Forces acting across membranes Flashcards
Describe the basic structure of membranes.
The cell membrane is made up of a phospholipid bilayer.
o Freely permeable to some substances, but permeability is selective.
o Permeability can vary (increase or decrease)
o Membranes provide binding sites for chemical recognition (e.g. Hormones)
o They are Dynamic (constantly formed and maintained, or dismantled and metabolized depending on needs of the cell.
o Very flexible due to fatty acids (may rupture if overstretched)
o Insulators (hydrophobic hydrocarbon tails)
List the functions of membrane proteins.
There are two classes of membrane proteins.
1) Integral Membrane Proteins
These cannot be removed without disrupting the membrane. They are amphipathic with the same orientation as the phospholipids and may span the membrane. They act as:
i) Channels, through which ions can cross the membranes
ii) Carriers, to transport substances across membranes (e.g. pumps)
iii) Enzymes, with binding sites at the surface
iv) Receptors, recognition sites for chemicals
2) Peripheral Proteins
These can be removed without major disruption of function. They are not amphipathic. They contact IMP’s on the intracellular side of the membrane and tend to have enzymatic function. They are important for cell shape and motility (e.g. Dystrophin)
Membranes can vary in protein content.
Schwann Cells in nerves have very little protein (18%) as they are mainly composed of lipid, to provide insulation against electrical signals.
Most other cells have greater activity and protein is around 50%
Mitochondria are incredibly active and have protein content of around 75%
Define what is meant by diffusion across membranes.
In the body, diffusion occurs between compartments, from a high concentration to a low concentration, provided the barrier between the two is permeable to the diffusing substance.
List the factors which favor diffusion through the membrane.
Diffusion is aided by :
o A large surface area
o High permeability
o High conc. Gradient
To diffuse through the lipid bilayer, molecules need to be:
o Small
o Uncharged
o Hydrophobic (Lipophilic)
e.g. O¬2 and N2 because they are small, uncharged and lipophilic. CO¬2 and Urea are small, uncharged and polar, but can still pass through.
Describe the different general types of protein channels.
Protein channels are trans-membranous IMP’s that act as an aqueous route for the diffusion of ions.
H2O passes through aquaporins, a family of water channels.
Some channels are always open, whereas others are “gated”…
o Voltage Gated Channels
The changes in electrical potential act on the charged regions of the channel proteins, producing a change in the configuration in their shape, opening or closing the channel.e.g. Na+ channels in nerve cells.
o 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, we need to consider the electrical gradient as well as the concentration gradient. There is a separation of charge across most cell membranes so that the inside of the cell carries a relative negative charge in respect to the outside. This membrane potential can affect the diffusion of ions across the membrane. There is thus, an electrochemical gradient.
Explain what is meant by carrier-mediated transport systems.
Not all ions cross cell membranes via channels, nor can all uncharged molecules, such as glucose, diffuse across the membrane.
Carrier mediated transport proteins have binding sites for such substances. When they bind the solute, they undergo a change in a configuration that exposes the site on the other side of the membrane. From here, the solute diffuses into the cell and the protein returns to its normal shape.
What are the two types of carrier-mediated transport systems
Facilitated diffusion - Transport of solutes down their conc. Gradient. Needs no direct energy source.
Active Transport- Requires energy to move substances against. This energy comes from ATP, so these pumps are known as ATPases and are sometimes called “pumps”. e.g. Na/K ATPase is in all cells. It extrudes 3 Na and takes in 2K for each molecule of ATP hydrolysed. Since it produces a net +ve charge out of the cell, it is an electrogenic pump. It uses about 40% of the resting energy of the body.
Define osmolarity and osmolality
Osmolarity is the measure of solute concentration.
o The number of osmoles of solute per litre of solution.
Osmolality is the measure of osmoles of solute per kilogram of solvent.
e.g. 1M of glucose has an osmolarity of 1 osmole/L
1M of NaCl has an osmolarity of 2 osmole/L
The volume of the cell depends on the conc. of non-penetrating solutes on the 2 sides of the membrane. This determines the tonicity.
If the ECF has higher tonicity than the ICF, the solution is hypertonic and the cell will shrink as water leaves via osmosis to compensate.
If the ECF has lower tonicity than the ICF, the solution is hypotonic, and the cell will swell as water enters the cell.
Explain the difference between an isosmotic solution and an isotonic solution.
An isosmotic solution is one in which there is an equal number of both penetrating and non-penetrating solutes on either side of the cell membrane.
An isotonic solution is one in which there is an equal number of non-penetrating solutes on either side of the cell membrane.
Describe the process of endocytosis and exocytosis.
In endocytosis, there is an invagination of the membrane to form a vesicle around the target substance. It eventually separates from 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.
