Cells and Their Membranes
Cells, small membrane-bound compartments of chemicals and water, are truly
remarkable. All organisms consist of one or more cells, thus cells are the fundamental
units of life. All cells arise from the division of pre-existing cells. In complicated
organisms, such as ourselves, intricate systems of communication link together
specialized groups of cells performing various functions. The plasma membrane that
surrounds each cell creates a division between the living (biotic), and nonliving (abiotic)
worlds. In this laboratory you will learn the structure and function of cells and their
membranes.
Objectives
When you have finished this lab, you will be able to:
1. describe the processes of diffusion and osmosis.
2. predict the effect of isotonic, hypertonic, and hypotonic solutions on
plant and animal cells.
3. recognize cell structures in living cells, models, and electron
micrographs.
4. distinguish between prokaryotic and eukaryotic cells.
5. distinguish between plant and animal cells.
6. define all boldface terms.
How Small Molecules Enter and Leave Cells
Diffusion is the movement of molecules from an area of greater concentration to an area
of lesser concentration. To demonstrate the process of diffusion, your instructor has
prepared two petri dishes of agar through which certain ions will diffuse (see Figure 1).
Four holes (A, B, C, and D) are filled with equal concentrations of the following
solutions:
A:
B:
C:
D:
silver nitrate
sodium chloride
potassium bromide
potassium ferricyanide
AgNO3
NaCl
KBr
K3Fe(CN)6
One petri dish will be kept at room temperature while the other will be refrigerated.
Spring 2006
1
K3Fe(CN)6
AgNO
NaCl
3
KBr
Figure 1. Petri dish prepared to demonstrate diffusion of ions through a gel.
The positive Ag+ ion will react with the negative Cl-, Br-, and [Fe(CN)6]-3 ions resulting
in a colored combination (a chemical precipitate.) At the end of the experiment record
the distance each ion moved in mm. Measure from the edge of the filled holes to the
center of the colored band. Record your data in Table 1 below. As you might expect, the
larger an ion is (meaning that it has a higher molecular weight, MW), the slower it will
diffuse through the agar.
Given the MW of the ions in Table 1, predict which ion should move the fastest.________
Which ion should move the slowest? __________________________________________
Did your results agree with your predictions? __________________________________
What effect did cold temperatures have on diffusion? _____________________________
Ion
Table 1. Results of Diffusion of Liquids in Agar
Weight
Reaction
Band Color Distance Traveled (mm)
Ag+
108
-----
Cl-
35
AgCl
Br-
80
AgBr
[Fe(CN)6]-3
212
Ag3Fe(CN)6
Spring 2006
------
Warmer
Cooler
2
Osmosis:
Osmosis is the diffusion of water across a selectively permeable membrane. A selectively
permeable membrane, like the cell's plasma membrane, permits some substances through but
not others. To demonstrate osmosis, you will perform only one of the following two
experiments.
Option 1. Artificial cells.
In this experiment you will use dialysis tubing to simulate the selectively permeable membrane
of an ‘artificial cell’. Dialysis tubing permits the passage of water but obstructs larger molecules
like sucrose (table sugar).
Procedure:
1. Obtain 4 pieces of dialysis tubing that have been soaked in distilled water (dH20).
2. To form a dialysis bag, fold over one end of the tube and tie it tightly with a string. See
Figure 2. Any leakage will spoil your results!
Figure 2. Procedures for filling dialysis bags.
3. Attach one tag labeled #1 to #4 at the end of each tube.
4. Slip the open end of the bags over the bottom of a funnel and fill the bags with the
following solutions. Use a clean graduated cylinder to measure the amounts. See Figure
2C.
Bag
#1
#2
#3
#4
Spring 2006
Contents
15 mL of dH2O
15 mL of 10% sucrose
15 mL of 40% sucrose
15 mL of dH2O
3
5. Force the excess air out of each bag by gently squeezing the bottom end of each bag. Be
careful not to squeeze out any of the liquid!
6. Fold over the open end of the bag and tie it securely with a piece of string.
7. Rinse the filled bag in dH2O and gently blot off any excess water with paper toweling.
8. Weigh each bag to the nearest 0.01 g. and record the weights in the column labeled “O
min” in Table 2.
9. Label 4 beakers with a wax pencil. Fill beakers #1 - #3 with 400 mL of dH2O; fill beaker
#4 with 400 mL of 40 % sucrose solution.
10. Place each bag in the correspondingly numbered beaker and set your timer for 40
minutes. Proceed with the next section and return here for the rest of the directions after
your timer goes off.
……………………………………………………………………………………………
11. After 40 minutes remove each bag from its beaker, blot off the excess fluid, and weigh
each bag. Record the weights in the “40 min” column in Table 2.
12. Calculate the % change in the weight of each bag using the following equation:
% weight change = weight at 40 min – weight at 0 min
weight at 0 min
Table 2. Change in weight of dialysis bags as a result of osmosis.
Bag #
Bag Contents
Beaker Contents
Bag Weight (g)
0 min
1
dH2O
dH2O
2
10% sucrose
dH2O
3
40% sucrose
dH2O
40 min
+ % wt. change
4
dH2O
40% sucrose
Was the direction of the net movement of water into or out of . . .
Bag 1? ____________________________ Why? ____________________________________
Bag 2? ____________________________ Why? ____________________________________
Bag 3? ____________________________ Why? ____________________________________
Spring 2006
4
Bag 4? ____________________________ Why? ____________________________________
Which bag gained the most weight? ________________________________________________
Why? ________________________________________________________________________
Option 2. Eggs as model cells.
You will be given 4 chicken eggs from which the shell has been dissolved away. In this state,
they are quite rubbery, but you don’t want to try bouncing them! What holds them together is
one remaining membrane that is fairly tough, yet selectively permeable. We will use the eggs as
models of large “cells”. The eggs have been stored in a solution that has the same concentration
as the contents of the egg.
Procedure
1.
Rinse the egg in dH2O and gently blot off any excess water with paper toweling.
2.
Weigh each egg to the nearest 0.01 g. and record the weights in the column
labeled “O min” in Table 3.
3.
4.
Label 4 beakers. Fill them with 400 mL of the following solutions:
Beaker 1:
dH2O
Beaker 3:
20% sucrose
Beaker 2:
10% sucrose
Beaker 4:
30% sucrose
At the same time place each egg in the correspondingly numbered beaker and set
your timer for 15 minutes. Proceed with the next section and return for the rest of
the directions after your timer goes off.
……………………………………………………………………………………………
5.
After 15 minutes remove each bag from its beaker, blot off the excess fluid, and
weigh each egg. Record the weights in the “15 min” column in Table 3.
6.
Record the weights of the eggs 30 and 60 minutes from the time the eggs were
immersed.
7.
Calculate the % change in the weight of each egg using the following equation:
% weight change = weight at 60 min – weight at 0 min
weight at 0 min
Spring 2006
5
Table 3. Change in weight of eggs as a result of osmosis
Egg Weight (g)
Beaker
Beaker Contents
Was the direction of the net movement of water into or out of . . .
Egg 1?
_________________________
Why? ________________________
Egg 2?
__________________________
Why?_________________________
Egg 3?
__________________________
Why? ________________________
Egg 4?
__________________________
Why? ________________________
Which egg gained the most weight? ________________________________________
Why? _______________________________________________________________
What would you expect to happen if an egg were put into a fifth breaker containing an 80%
sucrose solution? _______________________________________________
Osmosis in living cells:
Tonicity refers to the concentration of dissolved molecules in water compared to the
concentration of another solution. A solution which has a lower concentration of dissolved
substances than those inside the cell is said to be hypotonic compared to the cell. If a cell is
placed in a hypotonic solution water passes through the plasma membrane into the cell. This
movement of water may cause the cell to swell and burst. If the concentrations of dissolved
substances are equal, the solutions are said to be isotonic to each other, and a cell would
maintain its normal appearance. In hypertonic solutions, where the concentration of dissolved
substances is greater outside than inside the cell, cells lose water and shrivel.
Spring 2006
6
Procedure
Elodea Cells:
1. Remove two young leaves from the top of an Elodea plant.
2. Place the leaves on separate slides. Place a drop of distilled water on one leaf and a
drop of 10% NaCl on the other. Place a coverslip on both.
3. Observe the leaf mounted in distilled water under high power. Draw one cell as it
appears in the space below labeled hypotonic solution. Note that the cell is swollen
or turgid, but the rigid cell wall has prevented it from bursting. If possible locate the
following structures then draw and label them: cell wall, chloroplasts (small green
structures), and the central vacuole.
4. Observe the leaf mounted in 10% NaCl using high power. Draw one affected cell as
it appears in the space below labeled hypertonic. Label the same structures if
possible. Note the shrunken appearance of the cell. This plasmolysis is due to water
loss.
Where do the chloroplasts appear in the Elodea cell when it is placed in an:
hypotonic solution? _______________________________________________
hypertonic solution? ______________________________________________
Elodea leaf cells:
Hypotonic solution
Spring 2006
Isotonic Solution
Hypertonic solution
7
Red Blood Cells
(Modified from Perry, J. and D. Morton. 1987. Laboratory manual for Starr & Taggart’s
Biology, The Unity and Diversity of Life.)
Animal cells lack the rigid cell wall found in plant cells. Consequently when animal cells are
placed in a hypotonic solution the swelling breaks the plasma membrane and lysis, or bursting,
occurs. The cells of your body live in an environment where the internal concentration of salt is
about 0.9%. This concentration is known as a normal saline solution.
Red blood cells will be used in this procedure to demonstrate the effects of osmosis on animal
cells.
Blood can be contaminated with disease-causing organisms. Be sure to follow the
sterile procedures explained to you.
Procedure:
1. Obtain 3 clean screw-cap test tubes and label them 1, 2 and 3. Fill each tube as
follows:
Tube 1
25 mL of normal saline solution (0.9%)
Tube 2
25 mL of 10% salt solution
Tube 3
25 mL of dH20.
2. Place 2-3 drops of animal blood into each tube trying not to get any on the
sides of the tube.
3. Replace the caps and mix the contents of each tube by inverting it gently
several times.
4. Hold each tube flat against the printed page of this lab handout.
5. Record your observations on Table 3 under the column “Print Visible?” Only if the
blood cells have burst (hemolyzed) should you be able to read the print.
6. Label 3 clean microscope slides.
7. With three separate disposable pipettes place a drop of blood solution from each tube
on the corresponding slide (blood from Tube 1 goes on slide 1, etc.). Cover each
drop with a coverslip.
8. Observe the slides with a microscope using the high power.
Spring 2006
8
9. Record your observation in Table 3 (under the column “Microscopic Appearance”)
indicating whether the cells are normal, shrunken (crenated), or burst (hemolyzed).
Usually hemolyzed cells cannot be seen because they have blown up!
10. Record in Table 3 the tonicity of the NaCl solution (external solution) added to the
test tubes. Is it isotonic, hypotonic, or hypertonic?
Table 3. Effect of salt solutions on red blood cells.
Tube
Number
Contents of
Tube
Print Visible?
(yes or no)
Microscopic
Appearance
Tonicity of External
Solution
1
2
3
Why did the red blood cells burst in the hypotonic solution whereas the Elodea cells didn’t?
______________________________________________________________________________
After completing this exercise immediately clean up all equipment contaminated with
blood. Put all bloody materials into the hazardous waste bag.
CELL STRUCTURE
In this section of the laboratory the light microscope will be used to observe living cells and
prepared slides of cells. You will examine both prokaryotic and eukaryotic cells. A
prokaryotic cell has no nucleus whereas a eukaryotic cell does.
Prokaryotic Cells
Procedures:
A prepared slide of the 3 morphological types of bacteria is available. Observe it and draw a
picture of each type of bacteria in the circle below. Note: these cells are very small and have
been stained.
Spring 2006
9
Coccus
(round cells)
Bacillus
(rod-shaped cells)
Spirillum
(spiral-shaped cells)
Eukaryotic Cells
Protists are unicellular eukaryotic organisms. The protist available for this exercise is
Paramecium. Paramecium moves constantly by the rowing action of hairlike projections called
cilia.
Procedures:
1. Place a small drop of the Paramecium culture on a clean slide and add a drop of
Protoslo. Stir the drop with a toothpick and add a coverslip.
2. View the slide under low power and then switch to high power.
3. Make a drawing of Paramecium in the space provided below. If your specimen won’t
hold still, obtain a prepared slide. Using the protist guide provided, see if you can locate
and draw the following structures: oral groove, large nucleus, contractile vacuoles.
Eukaryotic Plant Cells: Elodea
You have already observed a eukaryotic plant cell. Look back to the picture which you have
already drawn to familiarize yourself of its eukaryotic nature.
Spring 2006
10
Eukaryotic Animal Cells: Human Cheek epithelium
Procedure:
1. Use the flat edge of a clean toothpick to gently scrape the inside of you cheek.
2. Stir the scrapings in a drop of water on a clean slide. Add a drop of methylene blue
stain and a coverslip.
3. Observe the slide under low power. Draw a cheek cell in the space provided below.
Label the nucleus and plasma membrane.
Paramecium
Your cheek cells
List two differences you have observed between plant and animal cells.
1. ___________________________________________________________________________
2. ___________________________________________________________________________
List two differences you have observed between prokaryotic and eukaryotic cells.
1. ___________________________________________________________________________
2. ___________________________________________________________________________
Spring 2006
11
Electron Micrographs
Electron micrographs of labeled cell organelles are shown below. Use the chapter on cell
structure in your textbook to identify the organelles and your answers the questions.
a) Name of this organelle ________________
Sketch of an animal cell
Found in: plants? _______ animals? ________
a
b
b) Name of this organelle ________________
Found in: plants? _______ animals? ________
c) This is an entire cell. Name these organelles
a) _______________ b) _______________
Found in: plants? _______ animals? ________
d) Name of this organelle ________________
d) Name of this organelle ________________
Found in: plants? _______ animals? ________ Found in: plants? _______ animals? ________
Spring 2006
12