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WARD'SOsmosis and Diffusion

Lab ActivityStudent Study Guide

BACKGROUND

If a bottle of perfume is opened at one end of the room, in a fairlyshort time someone at the other end of the room will be able to detectthe scent. How does this happen? It depends on the movement ofmolecules. All molecules in gases and liquids are in a state of con-stant, random motion. The molecules in the perfume, released fromthe bottle, travel through the air. When molecules are closely packedtogether, they collide with other molecules, their kinetic energy is re-directed, and the molecules move away from the point of the colli-sion. As a result, molecules tend to distribute themselves evenlythroughout the area they occupy. When the molecules are evenly dis-tributed, they reach dynamic equilibrium. Individual molecules arestill in motion, but there is no net movement of molecules; as manymolecules move in one direction as they do in the opposite one. Thismovement from a higher concentration to a lower concentration isknown as diffusion.

Figure 1

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Diffusion also occurs in water separated by a selectively permeablemembrane, with different solute concentrations on either side of themembrane; this special kind of diffusion is known as osmosis. In os-mosis, water moves from regions of low solute concentration to re-gions of high solute concentration. Because systems always tend to-ward equilibrium, this movement occurs to even out the concentra-tions on the two sides of the membrane (Figure 1).

Only a solute's relative concentration, or water potential, affects therate of osmosis; the size of the solute molecules does not matter. Thehigher the concentration of solutes, the faster water will flow throughthe membrane to equalize the concentration. This is due to the higherwater potential of the solution.

Osmosis occurs in both plant and animal cells, across a cell's plasmamembrane. The membrane allows the free passage of some materialswhile restrictingJhe diffusion of others. Small, neutrally chargedmolecules such as oxygen, carbon dioxide, water, and glucose canpass freely through the plasma membrane.

Diffusion is vital to many life functions of a cell, it allows the trans-port of vitally important nutrients and compounds without the ex-penditure of excess metabolic energy. Diffusion is also responsible forthe transport and exchange of oxygen and carbon dioxide in thelungs. During respiratory gas exchange, oxygen is broken down andcarbon dioxide produced. As a result, the concentration of oxygen isalways lower inside the cell than outside, while the concentration ofcarbon dioxide is higher inside. The gases travel down their respec-tive concentration gradients through the body's intercellular fluid.Oxygen is picked up by the oxygen-poor blood as it passes throughthe lungs and diffuses into the body cells as carbon dioxidediffuses au t.

In plants, diffusion aids in the transport of water and nutrients in thexylem. 1<\ plant's root cells expend energy to maintain a high soluteconcentration in the xylem, which in turn forms a concentration gra-dient wjithin the roots for water; a higher solute concentration equalslower rater concentration. Water flows down the concentration gra-dient, ,ausing it to diffuse into the roots. Water pressure builds upwithin fthexylem, forcing it up the stem. Osmosis allows for the ab-sorptiop and early transport of water into the root system of plantsand, 1th transpirational pull, helps transport water in the xylem.

osmos~' also has a major impact on living cells. Organisms rarely ex-ist in e vironments with solute concentrations that match their cyto-plasm; there are usually more or fewer dissolved particles in one oftwo co pared solutions separated by a membrane, such as a cell andthe m~dia in which it exists. A solution is hypotonic if it has fewerdissolved particles than the other solution and hypertonic if it hasmore particles than the other solution. If the number of dissolved par-ticles are the same in the two media, it is called isotonic.

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If a cell is hypertonic to its surroundings, water rushes into the cell,causing it to expand. If this occurs in an animal cell, the cell bursts, orcytolizes. In a plant cell, however, the rigid wall of the cell holds thecell together. Eventually the pressure exerted by the wall on the wa-ter in the cytoplasm will equal the osmotic potential of the solution,and the net movement of water will become zero. Water will still bemoving, but the am.ount moving into the cells from osmosis is thesame as the amount being forced out by the cell wall pressure. Thispressure exerted on the cell wall by the water is known as turgor; it isresponsible for supporting the stems and leaves of plants.

If a cell is hypotonic to its surroundings, it loses water, causing it toshrink. This is called plasmolysis. In plants, the plasma membranewill pull away from the cell wall; the loss of pressure on the cell wallis what causes plants to wilt. Animal cells, when plasmolyzed, loseshape and support, and shrivel up.

Animals in hypertonic media, like saltwater fish, must find ways toretain their water, while animals in hypotonic media, like parameciain freshwater ponds, must develop mechanisms to pump out the ex-cess water they absorb.

Rates of diffusion and osmosis, and the water potential of a solution,can all be determined through lab activities and calculations.

The osmotic potential is calculated using known concentrations ofsolution. A solution of sugar and water is placed in a tube and thetube is covered by a selectively permeable membrane. If this tube isplaced in a beaker of pure water, the water will diffuse into the tubeand the level of the water will rise. Determining how high the waterrises helps ascertain the water potential of the solution. Water poten-tial determines the direction and rate of osmosis. It consists of twocomponents: pressure potential, which is the exertion of pressure on asolution, and osmotic potential, which is the relative concentration ofsolutes within the two solutions.

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OBJECTIVES

• Demonstrate diffusion across a semi-permeable membrane• Measure the effects of various concentrations of solute on the

process of osmosis• Differentiatebetween hypotonic, isotonic,and hypertonic environments• Measure and calculate water potential• Examine the effects of osmosis on plant cells

MATERIALS

MATERIALS NEEDED PER GROUP

4 Glucose indicator strips1 Graduated cylinder1 Plastic cup, 250 ml dialysis tubing, 7 ft.6 Plastic cups, 250 ml

Glucose starch solution, 15 mlIKI (potassium iodide) solution, 1 mlSucrose solution, 0.2 M, 175 mlSucrose solution, 0~4.M,175 mlSucrose solution, 0.6 M, 175 mlSucrose solution, 0.8 M, 175 mlSucrose solution, 1:9 M, 175 mlDistilled water, 175 mlPaper towels

SHARED MATERIALS

BalanceCompound microscope

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PROCEDURE

As a general laboratory practice, it is recommended that stu-dents wear proper protective equipment such as gloves, safetygoggles, and a lab apron to avoid staining any clothing or skin.

A. Diffusion

1. Pour 15 ml of the prepared glucose/starch solution into a gradu-ated cylinder.

• ti« pores in the dialysis tubing ar. '~tremely small, and. __ can be easzly clogged by any oil or dirt on your fingers--~ and hands. Wash your hands before handling the dialysis

-- _ tubing, and keep physical contact with the tubing toa minimum.

2. Obtain a piece of dialysis tubing that has been soaking in water.Tie a tight knot in one end of the tubing, or use a piece of string totie off the end .

•••••••••

. If you choose to tie off the end of the dialysis tubing with~ string, tie two knots, about 1/4" apart, to prevent leaking.

3. Open the tubing by rubbing the untied end between your fingers.Pour 15 ml of the glucose/ starch solution into the tubing.

4. Note the color of the solution in the bag. Record the color in Table1in the Analysis section.

5. Determine if glucose is present in the tubing by dipping a glucoseindicator strip into the solution. Record the data in Table 1.

6. Carefully tie a knot in the open end to form a bag. Be sure toleave enough space in the bag for expansion./-:---7~·· -,

( J<:.." ..<'H_--7. Fill a 250 ml beaker or a provided plastic cup appro~~iy"'213'

full with distilled water. Add 1 ml of potassium iodide (IKl) tothe beaker.

~I~ Th, lKl solution is an irritant; it affects skin and eyes,and can stain clothing. Handle the solution with caution.

• Wash off spills and splashes with water.CAunON

8. Note the color of the solution. Record the color in Table 1.

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9. Determine if glucose is present in the beaker by dipping a glucoseindicator strip into the solution in the beaker. Record the data inTable 1.

11. Wait 30 minutes .

• Though a color change may become apparent after a few.... .", minutes, be sure to allow the tubing to remain in the

beaker for 30 minutes to allow the glucose to diffuse out..::. into the medium.

12. Remove the dialysis bag from the beaker. Record the final colorof the solutions in the bag and the beaker in Table 1.

13. Determine the glucose content in the beaker and in the dialysisbag using glucose indicator strips. To test the solution in the bag,make a small cut in the bag with a pair of scissors, and insert theindicator strip through this hole. Record the presence or absenceof glucose in Table 1.

• Be sure to toash your hand, tlwroughly before leauingthe laboratory.

B. Osmosis

1. Obtain six plastic cups and label them as follows: water, 0.2 M,0.4 M, 0.6 M, 0.8 M, and 1.0M.

• The PO'" in the dialusis tubing are extremely small, and..... .... can be easily clogged by any oil or dirt on your fingers

and hands. Wash your hands before handling the dialysistubing, and keep physical contact with the tubing to~ . .a mtntmum.

2. Obtain six pieces of dialysis tubing from the beaker of water. Tiea tight knot in one end of the tubing, or use a piece of string to tieoff the end.

,~/~3. Open one ;piece o~bing by rubbing the untied end between

your fing~rs. Pou~ of distilled water into the tubing andcarefully tie a knot in the open end to form a bag. Be sure to leaveenough space in the bag for expansion. Blot the sealed tubing dryand place it in the cup labeled "water".

• If at any time the dlaly,is tubing becomes too dry duringthe procedure, simply reuei the tubing by placing in wa-ter for several seconds and continue setting upyour experiment.

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.' 4. Repeat this procedure with the remaining five pieces of dialysistubing, adding a different sucrose solution to each bag: 0.2 M,0.4 M, 0.6 M, 0.8 M, and 1.0 M. Be sure to blot each piece of tub-ing dry and place it in the proper cup before proceeding with thenext concentration.

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Carefully ~~Ag~.clry and w~~=~09Jl~.- Record eachbag's initial mass in Table 2 in the Al1alysissection.

.~) \_~ c;...:::;~, ':~rI·~~..rT;~ .) ~6. Fill the six plastic cups approximately 3/4 full of distilled water.

Immerse one bag in each of the cups. Be sure each bag is in theproperly labeled cup.

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10. Graph J '"jc",; "~"Clcf 'sgraph paper in the Analysis section.

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ANALYSIS 8/'<-'I&:.tl~~:;/l.~~,~-- A/~}'~·h\i<:_fi

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