I A What is a Cell?


[2]  Welcome to this week's laboratory, "The Organization of Life: From Cells to Organisms".  Now you have finished an introduction to chemistry and you are prepared to look at some higher levels of biological organization.


[3]  An important property of life is that it is modular.  This is the smallest unit of life, called the cell.  The closed boundaries of a cell permit concentrations and arrangements of chemical components that would be highly improbable in a test tube.


[4]  Large organisms do not consist of a single large cell, but of many small ones.    Molecules can only be efficiently moved over very short distances.  The biochemical machinery in the cell must carry out thousands of chemical reactions per second.  If cells were too large, they would not have adequate surface area for exchange of gases, nutrients and waste products.  The material in the center of a large cell would either suffocate or starve itself.


[5] The cell has been called a miniature machine for living--a chemical factory that takes both small and large molecules from the environment and rearranges them into living matter.   As you remember from Lab #1, cells are small, even though there is quite a bit of size variation.  A period in your textbook is about 400 micrometers in diameter and most cells are smaller than this.


[6]  Now, turn to your lab book and we will begin with an observation of some of the cells nature has produced.  Read the introduction on page 4.2, and read over the definition of a cell on the next page. Then do exercise 1 on pages 4.3 and 4.4.


[7]  Now, I would like you to check your sketch.  You should have labeled the nucleus, the cytoplasm, and the outer boundary--the cell wall.  In all of the advanced organisms the hereditary material, or DNA, is separated from the rest of the cell by a membrane.  The rest of the material inside the cell is called the cytoplasm.  Within the cytoplasm are a variety of smaller structures called organelles.




I B What is an Organelle?


[8]  Here is a view of the cell membrane which surrounds the cytoplasm.  In Table 1 on page 4.6, I would like you to write a very short summary of the function of each organelle.  These are the most common organelles found in plant and animal cells.  Note that the "cell membrane" is also called the "plasma membrane".  The function of the cell membrane is to regulate the passage of materials into and out of the cell.


[9]  You can see in the diagram that the two main chemicals in the cell membrane are protein and phospholipid.  Pores allow for the passage of the large molecules.  Whether or not a molecule is admitted to the cell depends on its size, shape, electrical charge and chemical properties.  To see the cell membrane in detail, we must use the electron microscope, since the membrane is only about 70 angstroms thick.


[10]  The nucleus is the next organelle listed.  The nucleus is the information center of the cell, storing genetic messages.  The nucleus contains the nucleic acids DNA and RNA.


[11]  ER or endoplasmic reticulum is a system of double membranes in the cytoplasm which serve as internal channels through which various materials can be transported. The endoplasmic reticulum also functions to package various secretions by surrounding them with a membrane.  Let me summarize the function of the endoplasmic reticulum: it serves to transport and package materials. 


[12]  The endoplasmic reticulum may also provide a structural framework for the attachment of the ribosomes.  In this case it is called rough endoplasmic reticulum. The dots in this picture are ribosomes which serve the function of synthesizing, or assembling, proteins.  The role of protein synthesis is crucial to the cell, as you learned last week in chemistry.  Endoplasmic reticulum without ribosomes is called smooth endoplasmic reticulum.


[13]  This system of flattened membranes is called the Golgi complex.  The Golgi complex may also be called the Golgi apparatus or the Golgi body.  It functions to assemble and package secretions, such as digestive enzymes.


[14]  Next, we have lysosomes.  Here you see lysosomes labeled on the right edge of the picture. Can you find other lysosomes in the picture?  The function of the lysosome is digestion.  Cell organelles which are worn out are digested and recycled in the lysosome.  Lysosomes are found mainly in animal cells.  Lysosomes contain hydrolytic enzymes, that is, they catalyze the hydrolysis of large molecules into smaller ones.  You can see in this picture some of the other organelles we have discussed.  The cell membrane regulates the passage of materials and the nucleus is the control center of the cell.  The endoplasmic reticulum transports and packages proteins and fat droplets store energy.  Would this be a plant cell or an animal cell?  Animal is correct.  Specifically, this cell is from the liver of a rat.

[15]  Here is a plant cell.  The dark ovals in this electron micrograph are chloroplasts.  The function of the chloroplast is to convert light energy to chemical energy.  The name for this process is photosynthesis. The end product of photosynthesis is glucose, which you constructed in laboratory 3.


[16]  At the center of the cell is a large space called a vacuole--an organelle common in plant cells.  The vacuole is surrounded by a membrane and serves to hold fluid within the cell.  This vacuole holds water, which saturates the cytoplasm and gives the cell shape.  Vacuoles are also found in animal cells.  Vacuoles may serve a variety of functions.  They can serve as a chamber for digestion, storage or waste collection.  Certain molecules called pigments are sometimes stored in vacuoles, such as the red pigment you find in roses, red onions, and beets.  You can see small vacuoles near the edge of the cell.


[17]  Also at the edge of the cell, you will observe a structure labeled the cell wall.  The cell wall gives support to plant cells.  A polysaccharide called cellulose is the main structural material in plant cell walls.  The cell wall is found only around the plant cell--it is secreted from within the cell, but it actually lies outside of the cell membrane.  Animal cells have other means of support, such as an internal or external skeleton, which maintains a fairly constant shape for the animal.


[18]  Here you see a small but by no means insignificant organelle bounded by a double membrane.  This is a mitochondrion.  The plural form of the word is mitochondria, which you see in your lab guide.  Mitochondria are known as the power plants of the cell.  They carry out a complex set of chemical reactions which produce the energy compound called ATP.  ATP stands for adenosine triphosphate.  But for now, simply make a note of the capital letters ATP.  The mitochondria provides 90% of a cell's energy.  Many metabolic reactions take place here.  Notice that the mitochondrion has internal membranes which are involved in the chemical reactions of energy production.  Mitochondria are more abundant in the cells that consume large amounts of energy.  They are also situated near the structures within the cell which utilize the energy.  For example, in muscle cells, the mitochondria can be associated with the words metabolism, energy conversion, and ATP.


[19]  These are ciliated cells which line the air passageways into our lungs.  Dust and foreign matter are trapped in the film of mucus secreted by goblet cells.  The rhythmic beating of cilia sweeps mucus upward toward the throat.  Cilia and flagella function in movement.  Write the term "movement" after cilia and flagella.


[20]  For some cells, the whip-like action of cilia and flagella is the only means of locomotion for the cell.  In multicellular organisms, the cilia may function to move a liquid past the surface of the cell the way these ciliated cells of the trachea push mucus past their cell surface.  Ciliated cells also circulate water through the gills of a clam and through the bodies of living sponges.  Ciliated cells also line our nasal passages, carrying dust and mucus downward toward the throat.


[21]  To summarize, cilia and flagella function in movement...whether to propel cells through water or to propel water past the surface of the cell. Remember that  these organelles are not just isolated structures which work independently of each other in the cell. They all function together as a working cellular unit.


[22]  This is a goblet cell, which secretes mucus.  Mucus is a vital biochemical secretion. Beginning at number one in this diagram, amino acids, sugars and sulfate are transported from the bloodstream into the cell, and then they are packaged together at the Golgi complex.  Note that the amino acids are first assembled into protein at the ribosomes, numbers two and three.  Packaging and transport takes place in the Golgi complex at four, five and six, finally producing mucus in vacuoles at number seven.  Mucus is a complex chemical, a combination of carbohydrate and protein--thus the name glycoprotein.  Finally, the mucus is released, or secreted, at number eight.  This is just one example of how organelles function together to perform a specific task.


[23]  Now I would like you to try an easy question.


[24a]  Following Table 1 in your lab book is exercise B-2  "Organelles in Elodea".  Elodea is a common freshwater plant you may have seen in an aquarium.  The common name is also elodea.  You may now follow the directions for exercise two.


[24b]  Be sure to save your slide.  We will use Elodea for a demonstration of diffusion and osmosis.


I C Diffusion and Osmosis


[25]  Now look at part C on page 4.7 - "How do small particles disperse and gain entrance to cells?" You may know this answer already.  Small particles, like minerals and ions, enter cells by the process of diffusion. The definition of diffusion is: "the random movement of particles from a region of high concentration to a region of low concentration."


[26]  This diagram is an example of diffusion.  Diffusion occurs every time you add sugar or cream to coffee, or when you wear a perfume or cologne.


[27]  Osmosis is "the diffusion of water through a semipermeable membrane."  As with diffusion, osmosis will occur if there is a higher concentration of water on one side of the membrane than on the other.


[28]  Here, the tiny particles represent water molecules which are able to diffuse through the pores in the cell membrane.  Now read the next section in your lab book:  "How is a cell affected by its environment?"


[29]  Here you see the effect of putting a cell--in this case a sack with a semipermeable membrane--into a hypertonic solution.  Fill in Table 2 accordingly. The solute is the material that has been dissolved in the water.  In this case the solute is sucrose, or table sugar.  Next to the term "hypertonic", you should indicate that the water concentration compared to the cell would be lower in the hypertonic solution--that is, 90% as compared to 98%.  Note that water diffuses out of the sack when it is put in the hypertonic environment.  You should also indicate that the solute concentration in the hypertonic solution is higher when compared to the cell.  In this example, it is 10% sucrose, compared to 2% sucrose in the cell.


[30]  Here you see an isotonic solution on the right, because it is the same as the cell:  98% water.  The solution on the left is hypotonic because it is 100% water.  Now, fill in the table again for the isotonic solution on the right and the hypotonic solution on the left. Now, next to the word "isotonic", you should have the water concentration and the solute concentration the same as that within the cell.  For the hypotonic solution, you should have noted that the water concentration is higher than in the cell and the solute concentration is lower.  Note that the water concentration does not have to be 100%, but 100% is the most hypotonic solution possible, such as rainwater or distilled water.  The arrows emphasize that in the hypotonic environment, water diffuses into the sack, and in the isotonic environment, there is no net change in water movement. Complete the rest of this page and then complete   "Elodea in a different environment".  Follow the steps and obtain the instructor's initials.


[31]  Your sketch should indicate that the cell contents, or cytoplasm, have shrunk to some extent and pulled away from the cell wall--similar to the diagram shown here.  Specific cells are adapted to specific environments.  The normal environment for Elodea cells is hypotonic, such as a freshwater pond.  The environment of organisms in the ocean or in saltwater lakes is hypertonic.  The cells which live in your bloodstream require an isotonic environment.


[32]  A picture taken with a scanning electron microscope will give a somewhat 3-dimensional view such as this.  The shape of the red blood cell is described as a biconcave disk, which gives it a high surface-to-volume ratio.  This allows for rapid and efficient diffusion of oxygen molecules through the cell membrane.


II   Tissues


[33]  We group similar cells together into units we call tissues.  Read over the definition of a tissue in your lab guide. These cells are a sample of human muscle tissue.  The protein molecules in skeletal muscle are organized into very regular patterns, thus we see dark bands as striations.  All of these cells are similar, and they function together to contract a muscle which moves the skeleton.


[34] Nerve tissue is found in the brain, spinal column, and the nerves of higher animals, such as most of us humans.  This is an individual nerve cell, or neuron.  The dark spots are the connections or synapses with other nerve cells.


[35]  The structure of epithelial tissue is such that the cells are closely spaced and are arranged in thin layers or sheets.  Epithelial tissues make up the covering of the body, which is called the epidermis or skin.  Epithelial tissue also lines the internal cavities and ducts, forming glands.


[36]  In exercise B-1 you will observe some of your own human epithelial tissue.  You will observe cells like these called squamous epithelium, which consists of thin, flattened cells arranged in one or several stratified layers.


[37]  You will obtain epithelial cells in this manner.  Be gentle when you scrape your cheek--you will not see any cells on your toothpick with your naked eye, but they will be there nonetheless.  Now follow the steps for Human Epithelial Tissue under Roman numeral II.


[38]  Connective tissue includes a variety of cell types. Dense connective tissue forms tendons and ligaments. Cartilage, bone tissue, and blood, or vascular tissue are considered  to be connective tissues. The white blood cells in this picture are marked with an "ly" for lymphocyte.  "Er" is for erythrocyte, or red blood cell, and "pl" is for the platelets--involved in the clotting mechanism.  Unlike epithelial cells, the cells of connective tissue are scattered through an extensive extracellular matrix.


[39]  Macrophages and white blood cells called lymphocytes are housed in filters in the body called lymph nodes, which you see here.  "Ly" stands for lymphocyte, "ma" is for macrophage.  These are cells which engulf invaders and clean up debris in the body tissues.  They engulf particles by a process called phagocytosis.


[40]  This depicts the process of phagocytosis.  The cell forms a vacuole around the food particle.  Later in this lab you will observe an organism which looks very closely related to our white blood cells.  You may even be able to observe the process of phagocytosis.


III Organs


[41]  Here you can see a variety of organs which belong to our distinctive animal friend, the frog.  The organs of the frog are very similar to the organs of the human.  Thus when you study the frog, you are also studying yourself.  Some organs in the frog and the human are composed mainly of one type of cell.  For example, the brain is made up of neurons and muscles are made up of muscle cells.  Other organs, such as the small intestine, are a composite of different types of tissues.


[42]  Here is an electron micrograph of a cross-section of small intestine.  The projections into the opening, or lumen, of the small intestine are called villi.  On the surface of each epithelial cell are tiny extensions of the cell membrane called microvilli.


[43]  Together, these two types of projections produce a tremendous surface area for the small intestine.  It is in the small intestine that digestion and absorption of most nutrients occur.  Do exercise A-1 on pages 4.10 and 4.11.


[44]  Look at your answer to the question in step "e". Be  sure to locate the epithelial cells because of their vital importance in the absorption of digested food in the form of simple molecules.  The size of the epithelial layer is generally from 30 to 70 micrometers in thickness.  Your answer should be in this general range. Now turn to the top of page 4.12.


[45]  This is the small intestine again, and these projections, called villi, function to increase the surface area of the small intestine for the absorption of food.  Write the function next to the word "villi" at the top of the page.  Again, the villi increase surface area for absorption.


[46]  The microvilli are microscopic projections on each epithelial cell in the small intestine.  Their function is the same as that of the villi.  In fact, the entire surface area in the human small intestine is equal to 300 square yards.  This is enough surface area to cover the floor of a 2700 square foot house.  This would be a cheap floor covering--but it would be extremely thin...   The function of the epithelial tissue is for the absorption of nutrients into the bloodstream.


[47]  The muscle tissue provides waves of contraction, called peristaltic waves.  This constantly moves more nutrients past the cells which do the absorbing.


[48]  Connective tissue like this binds other tissues together.  In the small intestine, it binds the muscle cells to the epithelial cells.  This is a sketch of loose connective tissue, which provides a framework for every organ in the body.


[49]  The nervous tissue was not visible in your microscope section.  Nerve tissue provides stimuli to control the muscle cell contractions. 


V Organ Systems


[50]  Read the definition of an organ system in Roman Numeral IV.


[51]  The organ system you will now observe is the digestive system.  We will examine the organs which work together to accomplish digestion.  Your frog is a fine example of organization and adaptation.


[52]  There are steel probes on the demonstration table, so you don't have to use your fingers.  Find the tongue and see if you can notice anything unusual about it.


[53]  You might have noticed that the tongue is attached at the front of the mouth.  Why?  If we could observe a living frog catch an insect, the tongue would unroll.  Of course, the living tongue was more elastic.  Run your dissecting probe around the edge of the mouth to detect the tiny row of maxillary teeth, which, along with the two vomerine teeth, help to keep the prey in the mouth.  In Table 4, the function of the tongue is to taste and manipulate food.  The frog's tongue also is used to catch its food.


[54]  The food passes through the esophagus to the stomach.  Only the beginning and the end of the esophagus will be visible to you.  The esophagus functions to pass the food from the mouth to the stomach.


[55]  The function of the stomach is to begin the digestive process.  With the action of acid and enzymes and a churning motion, the solid food is converted into a semi-liquid blob.  The stomach of your frog has been cut open, and you can look in to verify the fact that the frog swallows its food whole.


[56]  Follow the stomach downward and you will find a narrow constriction between the stomach and the small intestine called the pyloric sphincter.  The pyloric sphincter regulates the movement of food from the stomach into the small intestine.  The small intestine completes the digestive process.  It is here that the food is completely digested into molecules and absorbed into the blood stream. The small intestine will eventually run into an enlarged portion.  This is the large intestine.


[57]  The function of the large intestine is to absorb water from the digested mass of food.  The large intestine also absorbs minerals, such as sodium, and vitamins which are manufactured by the bacteria which live there.


[58]  Remember the process of osmosis?  A great deal of osmosis takes place between the body fluids and the small intestine.  The water moves by osmosis from the body fluids into the small intestine. Eventually this water must be reabsorbed into the body to prevent dehydration.  The large intestine reabsorbs water--a homeostatic function that maintains water balance in the animal.


[59]  The liver is easy to find.  It is dark brown and has three lobes.  The liver produces a liquid called bile that helps to digest fat by breaking it into tiny droplets. Remember the emulsified fat in milk in lab 3. The liver is a metabolic factory, doing an amazing variety of jobs including hormone and glucose regulation, detoxification of poisons, removing nitrogen from amino acids, and storing vitamins. 


[60]  The next organ is the gall bladder.  The gall bladder stores the liver bile until it is emptied into the small intestine.  Look under the lobes of the liver and you will find a small greenish or clear sack.  This is the gall bladder.


[61]  The next organ, the pancreas, is a thin, light-colored organ located between the lower end of the stomach and the first loop of the small intestine.  The pancreas produces digestive enzymes.  Humans produce more kinds of enzymes than a frog does.  Why is that?  We have to be able to chemically process a number of different kinds of foods.


[62]  The next item on your list is the fat bodies.  The fat bodies are long, yellow structures, located under the digestive organs.  Fat is the most efficient way to store energy--and that is the function of fat bodies.


[63]  Finally, the mesenteries are the connective tissue membranes which hold the small intestines and the pancreas in place.  We have seen microscopic views of connective tissue-- this will give you a chance to observe the overall appearance of this tissue type.


[64]  Now answer this practice question.


[65]  Now I would like you to do exercise number three at the top of page 4.14


[66]  Now we will do exercise number four, and look at some other organ systems.  The main organ of the circulatory system is of course, the heart, which functions as a pump.  The heart is a firm muscular organ, located right under the liver.


[67]  The spleen is a dark, round sack located in the mesenteries of the small intestine.  Its function is to process red blood cells, and to serve as a reservoir of red cells for emergencies.  The white organ next to this spleen is one of the testes.


[68]  By moving the heart gently to the right and then to the left you will expose the lungs, the major organs of the respiratory system.  The lungs may be prominent, or they may appear like this, as thin, flattened sacks.


[69] The urinary bladder, which has probably collapsed in your frog, and will appear as a thin sheet of connective tissue.  The bladder and kidneys are important organs of the excretory system.  Gently push the digestive organs to one side and you will find two flat kidneys lying against the body wall.


[70]  Notice that the kidneys are covered by a membrane called the peritoneum.  At the anterior or front end of each kidney may be found the long, yellowish fat bodies.  On the midline of each kidney should be a yellowish band of tissue called the adrenal gland.  This is an endocrine gland that secretes hormones.


[71]  If your frog is a male, you will be able to find one of the small oval, whitish testes at the anterior or front end of each kidney.


[72]  If your frog is a female, you will see an obvious, white mass of coiled oviducts and you may find a mass of dark eggs.  When the eggs mature, they pass through the oviducts and are coated with several layers of jellylike material. Now you may return your frog to the demonstration table.





V Organisms


[73]  Now we are on Roman Numeral V, the last stretch of laboratory four.  The organism is an easy level of organization to comprehend.


[74]  Here is another organism,


[75]   and another.


[76]  You have looked at the frog, a multicellular, but dead, organism.  How about a unicellular, but live one?


[77]  This is one of the neatest, the amoeba--an organism that consists of only one large cell.  Its unique motion consists of extending lobe-shaped projections called pseudopods and having the cytoplasm of the cell flow into them. White blood cells, such as lymphocytes, use amoeboid movement and phagocytosis to capture infectious microorganisms in the blood.


[78]  Here you see a large amoeba capturing a paramecium.  If you recall, this event is called phagocytosis.  You may be able to observe this event in the pond water sample we have provided.


[79]  Now complete the last page of laboratory four. Please be sure your microscope and lab space is in order. Thank you.