Water Lab Report

Water is a charged or polar molecule (H+ - O- - H+) that is always

moving across cell membranes. Scientists theorize that this is

possible because it is such a small molecule or because there are

special gap or pores that allow water movement through the cell

membrane. The predominant direction of water flow is determined by the

concentration of the solutes (non-water molecules) inside and outside

of the cell. Water molecules will show a new movement from an area of

higher water concentration (& lower in solutes) to an area of lower

water concentration (& higher in solutes). In other words the net

water flow tends to dilute an area of higher solute concentration.

When water moves by diffusion through a semi-permeable membrane it is

called osmosis. This is a type of passive transport because no

cellular energy (ATP) is involved in the movement of water.

For convenience we will use tonicity & osmolarity as interchangeable

terms. In fact, there are exceptions when these terms do not have

identical meaning.

An extracellular solution is isotonic ["iso" = same, tonicity = tone

or tension] or iso-osmotic to a cell if the cell has no net gain or

loss of water. This is a dynamic equilibrium. The cell & the

extracellular solution have the same concentration of water & the same

concentration of solutes. Our extracellular fluids need to stay

isotonic in order for cells to survive. If cells are place in a

solution that contains a higher concentration of solutes than the

cell, cells suffer a net loss of water and appear crenated ["cren" =

notched] or wrinkled. These cells are in a hypertonic or hyperosmotic

solution. Cells in a highly hypertonic solution may die. A solution

that has a lower solute concentration than is present in cells is said

to be a hypotonic or hypo-osmotic solution. In this case, excess water

flows into the cells and the cells swell. These cells may eventually

rupture or burst open. This process is called lysis.

Although we simplify osmolarity problems by using the % of a solute to