Passive TransportHonors Biology

Recall: It’s all about homeostasis Activities of a cell depend on materials that enter

and leave it To stay alive a cell must exchange materials (food,

oxygen, waste) with its surroundings These materials must cross the cell membrane Small molecules (water, oxygen, carbon dioxide)

move in and out freely since they can squeeze between the molecules of the membrane

Large or charged molecules (proteins, sugars, ions) cannot

The cell membrane is said to be selectively permeable and only allows certain molecules to pass through

Quick Chemistry Review Solvent: does dissolving Solute: gets dissolved Solution: combined mixture Concentration gradient: difference

between the concentration of a solute in one place and its concentration in an adjacent area

Diffusion

Simple Diffusion Movement of molecules

from an area of high concentration to one of low concentration

Does not require cell to use energy

Simplest type of passive transport

Only small molecules can pass membrane by simple diffusion

Kinetic energy drives diffusion Temperature, size and type of molecules affect rates of

diffusion Movement is always with concentration gradient Equilibrium: when concentration of molecules is the

same throughout the space the molecules occupy

Diffusion in action How Diffusion Works

Facilitated Diffusion Most molecules cannot

cross the membrane by simple diffusion• Either too big or are ionic

Carrier proteins help molecules cross the membrane that otherwise could not• Example: glucose

Still high low concentration, so it is passive transport

Facilitated Diffusion in action

How Facilitated Diffusion Works

Osmosis

Passive transport does not require energy

Molecules move from high low concentration

Simple diffusion and facilitated diffusion are examples

Review:

Diffusion of water across a cell membrane

A form of passive transport

Water moves from high low concentration

3 osmotic environments:1. Hypertonic2. Hypotonic3. Isotonic

Osmosis

Less solvent (more solute) on the outside of the cell, greater on the inside of the cell

Water moves out of cell

“Hyper kids run out of school”

Animal and Plant cells:• Plasmolysis – cell shrinks

as water moves out

Hypertonic Environments

More solvent (less solute) outside of the cell, less on the inside of the cell

Water moves into the cell “HypO cell fills with water” Animal cell:

• Cytolysis – cell bursts Plant cell:

• Turgor pressure on cell wall • Best environment for

plants

Hypotonic Environments

Equal amounts of solvent (and solute) inside and outside of cell

Water moves in and out of cell at an even rate

“Like a full restaurant – one family must leave for another to enter”

Dynamic equilibrium – equally dispersed solvent on both sides of the membrane

Animal cell:• Best environment

Plant cell:• Flaccid (wilts)

Isotonic Environments

Animal Cells in Osmotic Environments

Plant Cells in Osmotic Environments

Molecules and solvent (water) are always moving

Movement is from high concentration to low concentration

All movement is in attempt to reach equilibrium

Homeostasis!

Important to Remember…