Diffusion

• Diffusion – net movement of particles from an area of higher concentration to an area of lower concentration

• Caused by random movement

• Is a slow process because it relies on the random molecular motion of atoms

Rate of Diffusion• Three key factors affect

the rate of diffusion:– 1. Concentration.

The most important factor. The more the concentrated the substances, the faster diffusion occurs.

– 2. Temperature. Increased temperature can speed diffusion because of more rapid molecular movement

– 3. Pressure. Increased pressure will accelerate molecular movement and speed up diffusion.

The Result of Diffusion

• Eventually, the two different concentrations (the concentration gradient) will be distributed evenly. – This is called dynamic

equilibrium.

• Diffusion is one of the methods that cells move substances in and out of the cell

Osmosis

• Osmosis – the diffusion of water across a selectively permeable membrane

• Regulating water is a very important factor in maintaining homeostasis in the cell.

What controls

Osmosis?

• If two solutions are separated by a selectively permeable membrane…– Water will flow to the side of the membrane where the water

concentration is lower– However, the substances in the water are prevented from moving and

keep their original concentrations.• Therefore, osmosis is controlled by a concentration gradient,

that is an unequal distribution of particles across a membrane.– Movement from a low solute concentration (high water potential) to a

solution with high solute concentration (low water potential)

Your Turn! Which way will the water go?

On your sheet, record what will happen to the red blood cells in each of these three cases:

Red Blood Cells (5% salt) in40% Salt Solution

Red Blood Cells (5% salt) in 5% Salt Solution

Red Blood Cells (5% salt) in Pure Water (0% salt solution)

Check your work!

How Osmosis affects cells: Isotonic Solution

• All cells are subject to osmosis since they are surrounded by water.

• In an Isotonic Solution, the concentration of dissolved substances in the solution is the same as the concentration of dissolved substances inside the cell.

• There is equal flow and water goes in both directions.

• Therefore, cells retain their original shape since the water concentrations are equal.

How Osmosis affects cells: Hypotonic Solution

• In an Hypotonic Solution, the concentration of dissolved substances in the solution is lower than the concentration of dissolved substances inside the cell.

• Therefore, there is more water outside the cell than inside & water flows in! – Animal cells may burst– Plant cells become more

rigid… • how does this explain the

misting machines in the produce section of grocery stores?

How Osmosis affects cells: Hypertonic Solution

• In an Hypertonic Solution, the concentration of dissolved substances in the solution is higher than as the concentration of dissolved substances inside the cell.

• Therefore, there is less water outside the cell than inside & water flows out! – Animal cells shrink– Plant cells become less firm

called plasymolysis…• How does this explain why

plants will wilt without water?

Passive Transport• Passive Transport

When the cell uses NO ENERGY to move particles across the membrane via DIFFUSION

• Includes Water, Lipids, and Lipid-Soluble proteins

• Why? The membrane is attracted to them.

Facilitated Diffusion

• Type of Passive Transport with the aid of Transport Proteins is called Facilitated Diffusion

• Includes Sugars and Amino Acids

• Fast and specific, but does not use energy!

• Driven by a concentration gradient

Active Transport• Active Transport is the

movement of materials through a membrane against a concentration gradient.

• Often called “pumps” such as Calcium, Potassium and Sodium

• Requires ENERGY from the cell– Which organelle supplies

this energy?– MITOCHONDRIA

How Active Transport Works• Each Transport Protein (also called a

Carrier Protein) binds to its unique ion• Energy is supplied to change the

shape of the protein to allow the ion to pass

• Types of Active Transport:Endocytosis = cell surrounds and takes in material from the environment (via vacuoles)Phagocytosis = means “cell eating” The Cytoplasm surrounds a particle and packages into a vacuole.Exocytosis = expulsion or secretion of materials from the cell – Used to expel wastes or indigestible

materials, release hormones