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Block 1 Week 1 > Movement Across Cell Membrane > Flashcards

Movement Across Cell Membrane Flashcards

1
Q

3 types of movement across membrane

A

1) bulk flow (convection)
2) diffusion
3) transport across a barrier

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2
Q

Bulk flow (convection movement)

A
  • movement of fluid/secretions and all the molecules dissolved in the substance
  • pressure gradient drives bulk flow (dense molecules sink, less dense rise)
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3
Q

Diffusion

A
  • passive, random thermal motion
  • used concentration gradients (high–>low concentration)
  • occurs in complete absence of convection
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4
Q

Transport across a barrier

A

1) solubility/diffusion across a lipid bilayer
2) carrier proteins mediate transport (proteins are extremely selective)
3) endocytosis/exocytosis

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5
Q

How determine rate of diffusion?

A

-Fick’s Law of Diffusion tells us rate of substance moving through medium

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6
Q

Equation for Fick’s law of diffusion?

A
  • rate of diffusion measured in (moles/sec)
  • J=DA(dc/dx)

1) J= flux (moles/sec) how many moles of a substance pass through a designated area per second
2) D= diffusion coefficient, takes into account temp, viscosity of medium, molecular size of solute (cm^2/sec)
3) A= (cm^2) area available for diffusion
4) dC/dx= the energy/driving force pushing the molecule, the concentration profile.

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7
Q

How does concentration gradient effect flux (rate of diffusion)?

A
  • concentration profile denoted at dC/dx

- as the concentration gradient/profile INCREASEs, rate of flux increases

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8
Q

What is the diffusion coefficient in Fick’s law of diffusion proportional to?

A

1) D proportional to temperature
2) D inversely proportional to viscosity of liquid
3) D inversely proportional to molecule sie

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9
Q

Diffusion and distance? Why?

A

1) diffusion only efficent/sufficently rapid over short distances
2) time required to diffuse increases with the square of distance
- is why no cell is > ~20um
- why no cell is farther than ~100um from a capillary

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10
Q

Distance of a cell? distance from cell to capillary?

A

1) 10um (50msec to diffuse)

2) 100um (5 sec to diffuse)

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11
Q

How do substances enter/exit cell?

A
  • need to cross the plasma membrane that separates cell from outside environment
  • can do it through diffusion, active transport, or chanels
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12
Q

The plasma membrane?

A
  • phopsholipid bilayer made of amphipathic phospholipids makes a thin film of oil around cell
  • 7nm, extremely thin
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13
Q

What are amphipathic phospholipids?

A
  • have hydrophobic & hydrophilic regions
    1) hydrophilic head made of glycerol phosphate, and choline/serine
    2) hydrophobic fatty acid tails (hydrocarbons)
  • tails are esterified to head group
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14
Q

Amphipathic phospholipids in water?

A
  • in presence of H20 phospholipids spontaneously assembly into a thin film/ bilayer w/ charged heads pointing out, hydrophobic heads facing in
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15
Q

Where are the proteins responsible for carrying out transport functions in cell?

A
  • within the lipid film are integral membrane proteins responsible for carrying out all specific functions of membrane
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16
Q

Diffusion through the lipid bilayer? (3 steps)

A

1) partition into lipid (most important step)
2) diffusion through lipid
3) partition into cytoplasm

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17
Q

What is diffusion through the bilayer largely dependent on?

A
  • the ability of the solute to partition into the lipid core of the membrane (hydrophobic space)
  • use the partial coefficient
18
Q

What is the partition coefficient? Equation?

A
  • the degree to which molecules can partition into the lipid phase
    (degree to which molecules favor hydrophobic space)
  • Denoted as K
  • S= separation

K= [S] in lipid/ [S] in water

19
Q

Why is K important? What does

A

-k= partition coefficient, determines whether molecules (drugs) can penetrate cell membrane through lipid phase easily

K>1 enter lipid phase readily (hydrophobic molecules)
K<1 doesn’t enter lipid phase readily (hydrophilic molecules)

20
Q

How K>1 molecules move into the cell?

A

since have high K (partition coefficient) they readily move into the hydrophobic space, accumulate and cause steep conc. gradient. Therefore readily move down gradient into the cell

21
Q

Partition coefficient and drugs? What does it determine?

A
  • K determines how fast drugs reach biological target
    1) rate of absorption from GI tract (large K so can go from belly–> into specific cells)
    2) whether accumulate in body fat (large K=hydrophobic=acucmulate in fat and escape metabolism/ kidney excretion)
    3) persistence in body
22
Q

What alter Fick’s law to determine flux per membrane area? (as opposed to just rate of diffusion)

A

-Ficks Law=DA(dC/dx)

  • diffusion rate across a membrane:
    • (J/A)= P(C(out)-C(in))
    • y=mx

-P= permeability coefficient & is experimentally measured

23
Q

most biological important molecules are hydrophilic (have very low K) how do they get into the cell readily?

A
  • transported by on or more membrane proteins

- either channels, transporters or pumps

24
Q

4 functions of membrane proteins?

A

1) gatekeepers (channels/pumps)
2) anchors (structural proteins)
3) sensors (receptors)
4) biochemical hubs (enzymes)

25
Q

What are the 4 modes of membrane transit?

A

1) simple diffusion
2) ion channels
3) transporters
4) Pumps

26
Q

Ion channel composition and movement?

A
  • is a water filled pore
  • molecules move down electrochemical gradient via diffusion
  • is an integral membrane protein
27
Q

Types of Transporters (3)?

A

1) uniporter: facilitate single molecule
2) cotransporter: use ion gradient to move 2 substances across cell
3) Exchanger: exchanges for one extracellular molecule for one intracellular molecule

ARE NOT CHANNELS

28
Q

What are pumps?

A
  • move solute from low–> high conc. (again gradient)

- ATPase

29
Q

What do ion channels do?

A

-creates a voltage difference between inside & outside of cell which can be used for other things

30
Q

Ion flow through a channel is?

A

1) conductive
2) passive
3) rapid
4) bidirectional based on gradient
5) highly selective
6) gated so opening is regulated

31
Q

What does it mean that ion channels are “slaves to two masters”?

A
  • that have high concentrate of K+ in the cell, low conc out side of cell. So K= wants to move out even though outside the cell is also +. Meaning that electrochemically K+ wants to enter the cell
  • have electrochemical gradient (voltage across membrane) as well as concentration gradient, tend to be opposite
32
Q

How do ion channels open/close (4 ways)?

A

1) ligand bindings extracellulary
2) phosphorylation
3) voltage/depoalrization
4) membrane deformation (stretch)

33
Q

How cll membranes permeable to H20 despite its hydrophilic nature?

A

-aquaporins (membrane spanning protein channels) facilitate H2O movement

34
Q

Aquaporin structure?

A
  • 6 transmembrane alpha helices, 2 central loops
  • form homotetramers (from 4 identical monomers)in cell membrane
  • each monomer conducts H2O
  • the tetramers form a pore in center only fits H2O in single file line
35
Q

Uniport?

A
  • a transporter
  • facilitated diffusion
  • high selective substrate binding pocket facing extracellualry.
  • when ligand binds conf. change occurs, open intracellularly releases molecule
  • conf. change open back up extracellular
  • ligand binding/unbinding causes conf. change
36
Q

outer vs inner cellular env?

A

1) outer= similar to sea water but diluted 3 fold (highly Na+)
2) high in K+, poor Na+

  • causes a cation gradient
  • is due to continuous activity of sodium-poitaasium pump
37
Q

Na/K pump?

A
  • example of active transport

- 3 Na+ into cell and 2 K+ out of the cell

38
Q

Cation gradients?

A

are generated by Na/K ATPase pump

  • store ENERGY that can be used to perform WORK
    ex: secondary active transport
39
Q

secondary active transport?

A

-use of cation ion gradients (active transport) to move molecules into the cell

40
Q

Na+/ glucose pump?

A
  • co-transporter
  • moves Na+ and glucose into cell together
  • relies on Na/K pump cation gradient
  • Na+ moves down cation gradient into cell (negative charge)
  • glucose move down concentration. gradient

Block 1 Week 1 (9 decks)

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