Cell Membrane & Transport

Cell Membrane

Transport In & Out of the Cell

Cell membrane is semi-permeable, meaning it is a barrier to most, but not all molecules

Cell membranes are bilayers of phospholipids with the hydrophobic regions facing each other and the hydrophilic regions facing out

The Fluid Mosaic Model is the theory by which the properties of the plasma membrane are explained.

Mosaic: mixture of phospholipids, cholesterol, & proteins

Fluid: components may move or shift but are kept ordered due to hydrophobic forces

Osmosis

The

Diffusion of Water

Through a Biological Membrane.

Cell Diffusion

H2O, CO2, & O2 are among the few molecules that can pass

across a cell membrane by diffusion

Diffusion

The net movement of a substance from an area of high concentration to an area of low concentration.

Substances move down their concentration gradient

Osmosis: Diffusion of H2O across a semi-permeable membrane

As the concentration of solute increases the concentration of solvent (H2O) decreases

Compared to an equal volume of water, the solute/H2O mixture

has less H2O molecules as that space is taken up by the solute

What is Osmosis?

• Osmosis is a form of diffusion.

• It is a passive process because it does not require outside energy to start the process.

• It is the movement of water molecules from a higher concentration to a lower concentration through a semi permeable membrane.

How does it Work?

• The cell is a closed structure protected by its semi permeable plasma membrane.

• This membrane will allow certain molecules to enter or leave the cell, depending on their size and or charge (+ or -).

• Water will to enter or leave the cell based upon its concentration on either side of the cell’s membrane.

Osmosis Begins

• Notice the higher concentration of water on the left of the cell membrane

• Random Molecular motion will allow the more concentrated water molecules (left) to move toward the right.

Osmosis Occurs in Both Directions

• Osmosis is an on going process.

• Water is always moving in both directions.

• The net movement of water is always in the direction from the higher concentration to the lower one.

Balance

• Even when the concentration of water is equal on both sides of the membrane, osmosis still continues.

• This occurs at a slower rate maintaining the balance.

Let’s Apply this to a Real Situation

• What can cause a human blood cell to swell and burst or shrink?

Hypertonic Solution

• When the solution inside the blood cell has a high solute concentration than its environment, the cell is said to be hypertonic to its environment.

• Water will diffuse in. The cell cannot get rid of the water fast enough, and the cell swells and lysis.

Hypotonic Solution

• When the solution inside the cell has a lower concentration of solute than its environment. The cell is said to be hypotonic to its environment.

• Water will diffuse out of the cell, causing the cell to shrink or crenate.

Isotonic Solution

• When the solute concentration outside the cell equals the solute concentration inside the cell, the cell retains its shape.

Osmosis in Plant Cells

• The cell wall of a plant will prevent the cell from exploding if placed in a hypotonic solution.

• Instead the extra water will push against the wall making the cell stiff or turgid.

Cell Membrane

Cell Wall

Tonoplast

Chloroplast

Nucleus

Why Salt Water Fish die in Fresh Water.

• The cells of a salt water fish are hypertonic to the fresh water that surround them.

• The water will move into the cells causing them to swell and lyse.

• The gill and blood cells die, eventually killing the fish.

Why a Fresh Water Fish dies in Salt Water.

• The cells of a freshwater fish are hypotonic to the salt water that surround them.

• The water will move out of the cells causing them to shrink or crenate.

• The gill and blood cells die, eventually killing the fish.

Practice

• Why is the water leaving the cell?

• What must be done to allow the water to reenter the cell?

• What does the 3 and 5 % represent?

3% salt

5% salt