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1. foundations of medicine ponf > Forces Acting Across Membranes > Flashcards

Forces Acting Across Membranes Flashcards

1
Q

Why does difference in chemical composition of ECF and ICF exist?

A

Presence of cell membrane

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2
Q

Which membranes divide the different water compartments and what are they permeable to?

A

Capillary wall (separates Plasma and ISF within the ECF)- most things apart from plasma proteins and blood cells
Plasma (cell) membrane - selective barrier which only allows for passive diffusion of gases and water by osmosis

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3
Q

What is the cell membrane studded with and what are they always?

A

Aquaporin channels - permanently open to water, allowing its movement between ECF and ICF

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4
Q

What else can freely pass between compartments

A

Gases

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5
Q

Explain ions in relation to movement between compartments?

A

Pass freely across capillary wall so exchange readily between plasma and ISF, but don’t penetrate cell membrane

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6
Q

What are the concentrations of K and Na either side of the membrane?

A

K+ high in ICF but low in ECF
Na+ high in ECF but low in ICF

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7
Q

What is the distribution of solutes like between compartments in terms of concentration?

A

Identical (apart from plasma proteins) between Plasma and ISF, however these vary greatly from ICF

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8
Q

What are the differences in composition of fluid essential for and how are they possible

A

Life (biochemical and physiological processes relying on conc gradients)
Presence and properties of cell membrane

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9
Q

What are the 7 important things about membranes?

A
  1. Selctive barrier
  2. Permeability can vary
  3. Membranes are dynamic
  4. Thin double layer of sheets of lipids (lipid bi-layer)
  5. very flexible
  6. Exellent insulators
  7. Embedded with proteins (and associated with carbs)
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10
Q
  1. Cell membrane is a selective barrier
A

Freely permeable to some substances (O2/CO2) but difference in composition of ICF and ECF shows that permeability is selective

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11
Q
  1. permeability can vary
A

May inc/dec at different times which is important for various cell functions (e.g. transmission of nerve impulses)

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12
Q
  1. membranes are dynamic
A

Continually being formed and maintained or dismantled and metabolized according to the needs of the cell

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13
Q
  1. Very thin double layer of sheets of lipids (lipid bi-layer)
A

Phospholipids: hydrophilic (lipophobic) head and hydrophobic (lipophilic) fatty acid tail. In aqueous environment they arrange themselves so the head is pointing outwards and tail inwards, with the FA chains on the inside of the membrane away from the H20

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14
Q
  1. Membranes are flexible
A

FA behave like oil in vivo, so can stretch (although may rupture if overstretched)

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15
Q
  1. Membranes are insulators
A

AGAINST MOVEMENT OF ELECTRICAL CHARGE. This prevents the passage of electrons which maintains the electrical stability of the cell

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15
Q
  1. Membranes are insulators
A

AGAINST MOVEMENT OF ELECTRICAL CHARGE. This prevents the passage of electrons which maintains the electrical stability of the cell

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16
Q
  1. embedded with proteins (and associated carbs)
A

Can have integral (all way through) or peripheral proteins embedded in the membrane carrying out tasks as enzymes, channels, transporters…

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17
Q

Explain membrane carbohydrates

A

Carbs may attach to phospholipids or proteins embedded in the lipid bilayer - known as glycolipids or glycoproteins - ALL are extracellular
Important in CELL COMMUNICATION (e.g. self vs non-self recognition by immune system). Flag on surface of cell with tell the immune system something about the identity of that cell

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18
Q

Integral membrane protein

A

Span hydrophobic core of lipid bi-layer

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19
Q

Peripheral protein

A

Associated only with phosphate head of lipid bi-layer; doesn’t penetrate hydrophobic core

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20
Q

Give 5 common functions of membrane proteins

A
  1. Receptors
  2. Transport (transporter or channel proteins)
  3. Enzymes
  4. Maintenance of cell structure (anchorage)
  5. Communication
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21
Q

What can membrane protein functions often be?

A

Linked

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22
Q
  1. Receptors
A

Penetrate cell membrane from ECF to ICF which allows for communciation of an extracellular signal (neurotransmitter/hormone) to the intracellular space, creating a cellular response. Only tissues that express receptors for that particular ligand can respond to it.

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23
Q
  1. Explain receptor action in more detail
A

The cell responds to this extracellular chemical signal because the ligand binds to a receptor protein which then causes a confirmation of change in that protein that is transmitted to the intracellular side. Once that signal reaches the intracellular side, it can then change events in the cell and it changes the cell behaviour.

Effectively, extracellular signal brings about change in intracellular activity of cell through the presence of receptor proteins which act to translate that extracellular signal to an intracellular response

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24
Q
  1. Transporter proteins (brief) and 2 forms
A

Integral proteins allowing movement of ions or (large) molecules across the membrane
1. Channel proteins
2. Carrier mediated (transport proteins)

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25
Q
  1. Transporters - a) explain channel proteins
A

Pore through membrane where water/ions/small molecules flow. Can be open (e.g. water) or gated (for ions)

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26
Q
  1. Transporter proteins - b) transporters in more detail
A

Don’t create pore from ECF to ICF but only open to one side of the membrane at a time (intracellular/extracellular). Typically move larger molecules such as glucose.

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27
Q
  1. Enzymes
A

Membrane enzymes catalyse chemical reactions on the cell membrane. Can be found in extracellular side (breaking down nutrients) or intracellular side (converting signals carried from receptors to intracellular responses)

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28
Q
  1. Structural Proteins
A

ANCHOR cell membrane to intracellular skeleton, to ECM (collagen) and/or to other cells.

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29
Q
  1. Communication
A

Between cells or within cells

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30
Q

Explain communication between cells further with example

A

Glycoproteins (membrane P with carb molecule attached to external surface) act as MARKERS which tell the immune system whether a cell is one of our own or foreign - communicate cell identity to immune system

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31
Q

Explain communication within the cell with examples

A

Communicate extracellular message/signal carried by hormones/drugs to intracellular environment and initiate a response. Ex inc receptor/enzyme complexes and G proteins (peripheral inside proteins that relay signal along membrane from integral receptor protein to an integral enzyme protein).

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32
Q

Give action of G protein (briefish)

A

The receptor is integral protein spanning the membrane. Ligand (hormone/neurotransmitter) bind here, causing a change in the receptor, allowing the G-protein to interact with it. That g-protein interacts with the active receptor, which activates the g-protein and allows it to move along the membrane (along the internal leaf). It then interacts with an effector protein (another integral membrane protein which is often an enzyme). G-protein changes that enzyme’s activity or if its an ion channel, will open/close it.

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33
Q

What can enzyme proteins combine with/also do

A

receptors, forming enzyme-receptor complexes

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34
Q

How can membranes differ

A

In protein content: - Myelin is insulator so has low P but high lipid content
- membranes involved in energy transduction (more active membrane e.g. mitochondria) have higher protein content

Protein might be receptor, might be enzyme; it has a function. And the more functions that membrane has, the more protein needs to be embedded in it. Membranes differ in function and thus, differ in protein content.

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35
Q

What are the forces that produce movement of H20 and other molecules across barreirs/membranes driven by?

A

concentration gradients - NO barriers to water movement however

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36
Q

Why doesn’t Na enter the cell and K exit to restor the imbalance?

A

Cell membrane non-permeable to ions - which have to move down channels which are normally closed

37
Q

What happens for charged particles?

A

Not only is there a conc grad across the membrane but due to the diff in charge, there is an electrical grad. These combine to produce and ELECTROCHEMICAL gradient which drives in the direction of passive movement.

38
Q

What is ion movement determined by

A

which grad (elecric or conc) is dominating/greater if against or the direction they are going if together

39
Q

what is independent of electrochemical grad?

A

Endocytosis and exocytosis

40
Q

what are endocytosis and exocytosis

A

Mechanisms of moving macromolecules across the membrane without disrupting them

41
Q

Describe endocytosis

A

INVAGINATION of the cell membrane to form a vesicle which eventually disintegrates on the cytoplasmic (inside) surface of the membrane to release the contents into the cell - to reach destination/be metabolised by enzymes

42
Q

What are endocytosis membrane receptors a common mechanism of?

A

terminating signals from extracellular ligands

43
Q

Define exocytosis

A

proteins /some hormones manufactured in cell are released from said cell. vesicles merge with cell membrane, releasing the protein into extracellular fluid

44
Q

what is one way the cell regulates how much extracellular signals allowed to activate/inhibit whats going on inside the cell?

A

endocytose those receptors into the cell itself - membrane wraps itself around repectors and pulls them into the intracellular compartment where they can then be dissassembled and put back together again on the cell membrane of broken down/recycled.

45
Q

Define diffusion

A

Process by which gas/substance (solute) in solution expands to fill all available volume meaning molecules spread from regions of HIGH CONC to regions of LOW CONC until conc is uniform either side of the membrane (equilibrates)

46
Q

What is diffusion dependent upon?

A

membrane permeability - cell membrane impermeable to intracellular P/organic anions so can’t move out of cell

47
Q

Differentiate between passive and facilitated diffusion

A

Passive - substance moves directly through lipid bilayer
facilitated - substance requires assistance from membrane P to cross lipid bi-layer

48
Q

Passive diffusion

A

Molecules must be:
* small
* uncharged
* lipophilic (hydrophobic) - not so important

gasses pass readily through although O2 and N2 (lipophilic) pass through faster than CO2 (lipophobic)

49
Q

Facilitated diffusion

A

Diffusion of ions/glucose through lipid bilayer is slow so through facilitated diffusion utilising MEMBRANE PROTEINS they travel at a much faster rate than predicted by their lipid permeability

50
Q

What can facilitated diffusion be through

A

channel or mediated transport proteins

51
Q

Summarise P diffusion through channel proteins

A

Transmembrane (integral) proteins providing aqueous route through membrane as a continuous pore for the diffusion of ions in solution (e.g. Na/K/Cl/Ca accompanied by H2O) - mostly closed

52
Q

What can cause gated ion channels to open/close

A

Electrical or chemical (ligand) stimulus - channels are unique for each ion

53
Q

Explain voltage gated ion channels

A

Open/close in response to alterations in MEMBRANE ELECTRICAL POTENTIAL - often found in muscle/nerve cells

54
Q

Explain ligand gated channels

A

Open/close when bind to extracellular chemical such as neurotransmitter/hormone to receptor binding site of channel protein

55
Q

multitasking membrane protein

A

act as both receptor and transporter (multiple function)

56
Q

What does the stimulus cause for all channel proteins

A

Conformational change in the configuration of protein causing them to open/close

57
Q

Membrane Potential

A

Magnitude of difference in separation of charge across membranes so inside carries -ve charge - creates potential gradient which ions flow down

58
Q

When is electrochemical gradient reached?

A

when chemical and electric gradient are in balance - rarely happens as ion channels only open for shor time and Na/K pumps for to repolarise membrane

59
Q

Explain facilitated diffusion through carrier-mediated transport proteins

A

Molecules too big for channel proteins use transporters/
Proteins have binding site for solute they transport (e.g. glucose). When solute binds they change shape and expose the binding site to the other side of the membrane. Conformation changes, solute has low affinity and is released and transporter moves back to original conformation

60
Q

Active transport

A

Molecules transported AGAINST electrochemical gradient which requires energy (ATP). - transporter protein functions as enzyme (ATPase) and hydrolyses ATP, releasing energy.

61
Q

What are active transporters also called

A

pumps (Na/K ATPase or Na/K pump)

62
Q

Explain Na/K pumps

A

Na conc higher outside, K higher inside - cell wants to keep it that way. Na/K ATPase maintains this difference by continually pumping 3 Na ions out and 2 K ions in against their conc grad for each molecule of ATP hydrolysed. Thus there is NET MOVEMENT OF +VE charge out of cell, creating a charge difference across the membrane = electrogentic pump.

63
Q

Why is conc grad important for Na and K?

A

overall electrochemical grad allows for normal neve and muscle function

64
Q

How much of body’s energy used by Na/K ATPase?

A

40%

65
Q

What does Na/K ATPase have

A

Both enzymatic and transporter functions

66
Q

What is filtration

A

movement between plasma and ISF

67
Q

Define osmosis

A

Net movement of water from regions of high conc to regions of low conc

68
Q

Summarise water movement

A

Can move to all body compartments so body in osmotic equilibrium - all movements passive (via aquaporin protein water channels)

69
Q

What happens to a solution of diff conc is separated by membrane permeable by H2O and solute

A

Will be equal conc and equal volume (equilibrium) either side of membrane - diffusion

70
Q

What happens to a solution of diff conc is separated by membrane permeable by H2O but NOT the solute

A

End up with EQUAL CONC but DIFF VOL - water moves to equalise conc (by diluting one compartment assuming it is expandable)
- pressure required t oppose the inc in vol = osmotic pressure (P required to prevent water movement)

71
Q

Describe relationship between osmosis and diffusion

A

When have diffusion (e.g. ion/glucose movement causing solute conc to change) we also have osmosis.
When have osmosis we may not have diffusion if the membrane is non-permeable to ions - this will result in a change in volume (swell/burst or shrink)

72
Q

Mole

A

6.02x10^23

73
Q

What happens to conc if 1 Mole of NaCl (salt/saline) is added to to 1L of water

A

C=2mol/L as 1 mole of Na and 1 mole of Cl produced

74
Q

Osmolarity

A

Conc of biological solutions in OSMOLES (or miliosmoles) and describes number of particles/L in solution

75
Q

What is osmolarity of normal huma plasma (and also tonicity)

A

285 mosmol/L but remember 300

76
Q

What does osmolarity only describe

A

Number of particles in solution, not their NATURE (if they can cross cell membrane)

77
Q

What is the volume of a cell dependent upon

A

Conc of NON-PENETRATING solutes on either side of the membrane (e.g. ions) - as can’t cross any change in conc produces osmotic flux, causing net movement of water in one direction; changing cell volume

78
Q

Give ex. of NP solutes

A

Na+, Cl-, K+, organic ions

79
Q

Define osmolarity

A

Total number of particles in solution (NP and P particles)

80
Q

Define Tonicity

A

Total number of NON-PENETRATING particles in solution (e.g. ions)

81
Q

Define isosmotic, hypo-osmotic, hyper-osmotic

A
  • Isosmotic - same total number of solute particles as normal ECF (plasma)
  • Hypo-osmotic - solution with fewer total solute particles
  • Hyper-osmotic - solution with greater number of toal solute particles
82
Q

Define isotonic, hypotonic, hypertonic

A
  • Isotonic - same number of NP solute particles as normal ECF (plasma)
  • Hypotonic - Solution with fewer NP solute particles
  • Hypertonic - Solution with greater number of NP solute particles
83
Q

What can a) isosmotic, b)hyperosmotic, c) hypoosmotic solutions be with relation to tonicity?

A
  • Isosmotic - isotonic or hypotonic (if not all solute is NP) but NOT hypertonic meaning greater number of NP particles than ECF
  • Hyperosmotic - all 3
  • Hypoosmotic - only ever be hypotonic
84
Q

what are we more interested in (osmolarity or tonicity)?

A

Tonicity as determines cell volume

85
Q

Describe osmolaric equilibrium

A

Number of particles inside cell = number of cells outside cell, per unit volume - but COMPOSITION of particles (NP ion species) differs vastly

86
Q

Predict effect on cell for hypotonic and hypertonic solutions

A

Cells in hypotonic solutions (fewer ions, more water) - SWELL as water enters down chemical grad
Cells in hypertonic solutions (more NP particles, less water in ECF) - SHRINK as water leaves down chemical grad

87
Q

Describe osmolarity/tonicity’s importance in a clinical situation

A

When transferring patients fluid we only have access to plasma so need to think about tonicity to get fluid into cells

88
Q

What happens if place RBC in hyperosmotic urea (penetrating) solution?

A

urea in aqueous hypotonic solution (as no NP particles), meaning urea enters cell, but nothing leaves and water will then enter to equilibrate solution as conc inside has also inc. Cells swells and bursts
- only does so due to hypotonic solution from hyperosmotic urea (pure water once urea move into cell). urea had no impact on cell volume. In living people it is different, very little will happen as plasma more likely to be isosmotic/isotonic.

89
Q

What has NO impact on cell volume?

A

penetrating particles

90
Q

What is the most sensitive organ to changes in tonicity

A

brain - cells can swell and with nowhere to go (skull) brain pressure inc —> death

91
Q

Explain process of glucose symport

A

Passive transport of Na into the cell provides energy/drives and active transport of glucose into intestinal cells (already have high glucose conc). Na/K ATPase then restores Na conc gradient.

1. foundations of medicine ponf (22 decks)

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