Forces Acting Across Membranes Flashcards
Describe the basic structures of membranes
- cell membrane is made of phospholipid bilayer
- freely permeable to some substances, permeability is selective
- membranes provide binding sites for chemical recognition
- dynamic - constantly formed and maintained or dismantled and metabolised depending on needs of cell
- very flexible due to fatty acids
- insulators
What are the two classes of membrane proteins?
- Integral
- Peripheral
Describe integral membrane proteins
- cannot be removed without disrupting membrane
- amphipathic with the same orientation os the phospholipids
- may span the membrane
Give four examples of how an integral membrane protein might work
As
- channels - through which ions can cross the membrane
- carriers - to transport substances across i.e. pumps
- enzymes - with binding sites at the surface
- receptors - recognition sites for chemicals
Describe peripheral membrane proteins
- can be removed without major disruption of function
- not amphipathic
- contact IMPs on the intracellular side of the membrane and tend to have enzymatic function
- important for cell shape and motility
In what cells do membranes have very little protein content (18%) as they are mainly composed of lipid to provide insulation against electrical signals?
Schwann cells in nerves
What organelles are very active and have a membrane protein content of around 75%?
Mitochondria
What is diffusion across membranes in the body?
Diffusion occurs between compartments in the body, from a high concentration to a low concentration, provided the barrier between the two is permeable to the diffusing substance
List the factors which favour diffusion through the membrane
- a large surface area
- high permeability
- high concentration gradient
To diffuse through the lipid bilayer, molecules need to be
- small
- uncharged
- hydrophobic (lipophilic)
Give four molecules which can diffuse through the lipid bilayer
- O2
- N2
- CO2
- urea
What are protein channels?
Trans-membranous IMPs that act as an aqueous route for the diffusion of ions
- H2O passes through aquaporins
- some are always open while others are gated
What are the two types of gated channels?
- voltage gated
- ligand gated
Describe voltage gated channels
- changes in electrical potential act on the charged regions of the channel proteins producing a change in the configuration in their shape
- this opens or closes the channel e.g. Na+ channels in nerve cells
Describe ligand gated channels
- when a certain chemical binds to the channel protein it produces a change in the configuration and opens or closes the channel
- e.g. Acetylcholine receptors
Define electrochemical gradients
- for ion diffusion, electrical and concentration gradients need to be considered
- separation of charges across most cell membranes
- so inside of cell carries a relative negative charge in respect to outside
- this membrane potential can affect the diffusion of ions across the membrane - electrochemical gradient
What is carrier mediated transport?
- carrier mediated transport proteins have binding sites for substances which cannot diffuse across or cross cell membranes via channels
- they bind to solute and undergo a change in configuration which exposes the site on the other side of the membrane so solute can diffuse into the cell
- protein returns to normal shape
What are the two types of carrier mediated transport systems?
- facilitated diffusion
- active transport
What is facilitated diffusion?
- transport of solutes down their conc gradient
- needs no direct energy source
What is active transport?
- requires energy to move substances against the gradient
- energy comes from ATP so these pumps are known as ATPases
- e.g. Na+/K+ ATPase is in all cells
Define osmolarity
The measure of solute concentration - number of osmoles of solute per litre of solution
Define osmolality
The measure of osmoles of solute per kilogram of solvent
Tonicity is determined by the volume of the cells which depends on
the concentration of non-penetrating solutes on the two sides of the membrane
If the ECF has a higher tonicity than the ICF, the solution is
hypertonic
- cell will shrink as water leaves via osmosis to compensate
If the ECF has a lower tonicity than the ICF, the solution is
hypotonic
- cell will swell as water enters the cell
What is an isosmotic solution?
A solution in which there is an equal number of both penetrating and non-penetrating solutes on either side of the cell membrane
What is an isotonic solution?
One in which there is an equal number of non-penetrating solutes on either side of the cell membrane
Describe the process of endocytosis
Invagination of the membrane to form a vesicle around the target substance
- it eventually separates from the membrane on the cytoplasmic side and migrates within the cell to its destination
- exocytosis is the reverse process
