Molecular Movement

Question 1

What are the key features of diffusion?

Answer 1

1- Occurs down a concentration gradient
2- Substances travel through the lipid bilayer or involve a protein channel or carrier
3- No additional energy is required

Question 2

What are the key features of active transport?

Answer 2

1- Occurs against the concentration gradient
2- Involved a protein carrier
3- Requires ATP

Question 3

What are aquaporins?

Answer 3

1- Protein carrier
2- Usually found in epithelial cells
3- One water molecule wide
4- 13 different types
5- Wide enough to allow one water molecule to pass at a time, but too narrow to allow passage to hydrated ions

Question 4

Describe the basic structure of an aquaporin.

Answer 4

1- 6 trans-membrane α-helices
2- 5 extra membrane loops
3- Tri-amino acid motif: asparagine-proline-alanine
4- Form tetramers in the membrane but each monomer acts as a water channel in intself

Question 5

Which aquaporins are present in the kidneys?

Answer 5

1- AQP1
2- AQP2
3- AQP3
4- AQP4
5- AQP6
6- AQP7
7- AQP8
9- AQP11

Question 6

How do levels of ADH affect aquaporin channels?

Answer 6

1- ADH, or Vasopressin, is a 9 amino acid long polypeptide
2- ADH increases water reabsorption in the kidney
3- This is done by up-regulating the AQP-2 channels in the epithelial cells of the late distal tubes, collecting tubules and collecting ducts
4- This reduces urine output
5- AQP-2 channels allow increased water uptake, but AQP-3 channels always remain constant

Question 7

Describe the mechanism of action of ADH in the distal and collecting tubule epithelium.

Answer 7

1- ADH, or Vasopressin, binds to V2 receptors

2- This triggers protein kinase A to synthesise more AQP-2 channels, which are inserted into the epithelial membrane

Question 8

What is the rate of diffusion dependant on?

Answer 8

Vmax of carrier protein

Question 9

What are the two types of ion channels?

Answer 9

1- Un-gated, always open

2- Gates channels, open and close

Question 10

What are the characteristics of un-gated channels?

Answer 10

1- Ion channels

2- Transport is determined by size, shape, and charge of both the channel and the ion

Question 11

What are the two types of gated channels?

Answer 11

Ion channels
1- Voltage gated
2- Chemical

Question 12

What is the mechanism of voltage gated sodium channels?

Answer 12

1- Sodium channels are lined with negatively charged amino acids
2- These pull the sodium ion away from its water shell, allowing it to diffuse through the channel

Question 13

What is the mechanism of voltage gated potassium channels?

Answer 13

1- Potassium channels contain carbonyl oxygens (C=O)

2- These pull the potassium ion away from its water shell, allowing it to diffuse through the channel

Question 14

When is active transport carried out rather than diffusion?

Answer 14

1- When the required intracellular concentration of a substance is higher than the extracellular concentration e.g. potassium
2- When the required intracellular concentration of a substance is lower than the extracellular concentration e.g. sodium

Question 15

What are ten examples of substances that require active transport across membranes?

Answer 15

1- Sodium
2- Potassium
3- Calcium
4- Iron
5- Hydrogen
6- Chloride
7- Iodide
8- Urates
9- Some sugars
10- Most amino acids

Question 16

What are the two types of active transport?

Answer 16

1- Primary

2- Secondary

Question 17

What are the characteristics of primary active transport?

Answer 17

1- Molecules are pumped against electrochemical gradient
2- ATP is used
3- Direct use of energy

Question 18

What are the characteristics of secondary active transport?

Answer 18

1- Molecules are driven against electrochemical gradient
2- This is done by the energy of another molecule- usually sodium- which was pumped into or out of the cell using active transport
3- Indirect use of energy

Question 19

What are three examples of primary active transport?

Answer 19

1- Sodium-potassium pump
2- Calcium ion ATPase transporter
3- Hydrogen ion ATPase transporter

Question 20

What is the role of the sodium-potassium pump?

Answer 20

1- Transporting sodium out of the cell
2- Transporting potassium into the cell
3- Maintains these gradients across cell membranes
4- Establishes a negative voltage inside the cell, which is vital for nerve function and signal transmission

Question 21

Describe the structure of a sodium-potassium pump.

Answer 21

1- Globular protein with two subunits
2- Subunit 1 is 100 kda: has three Na+ binding sites internally with ATPase activity and two K+ binding sides externally
3- Subunit 2 is 55 kda
4- Pump is responsible for secondary active transport due to its net potential

Question 22

How is the sodium-potassium pump activated?

Answer 22

1- Three Na+ bind to the internal sites and two K+ bind to the external sites
2- This activates ATPase
3- ATP is cleaved into ADP + Pi
4- Phosphorylation of the pump causes a conformational change leading to Na+ being extruded outside and K+ introduced inside

Question 23

Where are Ca++ ATPase transporters found?

Answer 23

1- Cell membrane

2- Sarcoplasmic reticulum

Question 24

What is the role of Ca++ ATPase transporters?

Answer 24

Maintaining a low cytosolic Ca++ concentration

Question 25

Where are H+ ATPase transporters found?

Answer 25

transporters found?
1- Parietal cells of gastric glands for HCl secretion
3- Intercalated cells of renal tubules to control blood pH

Question 26

What are the roles of H+ ATPase transporters?

Answer 26

H+ ATPase transporters?
1- HCl secretion
2- Controlling blood pH
3- Can increase the concentration of H+ ion up to one-million-fold

Question 27

Describe the mechanism of the Na+ symporter.

Answer 27

1- Secondary active transport
2- Dependant on energy from Na+-K+ pump
3- The high concentration of Na+ outside the cell creates electrochemical energy due to the pressure on Na+ to diffuse back into the cell
4- Na+ and a second molecule bind to the symporter and are transported back into the cell using this energy
5- Second molecule e.g. glucose, amino acids, or 2 x HCO3-

Question 28

Describe the mechanism of antiporters.

Answer 28

1- Secondary active transport
2- Dependant on Na+ electrochemical energy
3- The high concentration of Na+ outside the cell creates electrochemical energy due to the pressure on Na+ to diffuse back into the cell
4- Antiporter uses this energy to transport Ca++ or H+ out while Na+ enters