Biology, Answering the Big Questions of Life/osmosis
Contents |
Matter
There are three states of matter: Solid, liquid, and gas.
The difference between the three is how close together the atoms are and how much they move.
In a solid the atoms move very little. They move more in a liquid, and they bounce around the entire room as a gas.
Atoms bouncing around randomly is called BROWNIAN MOTION.
Diffusion
If you put a dye pill in a glass of water, it will start out concentrated in one place, but as the atoms of water move around by Brownian motion they knock the atoms of dye around until it spreads throughout the glass coloring the water evenly. This property of matter is called DIFFUSION.
Osmosis
Cells are surrounded by a plasma membrane that is semi-permeable. That means that some things can pass through it, and some things cannot.
OSMOSIS is the diffusion of water through a semi-permeable membrane. That is, the movement of water from an area of higher water concentration to an area of lower water concentration across a membrane.
Water goes from where there is less dissolved solutes and more water, to where there are more dissolved solutes and less water.
The water is trying to reach a state where the dissolved solute level is equal. EQUILIBRIUM.
The plasma membrane of a cell lets water pass freely. Water will flow toward the side that has the most dissolved molecules (called solutes). Said another way, water will flow from a hypotonic solution toward a hypertonic solution until both sides of the membrane reach equilibrium.
- a HYPOTONIC solution has less solutes and more water.
- a HYPERTONIC solution has more solutes and less water.
- an ISOTONIC solution has the same solute concentration on both sides.
Osmosis and cells
Osmosis is something that all cells must learn to deal with. Different cells use different strategies to avoid being damaged by osmotic pressure.
An animal cell in a hypotonic solution is in danger of lysis (breaking apart) because the water will flow into the cell making it bulge and expand.
A cell in a hypertonic solution will shrivel and lose water (plasmolyze).
Plant cells have a cell wall which prevents lysis when they are in a hypotonic solution. The cells expand until they hit the cell wall and then get no bigger. This is why we can put a flower in a glass of water without worrying about the cells bursting.
Plant cells, however, can plasmolyse. If you put a plant into salt water the cells will shrivel as water rushes out of the cells which have now become a hypotonic solution. Plant cells are not designed to work under such conditions, and so most plants will die if you try to grow them in salt water.
