Cell membrane - Passive transport Flashcards
Why the K+ ion channel can not carry a Na+ ion
First of all, the ions channels has a high specificity to their ions and it can differentiate between them by dimension and charge.
The structure that allows this distinction between them is the selective filter, it placed in the middle of the channel and only allows the passage of the right ion by their dimension.
- K+ is bigger than Na+
- Na+ has more charge density than K+
- Na+ is more solvated than K+
Mediated transport / facilitated diffusion
GLUT 1
GLUT 2
GLUT 3
The facilitated diffusion uses a transmembrane protein to allow the passage of bigger molecules across the membrane by the concentration gradient.
The graphic of transport kinetics correlates the glucose concentration and velocity, creating a hyperbolic curve.
Km = is the constant of dissociation between the protein and the ligand. Its the necessary glucose concentration to have a half of Vmax (1/2 Vmax). Small Kms means high affinity.
Vmax = is the diffusion velocity when the channels are saturated.
It depends on the ligand concentration and affinity.
FOR GLUCOSE
GLUT1 - We can find GLUT 1 in erythrocytes. They have high affinity to glucose, even when it has low concentrations (Km=1,5). In erythrocytes the glucose enter in phosphorylated form so th
GLUT2 - found on liver (hepatocytes). Km= 7 –> so the entrance of glucose on these cells only occur when we have elevated concentration of glucose in the blood so the liver can transform the glucose in glycogen by glyconeogenesis
GLUT3 - found in neurons. Km=1,8 which means high affinity and it allows big amounts of glucose to enter in the neurons because its the main source of nutrition for them.
WATER PERMEABILITY
The water permeability to the cellular membrane is very low because its a polar substance, so just a low percentage of water can cross freely the membrane
Most of them needs help of a protein channel like AQUAPORIN
AQUAPORIN is a transmembrane protein channel formed by 4 subunits. every subunit has a central channel that allow the water molecule to cross the membrane in a line, one molecule per time. the tetrameric structure can estabilize the subunits
IONIC CHANNEL
Ionic channels are formed by transmembrane proteins that allows ions to cross the cell membrane across the electrochemical gradient. Some of them have high specificity to an ion due to their selective filter inside the central canal that choose the right ion by their charge and dimension and the passage can be controlled also by permeability mechanisms (they work like doors) or they can keeps always opened (Leaky channels).
Ion channels with entrance control:
- Voltage dependent = change his conformation when the membrane potential is different than 70mV (resting potential)
- Chemo dependent = change his conformation by interaction with a ligand in his receptor
- Mechanic sensible = change his conformation by pressure alterations (ex. cellular volume)
- Temperature sensible = change with temperature variation
- Photo sensible = change with the radiation influence
OSMOSIS
Is the passage os the water across the semipermeable membrane (through aquaporins) from the lowest solute concentration side to the higher solute concentration side.
The liquid exerts an hydrostatic pressure in the membrane and the osmotic pressure is the opposite force caused by the solute concentration.
Osmolarity is the osmotic pressure exerted by the particle present in one liter of solution. It describes the quantity of the solute present in the solution.
Hyperosmotic -> high concentration
Isosmotic -> equal
Hyposmotic -> low concentration
Tonicity describes the behaviour of the cell face to the solution concentration.
Hypertonic -> the external solution have a higher osmolarity than the cell, so the cell starts to lose water by osmosis and become wrinkled
Isotonic -> same osmolarity, the cell keeps normal
Hypotonic-> the external solution has less osmolarity than the cell so the water migrate to the intracellular compartment and the cell swells.
SIMPLE DIFFUSION
equations
We can describe the diffusion across the cell membrane with the Fick’s FIRST LAW by using the permeability coefficient instead the diffusion coefficient (because isn’t a free move of solute so we need to consider also the permeability of the membrane not only the concentration)
Fi= Pi (C1-C2)
Pi: permeability coefficient( velocity = cm/sec), depends of lipid solubility (higher lipid solubility, higher permeability and higher velocity os diffusion)
The SECOND LAW depends also of the area and thickness of the membrane.
These equation describes the solute flux across the cell membrane
The permeability depends on:
- Area and thickness of the membrane
- Lipid solubility
- Size of the molecule
- Concentration gradient
- Composition of the double layer membrane
Permeability to lipid soluble substances - ACID AND WEAK BASIC MOLECULES
All lipid soluble substances can cross the membrane
The permeability speed depends on dimension and lipid solubility of the substance.
Acid or weak basic substances can be lipid soluble in the indissociated form and water soluble in dissociated form (can not cross the membrane)
The indissociated form can cross the membrane following the pH gradient. If the molecule has the pk=8, the closest pH is to 8, more indissociate and more lipid soluble the molecule is. But if the pH is 3 p.e. the molecule change to dissociated form, which is water soluble and it can not cross the membrane
