Osmosis Water crosses the membrane via facilitated diffusion. The tails of phospholipids are hydrophobic/non-polar, so water can’t cross the membrane without help.

Osmosis is only the diffusion of water. IT IS NOT OTHER LIQUIDS.

Movement during osmosis Like all other kinds of diffusion, water moves down its own concentration gradient. High water – low water

Movement during osmosis We don’t tend to think of water having a concentration. Instead we look at the concentration of what is dissolved in the water. ◦ E.g. salt, sugar ◦ If salt is at 30%, water is at 70%.

Movement in osmosis Water moves from low salt to high salt. Remember, salt makes you thirsty. Water goes to where salt is.

Example Problem

Checkpoint: How is osmosis related to facilitated diffusion?

Types of Solutions in Osmosis Hypertonic – cell has a greater water concentration (lower salt compared to outside environment) Hypotonic – solution has a greater water concentration (higher salt) Isotonic – solution and cell have equal water concentrations (equal salt)

Osmotic Pressure This is what causes cells to change shape when placed in hypotonic or hypertonic solutions. It can cause the cells to burst, therefore accommodations are needed to prevent cell death.

Accommodations AnimalsPlants Bathe cells in blood or other fluids Cells that come in contact with water lack proteins to transport water. Cell wall maintains rigidity, prevent the cell from expanding.

Checkpoint: When you receive fluids for dehydration via an IV, the fluid you receive is saline and not water. Why do you not receive water?