sign in sign up
<<
Home , Lipid bilayer, Membrane fluidity, Membrane transport, Phospholipid

A. Functions Biological are composed of… Membrane

Myelin Sheath 80% 20%

Plasma Membrane 50% 50%

Mitochondrial 25% 75% Inner Membrane

Fig. 5.12:

Hydrophilic head

2 Hydrophobic tails

Phospholipds are amphipathic (contain both hydrophilic and hydrophobic parts) Phospholipids form Membrane Bilayers

Bilayer consisting of two inverted layers (leaflets)

Hydrophobic ~30 Å ~45 Å Interior (3 nm) (4.5 nm)

Hydrophobic interior is an impermeable barrier to passage of hydrophilic molecules, but not to hydrophobic molecules has profound effects on Fig 7.8: Membrane Fluidity

(a) Phospholipid molecules move side-to-side within leaflet easily (lateral ) but do not “flip-flop” across bilayer (transverse diffusion)

(b) Phospholipids containing unsaturated acyl chains increase membrane fluidity by reducing packing efficiency

(c) Cholesterol reduces membrane fluidity at normal temperatures (reduces phospholipid movement) At low temperatures it keeps membrane fluid (disrupts packing) Membrane can Move Laterally Within the

Membrane proteins labeled with different color fluorescent dyes

Supports fluid-mosaic model of a dynamic membrane structure Three Types of Membrane Proteins

1. Integral membrane proteins (transmembrane proteins) Extracellular domain • span the bilayer

• has Transmembrane hydrophobic surface domain • cytosolic and extracellular Cytosolic domains have hydrophilic surfaces domain

2. Lipid-anchored membrane proteins - anchored via a covalently attached lipid

3. Peripheral membrane proteins - interact with hydrophilic lipid head groups or with integral membrane proteins How do proteins cross lipid bilayer membranes?

δ-

δ+ δ-

δ+

Even if the R-groups are hydrophobic, the peptide bond atoms are hydrophilic (polar) and will want to form Hydrogen Bonds; there are no H-bond donors or acceptors in the middle of a lipid bilayer. Fig 7.9: α-Helices Are Commonly Found in Membrane Proteins

EXTRACELLULAR SIDE Polar peptide bond atoms N-terminus H-bond with each other.

α-helix of 20 amino is long enough to α helix cross the bilayer. C-terminus

CYTOPLASMIC SIDE Fig 7.12: Membrane Synthesis & Sidedness Fig. 7.9: Functions of Membrane Proteins Signal

Receptor ATP

Transport Enzymatic activity

Glyco- protein

Cell- recognition Intercellular joining Attachment to the cytoskeleton and extra-cellular matrix (ECM) Fig 7.7: Overview of the Plasma Membrane

Fibers of extracellular matrix (ECM) bind to some membrane proteins

Glyco- protein EXTRACELLULAR SIDE OF MEMBRANE

Cholesterol

Microfilaments of cytoskeleton Peripheral linked to some membrane proteins proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Fig. 7.22:

Phagocytosis Receptor-Mediated Endocytosis

EXTRACELLULAR FLUID Solutes

Pseudopodium Receptor Plasma Ligand membrane Coat proteins

Coated “Food” or pit other particle Coated vesicle

Vesicle Food vacuole

CYTOPLASM Energetics of Diffusion

Why do molecules diffuse? A difference in contains chemical potential energy. Molecules diffuse to try to equalize .

[A] [A]

[A] [A] Fig. 7.13: Diffusion of Solutes Across A Membrane

Membrane (cross Molecules of dye section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion Net diffusion Equilibrium

Net diffusion Net diffusion Equilibrium

(b) Diffusion of two solutes Movement

If a membrane is permeable to water but impermeable to a solute with different concentrations in two compartments, water will move to try to equalize the concentrations on the two sides of the membrane.

Membrane permeable to water but impermeable to solute Fig. 7.15: Water Balance of Living Cells

Hypotonic Isotonic Hypertonic solution solution (a) Animal cell

H2O H2O H2O H2O

Lysed Normal Shriveled H O H O H O H2O (b) Plant cell 2 2 2

Turgid (normal) Flaccid Plasmolyzed

Osmosis