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. Because of their Kinetic Energy, molecules are in Constant Motion. Diffusion occurs when molecules move randomly away from each other in a liquid or gas.

RATE OF DIFFUSION

The Rate of Diffusion depends on the TEMPERATURE, SIZE and the TYPE OF THE MOLECULES THAT IS DIFFUSING. Molecules Diffuse FASTER at HIGHER

TEMPERATURES than at lower temperatures.

SMALLER Molecules Diffuse FASTER than LARGE Molecules.

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.

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 ENERGYAGAINST THE CONCENTRATION

GRADIENT low concentration --> high

concentration