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

Lecture 3: Diffusion Flashcards

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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
Diffusion equilibrium is achieved and can be defined as
- A condition in which there is no net movement | - The unidirectional movements are equal
26
Restrictions to diffusion
- Effective only over short distances - Mammals require huge efficient cardiovascular/renal/nervous systems to overcome severe limitations imposed by the laws of diffusion
27
Time for equilibration over distances
- Few microns = seconds | - Few centimeters = days
28
Diffusion imposes constraints on size of cells because
- Cellular metabolic rate in large cells would be limited by diffusion of nutrients from plasma membrane to interior of cell
29
Size of mammalian cells with high metabolic rates
- Less than 20 μm in diameter
30
No metabolically active cell in the body is
- More than 20 μm on average from a capillary bed
31
The capillary surface of 1gm of brain tissue is
- About 250 cm2
32
Factors influencing the magnitude of the permeability coefficient (Pm)
- Membrane partition coefficient, bp - Membrane diffusion coefficient, Dm - Membrane thickness, Xm
33
Membrane partition coefficient, bp
- Main determinant of Pm
34
Membrane diffusion coefficient, Dm
- Vary by less than a factor of 10
35
Membrane thickness, Xm
- Constant for any given cell
36
For any given membrane the range in Pm for a variety of molecules can be up to
- Eight orders of magnitude
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Other factors that affect Pm
- H bonding between a solute and water - Addition of one -OH group to solute - Addition of one -CH3 group to solute - Halogenation
38
H bonding between a solute and water
- Lowers bp and Pm | - i.e. reduces solutes lipid solubility
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Addition of one -OH group to solute
- Can reduce Pm by 100 fold
40
Addition of one -CH3 group to solute
- Can increase Pm by up to 10 fold
41
Halogenation
- Increases Pm
42
Examples of molecules that can permeate biological membranes
- 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
Rates of absorption of drugs from GI tract can be enhanced by
- Adding methyl groups | - Halogenation of the drug
44
Rates of absorption of drugs from GI tract can be slowed by
- Substituting OH groups for the H atoms
45
All small molecules are
- Highly permeable
46
The dominant force controlling permeation
- Molecular size - Where Pm alpha 1/molecular volume - (only for molecules smaller than glucose)
47
Glucose has a radius of 0.42
- 0.42 nm | - Pores are passive channels and are ubiquitous
48
Pores provide
- Diffusional pathways
49
Small molecules are carried via
- Mediated type of transport
50
Small molecules like water can squeeze between
- Hydrocarbon tails in lipid bilayer.
51
Osmosis (special case of diffusion)
- Net movement of water from a dilute solution (higher water potential) to concentrated solution (lower water potential) across a semi-permeable membrane
52
Osmolality
- Measure of the number of particles in a solution | - Not their size or weight
53
Osmolality can be determined by
- Using a property of the solution that depends on the solute concentration (colligative properties of the solution)
54
Colligative properties of the solution affecting osmolality include changes in
- Boiling/freezing point - Vapor pressure - Absolute osmotic pressure
55
Osmotic pressure is defined as
- The hydrostatic pressure required to stop water movement across a semipermeable membrane - Occurring because of an osmotic gradient (osmotic flow)
56
While the solute distribution in fluid compartments is specific, their total concentration
- Is similar | - Therefore, no great osmotic difference exists between the compartments
57
Diffusion occurs between compartments due to
- Solute imbalances | - Determined by properties related to the solute particle and the barrier across which it is diffusing
58
Functions of a membrane
- Structural (attach cell to ECM and adjacent cells) - Detect chemical signals - Signal transduction - Act as barriers - Compartmentalization
59
Membrane transport processes regulate
- Composition of body compartments - (Intra- vs Extracellular) - (Cytosolic vs intra-organelle)
60
Transport ACROSS not through a membrane
- Endocytosis (entry/engulfment) | - Exocytosis (exit or secretion)
61
Endocytosis (entry/engulfment)
- Phago- (solid) - Pino- (liquid) - Energy consumed - Can involve activation of inducer molecules that trigger the process (hence, receptor-mediated endocytosis)
62
Exocytosis (exit or secretion)
- Requires interaction of SNARE proteins | - Calcium also needed
63
Diffusion definition
- 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
Diffusion equilibrium characteristics
- 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
Diffusion limitations
- Very efficient over short distances (µm) - Cell Size - Distance from supply routes - Evolution of delivery systems to keep diffusion distances short
66
Influences on Pm
- Partition coefficient, bp - Membrane Diffusion coefficient, Dm - Membrane thickness, Xm
67
Influences that lower Pm
- H-bonding between solute and solvent | - Hydroxylation
68
Influences that increase Pm
- Methylation | - Halogenation
69
Osmosis (special case of diffusion of water)
- Same principles apply as for solute diffusion | - Movement FROM a region of high [H2O] TO a region of low [H2O]
70
Concentration of pure water
- 55.5 M | - Therefore any solution will have a lower [H2O] than pure water
71
Osmolarity v Osmolality
- Determined simply by total number of particles in a solution
72
Osmolarity & Osmolality
- 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
Body compartments
- 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
Total intra and extracellular ionic concentration is similar, but
- Many specific ionic concentration differences exist between the compartments
75
Osmosis of water follows
- Same principles as diffusion of solutes
76
Most body water is
- Intracellular
77
Total ionic concentration inside cells is
- Similar to that outside cells
Physiology I: Cellular Neuro
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