Lecture 16: Transport across membranes

Question 1

What is non-mediated transport?

Answer 1

Transport across the membrane pass the phospholipid bilayer - no involvement of a transport protein

Question 2

What is mediated transport?

Answer 2

Transport across the membrane via transport proteins

Question 3

What is passive transport?

Answer 3

Movement of substances down a concentration gradient or electrochemical gradient without the hydrolysis of ATP

Question 4

What is active transport?

Answer 4

Movement of substances against concentration gradients involving the use of ATP

Question 5

What is vesicular transport?

Answer 5

Movement of materials pass the membrane in small vesicles - process called exocytosis or endocytosis

Question 6

What substances can travel through the membrane via non-mediated transport?

Answer 6

Small, non-polar, lipid soluble substances such as

oxygen
co2
nitrogen
fatty acids
vitamins

Question 7

Why is non-mediated transport important?

Answer 7

Absorption of nutrients and excretion of wastes

Question 8

How do ions travel across membranes?

Answer 8

Through ion channels - mediated transport

Question 9

What is an ion channel?

Answer 9

A protein which has a channel forming a water-filled pore allowing ions to get past the hydrophobic lipid layer.

Question 10

What are the properties of ion channels?

Answer 10

Selective

Gating

Question 11

How are ion channels specific in the ions that pass through them?

Answer 11

The amino acids of the protein lining the water filled pore contains specific amino acids that allow specific ions to pass

Question 12

What is the importance of ion selectivity?

Answer 12

They are able to harness the energy stored in different ion gradients

Question 13

What is gating?

Answer 13

The idea that ion channels contain gates that control the opening and closing of the pore.

Question 14

How are ions opened and closed?

Answer 14

Via various stimuli

e.g. voltage, ligand binding, cell volume, pH, phosphorylation

Question 15

Is the diffusion through ion channels slow or fast?

Answer 15

Fast as once the water filled pore opens, ions are allowed to freely flow down their concentration gradient

Question 16

How can the ion channel function be measured?

Answer 16

The patch clamp technique - isolating of 1 channel by sucking through a glass pipette

Question 17

What does the patch clamp technique measure?

Answer 17

The current flowing through an individual channel - the fluctuations in current will show the opening and closing of the ion channels

Question 18

What is carrier mediated transport?

Answer 18

Transport across cell membrane involving a transporter protein in which the substrate directly interacts with the protein causing a conformational change to allow the substrate to pass through

Can involve both active and passive processes

Question 19

Is the diffusion via carrier mediated transport fast or slow?

Answer 19

Slow - the substrate must first bind to the transporter protein and cause a conformational change for the protein to pass through to the other side of the membrane

Question 20

What are the properties of the proteins involved in carrier mediated transport?

Answer 20

Carrier proteins have properties similar to enzymes

Specificity - specific substrates bind to specific proteins and are allowed through
Inhibition - certain substrates can cause the carrier proteins to be non functional by inhibiting the binding site
Competition - presence of similar shaped molecules in large concentrations can prevent the binding of a substrate and hence the transport of that substrate across the membrane
Saturation - when all carrier proteins are bound to a substrate the maximum rate of transport is reached

Question 21

How does facilitated diffusion of glucose occur?

Answer 21

1) Glucose binds to transport protein (GLUT)
2) Transport protein changes shape - glucose moves across cell membrane (down a concentration gradient)
3) Kinase enzymes reduce glucose concentration inside the cell by transforming glucose into glucose-6-phosphate

Hence the glucose concentration gradient is maintained and this allows continuous passive diffusion of glucose into the cell

Question 22

What is active transport?

Answer 22

A process of transport across membranes in which energy is required to move substances against their concentration or electrochemical gradient

Question 23

What are the two forms of active transport?

Answer 23

Primary active transport

Secondary active transport

Question 24

What are the properties of primary active transport?

Answer 24

Energy is directly acquired from the hydrolysis of ATP

Question 25

How much of a cell's energy from ATP is used on primary active transport?

Answer 25

30%

Question 26

What are the properties of secondary active transport?

Answer 26

Energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient Hence, energy from ATP is used indirectly

Question 27

What are examples of primary active transporters?

Answer 27

Na/K ATPase Ca/K ATPase (muscle) H/K ATPase (stomach)

Question 28

What is Na/K ATPase?

Answer 28

A type of primary active transporter - involving the movement of sodium and potassium ions

Question 29

Explain the step by step process of movement of ions by the Na/K ATPase

Answer 29

1. 3 Na+ ions in the cytosol of the cell bind to the transport protein 2. This causes ATP to break down and phosphorylate the Na/K ATPase causing conformational change and hence the 3 Na+ ions to leave the cell 3. 2 K+ ions from the extracellular fluid binds to the transport protein causing dephosphorylation of the transporter 4. The transport protein again changes shape allowing the 2 K+ ions to enter the cell

Question 30

What concentration gradient does the Na/K ATPase create?

Answer 30

3 Na+ ions are pumped out of the cell per 2 K+ ions into the cell hence low concentration of sodium ions in the cytosol and high concentration of potassium ions in the cytosol

Question 31

What are the ion concentration gradients created by the primary active transporters used for?

Answer 31

Maintaining resting membrane potential Electrical excitability Contraction of muscle Maintenance of steady cell volume Uptake of nutrients via secondary active transporters Maintenance of intracellular pH by secondary active transporters

Question 32

What is the pump-leak hypothesis?

Answer 32

The idea that the primary ion transporters are working constantly to maintain the concentration gradient as secondary active transporters make use of the gradient to carry out cellular processes

Question 33

What are examples of secondary active transporters?

Answer 33

Na+ antiporter (exchangers) | Na+ symporters (cotransporters)

Question 34

What does a Na+ antiporter do?

Answer 34

Na+ ions rush inward down a concentration gradient forming energy for Ca2+ ions to be pushed out against concentration gradient

Question 35

What does a Na+ symporter do?

Answer 35

Substances like glucose are allowed to travel inward with Na+ ions

Question 36

What is osmosis?

Answer 36

Net movement of water through a selective permeable membrane from an area of high water concentration to low concentration Water concentration is affected by solute concentration

Question 37

How can water travel through the membrane?

Answer 37

Through lipid bilayer and through water channel (called aquaporins)

Question 38

What are aquaporins?

Answer 38

Membrane proteins that allow movement of water There are 9 types

Question 39

What determines the overall permeability of a cell to water?

Answer 39

The permeability through bilayer and permeability through aquaporins Different isoforms of aquaporins are expressed meaning permeability through aquaporins vary by cell