Download AP Biology - Membrane Structure

Membrane Structure
& Function
RAVEN & JOHNSON CHAPTER 5
CAMPBELL CHAPTER 8
Membrane Structure
 Lipids (phospholipids) & proteins
They
are amphipathic – both
hydrophobic & hydrophilic
 Fluid-mosaic model
(Singer &
Nicolson,
1972)
Fluidity
 Held by weak hydrophobic interactions
 Movement is lateral
Fluidity
 Membrane must be fluid to work
properly
 Cholesterol is wedged into
phospholipid tails – restrains
movement in warm temp (less fluid),
but also lowers solidifying temp
(more fluid)
Fluidity
Mosaic
 Embedded proteins (over 50 types)
determine specific function of the
membrane
 Integral proteins
 Transmembrane
(-helical
amino acids) &
hydrophilic regions
 Hydrophobic
Mosaic
 Peripheral proteins
 On the surface of membrane
Protein Functions
Cell Recognition
 Crucial
 Determined by surface molecules
(carbohydrates) on plasma membrane
Cell Recognition
 Oligosaccharides (polysaccharide
fewer than 15 units) vary for
different types of cells
 Used as
markers to
distinguish
Example
– A, B, AB, O
Molecular Movement
 Hydrophobic molecules move
across easily
Hydrocarbons, CO2, O2
 Hydrophilic have trouble moving
through hydrophobic core (ions,
polar molecules, water, glucose,
sugars)
Need transport proteins
(facilitated diffusion)
Molecular Movement
 Proteins are specific to molecule it
transports
 Function: move molecules across the
membrane that would not be able to do
so otherwise
 Two types:
Molecular Movement
Molecular Movement
1) Channel Proteins
 Water or small ions
 If water, called aquaporins
 If needs electrical or chemical
stimulus to open & close, called
gated channel
Molecular Movement
 2) Other carrier proteins
Some
transport proteins change shape
to transport molecules
Molecular Movement
Review:
 Passive transport (diffusion)
 Active transport
 Concentration gradient
 Equilibrium
 Osmosis
 Hypertonic, Hypotonic, Isotonic ( &
what happens to the cell)
Molecular Movement
Molecular Movement
Molecular Movement
 Animal cells need isotonic environment
 If not, cells must adapt for
osmoregulation (water balance)
Ex – Paramecium’s
contractile vacuole
Active Transport
 Against concentration gradient –
requires energy (ATP)
 Ability to have cell with
concentrations very different from
surroundings
Ex) Animal cell has much K+ and
little Na+ compared to
surroundings
Active Transport
 Accomplished by membrane proteins
ATP
transfers P to transport protein,
protein changes shape, molecule
moves in or out
 Ex) Sodium-Potassium pump
One ATP pumps three Na+ out and
two K+ in
Active Transport
Cotransport
 Coupling of the “downhill” diffusion of
one substance to the “uphill” transport
of another against its own
concentration gradient
Exocytosis & Endocytosis
 For macromolecules
 Exocytosis – cell secretes
macromolecules by fusion of vesicles
with plasma
membrane
Exocytosis & Endocytosis
Endocytosis – cells takes in
macromolecules by forming new
vesicles
 Three types:




Phagocytosis (particles)
Pinocytosis (any liquid)
Receptor-mediated endocytosis
(specific)
Exocytosis & Endocytosis
Exocytosis & Endocytosis
Receptor-mediated endocytosis
 Proteins embedded in membrane –
specific receptor sites (area where they
are – coated pits)
 Extracellular substances (ligands) bind
to sites – triggers vesicle formation
 Process enables cell to acquire bulk
quantities of specific substances
(cholesterol)
Review
 Review Website
(http://www.wiley.com/college/pratt/0471393878
/student/animations/membrane_transport/index.
html)