Cells: Diffusion and Osmosis

Cells have to control what leaves and what enters the cytoplasm.

Water and solutes move according to the difference in concentrations EVERYTHING naturally moves away from where it is highly concentrated and towards where it is less concentrated This natural movement is called diffusion. Diffusion takes place until the concentration reaches equilibrium (equal concentrations inside and outside of the cell).

The farther you move from a source, the lower the concentration Distance from source The farther you move from a source, the lower the concentration

Diffusion naturally tries to equalize concentration of water and solutes

Animal Cell Example Carbon dioxide naturally diffuses out of animal cells because animal cells produce CO2 as they make energy from food. The concentration inside is higher, so the net movement of CO2 is outward.

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

Oxygen naturally diffuses into animal cells because they use up oxygen as they turn food into energy The concentration outside is higher, so the net movement of O2 is inward.

O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2

O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2

O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2

The cell membrane does not stop the diffusion of CO2, O2, or water molecules This is because CO2, O2, and H2O are small molecules These molecules will move across the membrane until equilibrium is reached.

Large molecules (like sugars) cannot get through the membrane, but water can. Because of this, we call the membrane semipermeable. A semipermeable membrane is selective about which molecules are allowed to pass Small molecules are allowed Large molecules (solutes) are not allowed. The movement of water through a semipermeable membrane that blocks large molecules from moving is called osmosis.

high concentration of solute lower concentration of solute = Solute (ex: sugar) membrane = water ↓ high concentration of solute lower concentration of solute

high concentration of solute lower concentration of solute = Sugar = water high concentration of solute lower concentration of solute

high concentration of solute lower concentration of solute = Sugar = water high concentration of solute lower concentration of solute

high concentration of solute lower concentration of solute = Sugar = water high concentration of solute lower concentration of solute

The net flow of water is towards the higher solute concentration high concentration of solute lower concentration of solute

Osmosis is the diffusion of water through a semipermeable membrane Osmosis is the diffusion of water through a semipermeable membrane. Water molecules move from areas of high concentration to areas of low concentration.

Osmosis can pull water into a cell if the cell is placed in an environment with a lower solute concentration

Hypotonic means lower solute concentration animal cells can rupture in hypotonic environments plant cells are protected from rupture by their strong cell walls

blood cell in hypotonic environment normal blood cell blood cell in hypotonic environment blood cell ruptures due to water flooding inward

Osmosis can also pull water out of a cell if the cell is placed in an environment with a higher solute concentration

Hypertonic means higher concentration of solutes A cell placed in a hypertonic environment will lose water and shrink. Plant cells plasmolyze when placed in very salty water.

Egg placed in hypertonic syrup Egg placed in syrup after 24 hours Osmosis dehydrated the egg.

chloroplasts Normal plant cell Plant cell placed in hypertonic cell wall cell membrane Plant cell placed in hypertonic environment causes water to move outward Plasmolyzed plant cell “drained” of water by osmosis Normal plant cell

Isotonic means balanced Cells in an isotonic environment experience no net flow of water due to osmosis. The water flowing in is balanced by the water flowing out

Summary Hypertonic = higher = dehydrating environment, cells shrink Hypotonic = lower = bloating environment, cells swell Isotonic = equal =homeostatic environment, cells “happy”