 |
Diffusion and Osmosis
Diffusion - the
process by which molecules spread from areas of high concentratiion, to
areas of low concentration. When the molecules are even throughout a space -
it is called EQUILIBRIUM
Concentration gradient
- a difference between concentrations in a space.
|
OSMOSIS
Watch this animation
of water molecules moving across a selectively permeable membrane. Water
molecules are the small blue shapes, and the solute is the green.
The solute is more
concentrated on the right side to start with, which causes molecules to
move across the membrane toward the left until equilibrium is reached.
Start Animation |
 |
Selectively Permeable - membranes that allow some things through, the cell
membrane is selectively permeable, water and oxygen move freely across the
cell's membrane, by diffusion
Osmosis -
the diffusion of water (across a membrane)
Water
will move in the direction where there is a high concentration of solute
(and hence a lower concentration of water.
A simple rule to
remember is:
Salt is a solute,
when it is concentrated inside or outside the cell, it will draw the water
in its direction. This is also why you get thirsty after eating something
salty.
Type of
Solutions
 |
If the concentration
of solute (salt) is equal on both sides, the water will move back in
forth but it won't have any result on the overall amount of water on
either side.
"ISO" means
the same |
 |
The word "HYPO"
means less, in this case there are less solute (salt) molecules outside
the cell, since salt sucks, water will move into the cell.
The cell will gain
water and grow larger. In plant cells, the central vacuoles will fill
and the plant becomes stiff and rigid, the cell wall keeps the plant
from bursting
In animal cells,
the cell may be in danger of bursting, organelles called CONTRACTILE
VACUOLES will pump water out of the cell to prevent this.
|
 |
The word "HYPER"
means more, in this case there are more solute (salt) molecules outside
the cell, which causes the water to be sucked in that direction.
In plant cells, the
central vacuole loses water and the cells shrink, causing wilting.
In animal cells,
the cells also shrink.
In both cases, the
cell may die.
This is why it is
dangerous to drink sea water - its a myth that drinking sea water will
cause you to go insane, but people marooned at sea will speed up
dehydration (and death) by drinking sea water.
This is also why "salting
fields" was a common tactic during war, it would kill the crops in the
field, thus causing food shortages. |
Diffusion
and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy
is required for the molecules to move into or out of the cell.
Sometimes,
large molecules cannot cross the plasma membrane, and are "helped" across by
carrier proteins - this process is called facilitated diffusion.
| |
ACTIVE
TRANSPORT
Active Transport - When cells must move materials in an opposite
direction - against a concentration gradient. It requires Energy.
Proteins or Pumps are found in the cell membrane transport molecules
across the membrane.
Molecular Transport - Proteins are used to move small
molecules such as calcium, potassium, and sodium ions across the
membrane
Endocytosis -
cell takes in large particles by engulfing them
Phagocytosis - "cell eating" - extensions off cytoplasm surround a
particle and package it within a food vacuole and then the cell
engulfs it. Ex. Amoebas use this process.
Pinocytosis - the process of taking up liquid from the surrounding
environment. Tiny pockets form along the membrane, fill with liquid,
and pinch off.
Exocytosis - cell gets rid
of particles, opposite of endocytosis |
|
|
