HOMEOSTASIS AND CELL
TRANSPORT
CH 5 - P. 99-104
OBJECTIVES
1. Explain how an equilibrium is established as a
result of diffusion.
2. Distinguish between diffusion and osmosis.
3. Explain how substances cross the cell membrane
through facilitated diffusion.
4. Explain how ion channels assist the diffusion of
ions across the cell membrane.
GENERAL INFORMATION
 1. The activities of a cell depend on the materials that enter




and leave the cell.
2. To stay alive, a CELL MUST EXCHANGE Materials such
as Food and Waste With Its ENVIRONMENT.
3. These materials must cross the Cell Membrane.
4. Small molecules like WATER, OXYGEN, AND CARBON
DIOXIDE can move in and out freely.
5. Large molecules like PROTEINS AND
CARBOHYDRATES CANNOT.
SEMIPERMEABLE
 The Cell Membrane is SEMIPERMEABLE.
 A SEMIPERMEABLE MEMBRANE OR
SELECTIVELY PERMMEABLE ONLY ALLOWS
CERTAIN MOLECULES TO PASS THROUGH.
DIFFUSION
 The Simplest type of PASSIVE TRANSPORT, is
DIFFUSION. Small molecules can pass through
the cell membrane this way.
 DIFFUSION IS THE MOVEMENT OF
MOLECULES FROM AN AREA OF HIGHER
CONCENTRATION TO AN AREA OF LOWER
CONCENTRATIONS.
 This difference in the Concentration of Molecules
across a space is called a CONCENTRATION
GRADIENT.
CAUSE OF DIFFUSION
 Diffusion is Driven by the KINETIC ENERGY the
molecules possess.
 Molecules are in constant random motion because of
their Kinetic Energy
 Diffusion occurs when molecules move randomly away
from each other in a liquid or gas.



Ie. Molecules in a high concentration will eventually be evenly
distributed within a volume over time due to their random motion
This means there “net” movement is from high to low concentration
***it is important to remember that when they reach “equilibrium”
the continue to move however there is no “net” movement
RATE OF DIFFUSION
 The Rate of Diffusion depends on:
 TEMPERATURE,


SIZE


Molecules Diffuse FASTER at HIGHER TEMPERATURES
than at lower temperatures.
SMALLER Molecules Diffuse FASTER than LARGE
Molecules.
TYPE OF THE MOLECULES THAT IS DIFFUSING.
HOMEOSTASIS
 A CONCENTRATION GRADIENT = difference
between the concentration of a molecule in one area
and another area.
 diffusion always occurs down a
concentration gradient, = from the area of
higher concentration to an area of lower
concentration.
 When molecules are dispersed
EVENLY=Homeostasis
OSMOSIS
 THE DIFFUSION OF WATER ACROSS A
SEMIPERMEABLE MEMBRANE IS CALLED
OSMOSIS.
 Like any other Diffusing Molecule, Water moves
DOWN its Concentration Gradient.

http://people.eku.edu/ritchisong/osmosis3.gif
Osmosis
SOLUTES
 OSMOSIS OCCURS IN RESPONSE TO THE
CONCENTRATION OF SOLUTES DISSOLVED
IN WATER.
 solutes are dissolved substances in a
solution.
DIRECTION OF OSMOSIS
 Water Moves
 LOW Solute Concentration to AREAS of HIGH Solute
Concentration.
 accross the Membrane toward the Higher Solute
Concentration until the Concentration Gradients of BOTH
WATER and SOLUTES EVEN OUT.
 The Net Direction of Osmosis DEPENDS on the
Relative Concentration of Solutes on the TWO
Sides of the Cell Membrane.
HYPERTONIC SOLUTION
 IN A HYPERTONIC SOLUTION, THE
CONCENTRATION OF SOLUTE MOLECULES OUTSIDE
THE CELL IS HIGHER THAN THE CONCENTRATION OF
SOLUTES INSIDE THE CELL CYTOSOL.
 In Hypertonic Solutions, WATER DIFFUSES OUT OF
THE CELL until Equilibrium is established.
 If the cell loses too much water, the cell will shrivel and
shrink.
 Cells in Hypertonic Solutions usually DIE because the cell's
activities are Disrupted by LACK OF WATER.
HYPOTONIC SOLUTION
 Hypotonic Solutions, WATER DIFFUSES INTO
THE CELL until Equilibrium is established.
 In Hypotonic Solutions, Water flows into the Cell
because there is a Greater Concentration of Solutes
inside than outside the Cell.
 The Flow of water into a cell causes it to swell.
 Animal cells placed in distilled water (Hypotonic
Solution) and they will swell and often burst
because of Osmosis. The Bursting of Cells is called
CYTOLYSIS.
ISOTONIC SOLUTION
 . IN AN ISOTONIC SOLUTION, THE
CONCENTRATION OF SOLUTES OUTSIDE AND
INSIDE THE CELL ARE EQUAL.
 Under these conditions, water Diffuses Into and Out
of the Cell at EQUAL RATES, so there is NO Net
Movement of Water.
FACILLITATED DIFFUSION
 REQUIRES NO ENERGY
 = Transport from High to low concentration down
the concentration gradient by Carrier Proteins
 for transporting larger molecules that can not pass
through the lipid bilayer
ACTIVE TRANSPORT
 REQUIRES ENERGY
 AGAINST THE CONCENTRATION GRADIENT
 low concentration --> high concentration
Make a Concept Map
Types of Cell Transport Across the Membrane
Passive
Facilitated
Active
Phagocytosis
Pinocytosis