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Diffusion
And
Osmosis
Homeostasis: A stable internal environment.
*In a cell, factors such as pH and the concentrations of other
substances must ALWAYS remain the same (even when the
external environment changes). It is the special properties of
the cell membrane that help the cell maintain homeostasis.
Quick physics lesson…
Atoms and molecules are
always in motion.
The atoms of gases move
about the most freely, but
even the molecules of
solids move about a fixed
point.
This energy of motion is called
kinetic energy.
Diffusion:
Movement from an
area of greater
concentration to an
area of lesser
concentration.
*Diffusion occurs because molecules are in constant motion…
Diffusion Animation
Question:
Do molecules diffuse faster or
slower in warm temperatures?
Faster
Turns out, temperature is
nothing more than a measure of
how fast molecules are moving.
Diffusion across membranes is determined
largely by the size and the chemical charge of
the molecule.
Oxygen, glucose, carbon dioxide, and
water can diffuse across a cell membrane.
* Osmosis *
the diffusion of water
across a semi-permeable
membrane from a region
of high water
concentration to and area
of low water
concentration.
What does “Water Concentration” mean?
-water concentration
refers to the number
of water molecules in
a given volume.
**Which has greater water concentration,
distilled water or salt water?**
Osmosis Animation
Osmotic Pressure:
the increase in pressure resulting
from the flow of water in osmosis.
Isotonic Solution: the environment has the Same concentration
of dissolved substances as inside the cell.
Hypotonic Solution: the environment has a Lower concentration
of dissolved substances than inside the cell.
Hypertonic Solution: the environment has a Higher concentration
of dissolved substances than inside the cell.
70% water
30% dissolved substances
Distilled water
(100% water)
80% water
20% dissolved substances
80% water
20% dissolved substances
Summed up:
“Bulk” Passage into and out of the Cell
•When the plasma
membrane is extended to
engulf large particles
•The reverse process.
Proteins or other
molecules are secreted
out of the cell this way.
Phagocytosis: solids/foods
Pinocytosis: liquids
Endo and exocytosis animation
What’s the difference between passive and active transpo
PASSIVE TRANSPORT: when materials move across cell
membranes without using cell energy.
*diffusion, facilitated diffucion, and osmosis are examples
*water, oxygen, carbon dioxide, glucose
http://fig.cox.miami.edu/~cmall
ery/150/memb/c8x16typestransport.jpg
ACTIVE TRANSPORT: the movement of materials across the cell
membrane that requires the use of cell energy (ATP)
Cells use active transport to:
•Transport molecules that are too BIG to diffuse~
AND
•Transport molecules from LOW concentration to
HIGH concentration (opposite of diffusion)
http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/12_Membrane_Transport_files/image032.jpg
Active transport animation