Diffusion and Osmosis

The Cell Membrane

The cell membrane is:Selectively permeable

Permeable = Pass through (Latin) Cell membrane = Gate-Keeper that

determines what can and can’t enter the cell.

The Gate Keeper: The Cell Membrane

Who comes in for the party?

Small molecules like water, carbon dioxide, or oxygen can easily pass through the pores of the membrane.

Who gets bounced?

Large molecules like glucose

Charged molecules like salts.

Molecule Movement

Recall cells are small because they need to obtain sufficient nutrients and dispose of their wastes.

This is done by the cell membrane by three means:

1. Diffusion

2. Osmosis

3. Active Transport (Senior Science)

Definitions.

Solution = A homogenous, liquid mixture of two or more substances.

Solvent = The dissolving agent of a solution. Water is the most versatile solvent known.

Solute = Substance that is dissolved in a solution.

Diffusion

The process by which molecules tend to move from an area of higher concentration to an area of lower concentration.

Concentration= amount of substance in a given volume.

Think of the perfume example (high low) Think of the food-dye example (high low)

Why Diffusion?

Kinetic molecular theory. Molecules are always moving Collisions occur between molecules, which

causes the molecules to move away from one another.

If there is space these molecules want to spread out evenly.

What direction will the molecules move?

Diffusion and the Cell

Movement of molecules by diffusion is how the cell fulfills its needs.

Nutrients: Low concentration in the cells, high concentration outside of the cells.

What direction will the nutrients diffuse? From outside the cell to inside the cell. High to Low Concentration!!

Diffusion and the Cell

Wastes: High concentration in the cell, low outside of the cell.

What direction will the wastes move? From inside the cell to outside the cell. High to Low concentration

A special case of diffusion: Osmosis

Water moves through the pores of the cell membrane like other molecules.

The movement of water from a high concentration to a low concentration (thru a semi-permeable membrane) is called: Osmosis

Osmosis = movement of water (the solvent) Diffusion = movement of dissolved materials

(the solutes)

Osmosis is the movement of water (red dots) through a semipermeable membrane to a higher concentration of solutes (blue dots).

Activity — Predicting Movement of Water

This “U” tube has a selectively permeable membrane separating side A and B.

Side A has a 40% solution of sugar and side B has a 60% solution of sugar.

Which side has more water?

Which way will the water move?

Selectively Permeable Membrane

Osmosis – a cure for the flu?

Who here has taken penicillin to combat sickness?

Penicillin weakens bacterial cell walls Water is at higher conc. outside the cell…

which way will water move? With a weak cell wall, the bacterial cell

swells and…POP!

**Summary**

Both osmosis and diffusion involve the movement of molecules from an area of high concentrations to a area of low concentrations.

Osmosis = movement of water (the solvent)

Diffusion = movement of dissolved materials (the solutes)

Water Balance The cell membrane doesn’t actually control the

movement of water. The environment around the cell determines the

direction of water movement. For example, a cell is placed in a salt water tide pool.

The sun evaporates some water. The concentration of water inside the cell is higher than

outside. What happens? Water leaves the cell. Possible shrinking of the cell.

Water Balance

On the contrary imagine it rains. A higher concentration of water outside

the cell is the result. What happens? Water moves into the cell Possible swelling of the cell.

Water Regulation

Without other mechanisms the cell would burst or shrivel.

Plant vacuoles store water even when the environment is dry.

The cell wall is rigid to prevent bursting or shriveling.

Animals can increase or decrease their amount of water intake and outtake (Urine concentration/amount of water that is drank)

Hypertonic Solution – More water inside cell than outside

Plasmolysis

Plasmolysis

Cells shrink and die

Iso-osmotic Solution – same concentration of water inside and outside cell

Animal cell

Plant cell

No net gain or loss of water

Hypotonic Solution – more water outside cell than inside

Cytolysis – cell swells and bursts

Builds up turgor pressure – cell becomes stiff, keeping plant upright

What would happen to a cell placed in pure water?

90% water

10% solute

100% water

Cell’s are about 90% water.

Human Red Blood Cell Before: After

What would happen to a cell placed in a 20% salt solution?

90% water

10% solute

Cell’s are about 90% water.

80% water

Human Red Blood Cell Before: After