Lecture 3: Transport mechanisms

Question 1

Define reabsorption:

Answer 1

The movement of a substance from the tubular fluid, back into the circulation

Question 2

Define secretion:

Answer 2

The movement of substances from the blood into the tubular fluid via tubular cells (active transport) or intracellular spaces (passive transport)
(not via glomerulus)

Question 3

Define excretion:

Answer 3

The removal of waste products from the blood and the net result of filtration, secretion and reabsorption of a substance

Question 4

How are solutes transported?

Answer 4

Paracellular movement: across the tight junctions connecting the cells, down the concentration gradient
Transcellular movement: through the cell, may be down/against the concentration gradient, water also follows the movement of solutes

Question 5

Give some examples of passive transport mechanisms:

Answer 5

Diffusion: movement of a substance down its electrochemical gradient
Facilitated diffusion: movement of a substance down its concentration gradient, relies on a carrier molecules to transport substances across the membrane (faster than diffusion)

Question 6

What is primary active transport?

Answer 6

An energy dependent process in which substances cross the cell membrane against their concentration/electrochemical gradient
-involves hydrolysis of ATP to ADP and Pi to provide chemical energy

Question 7

What is the most important active transporter in the kidney?

Answer 7

Na+/K+ ATPase pump found on the basolateral membrane of tubular cells (outside)
-transports 3 Na+ within cell and exchanging them with 2 K+ outside the cell so 99% of Na+ is reabsorbed (the 1% of Na+ is how we alter blood pressure)

Question 8

Give some active transporters on the tubular cell membrane:

Answer 8

Ca2+ ATPase
H+/K+ ATPase
H+ ATPase

Question 9

What is secondary active transport?

Answer 9

Uses energy produced from another process for transporting molecules. Carried out by specific carrier proteins embedded in cell membranes called transporters
(Na+/K+ ATPase is vital for pumping Na+ against its concentration gradient to form a driving force for other pumps)

Question 10

What is a symporter?

Answer 10

Moving things in the same direction as the Na+ gradient from the tubule into the tubular cell e.g. Na+/glucose

Question 11

What is an antiporter?

Answer 11

Moving things in opposite directions (other substance is moving in the opposite direction to the sodium gradient
e.g. Ca+/Na+ exchangers

Question 12

What are ion channels?

Answer 12

Protein pores found on epithelial cell membranes, allowing rapid transport of ions into the cell

Question 13

What ion channels do we find on the apical membrane of tubular cells?

Answer 13

Cl-
K+
Na+
(very fast, but not as many of these in comparison to other transporter types)