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Cellular and Molecular Biology > Lecture 37: Membrane Transport > Flashcards

Lecture 37: Membrane Transport Flashcards

Thursday 16th January 2025

1
Q

What are the two broad ways molecules can get across membranes?

A

Diffusion and active transport

Diffusion can be free or facilitated, while active transport requires energy.

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2
Q

What types of molecules can diffuse freely across a membrane?

A

Oxygen, carbon dioxide, steroid hormones, benzene. These are small hydrophobic molecules.

Examples include oestrogen, testosterone, and progesterone.

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3
Q

Which smsll, uncharged polar molecules can diffuse across the membrane, but not easily?

A

ethanol, glycerol, water

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4
Q

Why is it difficult for polar molecules to cross membranes?

A

They struggle to negotiate the hydrophobic core of the membrane

Water, ethanol, and glycerol are examples that cross inefficiently.

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5
Q

Which molecules do not cross membranes easily?

A

Large, uncharged, polar molecules, such as Glucose and sucrose

Despite being essential, these larger polar molecules have difficulty crossing membranes.

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6
Q

Can charged ions cross biological membranes ?

A

No

Charged ions like sodium, potassium, and hydrogen ions cannot cross membranes easily.

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7
Q

Can charged polar molecules cross membranes?

A

No

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8
Q

What does transport across a membrane continue until?

A

Transport continues to a dynamic equilibrium:
the same final concentration on each side

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9
Q
A
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10
Q

What is the role of transport proteins in membrane transport?

A

Facilitate the import and export of metabolites and ions

Transport proteins include channels and transporters.

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11
Q

What is the difference between channels and transporters?

A

Channels allow passive transport through an open passage, while transporters bind specific solutes and change shape to facilitate transport

Transporters can flicker between conformations without using metabolic energy.

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12
Q

How does the charge on a membrane affect transport rates?

A

Positive charges outside facilitate transport, while negative charges inhibit it (positive molecules are attracted to the negative inner membrane)

This is due to the electrochemical gradient influencing diffusion.

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13
Q

What is the electrochemical gradient?

A

The combination of concentration gradient and membrane charge influencing the transport of charged molecules

It acts as the driving force for facilitated diffusion.

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14
Q

What is the relationship between solute concentration and transport rate in simple diffusion?

A

Directly proportional

As concentration increases, the rate of transport also increases.

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15
Q

What happens to the transport rate when transporters are saturated?

A

It reaches a maximum rate

This saturation is analogous to enzyme kinetics and can be defined by Km.

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16
Q

What is the role of the GLUT family of transporters?

A

Facilitate glucose transport across membranes

There are 14 different GLUT transporters, each with specific functions.

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17
Q

Are the GLUT family integral transmembrane proteins?

A

Yes

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18
Q

Which GLUT transporter is responsible for basal glucose uptake in cells?

A

GLUT1

GLUT1 is essential for all cells to uptake glucose.

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19
Q

What is the significance of GLUT3?

A

It has high affinity for glucose, allowing the brain to scavenge glucose at low concentrations

This is critical for brain function during low glucose availability.

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20
Q

What is the effect of insulin on GLUT4?

A

Insulin promotes the movement of GLUT4 to the cell surface, reducing blood glucose levels

GLUT4 is key in glucose uptake in response to insulin signaling.

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21
Q

True or False: Transporters can have competitive inhibitors like enzymes.

A

True

Transporters can be competitively inhibited by molecules that resemble their substrates.

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22
Q

Fill in the blank: The maximum rate of transport in facilitated diffusion is analogous to the _______ of an enzyme.

A

Km

Km represents the concentration at which the transport rate is half of Vmax.

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23
Q

What is the specificity of D glucose in relation to transporters?

A

Highly specific for D glucose, not specific for L glucose

This specificity is crucial for understanding how glucose is transported across cell membranes.

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24
Q

Can we competitively inhibit transporters?

A

Yes

Competitive inhibition can affect the function of transporters similar to how it affects enzymes.

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25
Q

What is the function of the GLUT1 transporter?

A

Facilitates basal level glucose transport

GLUT1 is essential for glucose uptake in various tissues.

26
Q

What role does cytochalasin B play in glucose transport?

A

Competitively Inhibits glucose uptake by trapping the transporter in an inward open conformation

Cytochalasin B is an inhibitor of actin polymerization, affecting cell shape and transport dynamics.

27
Q

What is the approximate column measurement for glucose transport?

A

1.8 mm

This sensitivity is crucial for understanding transport mechanisms at the molecular level.

28
Q

What is the distinction between passive diffusion and active transport?

A

Active transport requires metabolic energy, passive diffusion does not

Understanding this distinction is fundamental in cell biology.

29
Q

What are uniporters?

A

Transporters that facilitate one-way transport of molecules. No metabolic energy is required, just the elctrochemical gradient

Uniporters transport molecules down their electrochemical gradient.

30
Q

What are the two main types of active transport mechanisms?

A

Primary active transport and secondary active transport

These mechanisms are essential for moving substances against their concentration gradients.

31
Q

What energy sources drive primary active transport?

A

ATP hydrolysis, light energy, or redox reactions

Each of these energy sources plays a vital role in cellular functions.

32
Q

What is meant by co-transporters?

A

Transporters that move two molecules in the same direction

Co-transporters utilize the energy released from one molecule moving down its gradient to transport another molecule.

33
Q

What is the function of sodium-potassium pumps?

A

Maintain sodium and potassium ion gradients across the cell membrane (contransport)

These pumps are crucial for cellular homeostasis and function.

34
Q

What is the role of calcium pumps in muscle cells?

A

Transport calcium ions into the sarcoplasmic reticulum

Calcium storage is critical for muscle contraction regulation.

35
Q

What is a P-type pump?

A

An uniporter that phosphorylates itself during the transport process. There are P-type pumps that can move:
H+ or K+ or Na+ or Ca2+ from the cytosol

This self-phosphorylation is a key feature of many ion pumps.

36
Q

Describe the P pump, with regards to muscle contraction and Ca2+ ions

A
  • When a muscle is relaxed, The P-type SERCA (sarcoplasmic/endoplasmic reticulum Ca - ATPase) pumps two Ca2+ ions per ATP into the sarcoplasmic reticulum (SR) lumen of myocytes
  • Release of Ca2+ ions from the SR via a Ca2+-release channel triggers a cascade of events that result in muscle contraction (details in LF129 for those taking it)
37
Q

What is the function of V-type pumps?

A

Pump protons into vacuoles or lysosomes to lower pH (uniporter)

This acidification is important for enzyme activation and cellular digestion.

38
Q

What is the significance of maintaining lysosomal pH?

A

Activates lysosomal enzymes for digestion

Proper pH levels are essential for lysosomal function.

39
Q

Describe how the lysosomal pH is maintained?

A
  • V-type ATPases pump protons into the lysosome lumen, lowering the pH.
  • This generates a transmembrane voltage, so another ion must move to dissipate this so that net pumping can continue.
  • The counterion may be either a cation (positive) moving out of the lysosome or an anion (negative) moving into the lysosome.
40
Q

What is a potent inhibitor of V-type pumps?

A

Bafilomycin

41
Q

Describe how parietal cells in the stomach maintain low pH?

A

① CO2 diffuses in from the blood

② CO2 combines with water to form bicarbonate, catalysed by carbonic anhydrase

③ Bicarbonate is exchanged for chloride

④ Protons (H+) are pumped into the stomach lumen by a P-type pump

⑤ Cl- enters the lumen via a Cl- channel (facilitated diffusion)

This process involves multiple transport mechanisms, including bicarbonate exchange.

42
Q

What is the role of carbonic anhydrase in parietal cells?

A

It splits water and forms bicarbonate from carbon dioxide

Carbonic anhydrase is a rapid enzyme crucial for bicarbonate production.

43
Q

How is bicarbonate exchanged in parietal cells?

A

Bicarbonate is pumped out and exchanged for chloride ions

This process raises the chloride ion concentration in parietal cells.

44
Q

What mechanism is used to remove hydrogen ions in parietal cells?

A

A P-type pump that uses energy to pump out hydrogen ions

The pump is activated by phosphorylation.

45
Q

What is the concentration of hydrochloric acid in the stomach?

A

About two molar hydrochloric acid

This results from the combination of hydrogen ions and chloride ions.

46
Q

How can a vacuole pump be repurposed in mitochondria?

A

By running it backwards to drive ATP synthesis

This mechanism involves pumping hydrogen ions into the matrix.

47
Q

What is the function of the electron transport chain in mitochondria?

A

It pumps protons into the membrane space

This creates a proton gradient used for ATP synthesis.

48
Q

What is the structure of A, B, C transporters?

A

They have a complex transmembrane structure and ATPase

This structure allows them to transport a wide range of molecules.

49
Q

Why are ABC transporters significant in bacteria?

A

They transport food molecules and are antimicrobial targets

Inhibiting ABC pumps in mycobacterium may reduce antibiotic use.

50
Q

What types of molecules do ABC transporters handle?

A

They transport a huge range of small molecules

Each transporter is specific to different sets of molecules.

51
Q

What distinguishes primary active transport from secondary active transport?

A

Primary active transport uses metabolic energy; secondary uses the energy of another molecule’s gradient (electrochemical)

Primary transport directly drives uphill transport, while secondary couples it with downhill transport.

52
Q

Fill in the blank: Large molecules and charged molecules do not get across a membrane easily, while _______ can diffuse passively.

A

[small hydrophobic molecules]

53
Q

What are uniporters?

A

Transporters that facilitate and increase the rate of transport

They allow specific molecules to pass through membranes more efficiently.

54
Q

True or False: Small hydrophobic molecules can pass through membranes easily.

A

True

55
Q

What is the main challenge for large and charged molecules in membrane transport?

A

They do not easily cross membranes

This is due to their size and charge, which affects their permeability.

56
Q

Do primary active transporters use ATP?

A

Yes, primary active transporters F-type and V-type use ATP

57
Q

How does the F-type pump work?

A

F-type pumps work in reverse: use proton gradients to synthesise ATP from ADP and Pi

58
Q

Is it true that Secondary active symporters and antiporters use energy stored in concentration gradients to couple the uphill transport of another solute?

A

Yes

59
Q

Is it true that Primary active transporters use metabolic energy to drive uphill transport?

A

Yes

60
Q

p type

A

primary active transporters

61
Q
A

Cellular and Molecular Biology (36 decks)

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