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).
