1. Lecture 3: Membrane Signaling and TransportLecture 3: Membrane Signaling and Transport
Reading: Ch 3, section: membrane transport
Ch 4, intercellular communication and signal transduction
6. How are extracellular chemicals secreted by one cell
detected by another?
1. Activation of cell surface receptors linked to
2nd
messenger pathways.
2. Activation of nuclear receptors that control gene expression.
3. Activation of ion channels in plasma membrane.
** especially important for brain function **
Need to understand the biophysics of membrane transport
7. Membrane Transport - movement of substances across the
plasma membrane. Used to maintain homeostasis, or for signaling.
Selective permeability - lipid soluble substances or small
molecules and ions can pass through the plasma membrane, but
polar molecules or macromolecules cannot.
Passive transport - the movement of substances through the
plasma membrane without the expenditure of cellular energy.
Active transport - the movement of substances through the
plasma membrane with the expenditure of cellular energy.
8. Unassisted Membrane Transport
1) Diffusion down a concentration gradient – net movement due
to random collisions between molecules. The greater the
concentration, the greater the likelihood of collision.
9. Unassisted Membrane Transport
2) Movement along an electrical gradient - electrostatic force
(voltage) caused by the separation of electrical charge.
*** Movement along an electrochemical gradient - the combined
force of concentration and electrical gradients.
11. How quickly a substance can diffuse across the
membrane is a critical factor for carrying out the
functions of homeostasis or signaling.
i.e. Rate of diffusion is important.
12. Rate of diffusion through a membrane depends on five
factors:
1) magnitude of the concentration gradient
as concentration gradient, rate of diffusion
2) permeability of the membrane
as permeability, rate of diffusion
3) surface area of the membrane
as surface area, rate of diffusion
4) molecular weight of the substance
as molecular weight, rate of diffusion
5) distance (thickness) over which diffusion takes place
as distance, rate of diffusion
13. Osmosis – a special case of passive transport occurring
when water diffuses down its concentration gradient.
14. Case 1- Membrane permeable to
both water and solute
Prior to Equilibrium:
- water moves from side 1 to side 2
- solute moves from side 2 to side 1
Initial State:
- water concentration higher on side 1
- solute concentration higher on side 2
Final State:
- water concentrations equal
- solute concentration equal
- no further net diffusion
15. Case 2 – Membrane permeable
to water, but impermeable to
solute
Prior to Equilibrium:
- water moves from side 1 to side 2
- no movement of solute
Initial State:
- water concentration higher on side 1
- solute concentration higher on side 2
Final State:
- water concentrations equal
- solute concentration equal
- no further net diffusion
16. Osmotic pressure – the magnitude
of opposing hydrostatic pressure
necessary to stop osmosis.
Case 3 – Membrane permeable
to water, but impermeable to
solute
Initial State:
- water concentration (100%) on side 1
- solute concentration higher on side 2
Prior to Equilibrium:
- water moves from side 1 to side 2
- no movement of solute
Final State:
- water concentration will never be equal
- net diffusion of water stops when
osmotic pressure is counterbalanced by
the hydrostatic pressure
17. Carrier Mediated Transport – method of moving substances across
a membrane when the substance can not simply diffuse “unassisted”
through the membrane. 2 General Types:
1) Facilitated Diffusion – occurs when a substance binds to a
transmembrane protein that undergoes a conformational change such
that the substance is released on the other side of the membrane.
Substances are transported down their concentration gradient. No
energy is required. Passive Transport
2) Membrane Pumps – transmembrane proteins that bind a
substance, undergo a conformational change, and move substances
across the membrane against their concentration gradient. Process
requires energy. Active Transport
18. 1 Facilitated Diffusion – binding of substance induces
conformational change of transmembrane protein
19. 1 Facilitated Diffusion – binding of substance induces
conformational change of transmembrane protein
20. 2 Membrane Pumps - conformational change of transmembrane
protein is induced by a separate energy-dependent process (e.g.
phosphorylation).
Hydrogen-Ion pump
22. Na+
/K+
ATPase - active transport protein that transports 3 Na+
out
of the cell for every 2 K+
pumped into the cell.
1) maintenance of Na+
and K+
conc. gradients
2) maintenance of osmotic balance
3) energy gradients used for co-transport
3 main functions:
23. Vesicular transport - transport of large molecules across the
plasma membrane by endocytosis and exocytosis.
Requires energy. Special case of “Active Transport”
24. SUMMARY: Membrane Signaling and Transport
1. Intercellular signaling
2. Membranes are selectively permeable
3. Passive transport:
- diffusion down concentration/electrical gradients
- osmosis (diffusion of water down its conc. gradient)
- facilitated diffusion
4. Active transport
- membrane pumps
- vesicular transport
