Post on 21-Jan-2016
Lecture #5
Membrane Transport
&
Cell Communication
Membrane Transport
i. diffusion
ii. facilitated diffusion
iii. active transport
Diffusion
O2
CO2
-molecules pass through membrane-movement is spontaneous-molecules travel down the concentration gradient-goal: equlibrium-speed is limited by rate of diffusion
INTRACELLULAR
EXTRACELLULAR
Osmosis
• movement of water down a concentration gradient
• difference in concentration of solute across a membrane
• intracellular [solute] vs. extracellular [solute]
• osmotic, hyperosmotic, hyposmotic
Facilitated Diffusion
molecule is transported down its concentration gradient
faster and more efficient than diffusion
uses transport proteins
2 kinds: carrier proteins channel proteins
GLUT – glucose transporter proteins
INTRACELLULAR
EXTRACELLULAR
GLUT1 – red blood cells, adipose cells, muscle cells
glucose
insulin
GLUT4 – liver cells, adipose cells, muscle cells
after glucoseuptake
Active Transport
• pumping a molecule against its concentration gradient
• requires energy (ATP)
Ca2+-ATPase – skeletal muscle
CYTOPLASM
ER LUMENCa2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+ Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+ Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+ Ca2+
ATP
Ca2+
P
Na+-K+-ATPase – nerve stimulation and membrane potential
INTRACELLULAR
K2+
Na2+
ATP ADP
EXTRACELLULAR
K2+
Na2+Na2+
resting stimulated
Na2+
Na2+Na2+K2+
K2+
Na2+
Na2+Na2+K2+
K2+
stimulation
release ofenergy
Active Transport and Co-transport
blood gut epithelial cell intestinal lumen
tight junctions
K2+
Na2+
ATP ADP
K2+
Na2+Na2+
Na2+ Na2+
glucoseglucose
glucoseglucose
Other forms of transport
• Exocytosis
• Endocytosis• phagocytosis• pinocytosis• receptor-mediated endocytosis
Cell Signalling
• Signals:• local regulators – e.g. growth factors• long-distance regulators – e.g. hormones
• 3 stages:• Reception• Transduction• Response
Non-hormonal cell-to-cell communication
Reception – Plasma Membrane Receptors
• 3 major types:• G-protein linked receptors• receptor tyrosine kinases• ion channel receptors
G-protein linked receptors
ligands: e.g. some hormones (epinephrine) neurotransmitters
Epinephrine – adrenergic receptor
• epinephrine – produced by the adrenal gland
• enters the bloodstream during short-term stress response
• liver - 1 receptors
• blood vessels - 2 receptors
Liver blood vessels - heart, lung, cerebral cortex
1 receptoradenylate
cyclase
GTP
1 receptoradenylate
cyclase
GTP
ATP
cAMP
PKA
glycogen breakdown – release glucose
2 receptoradenylate
cyclase
GTP
2 receptoradenylate
cyclase
GTP
ATP
cAMP
PKA
inhibits MLCK – vasodilation
blood vessels – skin and gut
2 receptoradenylate
cyclase
GDP
2 receptoradenylate
cyclase
GDPATP
cAMPX
vasoconstriction
Receptor Tyrosine Kinases
• kinase: an enzyme that phosphorylates another protein – catalyzes the transfer of phosphate groups
• RTKs – catalyze the transfer of phosphate to side chain of tyrosine amino acid in a protein
• ligands – e.g. growth factors – EGF, FGF, PDGF, etc.
Transduction
• protein phosphorylation – kinases
• second messengers:• cAMP• Ca2+
Transduction
• phosphorylation - kinases – growth factors» stimulate cell division
• cAMP – adenylate cyclase» activates PKA» various effects – e.g. glycogen breakdown or
vasodilation
• Ca2+ - PLC produces IP3 and DAG» activates PKC» various effects – e.g. muscle contraction
Response
• Cytoplasmic responses:– opening or closing of a gated ion-channel– alteration in metabolism– regulation of enzyme activity– change in cytoskeletal organization
• Nuclear responses– activation or repression of gene expression
» activation or inhibition of cell cycle» induction of differentiation» cell committment