6. Movement across Membrane
Membrane permeability varies for
different molecules & cell types
depends on??
Four movement categories
Active transport Passive transport Vesicular.Tra Osmosis
Primary A.T Secondary A.T Facilitated Dif. Simple Dif. Endocytosis Exocytosis
Carrier Mediated Transport pinocytosis Phagocytosis Receptor
mediate
7. Simple Diffusion (Passive)
•Uses energy of concentration gradient from high to low concentration
•Net movement until state of equilibrium reached (no more conc. gradient)
•Direct correlation to temperature (why?)
•Indirect correlation to molecule size
•Slower with increasing distance
•Lipophilic molecules can diffuse
through the phospholipid bilayer
Fick’s law of Diffusion
Rate of surface area x conc.gradient
diffusion membrane resistance x mem. Thickness
depends on size and solubility of molecule and
Composition of lipid bilayer
8. Facilitated Diffusion (as a form of carrier mediated transport)
Some characteristics same as simple diffusion
but also:
• specificity: Renal proximal tubule recognizes and transports the natural isomer d-glucose, but it does not
recognize or transport the unnatural isomer l-glucose.
• competition: the presence of d-galactose inhibits the transport of d-glucose by occupying some of the
binding sites and making them unavailable for glucoseView drug information
• saturation: When all of the binding sites are occupied, saturation is achieved at a point called the
transport maximum, or Tm.
9. Primary Active Transport
• Show specificity, competition and saturation
• ATP energy directly fuels transport
• Most important example: Na+/K+ pump = sodiumpotassium
ATPase (uses up to 30% of cell’s ATP)
10. Secondary Active Transport
• Uses ATP indirectly
• Involve two substances simultaneously
• One of the solutes, usually Na+, moves down its electrochemical gradient
(downhill), and the other solute moves against its electrochemical gradient
(uphill).
1. Co-transport (symport): movement to same direction.
• Na+/Glu, Na+/amino acids, Na+/K+ 2cl- co-transporter……
2. Counter-transport: opposite direction.
• Na+/K+ ATPase, K+/H+ pump…..
13. B. Receptor mediated endocytosis – highly
selective process to import imp. specific large
molecules. Requires energy & Ca++.
Coated pit
Clathrin, actin,
myosin
e.g. endocytosis of low density
lipoproteins
e.g. endocytosis of viruses such as
hepatitis, AIDS viruses & excess iron
14. C. Phagocytosis
• Internalization of large multimolecular particles,
bacteria, dead tissues by specialized cells e.g.
certain types of w.b.c.s ( Professional phagocytes)
• The material makes contact with the cell membrane
which then invaginates.
15. Osmosis
• Movement of water down its concentration gradient.
• Water moves freely in body until osmotic equilibrium is reached.
16. Passive transport Active transport
• No expenditure of
energy molecules
• Takes place along
conc., electrical, &
pressure gradient
• Carrier may or may not
be required
• Rate is proportional to
conc. difference
• Expenditure of energy
mol. ( ATP )
• Can take place against
conc. Gradient
• Carrier is always
required
• Rate is proportional to
availability of carrier &
energy. (Vmax)
17. Simple Diffusion Facilitated Diffusion
• Passive transport
• For small molecules
• No carrier required
• Rate of transport is
directly proportional to
conc. gradient
• Examples –
Lipid soluble –
O2, CO2, alcohol
Lipid insoluble –
urea, Na+, K+
• Passive transport
• For large molecules
• Carrier mediated
• Initially rate is
proportional to conc.
gradient till Vmax
( saturation of carriers)
• Examples –
glucose, amino acids
