Forces acting across membranes Flashcards
Describe the basic structure of membranes.
Phospholipid bilayer studded with aquaporin channels, proteins and associated carbohydrates.
Describe the function of the cell membrane.
- Allows water to move freely through aquaporin channels in/out of the cell
- Selective barrier so freely permeable to some substances such as O2 and CO2
- Permeability can vary at different times
- Excellent insulators against movement of electrical charge
Describe the different types of membrane proteins.
Integral proteins - span the hydrophobic core of the bi-layer (go all the way through)
Peripheral proteins - associated only with the phosphate head of the lipid bi-layer and do not penetrate the hydrophobic core
List and explain the different functions of membrane proteins.
Receptors - allow communication of an extracellular signal
Transporters - do not create a continuous pore from ECF to ICF, open to one then the other, never at the same time, typically move larger molecules such as glucose
Channel - create a pore through the membrane through which molecules, usually water and ions can flow, open = water, gated = ions
Enzymes - catalyse chemical reactions on the cell membrane
Structural - anchor the cell membrane to the intracellular skeleton, to the extracellular matrix (collagen) and/or to other cells
Communication - between cells or within cells, glycoproteins act as markers that tell the immune system whether a cell is one of our own or foreign
What is endocytosis?
Invagination of the cell membrane to form a vesicle which eventually disintegrates on the cytoplasmic (inside) surface of the membrane.
What is exocytosis?
Products being released from the cell in the form of a vesicle (opposite of endocytosis).
What is an electrochemical gradient?
The net effect of the chemical gradient and electrical gradient (difference in charge across the membrane) and ultimately drives the direction of passive movement
What is diffusion?
A process by which a gas or substance in a solution expands to fill all the available volume, therefore molecules spread from regions of high concentration to regions of low concentration.
What is the difference between passive and facilitated diffusion?
Both rely on concentration gradients however in passive diffusion the substance moved directly through the lipid bilayer, and in facilitated diffusion the substance requires assistance from membrane proteins to cross the lipid bilayer
What must be true for passive diffusion to take place?
To pass passively and rapidly through the lipid bilayer, molecules need to be:
1.) small
2.) uncharged
3.) lipophilic (hydrophobic)
Is there any barriers to water movement?
NAW
How to ions cross the membrane?
Using voltage gated or ligand gated channels.
How do voltage gated channels operate?
Open/close in response to alterations in the membrane electrical potential.
How do ligand gated channels operate?
Open/close when they bind a chemical such as a neurotransmitter or hormone to a receptor binding site on the channel protein.
What is active transport?
Movement of molecules through transport proteins against their electrochemical gradient which requires energy (ATP).
Explain the process of the sodium/potassium pump.
1.) [Na+] higher outside cell, [K+] higher inside cell
2.) Continuously pumps out 3 Na+ ions and pumps in 2 K+ ions for each ATP molecule hydrolysed
3.) Uses 40% of resting energy of the body
What is osmosis?
The net movement of water from regions of high concentration to regions of low concentration.
What happens to solutions of different concentrations separated by a membrane that is only permeable to water?
After a time will end up with equal concentrations on either side of the membrane but different volumes.
What is osmotic pressure?
The pressure required to prevent water movement/oppose water movement.
What is osmolarity?
Describes the total number of particles in solution.
Measures the concentration of biological solutions in units of “osmoles”, normally milliosmoles irl cause biological solutions are so weak.
What is the osmolarity of normal human plasma?
285mOsmol/l (often taken as 300)
What is the volume of a cell at any time dependent on?
The concentration of non-penetrating solutes on either side of the membrane.
What is tonicity?
Describes the number of non-penetrating particles in a solution.
What is hyper-osmotic?
Solutions with greater number of total solute particles.
What is hypo-osmotic?
Solutions with fewer total solute particles.
What is isosmotic?
Solution with the same total number of solute particles as normal ECF (plasma).
What is isotonic?
Solution has the same number of non-penetrating solute particles as normal ECF (plasma).
What is hypotonic?
Solutions with fewer non-penetrating solute particles.
What is hypertonic?
Solutions with greater number of non-penetrating solute particles.
What do cells in a hypotonic solution do?
Swell
What do cells in a hypertonic solution do?
Shrink
What should you never transfuse a patient with?
Pure water
What happens if you place RBC’s in hyper-osmotic aqueous urea solution?
Swell and burst because urea is a penetrating particle and in an aqueous hypotonic solution (no non-penetrating particles) so the water goes into the cell to try and even the concentration.
What will happen to the volume of RBC’s in patients with uraemia?
A - Cells will shrink
B - Cells will shrink
C - There will be no change
C
