DIFFUSION AND OSMOSIS 3.4

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.Diffusion 2.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 plays a key role in cells ability to move substances into and out of the cell Small lipids and nonpolar molecules like O 2 and CO 2 diffuse easily Cells continually consume O 2 Therefore the concentration of O 2 is almost always higher outside the cell Result O 2 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 H 2 0 concentration. Continues until dynamic equilibrium is reached Osmosis

Homeostasis Biological Balance Key concept of selectively permeable membrane and osmosis is to maintain homeostasis! Osmosis

Tonicity A measure of water pressure against a semipermeable membrane Three types of Tonicity 1. Isotonic Concentration of solute is equal inside and outside of cell

Cell in Isotonic Solution CELL 10% NaCl 90% H 2 O 10% NaCl 90% H 2 O 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% NaCl 85% H 2 O 5% NaCL 95% H 2 O 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% NaCl 90% H 2 O 20% NaCl 80% H 2 O 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