BIOM20002 Lecture Notes - Lecture 6: Active Transport, Resting Potential, Extracellular Fluid
Digestive movements are not rapidly regulated, blood pressure and balance are
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Positive feedback cannot maintain homeostasis
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Diffusion: passive, spontaneous movement of particles from a region of higher concentration
to lower concentration. Net movement down a concentration gradient (thereby eliminates it)
occurs until the concentration is equal everywhere. Faster over short distances, directly
related to temperature, inversely related to molecular weight/size
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Penetrating (can pass): lipid soluble, uncharged, small particles e.g. lipids, water
[traverses water-filled ion channels and aquaporins], O2, CO2
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Non-penetrating (cannot pass): lipid insoluble, charged, large particles e.g. ions, proteins
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Cell membrane: separates cell (intracellular) from environment (extracellular). Need energy to
maintain/control this difference
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Osmolarity: total concentrations of solutes, penetrating and non-penetrating. Normal
cell osmolarity is 300 mOsm
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Or: the number of osmotically active particles (ions/molecules) per litre of solution.
Expressed in osmoles per litre, osmol/L or OsM
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Comparing osmolarities of 2 solutions: isosmotic=equal number of solute particles per
volume. Hyperosmotic/hyposmotic=higher/lower number of solute particles per volume
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Tonicity: concentration of only the non-penetrating solutes. Always in reference of the
extracellular non-penetrating solute concentration compared to the cell's non-
penetrating solutes. A solution can be isotonic (same concentration of nonpenetrating
solutes; no change), hypertonic (higher concentration of nonpenetrating solutes;
shrinks) or hypotonic (lower concentration of nonpenetrating solutes; swells) with
respect to the cell
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Tonicity has no units; it is a comparative term that compares a solution to a cell
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Can a solution be both hyperosmotic and isotonic? Remember osmolarity is total
particles. Tonicity is non-penetrating particles. So yes you can.
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Osmolarity and tonicity
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Homeostatic control
6 Homeostasis, control, membrane potential
Wednesday, July 30, 2014
5:29 PM
HSF Page 1
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Q = net rate of diffusion
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ΔC = concentration gradient
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P = permeability of membrane to substance
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A = surface area of membrane
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MW = molecular weight of substance (√~molecular size)
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ΔX = distance (membrane thickness)
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Fick's Law
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Osmotic equilibrium
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HSF Page 2
Water moves between intra- & extra-cellular space
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Dilutes the more concentrated solution: osmosis
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Net movement stops when at equilibrium
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Osmolarity usually 300 mOsm
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e.g. Na+ higher outside than inside
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Chemical disequilibrium
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Does not mean an even distribution of different types of particle
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Body is in osmotic equilibrium
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Osmotic equilibrium
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i.e. simple negative feedback pathway
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Neural/endocrine/neuroendocrine
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Whole body integration
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Sometimes simple; sometimes complicated
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How is homeostasis achieved?
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particles influence solubility?
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membranes influence rate of passage?
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What properties of:
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Passive/facilitated diffusion; channel/carrier/pump proteins in membrane
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How do cells control entry/exit and therefore control their composition?
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What determines if a particle can enter a cell?
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Autonomic system
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Antagonistic but complementary
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Influence varies
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Maybe excitatory for one target; inhibitory for another
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Parasympathetic and Sympathetic
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HSF Page 3
Document Summary
Digestive movements are not rapidly regulated, blood pressure and balance are. Diffusion: passive, spontaneous movement of particles from a region of higher concentration to lower concentration. Net movement down a concentration gradient (thereby eliminates it) occurs until the concentration is equal everywhere. Faster over short distances, directly related to temperature, inversely related to molecular weight/size. Cell membrane: separates cell (intracellular) from environment (extracellular). Penetrating (can pass): lipid soluble, uncharged, small particles e. g. lipids, water. [traverses water-filled ion channels and aquaporins], o2, co2. Non-penetrating (cannot pass): lipid insoluble, charged, large particles e. g. ions, proteins. Osmolarity: total concentrations of solutes, penetrating and non-penetrating. Or: the number of osmotically active particles (ions/molecules) per litre of solution. Expressed in osmoles per litre, osmol/l or osm. Comparing osmolarities of 2 solutions: isosmotic=equal number of solute particles per volume. Always in reference of the extracellular non-penetrating solute concentration compared to the cell"s non- penetrating solutes.
