Semi-permeable membranes – allow some molecules to pass through, but not others
Cell membranes are semi-permeable. They allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but they do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.
Larger molecules can only get through the membrane by use of a membrane channel protein.
What is Osmosis?
Osmosis is the passage of water from a region of high water concentration, through a semi-permeable membrane, to a region of low water concentration.
Osmosis is the way that some particles (like dissolved salt) flow towards water.
A passive processes. Requires no added energy.
Below we see 2 compartments, separated by a semi-permeable membrane.
More water molecules on one side than on the other.
Water molecules flow from high concentration side to the low conc side
Continues until the conc of water on both sides is equal : dynamic equilibrium
Can cause water to move up agains gravity!
Adding sugars to water results in a decrease in the water concentration,
because the sugar molecules displace the water molecules
As water is pulled to one side, it’s level rises
Let’s see this in motion
Concentration of dissolved particles is super important in biology
solute conc. inside cell < solute conc. outside cell Hypertonic
solute conc. inside cell = solute conc. outside cell Isotonic
solute conc. inside cell > solute conc. outside cell Hypotonic
Diffusion and Osmosis <- document
AAAS Benchmarks for Science Literacy
5C/H1a ( Grades: 9-12 ): Every cell is covered by a membrane that controls what can enter and leave the cell.
Next Generation Science Standards
MS-LS1.A.3 ( Middle School Life Sciences ): Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell.
HS-LS1.A.1 ( High School Life Sciences ): Systems of specialized cells within organisms help them perform the essential functions of life.