Forces across membrane IA%% (+ Flashcards
Membrane info 1: selective barrier
- Cell membrane is a selective barrier. It is freely permeable to some substances e.g. O2 and CO2, but the difference in composition between ECF and ICF shows that permeability is selective and not universal.
Membrane info 2: permeability
. Permeability can vary, may increase/ decrease at different times, fundamentally important for various cell functions eg transmission of the nervous impulse.
Membrane info 3: chemical recognition
Membranes provide binding sites for chemical recognition; embedded in them are receptors for chemical signals such as hormones and neurotransmitters which regulate cellular activity.
Only tissues that express receptors for that particular hormone or neurotransmitter can respond to the chemical.
Membrane info 4: dynamic
Membranes are dynamic, continually being formed and maintained or dismantled and metabolized according to the needs of the cell.
Membrane info 5 : structure
- Very thin bi-layer of lipids, embedded with proteins
- Phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail
- In an aqueous environment they arrange themselves so that the polar, hydrophilic part is on the outside of the membrane and the fatty acid chains are on the inside, away from H2O.
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Membrane info 6: flexible
Membranes are very flexible, due to presence of fatty acids. Behave like an oil, so can stretch, although may rupture if over-stretched.
Membrane info 7 : insulators
Membranes are excellent insulators against movement of electrical charge. This prevents the passage of electrons – important in maintaining electrical stability of cell
Receptors
Receptors – integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal to the intracellular space to create a cellular response.
Transporters
Transporters – proteins which allow movement of ions or molecules across the membrane.
i) Carrier Proteins – do not create a continuous pore from ECF to ICF. Open to ECF, then ICF, but never at the same time. Typically move larger molecules than channels e.g. glucose.
ii) Channel proteins – create a pore through the membrane through which molecules, usually ions, flow. Can be open or gated.
Membrane Enzymes
Membrane enzymes catalyse chemical reactions on the cell membrane. Can be external e.g. those found in small intestine which break down nutrients into smaller units, or internal such as those associated with converting signals carried from receptors into an intracellular response.
Peripheral membrane proteins
Peripheral membrane proteins – associated with the cell membrane but not incorporated into it. They are proteins that adhere only temporarily to the membrane with which they are associated. These proteins attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer.
The have 3 main roles:
i.Maintain structure of cells by anchoring membrane to intracellular cytoskeleton (network of interlinking filaments and tubules that extend throughout the cytoplasm, from the nucleus to the plasma membrane). Dysfunction or loss can cause serious debility .
ii. Attach cells to the extracellular matrix e.g. collagen
iii. Perform signalling functions within cells e.g. G Proteins
Protein content difference in structures
Membranes differ in their protein content.
- Myelin: a membrane that serves as an insulator around myelinated nerve fibres has a low content of protein (18%), major component is lipid, very good insulator so ideal for function.
- Plasma membranes of most other cells have much greater activity and protein content is typically 50%.
- Membranes involved in energy transduction such as the inner membranes of mitochondria, have highest protein content, »75%.
Membrane carbohydrates
- Small amounts linked to proteins and lipids as glycoproteins and glycolipids.
- ALL extracellular.
- Important role in self vs non-self recognition by the immune system.
Diffusion definition
- Process by which a gas or substance in solution expands to fill all the available volume.
- molecules spread from regions of high concentration to regions of low concentration, until the concentration is uniform throughout the volume.
Magnitude of net flux
Diffusion through lipid bilayer
To pass rapidly through the lipid bilayer, molecules need to be:
- Small
- Lipophilic(hydrophobic)
- UNCHARGED
- a)Small, uncharged, lipophilic molecules such as O2 and N2 pass easily through the lipid bilayer.
- b)Small, uncharged, lipophobic molecules such as CO2 and urea, also diffuse easily across lipid bilayers.
- c)Large, uncharged lipophobic molecules such as glucose diffuse much more slowly through bilayer (neglible)
Diffusion through channels
- These are transmembrane Integral membrane proteins that act as an aqueous route through the membrane for the diffusion of ions.
- Only allow the passage of mineral ions such as Na+ and K+, Cl-, Ca2+, H+ and H2O. Too small to allow molecules such as glucose to go through.
- H2O passes through aquaporins, family of water channels, ubiquitously distributed. There is NO barrier to water
Gated channels
Ions can cross the membrane using (mainly) gated channels. They remain closed until a stimulus (chemical or change in electrical charge across the membrane) causes them to open.
Osmosis
Net movement of H2O from regions of high H2O concentration to regions of low H2O concentration
Note:
- ALL H2O movements in the body are passive, (via aquaporins, protein water channels).
- H2O concentration is inversely related to the concentration of solute, ie the more solute particles there are in solution, the more they will displace H2O molecules lowering the concentration of H2O
Plasma composition
- High [Na+] 3 Na+ out for every 2 K+ in
- Low [K+]
- High [plasma protein] too big to pass through capillary wall
ISF composition
- High [Na+] 3 Na+ out for every 2 K+ in
- Low [K+]
- low [plasma protein] too big to pass through capillary wall
ICF composition
- Low [Na+] 3 Na+ out for every 2 K+ in
- High [K+]
- High [protein]
Osmolarity
“Osmolarity” measures the concentration of biological solutions in units of “OSMOLES” and describes the number of particles/L of solution, (in reality use milliosmoles, because biological solutions are so weak).
285 —> 300 mOsmol/l in blood plasma (for ease)
Osmolarity vs Tonicity
- Osmolarity describes total number of particles in solution
- Tonicity describes the number of non-penetrating particles in solution
Note: the volume of a cell at any time is dependent on the concentration of non-penetrating solutes on the either side of the membrane
Osmolarity terms
- An isosmotic solution has the same total number of solute particles as ECF (plasma). 300 mOsmol/L
- Solutions with fewer total solute particles are hypo-osmotic.
- Solutions with greater number of total solute particles are hyper-osmotic
Tonicity terms
- An isotonic solution has the same number of non-penetrating solute particles as ECF (plasma).
- Solutions with fewer non-penetrating solute particles are hypotonic.
- Solutions with greater number of non-penetrating solute particles are hypertonic.
Facts
- Urea is a penetrating particle (disregard)
- (charged) Ions are non-pentrating (important)
- Brain is most sensitive organ to changes in tonicity.
Define electrochemical gradient
A gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane.
