Lecture 16: Transport across Membranes

Question 1

What is the difference between passive and active transport?

Answer 1

Passive transport moves substances down their concentration or electrochemical gradients using only kinetic energy

Active transport uses cellular energy to drive substances against their gradients.

Question 2

What is the difference between channel and carrier mediated transport?

Answer 2

Channel mediated transport involves the rapid movement of ions through a water-filled pore without binding

Carrier mediated transport involves binding of the substance to a carrier protein, which then changes shape to transport the substance across the membrane.

Question 3

Differentiate between primary and secondary active transport.

Answer 3

Primary active transport directly uses energy (e.g., ATP) to transport substances against their gradient

Secondary active transport uses the energy from the movement of another substance down its gradient to drive the transport of a different substance against its gradient.

Question 3

What properties drive water movement across cell membranes?

Answer 3

Water movement across cell membranes is driven by osmotic gradients, which depend on the concentration of solutes inside and outside the cell, as well as the presence of aquaporins that facilitate water transport.

Question 3

What is non-mediated transport?

Answer 3

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

Non-mediated transport refers to the diffusion of substances through the lipid bilayer without the assistance of transport proteins, important for the absorption of nonpolar, hydrophobic molecules.

Question 4

What is mediated transport?

Answer 4

Transport across the membrane via transport proteins

Question 5

What is passive transport?

Answer 5

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

Question 6

What is active transport?

Answer 6

Movement of substances against concentration gradients involving the use of ATP

Question 7

What is vesicular transport?

Answer 7

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

Question 8

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

Answer 8

Small, non-polar, lipid soluble substances such as

oxygen
co2
nitrogen
fatty acids
vitamins

Question 9

Why is non-mediated transport important?

Answer 9

Absorption of nutrients and excretion of wastes

Question 10

How do ions travel across membranes?

Answer 10

Through ion channels - mediated transport

Question 11

What is an ion channel?

Answer 11

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

Question 12

What are the properties of ion channels?

Answer 12

Selective via Gating

Question 13

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

Answer 13

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

Question 14

What is the importance of ion selectivity?

Answer 14

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

Question 15

What is gating?

Answer 15

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

Question 16

How are ions opened and closed?

Answer 16

Via various stimuli

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

Question 17

Is the diffusion through ion channels slow or fast?

Answer 17

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

Question 18

What is the role of ion channels in transport?

Answer 18

Ion channels facilitate rapid transport of ions across the membrane through a water-filled pore, allowing ions to pass without binding to the channel.

Question 19

What is ionic selectivity in ion channels?

Answer 19

Ionic selectivity is determined by specific amino acids lining the channel pore, which allow only certain ions to pass based on their charge and size.

Question 20

What is gating in ion channels?

Answer 20

Gating refers to the control of opening and closing of ion channels, which can be influenced by various stimuli such as voltage, neurotransmitters, hormones, and mechanical stress.

Question 21

What is the patch clamp technique used for?

Answer 21

The patch clamp technique is used to measure the electrical current generated by the diffusion of ions through ion channels, allowing for the recording of current fluctuations that represent the opening and closing of single ion channels.

Question 23

How does the current through an ion channel relate to ion movement?

Answer 23

The diffusion of over 1 million ions per second through a channel generates a measurable current (~10⁻¹² amp), with current fluctuations indicating conformational changes in the channel structure associated with gating.

Question 24

What are the characteristics of carrier-mediated transport?

Answer 24

Carrier-mediated transport is slower than channel transport, involves direct interaction between the substrate and transporter protein, and exhibits properties such as specificity, inhibition, competition, and saturation.

Question 25

What is the process of facilitated diffusion of glucose?

Answer 25

In facilitated diffusion, glucose binds to the transport protein (GluT), causing a conformational change that allows glucose to move across the membrane down its concentration gradient, while a kinase enzyme converts glucose to glucose-6-phosphate to maintain the gradient.

Question 26

What is saturation in the context of transporter proteins?

Answer 26

Saturation occurs when transport increases with substrate concentration until all binding sites on the transporter are occupied, displaying enzyme kinetics similar to that of enzymes.

Question 27

What is the significance of the energy expenditure in primary active transport?

Answer 27

A typical cell uses about 30% of its energy (ATP) on primary active transport, highlighting its importance in maintaining cellular functions.

Question 28

What is the function of the Na/K ATPase?

Answer 28

The Na/K ATPase pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, maintaining low Na⁺ and high K⁺ concentrations in the cytosol, which is electrogenic and generates a net current.

Question 29

What are the steps involved in the Na/K ATPase function?

Answer 29

The steps include Na⁺ binding ATP splitting and Na⁺ being pushed out K⁺ binding and phosphate release K⁺ being pushed in.

Question 30

Why is the Na/K ATPase important for cellular functions?

Answer 30

It maintains resting membrane potential, electrical excitability, muscle contraction, steady state cell volume, nutrient uptake via secondary active transporters, and intracellular pH.

Question 31

What is the pump-leak hypothesis?

Answer 31

The pump-leak hypothesis states that the Na/K ATPase continuously works to counteract the leakage of Na⁺ and K⁺ ions back into the cell down their gradients.

Question 32

What is secondary active transport?

Answer 32

Secondary active transport uses energy stored in ion gradients created by primary active transporters to move other substances against their concentration gradient, indirectly utilizing ATP hydrolysis energy.

Question 33

What are examples of secondary active transporters?

Answer 33

Examples include: Na⁺ antiporters (exchangers) that move Na⁺ inward while pushing Ca²⁺ or H⁺ out Na⁺ symporters (cotransporters) that transport glucose or amino acids inward along with Na+ ions.

Question 34

What is osmosis?

Answer 34

Osmosis is the net movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration, occurring when the membrane is permeable to water but not to certain solutes.

Question 35

What happens when an osmotic gradient exists?

Answer 35

When an osmotic gradient exists, water will move to eliminate it, balancing the concentrations on either side of the membrane.