Lab 5: Intro to Osmosis

Osmosis

Osmosis is the net movement of water across a selectively permeable membrane due to a difference in solute concentrations on the two sides of the membrane. Water always moves by osmosis from the solution with the lower solute concentration to the solution with the higher solute concentration. If the solutions have the same solute concentration, there is no net movement of water across the membrane.

Equailibrium

Osmosis will continue until sufficient water has moved to equalize the solute concentration on both sides of the membrane, which is a state of equilibrium. Water still flows back and forth across the membrane at equilibrium; there is just no longer any net water flow in one direction or the other. For example, it has been estimated that an amount of water equivalent to roughly 250 times the volume of the red blood cell moves across the cell membrane every second; the cell doesn't lose or gain water because equal amounts go in and out.

Hypertonic:

The extracellular fluid has a higher solute concentration than the intracellular fluid. Cell shrinks

Hypotonic:

The extracellular fluid has a lower solute concentration than the intracellular fluid. Cell swells

Isotonic:

The extracellular fluid has the same solute concentration as the intracellular fluid. Cell is normal

Bar Graphs

are used when you have categorical independent variables, such as type of fertilizer or seasons of the year

Line Graphs

are used when you have continuous independent variables, such as time intervals. In both types of graph, error bars are used to represent standard deviation.

Purpose of Lab

find tonicity o the solution

Extracellular Fluid Intracellular Fluid

fluid outside the cell fluid inside the cell

Solution Solute Solvent

A simple solution is basically two substances that are evenly mixed together. One of them is called the solute and the other is the solvent. A solute is the substance to be dissolved (sugar). The solvent is the one doing the dissolving (water).

Dependent Variable

(the variable we are measuring, which is % weight gain in today's lab). weight change in potato

Independent Variable

(the variable we are testing, which is sucrose concentration in today's lab) # of solutions (11)

Free Water Molecules

A difference in solute concentrations across the membrane leads to a difference in the number of free water molecules on either side of the membrane. Essentially we have a free water molecule concentration gradient that allows for the diffusion of water across the membrane - this is osmosis! There is an inverse relationship between solute concentration and free water molecule concentration. The higher the solute concentration, the fewer free water molecules there are, and vice versa. So, if we place a cell in a hypotonic solution, the greater number of free water molecules in the extracellular fluid (due to the lower solute concentration) will lead to the net diffusion of water into the cell, where there are fewer free water molecules (due to the higher solute concentration).

Lab Experiment

Today you will determine the approximate intracellular sucrose concentration of a potato cell by measuring the amount of water that moves across the membranes of potato cells placed in solutions of different concentrations. You will place pieces of potato in test tubes filled with sucrose solutions of known concentrations. Some of the potatoes may gain water weight, some may lose water weight, and some may remain unchanged. Any change in weight is due to the osmosis of water across the potato cell membrane - either in or out.

Aquaporings

Water moves by osmosis across the cell membrane via specialized water channels called aquaporins.