Becker's World of the Cell - Chapter 7 - Membranes

Fluid Mosaic Model

Model for membrane structure consisting of a lipid bilayer with proteins associated as discrete globular entities that penetrate the lipid bilayer to different degrees

What are the five functions of the cell membrane?

1. Boundary and permeability barrier

2. Organization and localization of function

3. Transport processes (regulation)

4. Signal detection (not a maain focus)

5. Cell-to-cell interactions (communications)

Permeability barrier

- hydrophobic membrane

- non polar

Why can't these molecules pass through the permeability barrier without assistance?

Hydrophillic molecule

Charged ions

Large molecules

Phospholipids

Lipid possessing a covalently attached phosphate group and therefore exhibiting amphipathic properties

Lipid Bilayer

Unit of membrane structure consisting of two layers of lipids arranged such that their tails face toward each other and the polar region of their heads face toward the aqueous environment

What is signal detection?

Membrane proteins allow for detection of extracellar signals

What is signal transduction?

External signal to internal response

Phosphoglycerides

Predominant phospholipid component of cell membranes consisting of a glycerol molecule esterified to two fatty acids and a phosphate group

Phosphosphingolipids

Amphipathic, phosphate containing sphingolipid that is a major phospholipid in some membranes

Glycolipids

Lipid molecule containing a bound carbohydrate group

Cerebrosides

Uncharged glycolipids containing the amino alcohol sphingosine

Gangliosides

Charged glycolipid containing the amino alcohol sphingosine and negatively charged silica acid residues

Sterols

Compounds consisting of a 17 carbon 4 ring system with at least one hydroxyl group and a variety of other possible side groups

- Includes Cholesterol

- Includes Sex Hormones

Stabilizes membranes

How are membrane proteins analyzed with TLC

Thinlayer chromatography (TLC)

- Glass plate coated with silicic acid

- Non-polar organic solvent

• Separation occurs due to polarity

Pigmens at top are less polar than ones at bottom

How are ABO blood types determined?

determined by glycosphingolipids

Cholesterol

Lipid constituent that serves as a precursor to steroid hormones

Acts a fluidity buffer

Decreases fluidity above the Tm

Increasing fluidity below the Tm

Also decreases membrane permeability

Saturated Fatty Acids

Fatty acid without double or triple bonds; every carbon on the chain has the maximum amount of hydrogens bonded to it

Unsaturated Fatty Acids

Fatty acid containing one or more double bonds

Membrane Asymmetry

Property based on differences between the molecular compositions of the two lipid monolayers and the proteins associated with each

Transverse Diffusion

Thermodynamically unfavorable movement of one lipid molecule from one monolayer of a membrane to the other

Lateral Diffusion

Diffusion of a membrane lipid or protein in the plane of the membrane

Phospholipid Translocators

Membrane protein that catalyzes the transverse diffusion of phospholipids

Fluorescence Recovery After Photobleaching

Molecules in the membrane of a cell are tagged by covalently linking them to a fluorescent dye. A laser is then used to bleach the dye in a tiny spot. Bleached molecules diffuse out while fluorescent molecules diffuses in, and soon the spot is indistinguishable

Transition Temperature

Temperature at which a lipid bilayer becomes fluid

Lipid distribution in animal cell membrane

- Outer membrane - Glycolipids

- Inner membrane - PI, PE, PS

Infrequent transverse movement (3 types)

Lateral diffussion - movement of lipid on top of layer

Rotation

Transverese diffusion- (flip flop) switch from layers of phospholipids

Transition temperature (Tm)

- Temperature where membrane becomes fluid

- Membrane stops functioning below Tm

Phase Transition

The change of a lipid bilayer from a solid to a fluid

Lipid Rafts

Localized regions of membrane lipids characterized be elevated levels of cholesterol and glycosphingolipids that sequester proteins involved in cell signaling

Integral Membrane Proteins

Hydrophobic protein localized within the interior of a membrane but possessing hydrophilic regions that protrude from one or both membrane surfaces

- 20-30 amino acids long a-helicies

- b sheets arranged into a barrel

Transmembrane Proteins

Integral membrane protein possessing one or more transmembrane regions that span the membrane plus hydrophilic regions protruding from the membrane on both sides

Peripheral Membrane Proteins

Hydrophilic protein bound through weak ionic interactions and hydrogen bonds to a membrane surface

Lack a discrete hydrophobic region

Lipid Anchored Membrane Protein

Protein located on a membrane surface that is covalently bound to one or more lipid molecules within the lipid bilayer

Linked to Fatty acids, Isoprenyl groups

Fatty Acid Anchored Membrane Protein

Protein located on a membrane surface that is covalently bound to a fatty acid embedded within the lipid bilayer

- Attached to a saturated fatty acid

- Usually myristic acid (14C) or palmitic acid (16C)

Isoprenylated Membrane Protein

Protein located on a membrane surface that is covalently bound to a phenyl group embedded within the lipid bilayer

Synthesized in the cytosol and then modified by addition

of multiple isoprenyl groups (5C)

- Usually farnesyl (15C) or geranygeranyl (20C) groups

GPI Anchored Membrane Protein

A type of single pass transmembrane protein whose transmembrane segments were cleaved and replaced with GPI anchors

Covalently linked to glycosylphosphatidylinositol

Hydropathy Plot

Graph showing the location of hydrophobic amino acid clusters within the primary sequence of a protein

Hydropathy Index

Value representing the average of the hydrophobicity values for a short stretch of contiguous amino acids within a protein

Glycoprotein

Protein with one of more carbohydrate group covalently linked to amino acid side chains

Glycosylation

Addition of carbohydrate sure chains to specific residues of proteins

Describe localization of function

Each membrane has a unique combination of molecules embedded within and associated with it

- site of localized functions

- used for identification / purification

Membrane lipids

One or two galactose linked to glycerol backbone and two polyunsaturated fatty acids

Abundant glycolipids

Role of lipids raft

Transport of nutrients and ions across membranes

Activation of signaling cascades after ligand binding

Caveolae

invagination of plasma membrane

"Drawing in of membrane"

How does temperature and saturation relate to each other?

More saturate, less fluid Tm = 70 degrees

More unsaturated, more fluid Tm = 16 degrees

E face

Exoplasmic

Exterior side of cell membrane

P face

Internal side of plasma membrane

Protoplasmic