Transcript Membrane Structure & Function

Chapter 7 ~
Membrane
Structure &
Function
Membrane structure, I
 Selective permeability – only
some substances can cross
 Amphipathic – has both
hydrophobic & hydrophilic
regions
 Singer-Nicolson: fluid mosaic
model
 Fluid structure w/ various
proteins embedded
Membrane structure, II
 Phospholipids – membrane fluidity

Lipids move along membrane lateral

May move in/out – rare – flipflop

Unsaturated – kinked tails –
increase fluidity
 Cholesterol – membrane stabilization
 “Mosaic” Structure – collection of
different proteins – unique to each
cell
 Integral proteins – transmembrane
proteins – cross entire membrane
 Peripheral proteins – surface of
membrane
 Membrane carbohydrates – cell to
cell recognition

Oligosaccharides (cell markers);
glycolipids; glycoproteins

Immune system, blood types
Membrane structure, III
 Membrane protein function:
 Transport – channel,
shuttle
 Enzymatic activity –
active site exposed
 Signal transduction
(receptor) – binds to chem
messenger – sends
message inside
 Intercellular joining –
gap/tight junctions
 Cell-cell recognition
 ECM attachment –
maintain cell shape and
stability
Membrane traffic
Passive transport~
diffusion of a substance
across a biological
membrane – no energy
Diffusion~ tendency of
any molecule to spread out
into available space

Moves down its
concentration gradient
Osmosis~ the diffusion of
water across a selectively
permeable membrane
Water balance
 Osmoregulation – control
of water balance
 Hypertonic – higher
concentration of solutes – cell
is hypotonic – cell shrinks
 Hypotonic – lower
concentration of solutes – cell
is hypertonic – cell swells
 Isotonic – equal
concentrations of solutes
 Cells with Walls:
 Turgid (very firm) - hypo
 Flaccid (limp) - iso
 Plasmolysis – plasma
membrane pulls away from
cell wall - hyper
Water Balance cont.
 Water potential (Ψ) - movement of water from an area of high concen. to low
concen.

Ψ = ΨP + ΨS

Water Potential = Pressure Potential + Solute Potential

At atmospheric pressure - Ψp = 0

Water potential of pure water in an open beaker is zero (ψ = 0)

Water will move from an area of high Ψ to an area of low Ψ
 Tonic refers to solute concen., potential refers to water concen.
 The Solute Potential of the Solution

Ψs = – iCRT

i = ionization constant (For sucrose this is 1.0 because sucrose does not
ionize in water.)

C = molar concentration

R = pressure constant (R = 0.0831 liter bars/mole K)

T = temperature in Kelvin (273 + °C)
Specialized Transport
 Facilitated diffusion~
passage of molecules and
ions with transport proteins
across a membrane down the
concentration gradient
 Channel proteins: ion
channels, aquaporins,
gated proteins, carrier
proteins
 Active transport~ movement
of a substance against its
concentration gradient with
the help of cellular energy
Types of Active Transport
 Sodium-potassium pump –
electrochemical gradient – nerve
cells
 Cotransport – solute pumped out
can do work as it diffuses back
 Exocytosis~ secretion of
macromolecules by the fusion of
transport vesicles with the plasma
membrane
 Endocytosis~ import of
macromolecules by forming new
vesicles with the plasma
membrane



Phagocytosis
Pinocytosis
Receptor-mediated
• Ligands – bind to receptor