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Homework answers / question archive / LAB TOPIC 3 Basic Chemistry: Atoms, Molecules, Electrolytes
LAB TOPIC 3
Basic Chemistry: Atoms, Molecules, Electrolytes. Acids, Bases, pH, Buffers. Organic Molecules.
Objectives:
* Before coming to lab, you should read through all of Lab Topic 3 and the Basic Chemistry chapter in your textbook.
Introduction, Rationale and Review
As you already know, all things in the world are made of atoms. Atoms in turn are composed of smaller building blocks called - believe it or not - leptons and quarks (and even quarks with "flavors"). Our objective today is to gain some working knowledge and familiarity with a few of the very fundamental chemical concepts. Today's exercises are designed to complement in a "hands-on" way your text readings and lectures. We will not delve deeply into chemical theory, but we will try to gain some information about atoms and molecules and how they behave under certain circumstances. Since living things may in part be viewed as chemical machines, some knowledge of basic chemistry is essential to understanding most biological phenomena.
There are 92 naturally occurring elements which make up the earth and perhaps the entire universe. There are more than ten additional elements which have been created in special atomic reactors. An element is matter made up of identical atoms. We give each element a different symbol. For instance, oxygen is given the capital letter O, hydrogen capital H, carbon capital C. It's easy to see that with 92 elements we quickly run out of letters, so some elements have symbols which are abbreviations of their names: magnesium is Mg; chlorine is Cl. Some elements are given a symbol which is the abbreviation for their Latin or Greek name -sodium in Latin is natrium, so the symbol for sodium is Na. Please note that the first letter is a capital and the second is lower case and the two letters together represent one element.
The structure of the atom is sometimes compared with the layout of the solar system - at the center of the atom, instead of the sun there is a nucleus. (This is not to be confused with the nucleus of a cell.) Within the nucleus there are two parts: protons which are particles with one positive charge and neutrons which, as their name implies, are neutral. Orbiting the nucleus are electrons -- these are much smaller particles with a negative charge. In an atom, there are always the same number of electrons as there are protons -- therefore atoms are electrically neutral [that is, they have the same number of positive (+) and negative (-) charges]. An element is defined by the number of protons in the nuclei of its atoms. The number of protons is called the atomic number. For example, the element carbon (C) has an atomic number of 6 because it has 6 protons in its nucleus (and therefore 6 orbiting electrons). Almost all the carbon atoms on this earth also have 6 neutrons in their nuclei. However some carbon atoms have 4, 5, 7, 8 or 9 neutrons -- they are known as isotopes of carbon. These atoms with the varying number of neutrons are all carbon regardless of the number of neutrons because they all have 6 protons. Just about every element exists in nature in various isotopic forms.
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Fig. 3.1 |
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As you know from lecture, electrons orbit the nucleus in distinct "shells". Figure 3.1 shows the electron shells, their names (which are letters starting with K) and the maximum number of electrons (e) each shell can hold.
Figure 3.2 gives examples of atoms of several different elements and shows the
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Fig. 3.2
Electrolytes and Non-Electrolytes
Atoms interact with each other to form molecules using three kinds of interactions called chemical bonds. The first of these, hydrogen bonds, do not involve exchanges of parts of the atoms. However, in both ionic and covalent bonds electrons are exchanged or shared. We will now examine a relatively simple chemical interaction that involves exchanges of electrons in the outermost shell - the nucleus and inner electrons do not become involved.
Because of their physical properties, the atoms of some elements have the ability to take electrons from atoms of other elements which tend to give up electrons. Sodium (Na) is an atom that tends to give up electrons while chlorine (Cl) is an atom that tends to take electrons. Sodium normally has 11 positively charged protons and 11 negatively charged electrons and therefore has no charge. Chlorine has 17(+) and 17(-) and therefore it is also neutral. If chlorine takes one electron from sodium, it now has 18 negative charges whereas it still has only 17(+) charges in the nucleus. It therefore has one extra negative charge and is now chlorine with a negative charge. When an atom has a charge other than neutral it is called an ion. Chlorine with a negative charge is called chloride ion and the symbol is Cl-. Similarly, sodium, after giving up an electron has 10 negative charges and 11 positive charges left in its nucleus and it therefore has one extra positive charge -- it is now sodium ion, written Na+. The sodium ion and the chloride ion, because they are oppositely charged, attract one another and form a molecule of sodium chloride -- common table salt. A molecule is two or more atoms or ions bonded together. The chloride and sodium ions are said to be held together by an ionic bond.
sodium atom (Na): chlorine atom (Cl):
11 protons (+), 11 electrons 17 protons (+), 17 electrons
one electron has moved from Na to Cl
sodium ion(Na+): 11+,10- chloride ion (Cl-): 17+,18-
Sodium chloride (NaCl) - held together by an ionic bond
Figure 3.3
It is interesting to note that sodium and chlorine atoms exchanged one tiny negatively charged electron and that exchange turned sodium and chlorine atoms, which are poisonous, to sodium and chloride ions, which are necessary for life. We will learn later in the semester some of the ways these two ions function in living systems.
When ionically bonded molecules are placed in water, the ions separate; and the molecule is said to have dissociated or ionized. When sodium chloride is dissolved in water, sodium ion separates from chloride ion. Water acts somewhat like a wedge separating the ions and then surrounding each ion to help keep them separated. You have undoubtedly discussed this ability of water to act as a solvent for many compounds.
The ions distributed in the water confer upon water the ability to conduct an electric current. Pure water cannot conduct electric currents. Therefore all molecules that ionize in water are called electrolytes, because the free ions allow the water to conduct electric currents.
molecule is dissolved in water, ions are "pulled" apart
NaCl Na+ + Cl-
molecule is dissolved in water, ions are "pulled" apart
MgCl2 Mg++ + 2 (Cl-)
Some atoms do not completely exchange electrons when they react as did sodium and chlorine. These atoms instead can share electrons in a molecule. Shared electrons take a position between the two atoms and are held onto by both atoms. Atoms that form a molecule by sharing electrons are held together by a covalent bond. Covalently bonded molecules do not ionize in water. The water containing these molecules does not have charged ions and therefore cannot carry an electric current. Covalent molecules are called non-electrolytes. In Figure 3.4 we see a central carbon atom surrounded by four hydrogen atoms. As you will recall, carbon has an atomic number of 6 -- it has 6 protons and 6 electrons. We are concerned with the outer 4 electrons in the outermost (L) shell, because these do the bonding with other atoms. Hydrogen has an atomic number of 1 -- it has 1 proton and 1 electron. Each covalent bond uses one electron from H, and one from C.
Fig. 3.4 CH4 (methane)
All life characteristics ultimately depend on chemical interactions. In the following exercises various chemical properties and reactions will be demonstrated.
Exercise 3.1
Work in groups of four. Please work carefully. Record your answers on the Answer Sheet for Exercise 3.1-3.3.
Fig. 3.5 Light apparatus.
Turn the apparatus off by pulling out the plug. Remove the beaker.
Turn the apparatus off. Remove the beaker with sugar water and place an empty waste container under the electrodes. Rinse the electrodes off by squirting them with distilled water from a squeeze bottle (allow the water to collect in the empty waste container). After rinsing, make sure the apparatus is off, then wipe the electrodes with a paper towel.
Explain your results. (c)
Turn the apparatus off. Again rinse and clean as above.
Acids and Bases
With the apparatus you just used you could test just about anything to see if it is an electrolyte or not. Among the things that are electrolytes are two special groups of molecules called acids and bases. Acids are electrolytes. When acids are dissolved in water, one of the ions formed is hydrogen ion, H+. Bases are also electrolytes.
When bases are dissolved in water, one of the ions formed is hydroxyl ion (OH)-. Bases are also called alkalies.
Acids are electrolytes which produce hydrogen ion (H+) in water. Following are some examples of acids:
(hydrochloric acid) HCl -------- > H+ + Cl-
(hydrofluoric acid) HF -------- > H+ + F- (sulphuric acid) H2SO4 -------- > 2 (H+ ) + S04-2
Bases are electrolytes that upon ionization in water give a hydroxyl ion (OH-). The hydroxyl ion is a complex ion made of one oxygen, one hydrogen atom and one extra electron. Following are some typical bases: (sodium hydroxide - lye) NaOH -------- > Na+ + OH-
(potassium hydroxide) KOH -------- > K+ + OH- (magnesium hydroxide) Mg(OH)2 ----- > Mg +2 + 2 (OH-)
Exercise 3.2
happens? (d)
Turn the apparatus off and rinse and wipe the electrodes as before.
happens when the apparatus is turned on? (e) Turn off the apparatus and rinse and wipe the electrodes.
Hydrogen ions confer acid properties upon water whereas hydroxyl ions confer basic properties. When equal amounts of both hydrogen and hydroxyl ion are in the same water the water is neither acidic or basic. Hydrogen ions and hydroxyl ions react with one another to form water; we call this reaction neutralization. The remaining ions -- those other than the hydrogen and hydroxyl -- form a salt. For example:
HCl -------- > H+ + Cl-
NaOH -------- > OH- + Na+
H2O (water) + NaCl (salt)
An acid and a base always neutralize one another to water and a salt. What would be the salt formed if we mixed HF and KOH?
(f)
If we were to take an acid and add some to an amount of water there would be a certain number of hydrogen ions (H+) in that water. If we kept adding acid the hydrogen ion (H+) concentration would go higher. The same can naturally be said of the hydroxyl ion (OH-) concentration if we keep adding base to an amount of water. There is a convenient way of measuring the amount of acidity or basicity (also known as alkalinity) of a solution; this acid/base measurement scale is called the pH scale. Look at the diagram below. At pH 1, a solution has a great quantity of hydrogen ions and no hydroxyl ions. As you go up the scale there are fewer hydrogen ions and more hydroxyl ions.
Hydrogen ion con- centration
Hydroxyl
ion con-
centration
pH of Solution
Figure 3.6 pH chart
When you reach seven there is an equal amount of both ions. Seven represents a neutral solution. If we continue to go up the pH scale the hydrogen ion concentration continues to fall while the hydroxyl ion concentration continues to rise until we reach pH 14 where there are virtually all hydroxyl ions and no hydrogen ions.
The numbers on the pH scale are the negative logarithms of the number of (H+) ions in a solution. The table below reminds us of how logarithms are related to numbers. The amount of (H+) ions is very small, so the pH scale starts at 0.1 (H+) ions. That is pH 1. The fewer the (H+) ions, the larger the pH becomes.
Number Representation and Logarithms
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103 |
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10-2 |
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0.001 |
10-3 |
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3 |
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0.0001 |
10-4 |
-4 |
4 |
There are several ways to determine the pH of a solution – that is, how acid or basic a solution is. One method is to use an instrument called a pH meter; another method is to use specially treated paper strips which turn different colors in solutions of different pH; a third is to use a computer. The computer records the pH and can also graph pH changes over time.
Exercise 3.3
paper container and record the pH. (g) Rinse the glass rod with distilled water and wipe.
paper. Record the result. (h)
Rinse and wipe the glass rod.
result. (i)
Rinse and wipe the glass rod.
HCl. Stir and test the pH. Record your result. (j)
Explain. (k)
You can see by the results of your acid and base experiment that water is neutral but can be made acidic or basic. The acidic and basic solutions can be mixed and the resulting solution will then be (close to) neutralized. Please realize that to come to perfect neutrality you must add exactly equal quantities of exactly equal concentrations of acid and base. Your result in (j) should be somewhere between your answers to (h) and (i).
Your instructor will tell you whether to continue with the exercises on organic chemistry (Exercise 3.5), or do the computer exercise on buffers (Exercise 3.4).
ANSWER SHEET EXERCISE 3.1 – 3.3
Exercise 3.1
Exercise 3.2
Exercise 3.3
ORGANIC CHEMISTRY
Exercise 3.5
We've just learned that atoms involved in a bond in which electrons were exchanged are said to be ionically bonded whereas those that share electrons are said to be covalently bonded. Carbon atoms form a great number of covalently bonded molecules, and form long chains and rings of carbon atoms to which many other atoms may be bonded. The area of chemistry that deals with carbon compounds is called organic chemistry. It is called organic chemistry because it was first believed that organic molecules could come only from living things, which of course we know now is not true. Organic chemistry is an immense area of study because of the almost endless variety of different compounds that can be formed by carbon and other elements sharing electrons with carbon. We will attempt to learn only some very basic concepts of organic chemistry, but be sure, organic chemistry is very relevant to the study of biology. In this section, presentation of a concept will be followed with a demonstration and/or exercise.
Figure 3.7
Hydrocarbons: Carbon atoms have four electrons in the outer shell that are available for sharing in covalent bonds. They can share these electrons with many other kinds of atoms.
Figure 3.8
The left carbon atom is sharing three electrons with three separate hydrogen atoms and the fourth electron with another carbon atom. This second carbon atom shares its three remaining electrons with three more hydrogen atoms. For convenience sake, we will henceforth show a shared pair of electrons as a simple straight line, and the above molecule will look like the following:
H – C – C - H
H H
Figure 3.9
We can build very simple to very immense molecules just with carbons and hydrogens; these molecules are known as hydrocarbons. One of these is a component of swamp gas: the other, part of a liquid known as gasoline.
H H H H H H H H H
H-C-H H – C – C – C – C – C – C – C – C – H
H H H H H H H H H
methane octane
Figure 3.10
As we add carbon and hydrogen atoms and change configurations, the physical and chemical properties of the molecule change. The smallest hydrocarbon molecules exist as gases, while those of intermediate size are liquid and the very large molecules are solids (for example, margarine or the fat on steaks). All hydrocarbons may be used as fuels – they burn in the presence of oxygen to form carbon dioxide (CO2) and water (H2O). The paraffin (“wax”) in candles is a large, solid hydrocarbon.
Some organic molecules may form branches and rings as below:
Figure 3.11
Alchohols. Molecules similar to hydrocarbons but with some other atoms involved in the sharing of electrons have major biological significance. For instance, if we add an oxygen atom into any of the hydrocarbons we have discussed on previous pages we form a group of molecules known as alcohols.
methane becomes methyl alcohol or methanol
ethane becomes ethyl alcohol or ethanol Figure 3.12
Unsaturated Hydrocarbons. Carbon has another important characteristic. Under certain circumstances, carbon atoms may share more than one electron with a neighboring carbon atom (it may even share as many as three electrons). When carbon shares more than one electron with a neighboring carbon it has that many less electrons to share with other atoms. Let us look at the organic molecule ethane in Figure 3.12, above. Each carbon is sharing its four electrons with four different atoms (three hydrogens and one carbon). The molecule is said to be saturated. If the two carbon atoms were to share more than one pair of electrons then ethane would look like the molecule below. Each carbon is sharing electrons with three other atoms (two hydrogens and one carbon). The molecule is said to be unsaturated.
Figure 3.13
The molecule in Figure 3.13 above is called ethylene. The sharing of two pairs of electrons by two carbon atoms is called a double bond. The occurrence of unsaturated (double) bonds in organic molecules has an interesting biological significance. Large organic molecules like long hydrocarbons and long fatty acids (about which you will shortly learn) are usually solids, but if any two carbons among the many in the chain have a double bond, the molecule exists in the liquid state. See Figure 3.14.
saturated tail:solid monounsaturated:fluid, oily liquid Figure 3.14
Some molecules may have several carbon atoms with double bonds and they, too, are liquids. These molecules with many double bonds are said to be polyunsaturated.
Organic Acids. The organic acids are among the few organic molecules that will ionize (partially) in water. Like all acids, they free a hydrogen ion (H+) when they ionize. Organic acids are hydrocarbons, but have an end carbon with a new electron sharing pattern.
Figure 3.15
The carbon with the double bonded oxygen and the OH group is the acid (proton (H+) donor) part of the molecule. We have seen two long organic acids (long hydrocarbons with the last carbon an acid group), called fatty acids, in Figure 3.14. They are solids (when they are saturated), and if they have double bonds in the hydrocarbon portion are called unsaturated fatty acids and are liquids As you have probably assumed from the name, fatty acids are important constituents in fats.
Amino Acids. The -next group of organic molecules we will study is amino acids.
Amino acids have two important groups sharing electrons with the carbon atoms. First, there is the acid group we just learned about; secondly, the next carbon shares electrons with a nitrogen atom which in turn shares electrons with two hydrogen atoms - this nitrogen with two hydrogens is called an amino group. Hence the name amino acid (Figure 3.16).
Figure 3.16
NOTE: The parts of the molecule contained in the box - the amino group and the acid group - are common to all the amino acids. The "R" above refers to any number of possible combinations of carbon, hydrogen and atoms of other elements.
You can imagine an almost infinite number of different amino acids according to what atoms and group of atoms (the "R" group) are bonded to the amino acid part. However, there are only twenty or so biologically important amino acids. These amino acids are subunits of proteins, important molecules that give structure and activity (as enzymes) to organisms. Each amino acid has the same basic structure: an acid group on the first carbon, an amino group on the second, and then a variety of twenty different molecular arrangements where the diagram shows R. Below are several examples:
Figure 3.17
Exercise 3.7
Go to the demonstration table and look at the examples and models of various organic chemical structures. There are extra model kits available; take one to your desk (one for every four students) and at the end of the period you will be asked to construct a model of several organic molecules.
Simple Sugars.The last individual group we will study are the simple sugars. Simple sugars can contain three to seven carbons; the sugar we will talk most about during the course is the six carbon simple sugar glucose. Don't let the word simple fool you, simple sugars are quite complex in structure and cannot be derived from the chains of hydrocarbons as the other organic molecules were. You will find two forms representing the glucose molecule in the figure below. You will not be required to reproduce from memory any of these organic structures - however, given a diagram of the structures, you will be asked to identity the type of organic molecule - i.e. whether it is a simple sugar, amino acid, fatty acid, alcohol and so on.
a) straight chain form b) ring and space-filling ring form
Figure 3.18
Even with all the "OH" groups the sugar molecule is not an alcohol. The top carbon in the straight chain form is not an acid group: it is known as an aldehyde. The combination of aldehyde and alcohol groups in the same molecule makes this a new chemical entity called a simple sugar or monosaccharide.
Exercise 3.8
Work in groups of four.
Butane is a hydrocarbon with a structure like octane, but it has only 4 carbon molecules and 10 hydrogen molecules. Your instructor will explain the proper use of the model making kits: please check with him/her after each assembly. With the molecular model kit construct the following four models. Make models 1-3 in order, then do model 4.
Hint: first draw the molecule in the space below and then construct the model. Your instructor may modify this list.
NOTE: Please make every effort to utilize lab time to do Exercise 3.8 -- it is an important learning experience.
Exercise 3.9
If you have time, go to the demonstration table labeled Exercise 3.9, and with pH paper, test the pH of the following items. Record the pH of each.
cola p)________________
distilled H20 q)________________
cleaner with ammonia r)________________
We can see that some common household items show a wide range of pH !!!.
Exercise 3.10
Again, if you have time, go to the demonstration table and look at the various hydrocarbons. Record your observations. With how many are you familiar? (s)
ANSWER SHEET EXERCISE 3.5 – 3.10
Exercise 3.6
Exercise 3.9
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LAB TOPIC 4
Part 2
Macromolecules(1)
Introduction
Living organisms are composed of molecules that come in diverse shapes and sizes and serve a variety of purposes. Some molecules form the structure of an organism's body -- for example, the cellulose that makes up the cell walls in plants, the proteins and phospholipids that comprise cell membranes, and the fibers that make up animal muscles.
There is also a wide array of molecules that perform all the functions of life. For example, enzymes catalyze the chemical reactions necessary for biological processes, neurotransmitters convey information from one brain cell to another, and visual pigments absorb light so that you can read the words on this page,
In this laboratory you will do further study on three classes of the largest biological molecules, called macromolecules: carbohydrates, lipids, and proteins. Your objectives are listed with each set of exercises. A fourth class of macromolecules, the nucleic acids, will be studied near the end of the semester.
Outline
Exercise 4.1: Carbohydrates
Activity A: Monosaccharides and Disaccharides
Activity B: Starch
Exercise 4.2: Lipids
Exercise 4.3: Proteins
Exercise 4.4: Macromolecules in Food
Note: As you do this investigation, please write your answers to the questions at the end of Lab Topic 4 Part 2.
Carbohydrates
Objectives
After completing this exercise, you should be able to
Most carbohydrates contain only carbon (C), oxygen (O), and hydrogen (H). The simplest form of carbohydrate molecules are the monosaccharides ("single sugars"). One of the most important monosaccharides is glucose (C6H1206), the end product of photosynthesis in plants. It is also the molecule that is metabolized to produce another molecule, ATP, whose energy can be used for cellular work. There are many other common monosaccharides, including fructose, galactose, and ribose.
Some disaccharides ("double sugars") are also common. A disaccharide is simply two monosaccharides linked together. For example, maltose consists of two glucose molecules, lactose (milk sugar) consists of glucose and galactose, and sucrose (table sugar) consists of glucose and fructose. Can you discern a rule used
in naming sugars? 4.1 a)
Carbohydrates are also found in the form of polysaccharides ("many sugars"), which are long chains of monosaccharide subunits linked together.
Starch, a polysaccharide composed of only glucose subunits, is an especially abundant component of plants. Most of the carbohydrates we eat are derived from plants. What was the last starch you ate? 4.1 b)
Starch is the plant's way of storing the glucose it makes during photosynthesis. When you eat starch, you are consuming food reserves that the plant has stored for its own use. The starch of potatoes and root vegetables, for example, would be used the next spring for the plant's renewed growth after the winter die-back. All perennial plants (those that come up year after year, such as tulips) have some kind of food storage for overwintering. Beans, on the other hand, contain starch in the seeds. Beans are annual plants; they will die at the end of the growing season. So the seeds are stocked with starch to use when they have a chance to germinate the next spring.
Animals store glucose in glycogen, which is another form of polysaccharide. Although starch and glycogen are both composed of glucose subunits, the glucose molecules are bonded together in different ways, so these polysaccharides are not identical. Glucose subunits are bonded together a third way in the polysaccharide cellulose. While starch and glycogen are meant to be metabolized for energy, cellulose, which is the most abundant carbohydrate in the world, is a structural molecule that is designed not to be metabolized. Cellulose makes up the cell walls of plants and is a primary component of dietary fiber. For most animals it is completely indigestible. Those that can digest it, such as termites and cows, do so only with the assistance of organisms such as bacteria, fungi, or protistans.
Most disaccharides and polysaccharides can be broken down into their component monosaccharides by a process called hydrolysis, which is accomplished in organisms by digestive enzymes. This process is important in seeds. If the seed's food resource is starch, it must be able to convert the starch to glucose. The glucose is then used to generate ATP which in turn is used to provide the growing plant embryo with energy for metabolic work. Hydrolysis of starch begins when the seed takes up water and begins to germinate.
Germination of barley seeds is part of the process of brewing beer. When the barley is germinated, the starch-to-sugar conversion begins. In the breakdown of starch, disaccharide maltose molecules are formed before the final product, glucose, is obtained. At a certain point in the germination, the barley is dried so that no further hydrolysis takes place. The maltose sugar is extracted and used in the brewing process. That's the "malt" listed on the beer can as an ingredient. The process of germinating the barley is called malting.
A chemical hydrolysis can be done in the laboratory by heating the molecules with acid in the presence of water. You will perform a chemical hydrolysis in this exercise.
* Wear safety glasses throughout the lab session.
Activity A: Monosaccharides and Disaccharides
You will use Benedict's reagent as a general test for small sugars (monosaccharides and disaccharides). When this reagent is mixed with a solution containing single or double sugars and then heated, a colored precipitate (solid material) forms. The precipitate may be yellow, green, orange, or red. If no monosaccharide or disaccharide is present, the reaction mixture remains clear. However, Benedict's reagent does not react with all small sugars. For example, sucrose gives a negative Benedict's reaction.
Glucose will be used in this laboratory to demonstrate a positive Benedict's test
(Figure 4. 1). What should be used as a negative control for this test? 4.1 c)
Figure 4.1. Benedict’s test for detecting small sugars.
Procedure
be used? 4.1 d)
4 e) Tube 1 (glucose). 4 f) Tube 2 (negative control)
Interpretation of Results
Describe a positive Benedict’s test. 4.1 g) What are the limitations of this test? 4.1 h)
Activity B: Starch
Starch is tested by using iodine reagent (I2KI – iodine potassium iodide). A dark blue color indicates the presence of starch (Figure 4.2).
You will use a solution of potato starch to demonstrate a positive test. What negative control should be used for this test? 4.1 i)
Figure 4.2. The iodine test for detecting starch.
Procedure
be used? Why? 4.1 j) 4. Put 3 or 4 drops of iodine reagent into each tube.
5. Record the results.
4.1 k) Tube 1 (starch): 4.1 l) Tube 2 (negative control):
Interpretation of Results
Describe a positive test for starch. 4.1 m)
What are the limitations of this test? 4.1 n)
EXERCISE 4.2
Lipids
Objectives
After completing this exercise, you should be able to 1. Define lipid and give examples.
2. Describe the test that indicates the presence of lipids.
Lipids are compounds that contain mostly carbon and hydrogen. They are grouped together solely on the basis of their insolubility in water. The lipids we will consider in this laboratory are fats and oils, which are generally used as storage molecules in both plants and animals. You are no doubt already familiar with the fact that your body converts excess food into fat. This fat is stored in your adipose tissue until your food intake is lower than your metabolic needs, at which time the fat can be metabolized to generate ATP, whose energy can be used for cellular work. Plants, too, can store fats. Seeds are often provisioned with fats that can be metabolized by the developing embryo when germination time comes. Thus we obtain corn oil, peanut oil, sunflower oil, and others by pressing the seeds.
You will use the paper test (Figure 4.4) to indicate the presence of lipids in various foods. Although this test is not very sophisticated, it is quick and convenient.
Rub sample on Observe
brown paper translucence
Figure 4.4. Brown paper test for lipids.
Proceedure
Interpretation of Results
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Describe a positive test for lipids. |
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4.2 a) |
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What are the limitations of this test? |
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4.2 b) |
Proteins
Objectives
After completing this exercise, you should be able to 1. Define protein and give examples.
A protein's structure is determined by the amino acid subunits that make up the molecule. Although there are only 20 different naturally occurring amino acids, each protein molecule has a unique sequence. The amino acids are linked by fairly tight bonds, and the side groups (R groups) that are part of the amino acids also interact with each other to help shape the molecule.
Proteins have a greater diversity of roles than either carbohydrates or lipids. The shape of a protein is key to its purpose: Proteins work by selectively binding to other molecules.
You will use biuret reagent as a test for proteins (Figure 4.5). This reagent, which is blue, reacts with proteins to give a light violet or lavender color.
You will use a solution of egg albumin (a protein extracted from egg whites) to demonstrate a positive biuret test. What negative control should be used for this
test? 4.3 a)
Figure 4.5. Biuret test for protein. Procedure
Why? 4.3 b)
4.3 c) Tube 1 (egg albumin): 4.3 d) Tube 2 (negative control):
Interpretation of Results
Describe a positive biuret test. 4.3 e)
What are the limitations of this test? 4.3 f)
Macromolecules in Food
Objectives
After completing this exercise, you should be able to
1. Interpret the results of tests that indicate the presence of sugar, starch, lipid, and protein in food samples.
We metabolize food in order to release energy to produce the ATP needed for cellular work. We also break down food molecules in order to use their subunits as raw materials for synthesizing our own macromolecules. In this exercise, you will investigate certain foods to learn which macromolecules are present in each.
Activity: Tests with Food
Test some or all of the items in Table 4.2 for the presence of simple sugars, starch, lipid, and protein. Your instructor may want to modify the list. The procedures for the tests are reviewed below.
Benedict's Test (sugar)
Put 1 pasteur pipetful of sample into a test tube. Add 2 droppersful of Benedict's reagent; mix. Heat in a boiling water bath for 5 minutes. Allow to cool and observe the precipitate.
*Some samples may require extra cooling time, so don't be too hasty in recording results.
Iodine Test (starch)
Put a pipetful of sample into a test tube and add 4 or 5 drops of iodine reagent; mix.
*In some foods, the starch is still contained in granules inside the cells. You may see these dark granules suspended in the yellow solution instead of seeing the entire solution turning blue.
Paper Test (lipid)
If the sample is whole (for example, a peanut), rub a piece of it directly on the paper. If the sample is liquid, put a small drop on the paper.
*Remember to wait for the paper to dry before you record the results.
Biuret Test (protein)
Put 1 pipetful of sample into the test tube and add 1 dropperful of biuret reagent; mix.
*Allow at least 2 minutes for the reaction to occur. Some samples may take 5 minutes to react.
Some of the foods to be tested are solids. Use a razor blade to mince approximately 1 cm3 (about the size of a pea) of the sample. Put it in a test tube with 10 mL distilled water. Put your thumb over the top of the test tube and shake it vigorously for 1 minute. Perform the tests using the liquid (except the lipid test). Record your results in Table 4.2. Be sure to rinse off the razor blade and cutting board between samples to avoid contamination.
Table 4.2 (at end of Lab Topic 4 Part 2)
Interpretation of Results
Which results confirmed your previous knowledge about the composition of foods?
4.4 a)
Which results were unexpected? 4.4 b)
What factors might result in a false negative test (that is, the food does contain a
molecule but the tests results are negative)? 4.4 c). Why might a plant storage organ (such as a fruit or tuber) contain both starch and
sugar? 4.4 d)
If you have tested foods in addition to the ones listed in Table 4.2, compare the results from those tests with the results for the foods listed in Table 4.2.
At the end of the Answer Sheet, please answer the Questions for Review
(1)Acknowledgments
This laboratory is from Jean Dickey, Laboratory Investigations for Biology 2nd ed.
San Francisco: Benjamin Cummings, 2003.
Procedures for the macromolecule tests were adapted from the following sources: Armstrong, W D., and C. W Carr. Physiological Chemistry Laboratory Directions, 3rd ed. Minneapolis: Burgess Publishing, 1963.
Dotti, L. B., and J. M. Orten. Laboratory Instructions in Biochemistry, 8th ed.
St. Louis: C. V Mosby, 1971.
Oser, B. L., ed. Hawk's Physiological Chemistry, 14th ed. New York: McGraw-Hill, 1965.
Answer Sheet
Exercise 4.1 Carbohydrates
4.1 A
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4.1 B
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4.1 C
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4.1 D
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4.1 E
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4.1 F
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4.1 G
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4.1 H
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Activity B: Starch
4.1 I
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4.1 J
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4.1 K
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4.1 L
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4.1 M
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4.1 N
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Exercise 4.2 Lipids
4.2 A
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4.2 B
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Exercise 4.3 Proteins
4.3 A
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4.3 B
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4.3 C
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4.3 D
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4.3 E
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4.3 F
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________________________________________________________________ Exercise 4.4 Macromolecules in Food
Table 4.2
Use + for positive, - for negative, or +/- if inconclusive
|
|
Benedict's |
Iodine |
Paper |
Biuret |
|
|
(sugar) |
(starch) |
(lipid) |
(protein) |
|
Banana |
|
|
|
|
|
Coconut |
|
|
|
|
|
Milk |
|
|
|
|
|
Peanut |
|
|
|
|
|
Potato |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
4.4 A)
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4.4 B)
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4.4 C)
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4.4 D)
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________________________________________________________________ Questions for Review 1. What subunits make up
What procedure could you use to check your answer to the previous question?
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