A. Membrane Functions

Biological Membranes are composed of…

Membrane Lipid Protein

Myelin Sheath 80% 20%

Plasma Membrane 50% 50%

Mitochondrial Inner Membrane

25% 75%

Fig. 5.12: Phospholipids

2

Phospholipds are amphipathic molecules (contain both hydrophilic and hydrophobic parts)

Hydrophilic head

Hydrophobic tails

Hydrophobic interior is an impermeable barrier to passage of hydrophilic molecules, but not to hydrophobic molecules

~30 Å (3 nm)

~45 Å (4.5 nm)

Hydrophobic Interior

Phospholipids form Membrane Bilayers

Bilayer consisting of two inverted phospholipid

layers (leaflets)

Cholesterol has profound effects on membrane fluidity

Fig 7.8: Membrane Fluidity

(a) Phospholipid molecules move side-to-side within leaflet easily

(lateral diffusion) 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 Proteins can Move Laterally Within the Lipid Bilayer

Supports fluid-mosaic model of a dynamic membrane

structure

Membrane proteins labeled with

different color fluorescent dyes

Three Types of Membrane Proteins

Transmembrane domain

1. Integral membrane proteins (transmembrane proteins)

•  span the bilayer •  transmembrane domain has

hydrophobic surface •  cytosolic and extracellular

domains have hydrophilic surfaces

Extracellular domain

Cytosolic 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

δ-

δ+ δ-

δ+

How do proteins cross lipid bilayer membranes?

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

Polar peptide bond atoms H-bond with each other.

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

N-terminus

α helix

C-terminus

EXTRACELLULAR SIDE

CYTOPLASMIC SIDE

Fig 7.12: Membrane Synthesis & Sidedness

Enzymes Signal

Receptor ATP

Transport Enzymatic activity Signal transduction

Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extra-cellular matrix (ECM)

Glyco- protein

Fig. 7.9: Functions of Membrane Proteins

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

Glyco- protein

Microfilaments of cytoskeleton linked to some membrane proteins

Cholesterol

Peripheral proteins

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

Glycolipid EXTRACELLULAR SIDE OF MEMBRANE

Carbohydrate

Fig 7.7: Overview of the Plasma Membrane

Fig. 7.22: Endocytosis

Solutes

Pseudopodium

“Food” or���other particle

Food ���vacuole

CYTOPLASM

Plasma���membrane

Vesicle

Receptor Ligand

Coat proteins

Coated ���pit

Coated ���vesicle

EXTRACELLULAR ���FLUID

Phagocytosis Pinocytosis Receptor-Mediated Endocytosis

Membrane Transport

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

try to equalize concentrations.

Energetics of Diffusion

[A] [A]

[A] [A]

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

Fig. 7.13: Diffusion of Solutes Across A Membrane

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.

Water Movement

Membrane permeable to water but impermeable to solute

Fig. 7.15: Water Balance of Living Cells

Hypotonic solution

Osmosis

Isotonic solution

Hypertonic solution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2O Cell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed