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