Water will move out of a cell when it is placed in which type of solution

Osmosis Lab

Introduction: Human blood, at 0.9% salt concentration, is a little less salty than seawater, which has a salt concentration of about 35 parts per thousand (3.5%). If we take seawater as an example of a solution, the salt is called the solute (the particles that are dissolved) and the water is the solvent (the liquid that dissolves the particles). Osmosis is the movement of a solvent across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. The water (the solvent) can move across the membrane but the dissolved solutes (the sodium and chloride ions that form salt) cannot. In such situations, water will move across the membrane to balance the concentration of the solutes on both sides. Cells tend to lose water (their solvent) in hypertonic environments (where there are more solutes outside than inside the cell) and gain water in hypotonic environments (where there are fewer solutes outside than inside the cell). When solute concentrations are the same on both sides of the cell, there is no net water movement, and the cell is said to be in an isotonic environment. In this lab we will test samples of potato tissue to see how much water they absorb or release in salt solutions of varying concentrations. This gives us an indirect way to measure the osmotic concentration within living cells.

Hypo=under, iso=equal, hyper=over

Compare initial and final states. Which way did the water move? Why?

Osmosis Lab Setup

electronic balance (0.01 g range)
metric ruler with mm scale
metric measuring cups
6 cereal bowls or shallow pans
a small piece of raw potato to cut into six ~5 mm cubes
(this square is 5 x 5 mm)
single edged razor or knife
paper towels
watch or clock
table salt, distilled or tap water
6 beakers (250 ml or larger) or cups

Methods:

Pre-mix 6 beakers of salt solutions (0%, 0.1%, 0.5%, 1%, 2.5%, 5%) in distilled water. You can use this solution calculator to help you make your solutions. Just enter the water volume of your container and the percentage of salt you want and it will tell you how many grams of salt to add. A 1% salt solution is 1 part salt to 100 parts water. To make a 1% salt solution, you could use a 100 ml bottle, add exactly 1 gram of salt (use your electronic balance) to your bottle, and bring the water volume up to 100 ml. To make a 0.1% solution, add 1 gram of salt to 1000 ml of water (or add 0.1 g salt to 100 ml of water). If you have more water than you need, just stir well and then discard the excess.
Prepare six small potato cubes with no skin that are all about equal in size (approximately 5 millimeters in length, width and height) and blot them dry on a paper towel. (Blot means just gently remove the surface water; no need to squeeze them!)
Mass (weigh) each to the nearest 0.01 grams, keeping them separate, and record each initial mass in Table 1. Don't wait too long before putting them into the solutions, as evaporation will occur.
Fill each bowl with one of the 6 stock solutions, keeping track of which is which! Label them. You won't be able to tell the salinity just by looking. Note which potato piece went into which bowl.
Leave one of the potato slices in each of the salt solutions for up to 24 hours so that they may gain (or lose) water by osmosis. (Keep them all in the salt water the same amount of time--leaving them overnight is likely to give the best results).
Remove the slices, blot them dry on a paper towel, carefully re-weigh them and record in the data table as final mass.

Click here to go to the calculator page, and thanks to the University of Oklahoma for this useful tool!

Results:1. Record your actual results in a table like this one:

Table 1	% Salt	Intitial Mass	Final Mass	Mass Change (g)
Sample 1	0.0%
Sample 2	0.1%
Sample 3	0.5%
Sample 4	1.0%
Sample 5	2.5%
Sample 6	5.0%

Table 1: Changes in potato mass as a result of immersion in salt solutions.

2. Prepare a graph showing change in mass as a function of % salt. Scale the x-axis of your graph in units of 0.5 percent. The y-axis has a zero line half way up, indicating whether the samples lost or gained weight. You will have to scale the y-axis according to your greatest and smallest changes in mass. Download this

Excel spreadsheet if you need help making a graph.

Figure 1: Change in mass of potato (g) due to water gain/loss as a function of salt concentration.

3. When completed, use a ruler to draw a straight line of best fit through your six data points, or use the computer to graph your data and calculate the line of best fit. Where the line of best fit crosses the horizontal zero line, draw a vertical line down to the x-axis. This is the point at which the potato is isotonic with its surroundings, and is therefore the estimated salt concentration of the potato.

Questions:

Why did some potato samples gain water and others lose water? Was there any pattern?
When you drew the best fit line through your data and dropped the vertical line to the x-axis, what salt concentration did you obtain (Estimate if it is between numbers)? What does this mean for the potato?
Why can't we use seawater to irrigate our crops?
What happens when a thirsty person drinks salt water to try to quench their thirst?
Why does salted popcorn dry your lips?
What happens to a cell's water when the exterior liquid is saltier than its interior?
What happens to water outside the cell when the interior is saltier than its surroundings?
When a cell gains water, what happens to its size and weight?
When a cell loses water, what happens to its size and weight?
When you put limp celery stalks in water, they firm up. Why?
Challenge question: Saltwater fish are hypotonic (less salty) to their surroundings while freshwater fish are hypertonic (more salty) to their surroundings. Assuming the salt can't move, what must each fish do with its fluids in order to compensate for the difference in salinity between the body and the surrounding environment?

The effects of isotonic, hypotonic, and hypertonic extracellular environments on plant and animal cells is the same. However, due to the cell walls of plants, the visible effects differ. Although some effects can be seen, the rigid cell wall can hide the magnitude of what is going on inside.

Osmosis has different meanings in biology and chemistry. For biologists, it refers to the movement of water across a semipermeable membrane. Chemists use the term to describe the movement of water, other solvents, and gases across a semipermeable membrane. Both biologists and chemists define diffusion as the movement of solute particles (dissolved materials) from an area of higher concentration to lower concentration until equilibrium is reached.

Osmosis is a passive transport system, meaning it requires no energy. It causes water to move in and out of cells depending on the solute concentration of the surrounding environment. This movement is caused by a concentration gradient created when there are different solute concentrations inside and outside the cell. It doesn’t matter what dissolved materials make up the solute, only the overall concentration. It is important to note that cells do not regulate the movement of water molecules in and out of their intracellular fluid. They rely on other systems in the body (such as the kidneys) to provide an isotonic external environment (see below).

A cell in an isotonic solution is in equilibrium with its surroundings, meaning the solute concentrations inside and outside are the same (iso means equal in Latin). In this state there is no concentration gradient and therefore, no large movement of water in or out. Water molecules do freely move in and out of the cell, however, and the rate of movement is the same in both directions.

A hypotonic solution has a lower solute concentration than inside the cell (the prefix hypo is Latin for under or below). The difference in concentration between the compartments causes water to enter the cell. Plant cells can tolerate this situation better than animal cells. In plants, the large central vacuole fills with water and water also flows into the intercellular space. The combination of these two effects causes turgor pressure which presses against the cell wall causing it to bulge out. The cell wall helps keep the cell from bursting. However, if left in a highly hypertonic solution, an animal cell will swell until it bursts and dies.

In Latin, the prefix hyper means over or above. Hypertonic solutions have a higher solute concentration than inside the cell. This causes water to rush out making the cell wrinkle or shrivel. This is clearly seen in red blood cells undergoing a process called crenation. Plant cells in a hypertonic solution can look like a pincushion because of what’s going on inside. The cell membrane pulls away from the cell wall but remains attached at points called plasmodesmata. Plasmodesmata are tiny channels between plant cells that are used for transport and communication. When the inner membrane shrinks, it constricts the plasmodesmata resulting in a condition called plasmolysis.

	Isotonic Solution	Hypotonic Solution	Hypertonic Solution
High level of solutes outside of the cell	No	No	Yes
Low level of solutes outside of the cell	No	Yes	No
Water movement depends on the type of solute	No	No	No
If uncontrolled, may lead to cell death	No	Yes	Yes
Can cause the cell to wrinkle/shrivel	No	No	Yes
Can cause the cell to swell/burst	No	Yes	No
In plants, results in plasmolysis	No	No	Yes
In plants, results in turgor pressure inside the cell	No	Yes	No
Causes water movement via osmosis	No	Yes	Yes
Represents a homeostatic state	Yes	No	No

The image above shows what happens to a cell in isotonic, hypertonic, and hypotonic solutions.

References

OpenStax College. (2018). Anatomy & Physiology. Houston, TX. OpenStax CNX. Retrieved from http://cnx.org/contents/

Tonicity. (n.d.). In Wikipedia. Retrieved April 17, 2018 from https://en.wikipedia.org/wiki/Tonicity