The document discusses interactions between cells and the extracellular environment. It describes the extracellular matrix as consisting of collagen, elastin, and ground substance containing interstitial fluid. It then covers various mechanisms of transport across the plasma membrane including diffusion, osmosis, carrier-mediated transport, and bulk transport. It also addresses concepts such as membrane potential, equilibrium potential, and cell signaling.

1. Interactions Between Cells and the
Extracellular Environment
Compiled and Presented by
Marc Imhotep Cray, M.D.
Basic Medical Sciences Professor
Companion Notes
http://www.slideshare.net/drimhotep/ivmsoverview-of-cell-biology

2. Extracellular Environment
 Includes all constituents of the body located
outside the cell.
 Body fluids are divided into 2 compartments:
 Intracellular compartment:
 67% of total body H20.
 Extracellular compartment:
 33% total body H20.
 20% of ECF is blood plasma.
 80% is interstitial fluid contained in gel-like matrix.
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3. Extracellular Matrix
 Consists of collagen, elastin and gel-like ground
substance.
 Interstitial fluid exists in the hydrated gel of the ground
substance.
 Ground substance:
 Complex organization of molecules chemically linked to
extracellular protein fibers of collagen and elastin, and
carbohydrates that cover the outside of the plasma membrane.
 Collagen and elastin:
 Provide structural strength to connective tissue.
 Gel:
 Composed of glycoproteins and proteoglycans which have a high
content of bound H20 molecules.
 Integrins are glycoproteins that serve as adhesion molecules
between cells and the extracellular matrix.
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4. Categories of Transport Across
the Plasma Membrane
 Cell membrane is selectively  May also be categorized
permeable to some
molecules and ions. by their energy
 Not permeable to proteins, requirements:
nucleic acids, and other  Passive transport:
molecules.
 Net movement down a
 Mechanisms to transport concentration gradient.
molecules and ions through
Does not require
the cell membrane:

metabolic energy
 Carrier mediated transport: (ATP).
Facilitated diffusion and
Active transport:


active transport.
 Non-carrier mediated  Net movement against a
transport. concentration gradient.
 Diffusion and osmosis.  Requires ATP.
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5. Diffusion
 Molecules/ions are in constant state of
random motion due to their thermal
energy.
 Eliminates a concentration gradient and distributes
the molecules uniformly.
 Physical process that occurs whenever there is
a concentration difference across the
membrane and the membrane is permeable to
the diffusing substance.
5

6. Diffusion Through Plasma
Membrane
 Cell membrane is permeable to:
 Non-polar molecules (02).
 Lipid soluble molecules (steroids).
 Small polar covalent bonds (C02).
 H20 (small size, lack charge).
 Cell membrane impermeable to:
 Large polar molecules (glucose).
 Charged inorganic ions (Na+).
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7. Rate of Diffusion
 Speed at which diffusion occurs.
 Dependent upon:
 The magnitude of concentration gradient.
 Driving force of diffusion.
 Permeability of the membrane.
 Neuronal plasma membrane 20 x more
permeable to K+ than Na+.
 Temperature.
 Higher temperature, faster diffusion rate.
 Surface area of the membrane.
 Microvilli increase surface area.
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8. Osmosis
 Net diffusion of H20 across a
selectively permeable membrane.
 Movement of H20 from a high[H20]
to lower [H20] until equilibrium is
reached.
 2 requirements for osmosis:
 Must be difference in [solute] on the
2 sides of the membrane.
 Membrane must be impermeable to
the solute.
 Osmotically active solutes:
 Solutes that cannot pass freely
through the membrane.
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9. Effects of Osmosis
 H20 moves by osmosis into the lower [H20]
until equilibrium is reached (270 g/l glucose).
 Osmosis ceases when concentrations are
equal on both sides of the membrane.
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10. Osmotic Pressure
 The force that would have to be exerted to prevent
osmosis.
 The greater the [solute] of solution, the > the osmotic
pressure.
 Indicates how strongly the solution “draws” H20 into it by
osmosis.
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11. Molarity and Molality
 One-molar solution:
 1 mole of solute dissolved in H20 to = 1 liter.
 Exact amount of H20 is not specified.
 Ratio of solute to H20 critical to osmosis.
 More desirable to use molality (1.0 m).
 One-molal solution:
 1 mole of solute is dissolved in 1 kg H20.
 Osmolality (Osm):
 Total molality of a solution.
 Freezing point depression:
 Measure of the osmolality.
 1 mole of solute depresses freezing point of H20 by –1.86oC.
 Plasma freezes at –0.56oC = 0.3 Osm or 300 mOsm.
11

12. Effects of Ionization on Osmotic
Pressure
 NaCl ionizes when
dissolved in H20.
Forms 1 mole of Na+
and 1 mole of Cl-, thus
has a concentration of
2 Osm.
 Glucose when
dissolved in H20 forms
1 mole, thus has a
concentration of 1
Osm.
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13. Tonicity
 The effect of a solution  Hypotonic:
on the osmotic  Osmotically active solutes
movement of H20. in a lower osmolality and
 Isotonic: osmotic pressure than
 Equal tension to plasma. plasma.
 RBCs will not gain or  RBC will hemolyse.
lose H20.
 Hypertonic:
 .  Osmotically active solutes
 RBC will crenate. in a higher osmolality
and osmotic pressure
than plasma
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14. Regulation of Blood Osmolality
 Maintained in narrow
range by regulatory
mechanisms.
 If a person is
dehydrated:
 Osmoreceptors stimulate
hypothalamus:
 ADH released.
 Thirst increased.
 Negative feedback loop.
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15. Carrier-Mediated Transport
 Molecules that are too large
and polar to diffuse are
transported across plasma
membrane by protein
carriers.
 Characteristics of protein
carriers:
 Specificity:
 Interact with specific
molecule only.
 Competition:
 Molecules with similar
chemical structures
compete for carrier site.
 Saturation:
 Tm (transport maximum):
 Carrier sites have
become saturated.
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16. Facilitated Diffusion
 Passive:
 ATP not needed.
 Powered by thermal
energy of diffusing
molecules.
 Involves transport of
substance through
plasma membrane down
concentration gradient by
carrier proteins.
 Transport carriers for
glucose designated as
GLUT.
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17. Primary Active Transport
 Hydrolysis of ATP directly
required for the function of
the carriers.
 Molecule or ion binds to
“recognition site” on one
side of carrier protein.
 Binding stimulates
phosphorylation (breakdown
of ATP) of carrier protein.
 Carrier protein undergoes
conformational change.
 Hinge-like motion releases
transported molecules to
opposite side of membrane.
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18. Na+/K+ Pump
 Carrier protein is also an ATP
enzyme that converts ATP to
ADP and Pi.
 Actively extrudes 3 Na+ and
transports 2 K+ inward
against concentration
gradient.
 Steep gradient serves 4
functions:
 Provides energy for
“coupled transport” of other
molecules.
 Regulates resting calorie
expenditure and BMR.
 Involvement in
electrochemical impulses.
 Promotes osmotic flow.
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19. Secondary Active Transport
 Coupled transport.
 Energy needed for “uphill” movement obtained
from “downhill” transport of Na+.
 Hydrolysis of ATP by Na+/K+ pump required
indirectly to maintain [Na+] gradient.
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20. Secondary Active Transport
 Cotransport (symport):
 Molecule or ion moving in the
same direction as Na+.
 Countertransport (antiport):
 Molecule or ion moving in the
opposite direction of Na+.
 Glucose transport is an
example of:
 Cotransport.
 Primary active transport.
 Facilitated diffusion.
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21. Transport Across Epithelial
Membranes
 Absorption:
Transport of digestion
In order for a

 products across the
molecule or ion to intestinal epithelium into
the blood.
move from the  Reabsorption:
external  Transport of molecules out
of the urinary filtrate back
environment into into the blood.
Transcellular transport:
the blood, it must 
Moves material through the
first pass through an

cytoplasm of the epithelial
cells.
epithelial  Paracellular transport:
membrane.  Diffusion and osmosis
through the tiny spaces
between epithelial cells.
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22. Bulk Transport
 Movement of many large molecules, that cannot be
transported by carriers, at the same time.
 Exocytosis:
 Fusion of the membrane-bound vesicles that contains
cellular products with the plasma membrane.
 Endocytosis:
 Exocytosis in reverse.
 Specific molecules can be taken into the cell because of the
interaction of the molecule and protein receptor.
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23. Membrane Potential
 Difference in charge
across the membrane.
 Cellular proteins and
phosphate groups are
negatively charged at
cytoplasmic pH.
 These anions attract
positively charged cations
from ECF that can diffuse
through the membrane
pores.
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24. Membrane Potential (continued)
 Membrane more permeable
to K+ than Na+.
 Concentration gradients for
Na+ and K+.
 K+ accumulates within cell
also due to electrical
attraction.
 Na+/ K+ATPase pump 3 Na+
out for 2 K+ in.
 Unequal distribution of
charges between the inside
and outside of the cell,
causes each cell to act as a
tiny battery.
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25. Equilibrium Potentials
 Theoretical voltage produced across
the membrane if only 1 ion could
diffuse through the membrane.
 If membrane only permeable to K+,
K+ diffuses until [K+] is at
equilibrium.
 Force of electrical attraction and
diffusion are = and opposite.
 At equilibrium, inside of the cell
membrane would have a higher
[negative charges] than the
outside.
 Potential difference:
 Magnitude of difference in charge
on the 2 sides of the membrane.
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26. Nernst Equation
 Allows theoretical membrane potential to be
calculated for particular ion.
 Membrane potential that would exactly balance the
diffusion gradient and prevent the net movement
of a particular ion.
 Value depends on the ratio of [ion] on the 2 sides
of the membrane.
 Ex = 61 log [Xo]
z [Xi]
 Equilibrium potential for K+ = - 90 mV.
 Equilibrium potential for Na+ = + 60 mV.
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27. Resting Membrane Potential
 Resting membrane potential is less than Ek
because some Na+ can also enter the cell.
 The slow rate of Na+ efflux is accompanied
by slow rate of K+ influx.
 Depends upon 2 factors:
 Ratio of the concentrations of each ion on the 2
sides of the plasma membrane.
 Specific permeability of membrane to each
different ion.
 Resting membrane potential of most cells
ranges from - 65 to – 85 mV.
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28. Cell Signaling
 How cells communicate with each other.
 Gap junctions:
 Signal can directly travel from 1 cell to the next through
fused membrane channels.
 Paracrine signaling:
 Cells within an organ secrete regulatory molecules that
diffuse through the extracellular matrix to nearby target
cells.
 Synaptic signaling:
 Means by which neurons regulate their target cells.
 Endocrine signaling:
 Cells of endocrine glands secrete hormones into ECF.
 For a target cell to respond to a hormone, NT, or
paracrine regulator; it must have specific receptor
proteins for these molecules.
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29. Cellular Transport
 Diffusion, Dialysis and Osmosis Tutorial by RM Chute
 Osmosis - Examples Colorada State University
 Osmosis by Terry Brown
 Interactive Cellular Transport by Rodney F. Boyer
 Hypotonic, Isotonic, Hypertonic by June B. Steinberg
 Osmosis McGraw-Hill Companies, inc
 Symport, Anitport, Uniport by University of Wisconsin
 Facilitated Diffusion by University of Wisconsin
 Passive and Active Transport from Northland Community and
Technical College
 The Plasma Membrane Dr JA Miyan at Department of
Biomolecular Sciences, UMIST, UK
 Endocytosis of an LDL EarthLink
 Osmosis (thistle tube)
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30. Cellular Structure and Function
 Comparison of Prokaryote, Animal and Plant Cells by Rodney F.
Boyer
 Flash animations of Biological Processes by John L. Giannini
 Organize It by Leif Saul
 Stem Cells Sumanas Inc.
 Membrane Structure Tutorial
 Various Cellular Animations University of Alberta
 Cellular Receptor Animations University of Oklahoma
 Cell Tutorial from "Cells Alive!"
 Simple cell by Terry Brown
 Kinesin - Molecular Motor Sinauer Associates Inc., W. H. Freeman
Co. and Sumanas Inc.
 Kinesin Movie RPI
 Cellular Animations by Donald F. Slish
 Flagella and Cilia from Northland Community and Technical College
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