When a cell is placed into a salt solution that has a salt concentration lower than the inside of the cell the solution is said to be?

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?

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Saltwater Fish vs. Freshwater Fish?

Fish cells, like all cells, have semi-permeable membranes. Eventually, the concentration of "stuff" on either side of them will even out. A fish that lives in salt water will have somewhat salty water inside itself. Put it in the freshwater, and the freshwater will, through osmosis, enter the fish, causing its cells to swell, and the fish will die. What will happen to a freshwater fish in the ocean?

Imagine you have a cup that has 100ml water, and you add 15g of table sugar to the water. The sugar dissolves and the mixture that is now in the cup is made up of a solute (the sugar) that is dissolved in the solvent (the water). The mixture of a solute in a solvent is called asolution.

Imagine now that you have a second cup with 100ml of water, and you add 45 grams of table sugar to the water. Just like the first cup, the sugar is the solute, and the water is the solvent. But now you have two mixtures of different solute concentrations. In comparing two solutions of unequal solute concentration, the solution with the higher solute concentration is hypertonic, and the solution with the lower solute concentration is hypotonic. Solutions of equal solute concentration are isotonic. The first sugar solution is hypotonic to the second solution. The second sugar solution is hypertonic to the first.

You now add the two solutions to a beaker that has been divided by a selectively permeable membrane, with pores that are too small for the sugar molecules to pass through, but are big enough for the water molecules to pass through. The hypertonic solution is on one side of the membrane and the hypotonic solution on the other. The hypertonic solution has a lower water concentration than the hypotonic solution, so a concentration gradient of water now exists across the membrane. Water molecules will move from the side of higher water concentration to the side of lower concentration until both solutions are isotonic. At this point, equilibrium is reached.

Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higher concentration to an area of lower concentration. Water moves into and out of cells by osmosis. If a cell is in a hypertonic solution, the solution has a lower water concentration than the cell cytosol, and water moves out of the cell until both solutions are isotonic. Cells placed in a hypotonic solution will take in water across their membrane until both the external solution and the cytosol are isotonic.

A cell that does not have a rigid cell wall, such as a red blood cell, will swell and lyse (burst) when placed in a hypotonic solution. Cells with a cell wall will swell when placed in a hypotonic solution, but once the cell is turgid (firm), the tough cell wall prevents any more water from entering the cell. When placed in a hypertonic solution, a cell without a cell wall will lose water to the environment, shrivel, and probably die. In a hypertonic solution, a cell with a cell wall will lose water too. The plasma membrane pulls away from the cell wall as it shrivels, a process called plasmolysis. Animal cells tend to do best in an isotonic environment, plant cells tend to do best in a hypotonic environment. This is demonstrated inFigure below.

Unless an animal cell (such as the red blood cell in the top panel) has an adaptation that allows it to alter the osmotic uptake of water, it will lose too much water and shrivel up in a hypertonic environment. If placed in a hypotonic solution, water molecules will enter the cell, causing it to swell and burst. Plant cells (bottom panel) become plasmolyzed in a hypertonic solution, but tend to do best in a hypotonic environment. Water is stored in the central vacuole of the plant cell.

When water moves into a cell by osmosis, osmotic pressure may build up inside the cell. If a cell has a cell wall, the wall helps maintain the cell’s water balance. Osmotic pressure is the main cause of support in many plants. When a plant cell is in a hypotonic environment, the osmotic entry of water raises the turgor pressure exerted against the cell wall until the pressure prevents more water from coming into the cell. At this point the plant cell is turgid (Figure below). The effects of osmotic pressures on plant cells are shown in Figure below.

The central vacuoles of the plant cells in this image are full of water, so the cells are turgid.

The action of osmosis can be very harmful to organisms, especially ones without cell walls. For example, if a saltwater fish (whose cells are isotonic with seawater), is placed in fresh water, its cells will take on excess water, lyse, and the fish will die. Another example of a harmful osmotic effect is the use of table salt to kill slugs and snails.

Diffusion and osmosis are discussed at http://www.youtube.com/watch?v=aubZU0iWtgI(18:59).

Organisms that live in a hypotonic environment such as freshwater, need a way to prevent their cells from taking in too much water by osmosis. A contractile vacuole is a type of vacuole that removes excess water from a cell. Freshwater protists, such as the paramecium shown in Figure below, have a contractile vacuole. The vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it pushes the water out of the cell through a pore.

The contractile vacuole is the star-like structure within the paramecia.

Osmosis is the diffusion of water.
In comparing two solutions of unequal solute concentration, the solution with the higher solute concentration is hypertonic, and the solution with the lower concentration is hypotonic. Solutions of equal solute concentration are isotonic.
A contractile vacuole is a type of vacuole that removes excess water from a cell.

Use this resource to answer the questions that follow.

What is osmosis?
What does salt do to water?
What is a hypotonic solution? What happens to water in a hypotonic solution?
What is a hypertonic solution? What happens to water in a hypertonic solution?
What happens to water in an isotonic solution?

What is osmosis? What type of transport is it?
How does osmosis differ from diffusion?
What happens to red blood cells when placed in a hypotonic solution?
What will happen to a salt water fish if placed in fresh water?

This page titled 2.1: Osmosis is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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