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FLS > Cell membrane transport > Flashcards

Cell membrane transport Flashcards

1
Q

What are the types of transport across cell membranes?

A

Active
Passive

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2
Q

What are the types of passive transport?

A

Simple
Facilitated
No energy input (ATP) is required for these

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3
Q

What is simple diffusion?

A

Movement down a concentration gradient.
No membrane proteins are involved.

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4
Q

What is simple diffusion dependent on?

A

Concentration gradient
Hydrophobicity / charge
Size

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5
Q

What is the relative ease of compounds simply diffusing across membranes?

A

Easy: Hydrophobic molecules - O2, CO2, N2 steroid homones
Small uncharged polar molecules - H2O, urea, glycerol
Large uncharged polar molecules - glucose, sucrose
Impossible: Ions - H+, Na+, HCO3-, K+ etc

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6
Q

What is transport of inorganic ions required for?

A

Regulation of intracellular ion concentrations.
Uptake of nutrients - e.g. glucose and amino acids.
Excretion of metabolic waste products.

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7
Q

What are the classes of facilitated diffusion?

A

Channels - discriminates on size and charge
Uniporter carrier proteins - involves a binding site for solutes.

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8
Q

What is facilitated diffusion?

A

Transports inorganic ions or small molecules across the membrane passively along their concentration / electrochemical gradients.

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9
Q

What is an electrochemical gradient?

A

Combines the concentration gradient and membrane potential.
This force drives a charged solute across a membrane.

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10
Q

What are ion channels?

A

Membranes are impermeable to ions so ion channels are used to transport ions across membranes.

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11
Q

What are the features of ion channels?

A

Exhibit ion selectivity.
Driven by concentration/electrochemical gradient.
Fast - transport 10^7 molecules per sec
May be regulated (open in response to stimulus)

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12
Q

How are ion channels regulated?

A

Voltage gated
Ligand-gated - extracellular ligand
Ligand-gated - intracellular ligand
Mechanically gated

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13
Q

What are K+ channels?

A

Most common ion channels.
Continuously open
Selective.
Quickly moves K+ out of the cell.

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14
Q

What are uniporter carrier proteins?

A

E.g. the Glucose transporter (Glut2) in gut epithelia.
Highly selective - transported molecule is bound to carrier
Relatively slow - <1000 molecules per second - because it requires conformational change.

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15
Q

What does a uniporter carrier protein look like?

A
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16
Q

What are glucose transporters?

A

They only transport glucose.
Expressed by most cell types
12 pass membrane spanning proteins
Alternate between 2 conformations.

17
Q

How is glucose transported into erythrocytes by Glut1?

A

Glucose concentration is higher in blood than in erythrocyte.
Transported into cell by Glut1 along concentration gradient.
Glut1 works in both directions so gradient needs maintaining - glucose is converted to glucose-6-phosphate.
Glucose-6-phosphate not recognised by Glut1 so one-directional transport.

18
Q

Why do cells maintain electrochemical gradients?

A

To drive transport across membranes.
To maintain osmotic balance
Electrical forces inside and outside of the cell must be balanced.

19
Q

What happens if there is no active transport?

A

Without active transport to maintain electrochemical gradients, ions would flow down their gradients through channels, disturbing osmotic balance.

20
Q

What is active transport?

A

It moves solutes against their electrochemical gradients.
This requires energy.

21
Q

How do cells carry out active transport?

A

ATP-driven pumps
Coupled transporters
Light-driven pumps

22
Q

What are ATP-driven pumps?

A

Couple the transport of a solute against its gradient to the hydrolysis of ATP - primary active transport.

23
Q

What are coupled transporters?

A

Couple the transport of one solute with the gradient to another solute against the gradient - secondary active transport.

24
Q

What are light-driven pumps?

A

Couple the transport of a solute against its gradient to the input of energy from light.

25
Q

What are the concentrations of Na+ and K+?

A

Na+ concentration is high on the outside of the cell.
K+ concentration is high on the inside of the cell.

26
Q

How is the Na+ electrochemical gradient maintained?

A

Na+/K+ ATPase:
Pump operates continuously to expel Na+ that enters through other carrier proteins and channels.
Hydrolyses ATP to ADP - it is an enzyme and carrier protein.
Couples the export of Na+ to the import of K+.

27
Q

What is the mechanism of Na+/K+ ATPase?

A

3 Na+ ions bind. Pump hydrolyses ATP and is phosphorylated.
Na+ dependent phosophorylation causes pump to undergo conformational change, and Na+ is transferred across membrane.
2 K+ ions bind and the pump is dephosphorylated.
K+ dependent phosphorylation causes the pump to return to its original conformation and K+ is transferred across the membrane.

28
Q

What are the types of coupled transporters?

A

Symport - transports ions or molecules in the same direction
Antiport - transports ions or molecules in opposite directions.

29
Q

What is the Na+/glucose symporter?

A

Na+ electrochemical gradient is used to drive the movement of glucose against its gradient.
[Na+] is high in the gut.
[glucose] is high in the cell.

30
Q

What is the Na+/glucose coTransporter (SGLT1)?

A

Expressed by epithelial cells lining the GI tract.
Glucose concentration is higher in the epithelial cells than in the lumen.
Symporter utilises the Na+ electrochemical gradient to transport glucose into the cells against its concentration gradient.

31
Q

How is glucose absorbed?

A

Na+ gradient is created by Na+/K+ ATPase (primary).
Na+/glucose symporter transports glucose into epithelial cells (secondary).
Glut2 transports glucose out of the cells along concentration gradient and into blood supply (facilitated diffusion).

32
Q

What is the Na+/Ca2+ antiporter?

A

Cardiac muscle contraction is triggered by a rise in intracellular [Ca2+].
Na+/Ca2+ antiporter reduces intracellular [Ca2+] and reduces the strength of cardiac muscle contraction.

33
Q

How is cardiac failure treated?

A

Ouabain and digoxin inhibit Na+/K+ ATPase.
This increases intracellular [Na+] and reduces the Na+ gradient.
The effectiveness of the Na+/Ca2+ antiporter is reduced, [Ca2+] is maintained and cardiac contraction is strengthened.

34
Q

What is the configuration of the Na+/glucose symporter?

A

Transporter can exist as different confugurations:
Either open to the inside of the cell, or to the outside, and it can flip between these forms.

35
Q

Why are glucose symporters required for the absorption of glucose from the GI tract?

A

There are tight junctions between epithelial cells so small molecules such as glucose cannot travel by paracellular movement.
They must travel through the cells - transcellular movement.

FLS (14 decks)

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