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Chapter Chapter 7

BIO SCI 93 Chapter Notes - Chapter Chapter 7: Facilitated Diffusion, Cell Membrane, Passive Transport


Department
Biological Sciences
Course Code
BIO SCI 93
Professor
Jorge Busciglio Kim Green Marcelo A Wood
Chapter
Chapter 7

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Chapter 7
Molecules have thermal energy due to their constant motion
Diffusion - movement of particles of any substance so that they spread out into the
available space
In the absence of other forces, a substance will diffuse from where it is more
concentrated to where it is less concentrated
Diffusion of a population of molecules may be directional
Any substance will diffuse down to its concentration gradient - region along which the
density of a chemical substance increases or decreases
Diffusion spontaneous - needs no energy
Each substance diffuses down its own concentration gradient
When a substance is more concentrated on one side of a membrane than on the other,
there is a tendency for the substance to diffuse across the membrane down its
concentration gradient - oxygen in cellular respiration - dissolved O2 diffuses into the cell
across the plasma membrane
Diffusion of a substance across a biological membrane is called passive transport
Diffusion of free water across a selectively permeable membrane is called osmosis
Tonicity - ability of a surrounding solution to cause a cell to gain or lose water.
o Tonicity of a solution depends in part on its concentration of solutes that cannot
cross the membrane relative to that inside the cell.
If an animal cell is immersed in an environment that is isotonic (same) to the cell, there
will be no net movement of water across the plasma membrane
In an isotonic environment - volume in animal cell is stable
Water diffuses across membrane but at same rate in both directions.
In a solution that is hypertonic (more nonpenetrating solutes) the cell will lose water,
shrivel, and probably die
In a solution that is hypotonic (less), water will enter the cell faster than it leaves, and the
cell will swell and lyse(burst)
A cell w/o a cell wall can tolerate neither excessive uptake nor excessive loss of water
In hypo- and hypertonic environments, cells w/o wall adapt thru osmoregulation - control
of solute concentrations and water balance
Water can only fill up so much inside with cell walls before the wall starts putting
pressure back on it (turgor pressure)
Turgid (firm) is a healthy state for most plant cells
If a plant’s cells and surroundings are isotonic, there is no net tendency for water to
enter and the cells become flaccid (limp)
If plant cell is in a hypertonic environment - it will lose water to its surrounds and the
plasma membrane will pull away from the cell wall at multiple places - plasmolysis
Facilitated diffusion - when many polar molecules and ions impeded by the lipid bilayer
of the membrane diffuse passively with the help of transport proteins
2 types of transport proteins: channel proteins and carrier proteins
Channel proteins provide corridors that allow specific molecules and ions to cross the
membrane
Aquaporins, water channel proteins, facilitate massive amounts of diffusion that occur in
plant cells and in animal cells
Channel proteins that transport ions are called ion channels
o Most are gated channels - open or close in response to a stimulus
Carrier proteins seem to undergo a subtle change in shape that somehow translocates
the solute-binding site across the membrane
Like ion channels, carrier proteins also don’t use energy to diffuse - passive transport
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