The Structure and Properties of the Animal Cell and the Transport Systems Within it

Chua, Angela Pauline B.

January 30 and February 4, 2015

Abstract

The objectives of this study is to locate the different parts of the animal

cell, to know the structure and mechanisms of the plasma membrane in

regulating the flow of materials to and from the cell and to distinguish

diffusion from osmosis. The cheek cells that were collected with a

toothpick showed the different parts of the cell namely the nucleus, plasma

membrane and cytoplasm under the microscope. The experiment with the

mixing of oil and water explained the lipid bilayer structure of the plasma

membrane while the thin sheet formed on the surface of the boiling milk

explained how the plasma membrane is composed of protein and fats or

the phospholipids. The ability of the egg albumin to retain its shape when

pricked displayed the ability of the plasma membrane to repair itself when

torn. The spread of ink throughout the water is an example of diffusion.

Osmosis was shown through the movement of water molecules from

outside the egg up to the thistle tube while passing through the membrane

lining of the egg. The two types of osmosis were explained through the

subjection of the blood cells to water and varying concentrations of NaCl.

When placed in water (hypotonic), the cells underwent hemolysis which

caused it to swell and burst. However, when the blood cells were placed in

10% NaCl (hypertonic), the cells became smaller and wrinkled and this is

called crenation.

Introduction

The discovery of cells started in the 17th century when the first microscopes were

used. The person who coined the word ‘cell’ was Robert Hooke and he used it to describe

the “small, box-like cavities” he saw in the microscope while examining a cork and other

plant materials9. As further studies were conducted, the cell theory was developed and

according to the book of Elements of Zoology 3rd edition, the cell theory began in 1824

with Rene Dutrochet but was stated for plants by M.J. Schleiden in 1838 and for animals

by Theodor Schwann in 18398. It was later improved on by Virchow in 1855. The cell

theory states that (1) The cell is the basic unit of life, (2) All living things are composed

of cells and (3) Cells come from pre-existing cells and do not arise spontaneously. There

are two categories of cell: prokaryotic and eukaryotic. The animal cell falls into the

eukaryotic cell which has a nucleus with a nuclear membrane enclosing the genetic

material unlike the prokaryotic cell which does not have a nucleus and its genetic

material is confined in the nucleoid. The parts of the cell that are visible under the

microscope are the nucleus, plasma membrane and the cytoplasm. The nucleus contains

the chromosomes which contains the genetic blueprint for the protein in the body. The

plasma membrane is the one dividing the cell from external environment and mediates its

interactions with it. The cytoplasm is the portion of the cell outside the nucleus where the

organelles are embedded. In this experiment, the different parts of the animal cell visible

under the compound microscope are going to be examined. Furthermore, the following

experiments will the structure of the plasma membrane, its composition and how it

repairs itself. It will also show how the cell membrane regulates the flow of materials in

and out of the cell. It is important that these things are studied because it gives further

understanding on how the human body works.

Diffusion is the net movement of materials from an area of high concentration to

an area of lower concentration. It is also responsible for the movement of cell membranes

across the cell and its dispersion within the cell. On the other hand, osmosis is a special

type of diffusion that involves the passage of a solvent (water) through a selectively

permeable membrane that allows the passage of water molecules but not of some solute

molecules. Examples of osmosis are hemolysis and crenation. Hemolysis is the swelling

or enlargement of blood cells when placed in a hypotonic environment while crenation is

the collapsing or shrinking of blood cells when placed in a hypertonic environment. The

following experiments will show the difference between diffusion and osmosis. These

experiments will help in explaining the different transport systems in the human body.

This study was conducted on January 30 and February 4, 2015 at the University of the

Philippines Los Baῆos, Biological Sciences Building, Wing A, Room 137.

Materials and Methods

The first part of the experiment involved the structure of animal cells. In section

1, the human cheek cells were examined under the microscope. The cheek cells were

obtained by scraping the inside of the cheek with a toothpick and smearing it on a glass

slide. After drying the specimen, a drop of water was added then a drop of methylene

blue to stain the cells. After one minute, the cover slip was placed and the cells were

examined under the microscope at first under low power objective then under high power

objective. The cells were then drawn and its parts were identified and labeled.

Section 2 involves the structure of the plasma membrane. Three experiments were

conducted using materials with similar structure and molecule-molecule interactions in

the plasma membrane. In the first experiment, oil and water were used. Equal portions of

oil and water were mixed in a test tube and was vigorously shaken. After a few minutes,

it was observed then shook again until the oil and water settled. A few drops was then

placed on a glass slide and examined under the microscope. The second experiment used

evaporated milk. A small portion of the evaporated milk was placed in a beaker and then

the beaker was placed in a hot plate to boiling. After heating the evaporated milk, it was

taken out of the hot plate then left to cool. Upon cooling, the surface of the milk was

observed. This process was repeated three times and observed. The third experiment used

oil and egg albumin. The bottom of a petri dish was covered with oil. Then a drop of egg

albumin was added on the petri dish. The formation of a thin membrane between the oil

and egg albumin was observed and then it was pricked with a toothpick. The result was

observed again.

The second part of the experiment involved the two transport processes of water

and substances across living cells namely diffusion and osmosis. In section 1, the

experiment was about diffusion. A petri dish was filled with water until the bottom is

completely covered and then a drop of ink was added. The ink was observed.

In section 2, the experiment was about osmosis and an egg was used to explain

how it works. To make a circular opening in the egg, the rounded end of the egg was

chipped out with a sharp point of a pair of scissors in a manner that only removes the

portion of the shell but not breaking the membrane lining. The opening was just enough

to fit the end of the glass tubing. In a 1000ml beaker, a clay triangle was placed then the

egg was placed on top of it with the round end facing down. Then a hole just enough to

fit the diameter of the glass tubing was made on the pointed end of the egg. A length of

the thistle tube was then clamped on the iron stand and inserted on the opening of the

egg. The thistle tube must be carefully inserted for it not to touch the yolk inside the egg.

After that, a candle was lit with a matchstick then its wax was poured on the opening of

the egg so that its contents will not escape. After drying the wax, water was poured in the

beaker completely submerging the egg. The level of the egg content was noted at the start

of the experiment. Every 30 minutes, the changes in the level of the egg content was

measured in centimeters using a ruler. The photo below shows the set-up.

Photo 1. Egg experiment set-up

Section 3 is about the two types of osmosis which are hemolysis and crenation of

the human red blood cells. Three glass slides were prepared by placing two drops of

distilled water, 0.9% NaCl and 10% NaCl on each slide, respectively. Then the tip of the

ring finger or the fourth finger of the hand starting from the thumb was cleansed with

70% ethyl alcohol. After that, it was pricked using a blood lancelet. A drop of blood was

placed to each of the three slides wiping the finger after every drop. The glass slides were

then covered with the cover slip and observed under the microscope at first in low power

objective and then in high power objective.

Results and Discussion

In Part 1 Section 1 of the experiment a drawing of the cheek cells can be seen in

Drawings 1 and 2. Drawing 1 shows the masses of cell under low power objective. The

visible parts of the cell are nucleus, cytoplasm and plasma membrane as labeled. On the

other hand, Drawing 2 shows a single cell under high power objective. The visible parts

of the cell are the same but appears larger. These structures are important to the cell

because of their different functions. The nucleus, usually found in the center, is where the

DNA of the cell is located4. It is important in heredity and the functional activities of the

cell. The cytoplasm, on the other hand, looks translucent and it refers to all the material

outside the nucleus. The plasma membrane is the outermost part of the living cell and it is

generally invisible under the microscope, however it can be demonstrated with a dye. The

dye when injected to the cell will spread in the cytoplasm but does not pass into the

boundary of the cytoplasm indicating the presence of the plasma membrane6.

Photo 2. Mixture of oil and water

under High Power Objective.

In the first experiment of Part 1 Section 2: Structure of the Plasma Membrane, the

water and oil did not mix as shown in Photo 2. They formed two separate layers. The

water droplet surrounded the oil droplet. This behavior is similar to the structure of the

plasma membrane. The structure of the plasma membrane is explained by the Fluid-

Mosaic Model. It states that the plasma membrane has two layers of phospholipid which

is called the lipid bilayer structure. To understand how this is formed, the composition

and behavior of a phospholipid molecule when placed in water must be known. The

phospholipid molecule has a polar head made up of phosphate-glycerol and nonpolar tail

made up of fatty acids. When placed in water, the polar head seek partners for hydrogen

bonding maximizing the number of hydrogen bonds while the nonpolar tails will pack

closely together with each other away from the water. When two layers with the tail

facing each other forms, it is called the lipid bilayer structure. In the experiment, the

water cannot pass through the oil since it is nonpolar while water is polar. Similarly,

water cannot pass through the plasma membrane since the nonpolar tails of the

phospholipid in the lipid bilayer serves as a protective barrier from the entry of water-

soluble or polar molecules1. That is why the behavior of oil and water is used to explain

the structure of the plasma membrane.

Photo 3. Evaporated milk before heating. Photo 4. Evaporated milk after

heating.

After heating the milk to boiling, a plastic-like sheet formed on the surface as seen

in Photo 4. The same result was acquired upon repetition of the process. Milk is

composed of a number of proteins but primarily of milk proteins called caseins which

helps in growth and development. It is also composed of milk fat. Caseins are made up of

similar proteins with a structure called casein micelle which also contains water and salts

such as phosphorus and calcium5. When milk is subjected to high temperatures, the

water evaporates which leaves the protein and fats to bind together and condense on the

surface. This is because casein proteins tend to clump at about 45-50⁰C7. The protein and

fat eventually dry out which causes the formation of a “skin-like film” on top of the

milk7. Similarly, the animal cell membrane is also made up of proteins and fats which are

found in the phospholipid molecules.

Photo 5. Drops of egg albumin on oil.

When the egg albumin was dropped on the oil, a globule was formed on top of the

oil and when it was pricked with a pen, it retained its shape as seen in Photo 5. This is

explained by the structure of the plasma membrane. Recall that the plasma membrane is

made up of lipids and proteins or phospholipids. Since the polar heads of the

phospholipids form simultaneously due to the tendency to maximize the hydrogen bonds

formed, once it is disrupted, it will quickly regain its place again. Furthermore, if the

position of the polar heads is disrupted, it will expose the nonpolar tails which can’t come

in contact with the cytoplasm, so the polar heads must fall back into its original place

again protecting the nonpolar tails. That is why when the plasma membrane is slightly

torn, it will quickly repair the damaged area6.

Photo 6. Ink and Water.

When a drop of blue ink was added to a petri dish filled with water, at first it was

dark blue on the spot it fell then it gradually spread throughout the water making the

water slightly blue. This phenomenon is explained by diffusion. According to Braungart,

“diffusion is the movement of molecules from a region of greater concentration to one of

lesser concentration”2. In relation to the experiment, when a drop of ink fell on one part

of the water, that part has a greater concentration of ink than the other parts of the water

so it will diffuse. The ink molecules will move to other parts of the water with low

concentration of ink and this evenly distributes the ink molecule throughout the water.

This movement of ink molecules can be seen in the change of color of the water. In the

same manner, the plasma membrane also undergoes diffusion until equilibrium is attained

when it separates two substances with different concentrations and when the membrane is

permeable to both.

01.5

5

7

10

0

5

10

15

11:20 11:50 12:20 12:50 1:20

lLev

el o

f eg

g co

nte

nt

Time

Graph 1. Increase of water level in the thistle tube every 30 mins (in cm).

cm

As seen in the graph, the water level increased throughout the experiment. The

water level rose up the thistle tube because of osmosis. Osmosis as defined by Braungart,

is the diffusion of water from a region of greater concentration to a region of lesser

concentration through a semipermeable membrane2. In this case, the semipermeable

membrane is the membrane lining of the egg. It allowed the passage of water molecules

and because the water molecule has nowhere else to go to once it is inside the egg, it will

go up the thistle tube. That is why the water level increased in the thistle tube. However,

in the experiment, it is not the water molecule that went up the thistle tube but the egg

yolk. The possible source of error is that the thistle tube was placed too low in the

opening of the egg which possibly touched the egg yolk causing the egg yolk to rise up

and not the water.

Photo 7. Blood in 0.9% NaCl (Isotonic).

In Photo 7, the blood cells are sphere-shaped and are not tightly packed with each

other. In an isotonic solution, the blood cells retain their original shape and size because

there is no transport of molecules needed. There is no transport needed because the

concentrations inside and outside the cell is in equilibrium.

Photo 8. Blood in 10% Nacl (Hypertonic)

In Photo 8, the blood cells look flat and smaller. This is because the salt

concentration of the plasma is too high3 or it is in a hypertonic solution. It means that

there is less water molecule outside the cell so the water molecules inside the cell will go

out to attain equilibrium. This explains the wrinkling of the cell. This phenomenon is

called crenation.

Photo 9. Blood in water (Hypotonic)

In Photo 9, the blood cells look larger. This is because it is in a hypotonic solution.

Since there is more water molecules outside the cell, it will move inside the cell to attain

equilibrium. This will cause the cell to burst or “puff out like pillows”3. This process is

called hemolysis.

Summary and Conclusion

In the experiment of the human cheek cells, the parts of the animal cell namely the

nucleus, cytoplasm and plasma membrane were identified. These parts must be identified

because they play important roles in the human body. Section 2 of the experiment

discusses the structure of the plasma membrane. This section is consists of three

experiments—the experiment with oil and water, boiling of milk, and pricking of egg

albumin. These three experiments displayed the composition and behavior of the plasma

membrane. For instance, the experiment with oil and water explained how the plasma

membrane is both polar and non-polar because it is composed of phospholipid that has a

hydrophobic tail and hydrophilic tail. This explains why water-soluble molecules cannot

pass through the plasma membrane similar as to how water does not mix with oil. The

boiling of milk showed that milk has some similar compositions as the plasma

membrane. Both are made up of proteins and fats. The experiment with albumin and oil

showed that the plasma membrane can repair itself once it is torn. This is because once

the plasma membrane is torn, the hydrophobic tail of the phospholipid will be exposed

and this cannot be so the hydrophilic heads will quickly fill in the torn part causing the

plasma membrane to repair itself.

Part 2 of the experiment differentiated diffusion from osmosis. In section 1, the

experiment with ink and water showed how diffusion works. The region where the ink

was dropped was the region of higher concentration and from there the ink molecules

spread out to the region of the water with lower concentration of ink molecules which

made the water slightly blue. Section 2 of the experiment involved the osmosis of water

through the membrane lining of an egg. The water rose up the thistle tube because the

water molecules outside the egg was transported inside the egg and through the thistle

tube with the help of the semi-permeable membrane that is the membrane lining of the

egg. Therefore, the difference between diffusion and osmosis is that in osmosis a semi-

permeable membrane is needed for transport of water molecules. Section 3 of the

experiment is about the two types of osmosis: crenation and hemolysis. Crenation was

explained when the blood was placed in a hypertonic solution. This caused the water

molecules in the blood cell to move outside where there is lower concentration making

the blood cell smaller therefore shrinking it. Hemolysis, on the other hand, happens when

blood is placed in a hypotonic solution. The water molecules outside the cell will move

inside where there is lower concentration making the cell burst and become larger.

The experiments done provided a better understanding on the parts of the animal

cell, the structure of the plasma membrane and the different transport systems inside of it.

These experiments may be further improved through the use of different materials as

substitute for the materials used in the experiment such as dye instead of ink.

References

1. Biology of the Cell.

http://mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06b_06.pdf

. Accessed February, 11, 2015.

2. Braungart, D. (1964). An Introduction to Animal Biology. Sixth edition. The C. V.

Mosby Company: p.29

3. Morgan, A. Kinships of animal and man. n.d. pp.23

4. Dorit, R. 1991. Zoology. International Edition. Saunders College Publishing. pp. 67-69

5. Hurley, W. n.d. Milk Composition & Synthesis Resource Library.

http://ansci.illinois.edu/static/ansc438/Milkcompsynth/milkcomp_protein.html.

Accessed February 12, 2015.

6. Johnson, W.H. 1902. Principles of Zoology. 7th edition. New York, Holt, Rinehart and

Winston. pp. 46-47

7. Mitchell, C. 2015. Why Does Milk Form a Skin when it is Heated

http://www.wisegeek.org/why-does-milk-form-a-skin-when-it-is-heated.htm.

………Accessed February 12, 2015.

8. Storer, T. 1968. Elements of Zoology. 3rd Edition. McGraw-Hill Book Company.p.11

9. Villee, C. 1978. General Zoology. 5th Edition. W.B. Saunders Company. pp.31-33