Physiology 1021 Study Guide - Final Guide: Colloid, Pneumothorax, Anterior Pituitary

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13 Apr 2012
Physiology 1021 1
The Excitable Cell
Homeostasis and Body Fluid Composition
Physiology: the study of function in living organisms, exploring the mechanisms by which the
organisms control their internal environments, and attempts to explain the physical and chemical
factors responsible for both normal function and pathology
External environment: continuous with the outside world
o Air coming into the lungs has crossed no cellular membrane to get in
o Contents of G.I. tract is part of external until it has been absorbed
Internal environment: things must cross cells (absorbed, diffused)
o Trillions of cells, each its own entity
Homeostasis: the maintenance of relatively stable conditions in the internal environment.
Homeostatic mechanisms are dynamic, self-regulatory mechanisms involving all of the organs and
tissues of the body. Uses two control systems (both use negative feedback):
o Endocrine system: slow, long-acting
o Nervous system: fast, short period
Negative Feedback Control Systems: operate to maintain homeostasis. Measures a value and
compares to set point value. If above or below, initiates a series of responses to get back to set point
Extracellular fluid: fluid found outside of the cell (such as plasma)
Intracellular fluid: found inside the cell
o For the average person, 60% is water
o For a 70kg person, the total body water is 42L (28L intracellular fluid, 14L extracellular fluid,
11L interstitial fluid, 3L plasma)
In a typical cell, concentration is a ‘salty banana’ salt, NaCl on the outside, K+ on the inside
The compositions of the interstitial fluid and plasma (ecf) are approximately the same
o However, intracellular and extracellular fluid are very different
o This is possible because the intracellular fluid is separated from the interstitial fluid by a cell
membrane which is selectively permeable and contains transport mechanisms
Cell membrane structure: selectively permeable membrane, made up of phospholipids arranged into a
selectively permeable bilayer
o The fatty acid chains of the phospholipid are hydrophobic (and are therefore the major
permeability barrier to water and water-soluble substances like ions, glucose …etc., but is
permeable to fat-soluble substances such as oxygen, alcohol and steroid hormones)
o Phospholipid: polar phosphate head, 2 fatty acid chains (lipid, nonpolar)
o In water, the O atom exerts a greater pull on the electron, so is negative. Because of this,
water attracts to phosphate head, making phosphate hydrophilic and lipid hydrophobic
Cell Membrane and Membrane Transport
Functions of membrane proteins
1. Receptors (e.g. for hormones,
2. Enzymes
3. Ion channels (pores)
4. Membrane carriers (e.g. for
transport of glucose)
Membrane transport
1. Endocytosis/exocytosis (pinocytosis for small molecules)
2. Diffusion through lipid bilayer (fat-soluble molecules)
3. Diffusion through protein channels (water-soluble molecules)
4. Facilitate diffusion
5. Active transport
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Physiology 1021 2
Simple diffusion: the movement of molecules from an area of high concentration to low concentration
(down the concentration gradient) due to the molecules’ random thermal motion
o Net movement of the substance from A to B
o Will continue until the concentration of the substances are the same (now equilibrium)
o Equilibrium, no NET movement, molecules are still constantly in movement, but as a whole,
they move from A to B
Substances must be lipid-soluble (nonpolar) in order to penetrate the lipid region of the lipid bilayer.
Lipid soluble molecules: oxygen, carbon dioxide, fatty acids, steroids
Factors influencing rate of diffusion for non-polar molecules:
1. Concentration gradient (larger the gradient, faster the movement as a whole)
2. Temperature: (higher temperature promotes faster diffusion)
3. Surface area available for diffusion (larger area means faster diffusion)
4. Size of the molecule (smaller means faster)
5. Viscosity (stickiness of the medium, lower means faster)
Ions and water-soluble molecules (including water) cannot diffuse directly through the cell membrane
due to the hydrophobic lipid region. These require a pore channel (driving force is the concentration
Factors influencing diffusion in polar molecules:
1. Concentration gradient
2. Size of the molecule
3. Charge (+/-)
4. Number of pores in the membrane (depends on cell, function, tissue)
Protein-lined pore: pore is very small, inside has small negative charges (net negative). Pore is
somewhat selective, being negative (Cl- wouldn’t through, but Na+ would)
Facilitated diffusion: attachment of a molecule to an integral protein will cause a conformational
change in that protein, moving the molecule across the membrane (driving force is the concentration
o Example glucose: glucose is too big to simply diffuse, so glucose binds to the carrier protein,
causing a conformational change. The carrier flips, now facing intracellular fluid. Glucose is
then released into the cell
Competitive inhibition: similar structure, such as maltose, can bind to the carrier, but will not cause a
conformational change as glucose did (blocking all the carriers, glucose can’t enter cell)
Active transport: a form of carrier-mediated transport chemically specific, can be competitively
inhibited. However, moves substances against the concentration gradient, and so requires energy in
the form of ATP
Na+/K+ Pump: K+ and Na+ passively diffuse through pores, active transport takes input of ATP and
pumps K+ back into the cell, and Na+ out (counteracting the diffusion occurring)
o 1 ATP 3 Na+ out, bring 2 K+ in
o Pump maintains concentration gradient
Osmosis and Tonicity
Osmosis: the net movement of water down its concentration gradient (water concentration
determined by the number of solute particles in solution, not their size)
o Requires a semi-permeable membrane which allows water across, but not the solute
o Water moves from dilute solution to concentrated one (more water less water)
Osmosis across the cell membrane is affected by:
1. The permeability of the membrane to the solutes in the intracellular and interstitial fluids
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Physiology 1021 3
2. The concentration gradients of the solutes in the intracellular and interstitial fluids
3. The pressure gradient across the cell membrane
Solution: what’s being dissolved
Solvent: what’s doing the dissolving
Solution: solute + solvent (in bio, solvent is water)
As a general rule, water will move into the area with a higher solute concentration
Osmotic pressure: the pressure which would have to be applied to the NaCl solution in order to stop
the net influx of water into that solution
o Depends on the number (rather than size) of particle in solution
Osmole: concentration of a solute expressed in number of particles
Osmolality: the number of osmoles per kg of water
Osmolarity: the number of osmoles per litre of solution (used interchangeably with osmolality)
Tonicity: the ability of a solution to cause osmosis across biological cell membranes
o Isotonic: same, no net movement, no osmosis
o Hypotonic: lower osmolarity outside, water enters cell (goes from dilute to concentrated, cell
o Hypertonic: higher osmolarity outside, water exits the cell (cell will shrink)
Resting Membrane Potential and the Action Potential
Molecules move from areas where they are in high concentration to low concentration, moving down
their chemical gradient
Ions move towards areas of opposite charge, or down their electrical gradient
o An ion will move down its electrochemical gradient until an electrochemical equilibrium is
Resting membrane potential: the potential difference across the plasma membrane present in resting
o The fluids inside and outside the cells are electrolytic (contain ions)
o Anions accumulate immediately inside the cell membrane along the inner surface, while
cations accumulate immediately outside the membrane this establishes an electrical
potential difference (inside negative with respect to the outside)
Pores are not created equally: while the membrane is permeable to Na+, it isn’t that great (leaks in
slowly), while the K+ pore is very permeable
More positive charge outside of cell (net and charge, while inside is negative)
Selective permeability results in a membrane potential: eventually, a positive electrical charge
prevents the further influx of positive charges, creating an electrical potential
The membrane is permeable to a variety of ions
Each membrane affects the resting membrane potential depending on the membrane permeability
and concentration gradient for each ion
Ion movement depends on both concentration gradient and electrical gradient
The sodium potassium pump is a form of carrier-mediated transport, requiring energy
o Establishes and maintains the resting membrane potential, maintains concentration gradient
Excitable cells: generate or respond to electrical signals nerve cell (neuron) and muscle cells
(skeletal, smooth, cardiac)
The neuron: cell body (soma) is where the nucleus is found, and where the working parts for the cell
are located. The membrane at rest is -70mV
The action potential: a rapid reversal of the resting membrane potential (negative-positive-negative)
1. Depolarization: take away the charge
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