Chemical Composition of Cells:
Pre-Lab
LEARNING GOALS
By the end of this unit, you should be able to do the following:
1. Describe the difference between organic and inorganic chemicals.
2. Relate oxygen content of a chemical to whether it is hydrophilic or hydrophobic.
3. Explain the relationship between starch, fiber, and complex carbohydrates and how
they relate to glucose, sugar, and simple sugars.
4. Describe how to use iodine, Benedict, and Biuret tests to identify starch, simple
sugars, and protein respectively.
5. Describe the color of a positive reaction and a negative reaction for iodine, Benedict,
and Biuret tests.
6. Describe how an emulsifier changes the mixing of oil and water.
7. Identify the Independent, Dependent, and Control Variables in your experiment.
All organisms either eat or create the chemicals they need in order to survive. In your
case, you eat the chemicals you need. Complex chemicals are composed of simple
substances called atoms. The majority of the chemicals you eat are made up of the atoms
carbon (C), oxygen (O), hydrogen (H), nitrogen (N), and phosphorous (P). The type
of chemical is determined by which specific atoms are used and in what order. A
chemical made out of two or more atoms is also called a molecule. Example - Hydrogen
and oxygen are atoms, but H2O is a molecule.
Chemicals that contain both carbon and hydrogen are known as organic. (You see the
word organic in the grocery store used to mean "all natural," but that is not the way we
will use the term here.) Your food contains some inorganic chemicals such as water
(H2O) and minerals like calcium (Ca) and iron (Fe). The organic chemicals in your food
fall into four basic categories: carbohydrates (sugars and starches), lipids (fats and oils),
proteins (muscle and enzymes), and nucleic acid (DNA).
CARBOHYDRATES
Carbohydrates such as sugars, starches, and fiber are composed of carbon, hydrogen,
and oxygen molecules in a 1:2:1 ratio. This leads to the basic formula of CH2O. The
"H2O" in the formula is where the "hydrate" comes from in the name carbohydrate. The
presence of so much oxygen in carbohydrates is what makes them unique. For example,
lipids contain almost no oxygen. The presence of oxygen is also what makes
carbohydrates hydrophilic or able to dissolve in water.
Most living organisms use carbohydrates to provide energy to cells. Plants also use
carbohydrates, in the form of fiber, to build their bodies. One molecule of carbohydrate,
glucose, is shown below.
Glucose drawn with all atoms.
Because carbon and hydrogen are present in all organic molecules, pictures of organic
molecules often leave them out for simplicity as in the image of glucose below.
Glucose drawn without carbon atoms.
Many organic chemicals can be used individually or in a chain. Individual molecules of
carbohydrate are called simple sugars or just sugars. Molecules that have been linked
up in pairs are also called simple sugars. Glucose is a simple sugar, as is fructose (found
in high fructose corn syrup). Regular table sugar that you buy in a store is a simple
sugar made by linking a molecule of glucose to a molecule of fructose.
Sucrose (or table sugar) is a combination of glucose and fructose.
Experiment Note - You will not be doing an experiment to identify simple sugars.
Benedict reagent, which is too toxic to be used at home, turns red and orange in the
presence of simple sugars. You will need to learn the colors of Benedict reactions for
your quiz.
Chains of carbohydrate molecules linked together are called complex
carbohydrates. Two common complex carbohydrates are starch and fiber. Both are
long chains of glucose. The glucose molecules are linked facing the same direction in
starch. They are linked in a back-and-forth (or flip-flop) direction in fiber. Even though
both are made of the same sugar (glucose), the alternating shape of fiber blocks the
enzymes in your body from digesting it. This is why most vegetables, which contain a lot
of fiber, are low calorie. Lettuce is a good example of a plant full of fiber, but no starch.
Starch, a chain of glucose facing the same direction.
Fiber, a chain of glucose facing in alternate directions.
Experiment Note - You will be using iodine disinfectant to detect the presence of
starch. It will turn black in the presence of starch, but not fiber. You will be using this
technique to analyze common foods, including lettuce.
LIPIDS
Lipids come in the form of oils, fats, waxes, and steroids. The chemical structure of a
lipid contains large chained areas of hydrogen and carbon molecules and almost no
oxygen. The absence of oxygen makes lipids hydrophobic or unable to dissolve in
water. Hydrophobic molecules will dissolve in other hydrophobic liquids, like oils.
Fats and oils are called triglycerides. They consist of three long chains of carbon and
hydrogen connected together by glycerol. The difference between a fat and an oil is that
oils are liquid at room temperature due to being shorter or having a kinked shape.
Generic Triglyceride, either a fat or oil.
Remember that carbohydrates had one oxygen for every carbon atom? The fat above has
one oxygen for every TEN carbon atoms, making it much less soluble in water or
hydrophobic. The lack of oxygen also doubles the energy storage per gram compared to
carbohydrates. This is one of the reasons why animals use fat to store energy long-term;
storing the same amount of energy in a carbohydrate would take twice the room and
weight.
Hydrophobic chemicals like lipids can be dissolved in water with help from an
emulsifier. Common examples of these are soaps and detergents which you use to "wash
off" oil from your skin, dishes, and clothes. Emulsifiers have a hydrophobic tail and a
hydrophilic head. Molecules of emulsifier surround the oil, touching it with their
hydrophobic tail, and then point the hydrophilic head outwards towards the water. This
allows the object to be dissolved.
Common soaps/detergents.
Many naturally occurring lipids and also some proteins act as emulsifiers. These are
common components of foods we eat that combine both lipids and water.
Experiment Note - You will be using hot water to separate fats and oils from foods that
are known to be high in fat. The lipids will float to the surface. If the food also contains
an emulsifier, the fats will remain mixed in the water.
PROTEINS
Proteins are long chains of amino acids connected end to end. The amino acids contain
a nitrogen or "amine" group, which is where they get their name. Each of the 20 amino
acids has a different variable group attached to the central carbon. The variable group
can be large or small, hydrophobic or hydrophilic, and charged or uncharged. Based on
the order of amino acids, the protein folds up into a complex 3-D shape that can perform
work.
In this generic amino acid, the "R" stands for the variable group. It can be as small as one
hydrogen atom or as large as a six-carbon ring.
Unlike plants, which are built from carbohydrates like starch and fiber, the majority of
your body is made of protein. That is because proteins can be used to move, like in your
muscles. You probably know that meat is a high-protein food. "Meat" is muscle tissue
from an animal; muscle tissue that can move. In addition to moving your body, proteins
called enzymes can also perform chemical reactions (see Enzyme unit).
Experiment Note - You will not be doing an experiment to identify proteins. Biuret
reagent, which is too toxic to be used at home, turns purple in the presence of proteins.
You will need to learn the colors of Biuret reactions for your quiz.
NUCLEIC ACIDS
DNA and RNA are the two nucleic acids. Compared to the other types of organic
molecules, nucleic acids have a large amount of nitrogen and phosphorous in them. DNA
and RNA are made of individual nucleotides strung together in a long chain. There are
four nucleotides each for DNA and RNA. The order of the different nucleotides in the
chain is a recipe for making a protein. It specifies which amino acids to place into the
protein and in what order (see DNA unit).
Experiment Note - You will not be doing an experiment to identify DNA in this unit, but
you will isolate DNA from cells at a later date (see DNA unit).