DIFFUSION AND OSMOSIS3.4

KEY CONCEPT • Materials move across membranes because of

concentration differences.

• Molecules can move across the cell membrane through passive transport.

• Passive transport does not require energy input from a cell

• There are two types of passive transport 1. Diffusion2. Osmosis

Diffusion and Osmosis

• Diffusion is the movement of molecules in a fluid or gas from high concentration to low concentration

• Molecules diffuse down a concentration gradient.

• “Move downhill” or from high to low

Diffusion

• Diffusion stops when dynamic equilibrium (spread evenly, but molecules still moving) is reached

Diffusion

• Concentration• Number of molecules in a substance in a given

volume

• Concentration gradient• Difference in concentration from one area to another

Diffusion

Diffusion• Diffusion plays a key role in cells ability to move substances into and out of the cell• Small lipids and nonpolar molecules like O2 and CO2 diffuse easily

• Cells continually consume O2 • Therefore the concentration of O2 is almost always

higher outside the cell• Result O2 diffuses into cell without the need of energy

• Osmosis is the diffusion of water molecules across a semipermeable membrane from high water molecule concentration to low H20 concentration.

• Continues until dynamic equilibrium is reached

Osmosis

• Homeostasis • Biological Balance

• Key concept of selectively permeable membrane and osmosis is to maintain homeostasis!

Osmosis

Osmosis

• Tonicity• A measure of water

pressure against a semipermeable membrane

• Three types of Tonicity1. Isotonic• Concentration of

solute is equal inside and outside of cell

Cell in Isotonic Solution

CELL

10% NaCl90% H2O

10% NaCl90% H2O

What is the direction of water movement?

The cell is at _______________.equilibrium

ENVIRONMENT

NO NET MOVEMENT

2. Hypertonic • The concentration of

solute is greater outside the cell

• Plasmolysis – The process by which the cell shrinks from losing water

Osmosis

Cell in Hypertonic Solution

CELL

15% NaCl85% H2O

5% NaCL95% H2O

What is the direction of water movement?

ENVIRONMENT

3. Hypotonic• The concentration of

solute is greater inside the cell

• Cytolysis – The process by which a cell bursts from water entering

• Turgor pressure – Pressure exerted on the cell wall of plants due to water pushing out

Osmosis

Cell in Hypotonic Solution

CELL

10% NaCl90% H2O

20% NaCl80% H2O

What is the direction of water movement?

Osmosis• Some single celled organisms and animals are adapted to survive hypotonic solutions

• They have structures (cell wall and vacuole) to store or remove (contractile vacuole) excess water

Practice• Draw arrows to indicate the direction of water movement

into cell, out of cell, or both!• The 10% solution represents a cell

• Tonicity and osmosis

Video

• Facilitated Diffusion• Some molecules cannot easily diffuse across the

cell membrane.• A transport or carrier protein provides a door for

a substance to enter the cell. Square peg in a round hole idea!

• Still No energy is used

Some Molecules diffuse through transport proteins