Human Physiology:
NOTES
Study Guide – Excitable Membranes & Signaling
Membrane Transport
structure of the cell
- homeostasis: maintenance of a stable internal environment
- cellular structures
- plasma membrane: barrier between internal and external environments, with selective control
- nucleus: double-layered membrane containing cellular DNA
- cytoplasm: contents of a cell, composed of organelles, cytosol, and cytoskeleton
- cytoplasmic contents
- organelles: specialized structures of the cell
- endoplasmic reticulum: extensive, continuous network of fluid-filled sacs
- rough ER (granular): associated ribosomes; functions in synthesis and distribution of proteins
- smooth ER (agranular): lacks ribosomes; has varying functions in different cell types
- packaging of lipid and protein products for vesicular transport
- steroid hormone synthesis (gonad cells)
- detoxification of foreign substances (liver cells)
- calcium storage (muscle cells)
- Golgi complex: closely associated with ER; functions in distribution of proteins
- mitochondria: catabolize molecules to produce energy as ATP (food + O2 à CO2 + H2O + ATP)
- lysosomes: waste disposal system of the cell; contain hydrolytic enzymes
- peroxisomes: additional waste disposal system of the cell; contain oxidative enzymes
- cytosol: highly organized gel forming cellular environment
- contents: metabolic enzymes, ribosomes, storage vesicles
- cytosolic ribosomes: produce soluble proteins, as opposed to extracellular or integral proteins
- cytoskeleton: forms cellular “bone and muscle”
- components: microtubules, microfilaments, intermediate filaments, microtrabecular lattice
- functions: imparts structure
allows organelles and components to move in an ordered manner
allows cell to move as a whole
structure of the membrane
- plasma membrane
- function: forms a selective barrier that regulates the contents of the cell
- components: lipids, carbohydrates, proteins
- lipids
- structure: hydrophobic molecules composed primarily of carbon chains or rings
- composition: non-polar hydrocarbon tail
polar head group
- consequences of polarity: formation of the lipid bilayer
- molecules with similar polar/non-polar properties interact more strongly
- lipids form structures that reduce interactions with polar solutions
- monolayer: skin on top of aqueous solution, with hydrophobic tails lined up to face air
- micelle: spherical structure with hydrophobic tails clumped in the center
- bilayer vesicle: bubble formed by lipids lining up tail to tail
- carbohydrates
- structure: found in the form of glycolipids or glycoproteins
- function: help protect and insulate the cell, and act as binding sites for external substances
- proteins
- structure: polypeptide chains
- function: channels or carriers
membrane structure and movement
signaling between cell and environment
- fluid mosaic: model depiction of the flexible membrane and its varied components
membrane transport mechanisms
- diffusion
- diffusion: the random motion of molecules
- above absolute zero (T = 0 K), molecules have energy that causes vibration and movement
- molecules constantly collide, causing alterations in kinetic energy that affect both molecules
- random walk: path taken by an individual molecule as part of diffusion
- predictable behaviors of diffusion processes
- flow down a gradient: net movement in the direction of the decreasing gradient
- gradient: difference in some quantity between one location and another
- concentration gradient: maintained by cells as a means of using energy to do work
- determinants of diffusion rate: Fick’s law of simple passive diffusion
- Fick’s law: 
- variables
- Jx: flux across the membrane (moles/sec)
- Dx: diffusion coefficient (cm2/sec)
- A: membrane area (cm2)
- βx: partition coefficient between solution and membrane (unitless)
- w: membrane thickness (cm)
- ΔCx: concentration difference across the membrane (ΔCx = [X]o – [X]i)
- permeability coefficient
- expression: 
(cm/sec) 
- consolidates terms and can be empirically determined from Jx measurements across a membrane
- values in pure lipid bilayers
- ionic substances: at or near zero
- organic compounds: vary, but can range quite high
- tendency toward equilibrium
- equilibrium: situation in which opposing forces acting on a system add up to zero net force
- equilibrium is force balance, rather than zero force
- forces due to diffusion: osmotic, electrochemical
- osmosis
- definitions
- osmosis: diffusion of water molecules
- osmotic gradient: difference in the concentration of water
- osmotic pressure: force created by water on a membrane
- functions
- movement of water within an organism
- impartation of structure through exertion of hydrostatic pressure
- osmotic pressure: 
- π: osmotic pressure (pressure necessary to prevent movement of water down [] gradient)
- n: number of solute particles
- V: volume
- C: concentration (n/V)
- R: gas constant
- T: absolute temperature
- φ: nonideality coefficient (deviation from ideal gas)
- physiology
- equilibrium: pressure within cell is balanced with diffusional force driving water down gradient
- hypertonic: high [solute], and thus low [H2O] (relative to solute)
- hypotonic: low [solute], and thus high [H2O] (relative to solute)
- lysis: rupturing of the cell membrane; can be caused by exposure to a hypotonic extracellular fluid
- electrochemical potential
- definitions
- ion channels: proteins that impart selective permeability on a membrane
- chemical gradient: diffusional gradient
- charge gradient: electrical gradient
- potential difference: voltage across membrane created by movement of ions
- equilibrium potential
- movement of ions will cause buildup of charge that resists further ion movement
- equilibrium potential: balance of chemical and potential forces
- carrier-mediated transport
- integral membrane proteins: proteins that form part of the membrane
- can be used to transport molecules that otherwise cannot diffuse
- by spanning membrane, can bind specific solutes and ferry them across
- characteristics
- specificity: transport of a specific substance or closely-related substances
- saturability: limitation (transport maximum, or Tm) on amount of solute transported per unit time
- competitive binding: binding of closely related substances can diminish rate of transport
- types
- facilitated diffusion: passive diffusion of substances down an established gradient
- ion channel: continuous pathway for the solute to flow through
- multi-state carrier: binding on one side of the membrane and release on the other
- active transport: energy-based transport of solutes against a concentration gradient
- primary (1º): energy derived directly from ATP hydrolysis
- secondary (2º): use of diffusion down an established gradient to impart solute movement against a gradient
- cotransport: transport of the two molecules in the same direction
- antiport: transport of the two molecules in the opposite direction
- vesicular transport
- function: movement of solutes of a large size or quantity
- types
- endocytosis: formation of a pocket in the membrane and fusion to create an intracellular vesicle
- exocytosis: fusion of a vesicle with the membrane that releases contents of the lumen into extracellular space
maintenance of concentration gradients by active transport
TABLE: Intracellular and Extracellular Ionic Concentrations
| outside | inside | ||
| Na+ | 145 mM | Na+ | 12 mM |
| K+ | 4 mM | K+ | 155 mM |
| Ca2+ | 1 mM | Ca2+ | 10-4 mM |
| Cl- | 115 mM | Cl- | 5 mM |
- Na+/K+ ATPase
- type: primary active transport
- function: use of ATP hydrolysis to establish Na+ and K+ gradients
- mechanism: transports 3 Na+ out and 2 K+ in
electrical elements of the membrane
- components
- cell membrane: capacitors
- membrane separates two electrical conductors (intra- and extracellular fluid compartments)
- with membrane potential, ions accumulate on either membrane interface, as on plates of a capacitor
- ion channels: resistors (typically defined as conductance, or g = 1/R)
- ionic gradients: batteries (driving force)
- note: the behavior is not simply analogous, but actually describes the behavior of excitable membranes
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