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)

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

What are the two ways in which solutes can be transported?

Answer 4

• Paracellular movement – across the tight junctions connecting the cells. Down concentration gradient
• Transcellular movement – through the cell. May be down or against concentration gradient. Water follows movement of solutes by osmosis

Question 5

Define diffusion

Answer 5

• The movement of a substance down their electrochemical gradient. Passive.

Question 6

Define facilitated diffusion

Answer 6

The movement of a substance down their electrochemical gradient, relies on a carrier molecule to transport substances across the membrane. Passive. Faster than diffusion

Question 7

Define primary active transport

Answer 7

An energy dependent process in which substances cross the cell membrane against their concentration and electrochemical gradients.

Question 8

What does primary active transport include?

Answer 8

• Involves hydrolysis of ATP to ADP and Pi to provide chemical energy for the transport mechanism.

Question 9

What is the most important transporter in active transport and where is it found?

Answer 9

Na+/K+ ATPase pump. Found on the basal and basolateral membranes of the tubular cells

Question 10

What is the action of the Na+/K+ ATPase pump

Answer 10

• Na+/K+ ATPase transports Na+ from intracellular to extracellular spaces. Allowing the nephron to reabsorb
99% of filtered Na+

Question 11

List other active transporters on tubular cell membrane

Answer 11

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

Question 12

Define secondary active transport

Answer 12

• Uses the energy produced from another process for transporting molecules.

Question 13

Give an example of secondary active transport

Answer 13

Na+/K+ ATPase as the driving force for the secretion and reabsorption of other solutes in which the energy is provide by the Na+ gradient.

Question 14

Define symport and antiport in relation to secondary active transport using Na+/K+ ATPase

Answer 14

• Symport – Same direction as Na+ gradient e.g. Na+/K+ cotransporter or Na+/glucose
• Antiport - Against electrochemical gradient, opposite to Na+ gradient e.g. Ca2+/Na+ and the H+/Na+ exchangers

Question 15

Give a list of transporter that are used to drive secondary active transport

Answer 15

• H+/K ATPase
• Proton pump
• Ca2+ ATPase
• Na+/K+ ATPase

Question 16

What are ion channels

Answer 16

• Protein pores found on the epithelial cell membranes.
• Allow rapid transport of ions into the cell.
• Channels specific to Cl- , Na+ and K+ are found on apical membrane of tubular cells.