteins interact

Hydrophobic ends of detergents bind to hydrophobic regions of membrane proteins and bring membrane proteins into solution

What are detergent and membrane interactions good for studying

Membrane proteins characteristics and activities

So proteins swim through lipids

No

How are membrane proteins held in lace

Tight junctions

Function of tight junctions

Keeps proteins AND lipids in appropriate places for specific functionality

Protein-protein interactions that keep proteins in separate domains

• self-aggregation

• Tethering

• Interaction with other cells

What do nucleated cells contain

More complex network that makes up the cortical region of the cytoplasm

Cortex

Covering

Cortical cytoskeleton network is analogous to

Spectrum

Where can cytoskeletal filaments be attached to

Cytosolic membrane and form barriers to membrane protein movement

Corralling proteins

Corralling helps to concentrate activated signaling complexes so to increase signal