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ESA2 - Membranes and Receptors > Role Of Membranes As Permeability Barriers > Flashcards

Role Of Membranes As Permeability Barriers Flashcards

1
Q

What type of molecules can passively diffuse across a lipid bilayer?

A
  • HYDROPHOBIC MOLECULES such as O2, CO2, benzene and steroid hormones
  • SMALL UNCHARGED POLAR MOLECULES such as water, urea and glycerol
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2
Q

Name 2 types of molecules that cannot passively cross a lipid bilayer and give examples of each

A
  • LARGE UNCHARGED POLAR MOLECULES e.g. Glucose, sucrose

- INORGANIC IONS (carry a full charge) e.g. Na+, Ca2+, K+, Cl-

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3
Q

Define passive transport and state what it is dependant on

A
  • Transport of substances across membrane without the use of protein transporters
  • Dependant on PERMEABILITY COEFFICIENT and CONCENTRATION GRADIENT
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4
Q

What equation would you use to calculate the net rate of transport across a membrane, J?

A
  • J = P (C1-C2)
  • P is the permeability coefficient
  • C1 and C2 are the concentration gradients on each side
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5
Q

What is the relationship between the the permeability coefficient (P) and the net rate of transport (J)?

A
  • The higher the permeability coefficient, the more permeable a substance is
  • Therefore the net rate of transport is greater (J is directly proportional to P)
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6
Q

Name 3 roles of transport processes in the membrane

A
  • Maintenance of intracellular ion concentrations and pH
  • Regulation of cell volume
  • Extrusion of waste products
  • Generation of action potentials through ion exchange
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7
Q

Give two examples of ligand-gated ion channels and how they can be activated

A
  • NICOTINIC ACH RECEPTORS on the post-synaptic membrane (binding opens voltage gated Na+ channels)
  • ATP SENSITIVE K+ RECEPTOR in pancreatic β cells (when intracellular ATP is high it binds and closes K+ channels)
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8
Q

Describe an experiment to test the permeability of a lipid bilayer to certain substances

A
  • BLACK FILM
  • Container of water with a septum that has a pinhole
  • Paint phospholipids over pinhole (form bilayer) so can investigate the relative permeability of substances
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9
Q

Define active transport

A
  • Transport of ions or molecules against an unfavourable concentration and/or electrical gradient
  • Requires ENERGY directly or indirectly by ATP hydrolysis
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10
Q

What is facilitated diffusion?

A
  • Diffusion of ions through SELECTIVE CHANNELS in the bilayer
  • Each channel is selective for the ion it transports and depends on a concentration gradient
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11
Q

Describe the process of membrane transport at a gated pore

A
  • PING PONG TRANSPORT
  • Binding of substrate to receptor causes the membrane protein to undergo a CONFORMATIONAL CHANGE
  • Substrate binds on outside and change in conformation allows the substrate to be released on the other side, so it can enter the cell
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12
Q

Describe how a ligand gated ion channel works and give an example

A
  • Ion channel is CLOSED AT REST
  • Binding of a transmitter/hormone opens the gated channel and allows the import of cations into cell
  • e.g. Nicotinic ACh receptor
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13
Q

Describe how voltage gated ion channels are stimulated

A
  • Channels do not interact with other molecules
  • Stimulated to open by change in electric potential across the membrane e.g. Na+ channel
  • Causes DEPOLARISATION or HYPERPOLARISATION of the membrane
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14
Q

Why is passive transport spontaneous?

A
  • Passive transport relies on a concentration gradient (C1 -> C2)
  • Energy is given off so -ΔG and it is spontaneous
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15
Q

Why does active transport require energy?

A
  • Active transport is AGAINST A CONCENTRATION GRADIENT

- +ΔG so requires an energy input via ATP HYDROLYSIS

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16
Q

How much of a cell’s ATP is used in membrane transport?

A

30-50%

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17
Q

What are the normal intracellular concentrations of Na+,K+, Ca2+ and Cl-?

A
  • Na+ = 12mM
  • K+ = 155mM
  • Ca2+ = 10^-7 M
  • Cl- = 4.2mM
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18
Q

Is the extracellular concentration of K+ greater inside or outside the cell?

A
  • GREATER INSIDE (155mM inside and 4mM outside)

- Net movement of K+ is OUT OF CELL

19
Q

What are the normal concentrations of Cl- inside and outside of the cell?

A
  • 4.2mM INSIDE

- 123mM OUTSIDE

20
Q

What is the net movement of Na+ across the bilayer?

A

OUTSIDE cell —> INSIDE cell

21
Q

Why is Ca2+ kept at a very low concentration inside the cell? (10^-7 M)

A
  • Increase in intracellular Ca2+ can activate proteins and enzymes within the cell
  • In some cells this is advantageous (stimulates muscle contraction) however in most cells this can lead to tissue necrosis
22
Q

Where in the cell would you find an example of active transport down a concentration gradient (in the opposite direction)?

A
  • ATP SYNTHASE enzyme in the inner mitochondrial membrane
  • Transports H+ down a concentration gradient
  • ACTIVE as it uses potential energy of the PMF
23
Q

Give an example of a primary active transporter and describe its function

A
  • PMCA (plasma membrane Ca2+ ATPase)

- Transports Ca2+ OUT OF CELL against the concentration gradient using ATP hydrolysis (requires Mg2+!!)

24
Q

Co-transport of ions/molecules can occur by which two mechanisms of transport?

A
  • SYMPORT (same direction)

- ANTIPORT (opposite direction)

25
Q

What is meant by co-transport?

A

More than one type of ion or molecule is transported on a membrane transporter PER REACTION CYCLE

26
Q

Describe the function of the Na+/K+ ATPase transporter and state which mechanism of transport it uses

A
  • ANTIPORT
  • Actively transports 3Na+ out of cell and 2K+ into cell using ATP hydrolysis
  • NET transport of +1 into cell; uses 25% of BMR to maintain ion gradients
27
Q

What is meant by ‘P-type ATPase’?

A

ATP phosphorylates ASPARTATE producing a phosphoenzyme intermediate

28
Q

Why do some membrane proteins have a β subunit?

A

β subunit glycoprotein that directs the protein transporter to the plasma membrane from the ER membrane

29
Q

Describe the action of the drug OUABAIN

A
  • Cardiac glycoside drug

- INHIBITS Na+/K+ ATPase

30
Q

Which pump is the most important when maintaining the resting membrane potential?

A
  • K+ pump

- Outflow of K+ leaves a NEGATIVE MEMBRANE POTENTIAL of -70-80mV

31
Q

What role does the Na+/K+ ATPase pump have in maintaining the resting membrane potential?

A
  • Na+/K+ ATPase sets up a high intracellular concentration of K+
  • This allows K+ to be transported out of the cell which maintains the -ve membrane potential
  • ONLY contributes - 5-10mV to the resting membrane potential through electrogenic pump activity
32
Q

Which two membrane transporters are involved in maintaining a low intracellular Ca2+ concentration?

A

NCX and PMCA

33
Q

What is the role of the Na+/Ca2+ exchanger, NCX?

A
  • ANTIPORT transport of 3Na+ into cell and 1Ca2+ out of cell (NET +1 into cell)
  • SECONDARY ACTION of cell to maintain low intracellular Ca2+
34
Q

What is the relationship between PMCA and NCX with respect to affinity and capacity for Ca2+?

A
  • PMCA has a high affinity but low capacity

- NCX has a low affinity but high capacity

35
Q

Which transporter is present in the gut cells (enterocytes) to aid glucose absorption?

A

Na+/Glucose co-transporter

36
Q

Name the 4 types of co-transporter systems that can occur in membranes

A
  • Na+/K+ ATPase
  • NCX
  • Na+/H+ exchanger
  • Na+/Glucose co-transporter
37
Q

Describe how the action of the Cl- membrane transporter in cystic fibrosis relates to its pathophysiology

A
  • DEFECTIVE Cl- transport due to mutation of CTFR gene
  • Decreased Na+ transport out of cell to maintain resting membrane potential
  • Less Na+ so LESS WATER so results in viscous dehydrated mucus
38
Q

Explain the function of the Cl- transporter is affected by Cholera

A
  • Cholera causes activation of transporter via phosphorylation
  • Cl- transporter is UPREGULATED so more Cl- transported into lumen of large bowel
  • Results in more Na+ transported out, water follows so causes DIARRHOEA
39
Q

Explain why the permeability for Cl- in an erythrocyte membrane is ~10^7 fold greater compared to a phosphotidylserine bilayer

A
  • Band 3 protein facilitates the transport of Cl- across membrane in exchange for HCO3-
  • This is essential for the function of the cell
40
Q

Why are co-transporters considered as secondary active transport?

A
  • Transport of one substance is linked to the concentration gradient of another
  • Primary energy source e.g. ATP hydrolysis is used INDIRECTLY
41
Q

What is the action of a UNIPORT transporter?

A

Transports ONE solute molecule across membrane from one side to the other

42
Q

Describe the action of the Na+/Glucose co-transporter and state which mechanism of transport this uses

A
  • SYMPORT
  • Transports Na+ into enterocytes down its concentration gradient
  • Entry of Na+ provides energy for entry of glucose
43
Q

Give 2 examples of ANTIPORT co-transport systems in cells

A
  • Na+/Ca2+ exchange (NCX) which transports 3Na+ in and 1Ca2+ out
  • Na+/H+ exchange (NHX) which transports 1Na+ in and 1 H+ out

ESA2 - Membranes and Receptors (6 decks)

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