Forces Acting Across Membranes Flashcards
Difference of chemical composition in ICF and ECF is maintained by
presence of cell membrane (aka plasma membrane; aka plasmalemma)
The ECF consists of
the plasma and interstitial fluid.
Material moving between cells and ECF must cross the cell membrane.
Cell membrane is between the interstitial fluid and intracellular fluid.
Capillary wall separates
plasma from ISF.
Gases pass
freely across both the capillary wall and cell membrane.
Nutrients and waste pass
easily but sometimes need help crossing the cell membrane.
There is no barrier to
H2O movement
Ions pass
freely across the capillary wall so exchange readily between plasma and ISF. They do not penetrate the cell membrane.
[K+] is
high in ICF, low in ECF
[Na+] is
low in ICF, high in ECF.
Plasma is high in
[Na+] and plasma protein.
It’s low in [K+].
Interstitial fluid (ISF) is high in
[Na+].
It’s low in protein and [K+].
Intercellular fluid (ICF) is high in
[K+] and protein.
It’s low in [Na+].
Plasma and ISF are identical in everything except
(plasma) protein concentration
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.
Permeability can vary,
may increase or decrease at different times, fundamentally important for various cell functions eg transmission of the nervous impulse.
Structure of membrane:
Very thin bi-layer of lipids.
Major membrane lipids are phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail
Hydrophilic =
lipophobic (lipid repellent);
Hydrophobic =
lipophilic (lipid loving)
Membranes are embedded with
proteins (and associated with carbohydrates).
Membrane carbohydrates:
small amounts linked to proteins and lipids as glycoproteins and glycolipids.
ALL extracellular.
Important roles in cell to cell communication including self vs non-self recognition by the immune system.
Main Functions of Membrane Proteins
- Receptors
- Transporters (Carrier and channel proteins)
- Enzymes
- Maintenance of cell structure
Receptors
Integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal e.g. neurotransmitter or hormone, to the intracellular space to create a cellular response.
ligand-receptor complex
triggers intracellular response
Transporters
are proteins which allow movement of ions or molecules across the membrane.
Come in two forms:
- Channels
- Carrier Proteins.
Channel proteins:
create a pore through the membrane through which molecules, usually water and ions, flow.
Can be open (water) or gated (ions).
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.
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.
Structural proteins
are peripheral proteins, which are associated with the cell membrane but not incorporated into it. They can anchor the cell membrane both to the intracellular skeleton and to the extracellular matrix (collagen). Dysfunction or loss can cause serious debility (weakness) e.g. lack of dystrophin protein in Duchenne’s Muscular Dystrophy.
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, roughly 75%.
Major barrier is the
cell membrane, (and to lesser extent, capillary wall).
Forces which produce movement of H2O and other molecules across these barriers are driven by concentration gradients.
Because the ions creating the concentration gradients are charged particles there is also a difference in charge across the membrane. This creates
an electrical gradient.
The net effect of these two forces create an electrochemical gradient which ultimately drives the direction of passive movement.
Any movement against this gradient requires energy (active transport).
Endocytosis and exocytosis are mechanisms for
moving macromolecules across membranes without disrupting them.
In endocytosis,
there is invagination of the membrane to form a vesicle which eventually disintegrates on the cytoplasmic (inside) surface of the membrane, releasing contents which then migrate within the cell to their destination.
Exocytosis involves the reverse process and so contents leave intracellular fluid.
In the body, diffusion occurs from
one compartment to another, provided the barrier between the 2 compartments is permeable to the diffusing substance.
There is considerable variability in membrane permeability for different substances.
Cell membranes are effectively impermeable
to intracellular proteins and organic anions. These items cannot diffuse in any capacity so they stay inside the cell.
For smaller molecules, it is important to distinguish between diffusion through
the lipid bilayer or via transport proteins embedded in it.
Diffusion through the Lipid bilayer:
- Small
- lipophilic (hydrophobic)
- UNCHARGED
Small, uncharged, lipophilic molecules such as O2 and N2 pass
rapidly through the lipid bilayer.
Small, uncharged, lipophobic molecules such as CO2 and urea
diffuse relatively easily across lipid bilayers.
Large, uncharged lipophobic molecules such as glucose
diffuse much more slowly through bilayer.
