Transport Across Cell Membrane Flashcards
three types of transports:
1) Passive transport
2) Active transport
3) Vesicular transport
Passive transport
-Along concentration gradient or electrical gradient.
-Doesn’t require energy
-Downhill movement
1. Osmosis 2. Diffusion
It depends on:
a) Concentration gradient
b) Electrical gradient
c) Pressure gradient
Active transport
-Against concentration gradient or electrical gradient.
-Require energy
-Uphill movement
1. Primary 2. Secondary
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Diffusion
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration.
* Simple diffusion - substances diffuse through the lipid bilayer or channel proteins of the plasma membrane.
* Facilitated diffusion - substances diffuse through the cell membrane with the help of some carrier proteins.
Osmotic pressure
The force with which a solution attracts water is measured by its osmotic pressure.
mosmol/L or mosmol/kg
normal osmolarity of plasma: 290 mosmol/L
1 mole in 1kg of water = 1 osmol/kg
* depends on whether the substance dissociates in water
* 1 mole of NaCl in 1kg of water = 2 osmol/kg (assuming complete dissociation
into Na+ and Cl-)
1) ISOOSMOLAR solution-Solution that has the same osmolality as plasma
2) HYPEROSMOLAR solution-Solution that has an osmolality higher than plasma
3) HYPOOSMOLAR solution-Solution that has an osmolality lower than plasma
Behavior of RBCs in different tonicity of solutions
HYPERTONIC-Solution has greater osmolarity than plasma.RBCs become crenated (shrink).Exosmosis.
ISOTONIC- Solutions have the same osmolarity as plasma.No change in cell volume
HYPOTONIC- Solution has lesser osmolarity than plasma.RBCs bulge & swell.Endosmosis
SIMPLE DIFFUSION
- Occurs from higher to lower concentration (downhill)
- No energy is required
- Rate of diffusion increases as concentration gradient increases
- Occurs through lipid bilayer / free open channels/gated channels
- May or may not be specific
- Used to transport nonpolar substances and ions
in bilayer -nonpolar and lipid soluble substances
* E.g: O2, CO2, fat-soluble vitamins
Factors affecting diffusion of a species across a membrane and Fick’s law of diffusion:
Diffusion rate (J) = D A dc/dx
D - diffusion coefficient: molecular radius of the solute and the viscosity of the medium.
A - area available for diffusion
Dc - concentration gradient: The larger the difference in solute concentration ,the greater the net diffusion.
Dx - thickness of membrane: The thicker the cell membrane, the greater the distance the solute must diffuse and the lower the rate of diffusion.
Simple diffusion through free (open) protein channels
-selectively permeable
E.g. : Na+ channels, K+ channels, Cl-channels, Na+/K+ channels, Water channels (aquaporin)
Simple diffusion through gated channels
i) Stretch or pressure gated channels- in muscle cells
ii) Ligand gated channels- neurotransmitter or hormone
iii) Voltage gated channels-
* Voltage gated Na+, K+ channels in nerve and muscle cells
* Voltage gated Ca++ channels in presynaptic membrane
Mechanism of facilitated diffusion: Glucose transporter (GLUT)
Glucose binds to its specific binding site on one side of the membrane of the carrier protein
* Binding causes the carrier protein to undergo conformational change
* This allows subst to move along the spaces in the protein molecule towards the other side of the
membrane
* Exposure of binding sites to the ICF
* Release of glucose in to the ICF
* Protein configuration returns to original – is able to bind another molecule(s)
Special features of facilitated diffusion
- carrier mediated transport.
- Specific
- High flux: the rate of diffusion is faster in facilitated diffusion compared to simple diffusion.
- Saturable: the number of carrier proteins available determines the rate of diffusion.
- transport larger molecules and polar substances.
- Passive mechanism, no energy is required as it transport substances along the gradient.
Types of carrier protein systems
Uniport: transport only one type of molecule
Symport: one substance is linked with another substance and both move in the same direction.
Eg: Glucose is linked with the transport of sodium Antiport: In this the carrier proteins exchange one substance for another.
Eg: Na+ - K+ exchange or Na+ - H+ ion exchange.
Learning Objectives
2.1 Classify and define different types of transport.
2.2 Describe osmosis, osmotic pressure, osmolarity and its significance in the body.
2.3 Enumerate the salient features of simple diffusion.
2.4 Describe the factors affecting the rate of diffusion with suitable examples. 2.5 Explain the mechanism and special features of facilitated diffusion.
2.6 Compare and contrast simple diffusion and facilitated diffusion.
