The Plasma Membrane
 Membrane Structure
& Function
 Cell Communication
Membrane structure, I
 Selective permeability
 Amphipathic~
hydrophobic & hydrophilic
regions
 Singer-Nicolson:
fluid mosaic model
Membrane structure, II




Phospholipids~ membrane fluidity
Cholesterol~ membrane stabilization
“Mosaic” Structure~
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
function:
•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
Passive transport~
diffusion of a substance
across a biological
membrane
Osmosis~ the diffusion of
water across a selectively
permeable membrane
Water balance
 Osmoregulation~ control
of water balance
 Hypertonic~ higher
concentration of solutes
 Hypotonic~ lower
concentration of solutes
 Isotonic~ equal
concentrations of solutes
 Cells with Walls:
 Turgid (very firm)
 Flaccid (limp)
 Plasmolysis~ plasma
membrane pulls away from
cell wall
Specialized Transport
 Transport proteins
 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
•pinocytosis
•receptor-mediated
endocytosis
Exocytosis
 secretes macromolecules by the fusion of
vesicles with the membrane
 Transport vesicles leave the trans face of
the Golgi apparatus
 Secretory Cells



Pancreas cells – Insulin
Neurons – Neurotransmitters
Plant Cells – Carbohydrates for cell wall
Endocytosis
 new vesicles from the exterior of the plasma
membrane form

Almost the reverse of exocytosis
 Three types:



Phagocytosis – “cellular eating”
Pinocytosis – “cellular drinking”
Receptor mediated – acquire specific substances
 Human cells:

Used to take in cholesterol for membrane synthesis
Endocytosis Continued
 Receptor-mediated endocytosis = process of
importing specific macromolecules into the cell
by inward budding of vesicles from coated pits;
response of ligands binding to surface
receptors
 More discriminating than pinocytosis
 Ligand = a molecule that binds to a specific
receptor site of another molecule
 Coated pits = membrane-embedded proteins with
specific receptor sites exposed to the cell’s
exterior
 Clathrin = layer of fibrous protein; lines and
reinforces the coated pit on the cytoplasmic
side; helps form vesicle
Signal-transduction pathway
 Def: Signal on a cell’s surface is converted into a specific cellular
response
 Local signaling (short distance):
√ Paracrine (growth factors)
√ Synaptic (neurotransmitters)
 Long distance: hormones
Cell Signaling
 Earl W. Sutherland, Jr.


Vanderbilt University
Nobel Prize, Physiology, 1971
 Epinephrine’s ability to convert blood
glycogen to glucose in the liver
Stages of cell signaling
 Glycogen depolymerization by epinephrine
 3 steps:
•Reception: target cell detection
•Transduction: single-step or series of changes
•Response: triggering of a specific cellular response
Receptors
 Monomers – single receptor molecules that
initiate signal with recognized binding; can
activate relay proteins or enzymes
 Dimers – both monomers must have a
recognized signal molecule which causes
them to form a dimer, which can activate
relay proteins
Protein phosphorylation
Protein activity regulation
Adding phosphate from
ATP to a protein (activates
proteins)
Enzyme: protein kinases
(1% of all our genes)
Example: cell
reproduction
Reversal enzyme:
protein phosphatases
Second messengers
 Non-protein signaling
pathway (
 Example: cyclic AMP
(cAMP)
 Ex: Glycogen breakdown
with epinephrine
 Enzyme: adenylyl cyclase
 G-protein-linked receptor
in membrane (guanosine
di- or tri- phosphate)
Cellular responses to signals
Cytoplasmic activity
regulation
Cell metabolism
regulation
Nuclear transcription
regulation