Describe the ion makeup of intracellular solutions.
High potassium, low sodium and very low calcium ions.
Describe the ion makeup of extracellular solutions.
Low calcium ions (but much higher than intracellular), low potassium and high (about 145mM) sodium ions.
List the factors affecting solute diffusion across membranes.
- Lipid solubility of the substance
- Presence of specific membrane proteins
- Transmembrane voltage gradients
- Molecular weight of the diffusing substance
- Diffusion distance
- Membrane surface area
What is simple diffusion and when does it occur?
- Solutes are passed down their concentration gradient across a partially permeable membrane.
- Happens directly through the lipid bilayer if the diffusing substance is lipid soluble e.g. oxygen, steroid hormones
- Passive process
What is facilitated diffusion and when does it occur?
- Non-energy requiring transporters including channels/carriers.
- Polar ions and molecules are hydrophilic so require specific membrane proteins to form a diffusion pathway.
- Occurs down concentration/electrochemical gradient.
What are 3 key differences between simple and facilitated diffusion?
- FD is specific
- FD is limited by the number of carrier proteins as they can become saturated
- FD can be affected by competitive inhibition (something else binds to carrier protein)
What is osmosis?
- Water will move across a membrane from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration).
- Largely depends on aquaporins
What is osmolarity and what are the units?
The number of particles per litre of solution (mosmol L−1)
What is osmotic pressure?
Exact amount of pressure required to STOP osmosis.
What are isosmotic solutions?
Both have the same concentration of dissolved solutes (the same osmostic pressure).
What is hyperosmotic?
Increased osmotic pressure (due to increased dissolved solutes; more concentrated).
What is hypo osmotic?
Decreased osmotic pressure (due to decreased dissolved solutes; more dilute).
What is tonicity?
The ability of a solution to cause a cell to gain or lose water.
What is the saturation difference between channels and carriers?
A “carrier” will saturate, but a “channel” will not.
What is active transport?
- Always requires carrier proteins
- Uses an energy source e.g. ATP to move molecules against their gradients.
What is primary active transport and name an example.
- Carrier protein hydrolyses ATP and utilizes part of the released energy for the
transport. - Proton pump in the stomach transports H+ from the blood to the
stomach against a huge electrochemical gradient.
What is secondary active transport and name an example.
- Uses the potential energy stored by a concentration difference of an ion. The
energy-requiring transport is coupled to simultaneous transport of an ion
(most commonly Na+) down its electrochemical gradient. - The movement of glucose and amino acids into the intestine.
What is endocytosis?
- Uptake of extracellular substances by vesicles.
- Requires energy.
What is exocytosis?
- Process by which a cell transports secretory products through the cytoplasm to the plasma membrane via vesicles.
- Requires energy
What is transcytosis?
Transports substances from one extracellular fluid compartment to another;
the endocytic vesicle is transported to the opposite side of the cell and content released by exocytosis.
Outline the pattern of K+ ions across the cell membrane and what movement this causes.
The concentration of K+ is higher inside the cell that outside and the membrane is more permeable to K+, so K+ diffuses out.
Define equilibrium potential.
The potential at which an ions chemical driving force in one direction is equal to the electrical force driving it in the other direction and bulk ion movement is thus in equilibrium.
Explain the term resting membrane potential.
- The membrane potential when a cell is at rest.
- This occurs because of large (membrane impermeable) charged molecules within the cell, the pumps and channels create an imbalance of ions inside and outside the cell.
- At rest the membrane is more permeable to K+ than Na+ - K+ will move out and no Na+ can move in (membrane impermeable).
Define hyperpolarisation.
The membrane becomes more -ve.
