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BIOSCI 107 > Lecture 16: Transport across membranes > Flashcards

Lecture 16: Transport across membranes Flashcards

1
Q

What is non-mediated transport?

A

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

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2
Q

What is mediated transport?

A

Transport across the membrane via transport proteins

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3
Q

What is passive transport?

A

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

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4
Q

What is active transport?

A

Movement of substances against concentration gradients involving the use of ATP

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5
Q

What is vesicular transport?

A

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

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6
Q

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

A

Small, non-polar, lipid soluble substances such as

oxygen
co2
nitrogen
fatty acids
vitamins
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7
Q

Why is non-mediated transport important?

A

Absorption of nutrients and excretion of wastes

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8
Q

How do ions travel across membranes?

A

Through ion channels - mediated transport

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9
Q

What is an ion channel?

A

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

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10
Q

What are the properties of ion channels?

A

Selective

Gating

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11
Q

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

A

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

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12
Q

What is the importance of ion selectivity?

A

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

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13
Q

What is gating?

A

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

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14
Q

How are ions opened and closed?

A

Via various stimuli

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

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15
Q

Is the diffusion through ion channels slow or fast?

A

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

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16
Q

How can the ion channel function be measured?

A

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

17
Q

What does the patch clamp technique measure?

A

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

18
Q

What is carrier mediated transport?

A

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

19
Q

Is the diffusion via carrier mediated transport fast or slow?

A

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

20
Q

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

A

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

21
Q

How does facilitated diffusion of glucose occur?

A

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

22
Q

What is active transport?

A

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

23
Q

What are the two forms of active transport?

A

Primary active transport

Secondary active transport

24
Q

What are the properties of primary active transport?

A

Energy is directly acquired from the hydrolysis of ATP

25
Q

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

A

30%

26
Q

What are the properties of secondary active transport?

A

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

27
Q

What are examples of primary active transporters?

A

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

28
Q

What is Na/K ATPase?

A

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

29
Q

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

A
  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
30
Q

What concentration gradient does the Na/K ATPase create?

A

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

31
Q

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

A

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

32
Q

What is the pump-leak hypothesis?

A

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

33
Q

What are examples of secondary active transporters?

A

Na+ antiporter (exchangers)

Na+ symporters (cotransporters)

34
Q

What does a Na+ antiporter do?

A

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

35
Q

What does a Na+ symporter do?

A

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

36
Q

What is osmosis?

A

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

37
Q

How can water travel through the membrane?

A

Through lipid bilayer and through water channel (called aquaporins)

38
Q

What are aquaporins?

A

Membrane proteins that allow movement of water

There are 9 types

39
Q

What determines the overall permeability of a cell to water?

A

The permeability through bilayer and permeability through aquaporins

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

BIOSCI 107 (29 decks)

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