Chapter 7
Membrane Structure &
Function
Membrane Structure, I
Shows Selective permeability
Known as the plasma
membrane
Amphipathic - hydrophobic &
hydrophilic regions
Singer-Nicolson developed the
fluid mosaic model
Membrane Structure, II
Phospholipids - membrane fluidity
Cholesterol - membrane stabilization
“Mosaic” Structure due to:
Integral proteins - transmembrane
proteins
Peripheral proteins - surface of
membrane
Membrane carbohydrates -~ cell to
cell recognition; oligosaccharides (cell
markers); glycolipids; glycoproteins
Membrane Structure, III
Membrane protein functions:
Transport
Enzymatic activity
Signal transduction
Intercellular joining
Cell-cell recognition
ECM attachment
Membrane Traffic
Diffusion - tendency of any molecule
to spread out into available space
Concentration gradient – moves from
high to low
Passive transport - diffusion of a
substance across a biological membrane
Osmosis - the diffusion of water
across a selectively permeable
membrane; DOWN the concentration
gradient
Water Balance
Osmoregulation - control of water
balance
Hypertonic - higher concentration of
solutes
Hypotonic - lower concentration of
solutes
Isotonic - equal concentrations of
solutes
Water Balance
Cells with Walls (plants, bacteria):
Require hypotonic external
environments to keep their turgor
pressure (water pressure pushing cell
membrane out against cell wall)
Become limp or flaccid when lose turgor
pressure
Plasmolysis - plasma membrane pulls
away from cell wall
Water Balance
Cells without Walls (animals, most
protist):
Require isotonic external environments
Hypertonic environments – cells swell &
may burst with too much water pressure
(Cytolysis)
May have contractile vacuoles (some
protists) to control internal water
pressure
Specialized Transport
Transport proteins (with or without
channels)
Facilitated diffusion - passage of
molecules and ions with transport
proteins across a membrane down the
concentration gradient
Active transport - movement of a
substance against its concentration
gradient with the help of cellular
energy
Types of Active Transport
Sodium-potassium pump
Exocytosis - secretion of
macromolecules by the fusion of
vesicles with the plasma membrane
Endocytosis - import of
macromolecules by forming new
vesicles with the plasma membrane
Phagocytosis –cell “eating”
Pinocytosis – cell “drinking”
Receptor-mediated endocytosis
(ligands)