Epithelial Transport Flashcards
- Describe the generic epithelial transport mechanisms for absorbing NaCl and water into the blood.
Unlike basolateral membrane, apical/luminal membrane is much more permeable to sodium instead of potassium. So there is no Na/K pump on the apical membrane. Sodium ions leak into epithelial cell across apical membrane down their electrochemical gradient, but then pumped across the basolateral membrane via the Na/K pump. Chloride follows passively; and net transport f NaCl produces an osmotic gradient which draws water along as well.
Pericellular and transcellular are types of transport
- Describe the basic transport mechanisms by which glucose and amino acids are absorbed into the blood.
AAs and glucose are pumped from GI lumen into blood by GI epithelium. Nutrient is pumped across the apical membrane and then passively moves out of cell into the interstitial fluid where it can then be picked up by other cells (?). Glucose and AA pumps are sodium-dependent secondary active transporters (symporters). As Na moves down its gradient into the cell, pump uses that energy to move sugar or AA against its gradient into the cells. Across non-epithelial cells, glucose is transported by facilitated diffusion (not a pump). That is on the basolateral side.
- Differentiate between ‘tight’ and ‘leaky’ epithelia.
Tight = more junction proteins, tighter seal between cells (used in distal tubes of kidneys or sweat glands). Used for fine-tuning and controlling substance transport at lower levels. oculidin and cloudin.
Loose = less junction proteins, looser seal between cells. Capable of moving a larger amount of material. Less tight junctions provide a pericellular shunt pathway for the movement of water and solutes across epithelia. Leaky epithelia can’t maintain energy gradients as large as those in tight epithelia because of the shunt.
Substance can either cross both membranes to get from one side to another, or cross no membranes by passing through the shunt pathway.
Na/K pump only on basolateral membrane.
- Calculate, given two of three variables (apical membrane potential, basolateral membrane potential, transepithelial potential), the third one.
TransPD = Vm (BL) – Vm (Ap)
Remember that MPs of apical and basolateral membrane will be different (different ion permeabilities, etc.). Also, membrane potentials are always written in the form of describing the inside of the membrane with respect to the outside. Ex: If MP = -10 mV, the inside of the cell is 10mV more negative than the outside.
- Describe the basic process by which some epithelial cells secrete (rather than absorb) fluid.
Secretion based on chloride channel in the apical membrane (normally closed, but when activated, cell secretes electrolytes and water into lumen). Cl is pumped into cell across the basolateral membrane (via Na-K-2Cl co-transporter driven by leaky Na) and then leaks into the lumen (drawing Na+ passively, and consequently water). Apical Cl- channels at rest unless epithelia receive a signal at basolateral membrane (like parasympathetic nerve stimulation during digestion) signal carried to apical membrane by ‘signaling mechanisms’ (Ca, cAMP, etc.)
Depends on Cl if closedabsorbed
Cl is opensecretion
- Identify four important substances (water, O2, CO2, and urea) that are never pumped across membranes, but always move passively down their concentration gradients.
These always follow their gradients. You can open and close aquaporin channels to control the flow of water sometimes, but you can never pump it.
Water, O2, CO2, urea = never pumped; always passively moving.
- Describe the main routes of excretion of metabolic wastes - CO2 and urea, in particular.
We have 15 moles of metabolic waste that we need to get rid of each day.
Most of it, ~14.5-15 moles of waste is gotten rid of via breathing out CO2, which is volatile so it is simple exhaled with no need for any special transporters (lipid soluble). Some water is also lost this way.
Rest of waste is non-volatile falls to the kidney to get rid of them. (If not uremia = urine in blood). Majority of non-volatile waste is urea, rest are protons. Like lungs, in kidney, blood capillaries pass close to ends of dead-end tubules (in glomerulus or alveoli) chemical substances move from blood into tubules. Instead of selectively pumping waste products, kidney forms an ultrafiltrate of plasma that contains all the good things (sugars, AAs, etc.) and reabsorbs then (pumps them back into the blood the tings it wants to keep). Wastes just pass on. Requires lots of ATP.
About 30 mmoles of waste excreted by GI tract, tends to be breakdown products of RBCs, which are toxic. Otherwise excretes things we couldn’t really absorb in the first place.
- Compare and contrast the relative roles of the G.I. tract (minimal) and kidney (extensive) in excreting non-volatile metabolic wastes and regulating ECF composition.
GI tract absorbs everything. has no job in exccreting metabolic waste. kidney does everything.
- Identify the main function of the kidney, and explain how it is designed to do this: “I know what I like.”
Most important job of kidney is to get rid of non-volatile metabolic waste since no other organ can do it. Instead of using transporters to pump out the waste products, the kidney does the opposite because it ‘knows what it likes’ and instead forms a huge ultrafiltrate of all beneficial compounds + waste products and selectively reabsorbs all the good stuff while letting waste pass on.
Its the house cleaning example. it takes everything out of the house first and then takes back what it needs.
