DIFFUSION AND OSMOSIS

DIFFUSION• is the movement of liquid or a gas molecules

from a high concentration to a low concentration until the substance is evenly spread.

• important as it lets food and oxygen into a cell and waste products such as carbon dioxide out of the cell

MOLECULES IN SOLUTION TEND TO SLOWLY SPREAD APART OVER TIME. THIS IS DIFFUSION.

How does a scent spread through a gas?

How Does a Dye Spread Through a Liquid?

How Does Heat Effect the Spread of Dye Through a Liquid?

OSMOSIS

• osmosis is the movement of water molecules,

• from a region of high water concentration

to a region of low water concentration

• across a selectively permeable membrane.

SELECTIVELY PERMEABLE• a membrane controls what goes in and out of the cell.

• permeable means it allows substances to go through it.

• a selectively permeable membrane is a membrane with holes in it small enough for water to go through it easily.

• big molecules like starch can not fit through easily

Model of a Selectively Permeable Membrane

Water

Starch

Small molecules, such as water can pass through it.

Large molecules like starch cannot pass through the membrane

Lots of water molecules

Few water molecules

Water StarchThe water molecules pass both ways through the membrane

The overall flow of water molecules is from a region with lots of water molecules to region of less water molecules.

Until both sides have an evenly balanced water concentration.

Osmosis – what will happen?

WHICH MOLECULES WILL DIFFUSE IN EACH OF THE FIGURES BELOW?

1 2 3 4

5 6

ANSWERS1 2 3 4

5 6

No Movement

No Movement

SUMMARY• osmosis is the movement of water

molecules across a selectively permeable membrane.

• the membrane only allows small molecules to pass through it, large molecules can not pass through the membrane.

• water molecules move into the region where there are less water molecules

• the water acts to even up the concentration of water on either side of the membrane.

PLANT CELLS IN WATER

in pure water – water will move in swelling the vacuole which pushes out against the cell wall.

the cell does not burst because of the cell wall.

Water movesInto the cell

Plant cell placed inpure water

PLANT CELL IN STRONG SUGAR SOLUTION

Strong Sugar/SaltSolution

In a strong salt or sugar solution – water will move out of the vacuole causing it to shrink.

The cytoplasm can also shrink away from the cell wall.

The plant cell is described as flaccid.

ANIMAL CELLS IN DIFFERENT SOLUTIONS

• a red blood cell in pure water, water will move into the cell and the cell will burst (there is no cell wall to prevent this happening)

• in a strong salt/sugar solution water will move out and the cell will shrink