Membrane transport Flashcards
4 functions of membranes
- Homeostasis/Compartmentalization 2. Transport 3. Intercellular communication 4. Excitability
Signal transduction
Communication between the outside and inside of cells
Na+, K+, Ca2+, Cl- concentrations in cell
Na+: 135-145mM K+: 3.5-5mM Ca2+: 2-2.6mM Cl-:98-106mM
Na+, K+, Ca2+, Cl- concentrations in ECM
Na+: 10-15mM K+: 140mM Ca2+: 50mM Cl-:10mM
Action potentials
Transient changes in membrane potential that spread or propagate
Synaptic junctions
Allow communication between cells
Bulk flow
Bulk movement of solutions by hydrostatic pressure
Diffusion
Random thermal motion of molecules resulting in directed net movement of solutes when concentration differences exist. Distances must be small to achieve rapid movement.
Time to travel a particular difference
x^2 where x is the distance (if it takes 10ms to move 5 micrometers then it will take (10^2) 1000ms to move 50 micrometers
Electrical migration (electrodiffusion)
Charge movement in response to electrical field
Flux
Quantity that moves over a specified period of time (quantity/time)
Fick’s law
Flux = P x A x deltaC where P=permeability A=area C=concentration difference across membrane Measures how easily substance can cross membrane
Conductance
Ions/sec
Facilitated diffusion
No metabolic energy used (unlike active transport). Only accomplishes what simple diffusion could have accomplished eventually. Two types: transport via carriers and via channels.
Carrier-mediated transport
Transmembrane protein (enzyme) which undergoes repetitive spontaneous conformational changes. Specificity, saturation (transport maximum), and competition (similar molecules competing to bind). Much less effective than channels
Anthracyclines
Used to treat breast cancer. Cause cardiotoxicity due to inappropriate opening of ryanodine receptors (SR Ca2+ channels). Ca2+ diffuses out into the cytosol. The calcium is probably behind the arrhythmias.
Active transport
Transport that can proceed against an electrochemical potential difference or from low concentration to high concentration. Requires metabolic energy. Two types: Primary and secondary
Transport via channels
Pore that spans membrane and can exist in at least 2 states (open and closed). Have specificity, saturation, and competition.
Cysteinuria
Elevated cysteine in urine caused by defects in cysteine carriers in nephron membranes. Normally kidneys remove cysteine from fluid passing through kidney to form urine and return to blood. With this defect, large amounts of insoluble cysteine remain in tubular fluid that becomes urine and creates kidney stones.
Diabetes
Reabsorbtion of glucose out of filtrate and back into the body occurs through glucose transporter within epithelia of renal tubules. In diabetes low blood insulin lead to high blood glucose. Because of high glucose in the blood, Tm is reached and no more glucose can be moved back into the body. Excess glucose left in tubules and excreted in urine (how diabetes can be diagnosed)
Transport maximum (Tm)
Point at which higher concentration does not lead to increase in transport
Primary active transport
Sometimes called ATPases or pumps. Directly utilizes metabolic energy in transport process. ATP is hydrolyzed.
Na+/K+ ATP pump (ATPase)
Splits a single molecule of ATP to move 3 Na+ ions out of the cell and 2 K+ ions into the cell. In both cases moving from lower concentration to higher concentration.
Ca2+ pump
In the ER of most cells and the SR of muscle cells. Sequesters Ca2+ within these organelles which can be released by various cellular processes. Also on surface membrane of some cells (cardiac muscle) where Ca2+ moved from cytoplasm to extracellular fluid to maintain very low cytoplasmic resting concentration of Ca2+.
H+ pump
On the basolateral membrane of specialized stomach cells (parietal) resulting in secretion of HCl during digestion. Also in some kidney tubular cells.
Secondary active transport
Also against electrochemical gradients but without direct coupling to the hydrolysis of ATP; instead it is indirectly linked. Couples uphill movement of transported solute to the downhill movement of another solute (often Na+) whose concentration gradient was estabilished by primary active transport (energy stored in concentration gradient used to transport other solutes). Two types: Co-transporters and counter-transporters (exchangers).
Co-transporters
Move both solutes in the same direction (one against electrochemical gradient and one with the energy from primary active transport)
Enterocyte co-transport
Na+/glucose co-transporter moves glucose into absorptive cells against its’ concentration gradient with the energy of Na+ moving down its’ gradient
Iodine transport for thyroid hormone
Co-transporter moves I- into thyroid through NIS transporter (an Na+/I- co-transporter) even though I- concentration very low in blood
Counter-transporters
Solutes are exchanged in opposite directions. Similarly to co-transporters one solute is moving against its’ concentration gradient while another is moving down its’ concentration gradient.
Na+/Ca2+ transport in cardiac and smooth muscle
Counter-transport with downhill movement of 3 Na+ ions into the cell and uphill movement of 1 Ca2+ ion out of the cell
Vesicle mediated transport
Includes exocytosis and endocytosis
Endocytosis
Extracellular substances are trapped within vesicles that are formed from envaginations of the surface membrane which pinches off from the membrane nd fuses with lysosome to release contents
Exocytosis
Intracellular solutes encapsulated within membrane vesicles which fuse with the membrane (usually in response to a stimulus) and release their contents into the extracellular fluid. Include synaptic transmitters, hormones, and digestive enymes.
Receptor-mediated endocytosis
Material to be transported binds to a receptor and then the substance-receptor complex is “ingested” by endocytosis
Osmosis
Diffusion of water. Can occur through the lipid bilayer, ionic channels, and through aquaporins. Most cell membranes highly permeable to water.
Water concentration gradient in cells
Can be described as concentration of dissolved particles on either side of the membrane. No significant water concentration gradient in steady state
What does cell volume depend on?
Number of dissolved particles in cell water and in the extracellular fluid
Osmolarity
Concentration of dissolved particles in solution (must account for ions which separate in water like NaCl by multiplying times the # of ions)
Iso-osmotic
Solution has the same number of dissolved particles (osmolarity) as a reference solution
Hypo-osmotic
Solution has a lower concentration of dissolved particles (lower osmolarity) than a reference solution
Hyper-osmotic
Solution has a higher concentration of dissolved particles (higher osmolarity) than a reference solution
Tonicity
Defined in terms of steady state cell volume. Concerned with the concentration of particles that can permeate the membrane because those that cannot permeate the membrane are unchangeable. Depends on osmolarity and permeability of cell membrane. What will happen to cell if pur in a particular solution.
Hypotonic
Test solution has lesser concentration of nonpermeating solute than reference solution
Hypertonic
Test solution has greater concentration of nonpermeating solute than reference solution
Isotonic
Steady state cell volume remains constant
What effect do permeant can cross the membrane) particles have on steady-state volume?
NO effect but can cause transient changes in cell volume
Osmolarity in steady state
Osmolarity of intracellular and extracellular fluid must be equal to maintain constant cell volume
What particles determine steady state cell volume?
Only impermanent particles which will cross the membrane until their concentration is the same on both sides
Concentration equation
Concentration = Amount/Volume
What happens if the Na+/K+ pump is inhibited?
Cell volume increases because Na+ will accumulate inside of cell and therefore water will enter to balance
Osmosis through capillary walls
Impermeant plasma proteins establish osmotic difference across capillary walls and promote movement of water into capillary lumen from interstitial space. Hydrostatic pressure moves water out of the capillary lumen. Because of the opposing forces, water flows out of capillaries at first by hydrostatic pressure and then drawn back in by osmotic pressure.
Hypoproteinemia
Reduction in concentration of plasma proteins. When concentration reduced osmotic pressure reduced (normally pulls water into capillaries) and causes edema and pleural effusion (fluid accumulation in base of lungs). Comes from increased permeability of nephron walls to plasma proteins leading to urinary excretion of plasma proteins.
Osmolarity vs tonicity
Osmolarity is a physical property of solution and depends only on the concentration of solutes. Tonicity is a biological property that depends on both osmolarity and permeability of the cell membrane.
