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Physiology I: Cellular Neuro > Lecture 3: Diffusion > Flashcards

Lecture 3: Diffusion Flashcards

1
Q

Biological membrane transport processes regulate

A
  • The composition of body compartments
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2
Q

Transport processes can be used to explain

A
  • Differences in composition between cytoplasm/extracellular fluid (ECF) and cytoplasm/cellular organelles
  • How nutrients enter and waste products leave
  • Regulation of substances in their absorption/reabsorption
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3
Q

Functions of membranes

A
  • Act as barriers

- Oppose free movement of substances (hence have a resistance)

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4
Q

Movement of material through membranes is always

A
  • Less than in free solution
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5
Q

Types of transport THROUGH membranes

A
  • Simple/passive/non-mediated

- Mediated

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6
Q

Passive/non-mediated processes

A
  • Simple diffusion (no energy required)

- Osmosis (no energy required)

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7
Q

Mediated transport processes

A
  • Facilitated diffusion (no energy required)
  • Exchange diffusion (no energy required)
  • Active transport/primary active transport
  • Secondary active transport
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8
Q

Active transport/primary active transport

A
  • Energy required at site of transport
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9
Q

Secondary active transport

A
  • Energy required but not at site of transport
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10
Q

Examples of transport ACROSS membrane

A
  • Endocytosis (receptor mediated)
  • Exocytosis (SNARE mediated)
  • Pinocytosis
  • Phagocytosis
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11
Q

Pinocytosis

A
  • Cell drinking
  • Engulfment of solute
  • Transport across membranes, but not through
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12
Q

Pinocytosis found in

A
  • Kidney tubular cells
  • Intestinal epithelia
  • WBC
  • Kupffer cells in liver
  • Some malignant cells
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13
Q

Characteristics of pinocytosis

A
  • Variety of substances can induce formation of vesicles

- An active process requiring energy for vesicle formation

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14
Q

Substances that can induce pinocytosis

A
  • Proteins
  • Viruses
  • Amino acids
  • Ions
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15
Q

Substances than cannot induce pinocytosis

A
  • Carbohydrates

- Nucleic acids

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16
Q

Active process requiring energy for vesicle formation during pinocytosis

A
  • Selective aspect is the binding of inducer molecules to cell surface
  • Coating protein on cytoplasmic side of membrane
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17
Q

Pinocytosis may involve

A
  • Regions of membrane containing coated pits

- High levels of clathrin protein

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18
Q

Endocytosis can occur in

A
  • Membrane areas lacking coated pits

- Sometimes called receptor mediated endocytosis

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19
Q

Other roles of pinocytosis

A
  • Transport of proteins into embryonic cells
  • Transport of B12 (intrinsic factor complex in ileum)
  • Transport of some ions
  • Absorption of antibodies in infants’ intestine (colostrum from breast milk after birth)
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20
Q

Phagocytosis

A
  • Another example of endocytosis

- Involves particulate/solid matter

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21
Q

Exocytosis (secretion) example

A
  • Nurotransmitter release (eurosecretion) of hypothalamic releasing hormones
  • Ca2+ ions are important trigger molecules
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22
Q

SNARE proteins

A
  • SNAP receptor protein
  • Found on vesicular (v-SNAREs) and on target (t-SNAREs) membranes
  • Calcium interacts with one of the v-SNAREs to permit vesicle to bind to cell membrane
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23
Q

Diffusion definition

A
  • The net movement of a solute from a region of high concentration to an area of low concentration (concentration gradient)
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24
Q

Diffusion occurs due to

A
  • Random thermal motion of atoms and molecules (i.e. Brownian movement)
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25
Q

Diffusion equilibrium is achieved and can be defined as

A
  • A condition in which there is no net movement

- The unidirectional movements are equal

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26
Q

Restrictions to diffusion

A
  • Effective only over short distances
  • Mammals require huge efficient cardiovascular/renal/nervous systems to overcome severe limitations imposed by the laws of diffusion
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27
Q

Time for equilibration over distances

A
  • Few microns = seconds

- Few centimeters = days

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28
Q

Diffusion imposes constraints on size of cells because

A
  • Cellular metabolic rate in large cells would be limited by diffusion of nutrients from plasma membrane to interior of cell
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29
Q

Size of mammalian cells with high metabolic rates

A
  • Less than 20 μm in diameter
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30
Q

No metabolically active cell in the body is

A
  • More than 20 μm on average from a capillary bed
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31
Q

The capillary surface of 1gm of brain tissue is

A
  • About 250 cm2
32
Q

Factors influencing the magnitude of the permeability coefficient (Pm)

A
  • Membrane partition coefficient, bp
  • Membrane diffusion coefficient, Dm
  • Membrane thickness, Xm
33
Q

Membrane partition coefficient, bp

A
  • Main determinant of Pm
34
Q

Membrane diffusion coefficient, Dm

A
  • Vary by less than a factor of 10
35
Q

Membrane thickness, Xm

A
  • Constant for any given cell
36
Q

For any given membrane the range in Pm for a variety of molecules can be up to

A
  • Eight orders of magnitude
37
Q

Other factors that affect Pm

A
  • H bonding between a solute and water
  • Addition of one -OH group to solute
  • Addition of one -CH3 group to solute
  • Halogenation
38
Q

H bonding between a solute and water

A
  • Lowers bp and Pm

- i.e. reduces solutes lipid solubility

39
Q

Addition of one -OH group to solute

A
  • Can reduce Pm by 100 fold
40
Q

Addition of one -CH3 group to solute

A
  • Can increase Pm by up to 10 fold
41
Q

Halogenation

A
  • Increases Pm
42
Q

Examples of molecules that can permeate biological membranes

A
  • Gases
  • Uncharged particles
  • Weak acids or bases in unionized form
  • Polar and lipid soluble molecules
  • (Rate of transport is directly proportional to the concentration gradient)
43
Q

Rates of absorption of drugs from GI tract can be enhanced by

A
  • Adding methyl groups

- Halogenation of the drug

44
Q

Rates of absorption of drugs from GI tract can be slowed by

A
  • Substituting OH groups for the H atoms
45
Q

All small molecules are

A
  • Highly permeable
46
Q

The dominant force controlling permeation

A
  • Molecular size
  • Where Pm alpha 1/molecular volume
  • (only for molecules smaller than glucose)
47
Q

Glucose has a radius of 0.42

A
  • 0.42 nm

- Pores are passive channels and are ubiquitous

48
Q

Pores provide

A
  • Diffusional pathways
49
Q

Small molecules are carried via

A
  • Mediated type of transport
50
Q

Small molecules like water can squeeze between

A
  • Hydrocarbon tails in lipid bilayer.
51
Q

Osmosis (special case of diffusion)

A
  • Net movement of water from a dilute solution (higher water potential) to concentrated solution (lower water potential) across a semi-permeable membrane
52
Q

Osmolality

A
  • Measure of the number of particles in a solution

- Not their size or weight

53
Q

Osmolality can be determined by

A
  • Using a property of the solution that depends on the solute concentration (colligative properties of the solution)
54
Q

Colligative properties of the solution affecting osmolality include changes in

A
  • Boiling/freezing point
  • Vapor pressure
  • Absolute osmotic pressure
55
Q

Osmotic pressure is defined as

A
  • The hydrostatic pressure required to stop water movement across a semipermeable membrane
  • Occurring because of an osmotic gradient (osmotic flow)
56
Q

While the solute distribution in fluid compartments is specific, their total concentration

A
  • Is similar

- Therefore, no great osmotic difference exists between the compartments

57
Q

Diffusion occurs between compartments due to

A
  • Solute imbalances

- Determined by properties related to the solute particle and the barrier across which it is diffusing

58
Q

Functions of a membrane

A
  • Structural (attach cell to ECM and adjacent cells)
  • Detect chemical signals
  • Signal transduction
  • Act as barriers
  • Compartmentalization
59
Q

Membrane transport processes regulate

A
  • Composition of body compartments
  • (Intra- vs Extracellular)
  • (Cytosolic vs intra-organelle)
60
Q

Transport ACROSS not through a membrane

A
  • Endocytosis (entry/engulfment)

- Exocytosis (exit or secretion)

61
Q

Endocytosis (entry/engulfment)

A
  • Phago- (solid)
  • Pino- (liquid)
  • Energy consumed
  • Can involve activation of inducer molecules that trigger the process (hence, receptor-mediated endocytosis)
62
Q

Exocytosis (exit or secretion)

A
  • Requires interaction of SNARE proteins

- Calcium also needed

63
Q

Diffusion definition

A
  • The net movement of a solute from a region of high concentration to a region of low concentration
  • Movement is said to occur downhill
  • Random elastic collisions are the motive force
64
Q

Diffusion equilibrium characteristics

A
  • Solute concentration are equal throughout the solution
  • Does not require outside energy in order to be maintained
  • No NET movement of particles within solution
65
Q

Diffusion limitations

A
  • Very efficient over short distances (µm)
  • Cell Size
  • Distance from supply routes
  • Evolution of delivery systems to keep diffusion distances short
66
Q

Influences on Pm

A
  • Partition coefficient, bp
  • Membrane Diffusion coefficient, Dm
  • Membrane thickness, Xm
67
Q

Influences that lower Pm

A
  • H-bonding between solute and solvent

- Hydroxylation

68
Q

Influences that increase Pm

A
  • Methylation

- Halogenation

69
Q

Osmosis (special case of diffusion of water)

A
  • Same principles apply as for solute diffusion

- Movement FROM a region of high [H2O] TO a region of low [H2O]

70
Q

Concentration of pure water

A
  • 55.5 M

- Therefore any solution will have a lower [H2O] than pure water

71
Q

Osmolarity v Osmolality

A
  • Determined simply by total number of particles in a solution
72
Q

Osmolarity & Osmolality

A
  • Effect on water movement can be “temporary”
  • Tonicity: determined by the number of impermeant particles in a solution
  • Tonicity has permanent effect on water distribution
73
Q

Body compartments

A
  • 66% body weight is water
  • 55% in women; ~75% in newborn
  • 66% intracellular (ICF)
  • 33% extracellular (ECF)
  • 75% ECF water is outside blood vessels (interstitial)
  • 25% ECF water is inside blood vessels (plasma)
74
Q

Total intra and extracellular ionic concentration is similar, but

A
  • Many specific ionic concentration differences exist between the compartments
75
Q

Osmosis of water follows

A
  • Same principles as diffusion of solutes
76
Q

Most body water is

A
  • Intracellular
77
Q

Total ionic concentration inside cells is

A
  • Similar to that outside cells

Physiology I: Cellular Neuro (8 decks)

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