Review Questions
Carbohydrates
1. What is the difference between an inorganic molecule and an organic
molecule?
Organic molecules always have carbon and hydrogen.
2. List the four classes of organic compounds.
Carbohydrates, Lipids, Proteins, and Nucleic Acids
3. Explain the difference between a molecule and a macromolecule.
A molecule is simply two or more atoms chemically bonded to one
another. A macromolecule is two or more molecules chemically
bonded to each other. Organic compounds include molecules and
macromolecules.
4. What are monomers and polymers?
In organic chemistry, a monomer is a molecule by itself. Monomer
means “a single part”. Some examples of organic monomers are a
glucose molecule, a nucleotide, or an amino acid. Polymers are
macromolecules built of two or more monomers. Polymer means
“many parts”.
Polymers are often long chains of monomers. For example, starch is a
polymer. It is a long chain of glucose molecules. Proteins are polymers
composed of chains of amino acids. DNA is a super- long polymer of
nucleotides. Even small macromolecules like a dipeptide (two amino
acids joined together) are polymers.
5. Explain the chemical reaction for building a polymer from a set of
monomers.
The process for connecting two monomers together (forming a
covalent bond) is called dehydration synthesis. Dehydration means
“removal of water” and synthesis means “to join together”. So in this
process, two monomers are covalently bonded by the removal of a
water molecule. Each organic monomer has a hydroxyl group (-OH) on
one side and a hydrogen (H) on the other. When two monomers line
up side by side, they will have these two functional groups facing one
another (H & OH). The H and the OH will break off of their respective
monomers and bond forming a water molecule. This is the dehydration
part of the process. Each monomer now has a carbon atom that needs
to covalently bond with something, so they bind to each other forming
a polymer. That is the synthesis part of the process. Dehydration
synthesis is an energy-requiring process. Every time a cell needs to
build a protein or a starch, it has to expend some energy to form those
chemical bonds. Dehydration synthesis is universal for building all
organic polymers.
6. Explain the chemical reaction for breaking down a polymer into
individual monomers.
The process of splitting the bond between monomers is called
hydrolysis. Hydrolysis means “to break with water”. Since a water
molecule was lost during dehydration synthesis, hydrolysis brings the
water back. To separate the monomers, the functional groups H and
OH that broke off in the bonding have to be reattached to their
respective monomers. So first the covalent bond is broken between the
two carbons holding the two monomers to each other and then a water
molecule is introduced. The water molecule splits into a H and an OH
and these attach to their respective monomer. Hydrolysis is an energyreleasing process. Living organisms harvest energy from chemical
bonds through hydrolysis. Like dehydration synthesis, hydrolysis is
universal for breaking down all organic polymers.
7. List the names of the monomers and polymers of the four classes of
organic compounds.
Class
Carbohydrates
Lipids (neutral fats)
Proteins
Nucleic Acids
Monomer
monosaccharide
glycerol and fatty acid
amino acid
nucleotide
Polymer
polysaccharide
triglyceride
polypeptide
polynucleotide
8. What is a carbohydrate?
Carbohydrates are the sugars and starches. Every kind of
carbohydrate has to have carbon, hydrogen, and oxygen. Plus, the
ratio of C, H, O is always 1:2:1. CH2O is a common way to represent
this ratio. The term “carbohydrate” comes from the observation that the
molecule is a carbon linked to a water molecule (C – H2O), “hydrate”
meaning water. You can often recognize a carbohydrate by its suffix –
ose (e.g. glucose, sucrose, cellulose, etc.)
9. Describe the monosaccharides.
Monosaccharides are the monomers of carbohydrates and are often
referred to as “the simple sugars”. There are dozens of them but they
are classified according the number of carbons they contain. For
example, glucose has 6 carbons and belongs to a group of
monosaccharides called the hexose sugars: “hex” meaning 6. Triose
sugars, like glyceraldehyde, have 3 carbons. Pentose sugars, such as
ribose, have 5. Monosaccharides range from 3 to 7 carbons.
We are going to focus on 3 hexose sugars that organisms use for fuel:
glucose, fructose, and galactose. Glucose, also known as dextrose,
has the empirical formula C6H12O6. Glucose is the main energy source
for life. We often call it blood sugar. Some organs in our body feed on
nothing but glucose; our brain, for instance. Fructose has the exact
same empirical formula as glucose: C6H12O6. Fructose is also known
as levulose. Sweeter than the other 2 hexose sugars, fructose is
commonly found in fruits and honey. Since fructose is sweeter, less is
required, and so it is cheaper. High fructose corn syrup is now the
major sugar source for soft drinks. Galactose, just like glucose and
fructose, is C6H12O6. Galactose is rarely found free. Bonded to glucose,
it forms lactose or milk sugar. Once ingested, both fructose and
galactose are converted into glucose for an organism’s energy needs.
10. What are isomers?
You may have noticed that glucose, fructose, and galactose all have
the same formula C6H12O6. Then why are they different? Well, it is
because they have different structures. Molecules with the same
empirical formula but different structures are known as isomers. Slight
changes in structure give these molecules unique properties. There
are a variety of different types of isomers: structural, geometric, and
enantiomers (optical), just to name a few.
11. Describe the disaccharides.
As the name implies, disaccharides are two monosaccharides
covalently bonded together. This covalent bond is called a glycosidic
linkage. Dehydration synthesis is how disaccharides are built. There
are 3 disaccharides we will review: maltose, sucrose, and lactose.
Maltose is a polymer composed of two glucose molecules. Maltose is
produced by germinating grains as they break down their starchy
endosperm. We feed this disaccharide to yeast when we make beer
and whiskey.
Sucrose is glucose and fructose bonded together. Sucrose is table
sugar or cane sugar. We also find it in honey and maple syrup.
Lactose is milk sugar and is built from glucose and galactose. Most of
the world’s population has some level of lactose intolerance (70%).
The lactose-intolerant lack the enzyme lactase to break down the
sugar in the small intestines. Lactose in the large intestine acts as a
sponge and water remains in the large intestine resulting in diarrhea
and abdominal cramping. Thanks to recent developments, dairysensitive folks can now enjoy milk by taking a pill containing lactase.
12. Describe the oligosaccharides.
Oligo” means “few”. So oligosaccharides, like raffinose and stachyose,
are small chains of monosaccharides. In the figure below, raffinose
(sucrose + galactose) is on the left and stachyose (sucrose + 2
galactose) on the right. We lack the enzyme to break these down but
the bacterial flora in our gut do. Certain foods are rich in these sugars
and create extra flatus. The gaseous effects of foods, like beans and
the cruciferous vegetables, can be reduced by taking a dose of the
enzyme (Beano).
13. Describe the polysaccharides.
Complex carbohydrates like starch, glycogen, cellulose, and chitin are
long polymers of monosaccharides. Starch comes from plants and is a
long-term storage form of sugar. Starch is also the major source of
food for every human culture. A relatively simple macromolecule, it is
built of many repeating units of glucose held together by glycosidic
linkages. If you examine the cells of a potato under a microscope, the
starch molecules are compacted into oval bodies called starch
granules. Adding a little iodine to the potato turns the starch blue-black
and the granules will pop into view.
Animals also store starch but we call it glycogen. Glycogen literally
means “agent that initiates sugar”. Just like plant starch, glycogen is a
long polymer of glucose. Glycogen is stored in our liver and muscles.
We have on hand about 24 hour supply of glycogen in our liver. If we
skip a meal, our blood sugar will drop and the pancreas will release a
hormone called glucagon. Glucagon stimulates the liver to break down
enough of its store of glycogen into glucose to return our blood sugar
levels to normal. Like plant starch, glycogen is stored as concentrated
granules in the cells of the liver.
Cellulose is another long polymer of glucose. Plant cells make their cell
walls out of cellulose. In fact, 100 billion tons of cellulose is made every
year on earth. Cellulose is indigestible in most animals, including us.
Ever eat a cardboard box? You get the picture. We simply lack
cellulase, the enzyme that can break it down. Some bacteria, some
single-celled protists, and fungi have the enzyme. Animals that feed on
cellulose harbor these microbes that help them digest it. Even though,
we cannot break down this molecule, we do need cellulose in our diet.
We call it “fiber”. Cellulose stimulates the colon to produce regular
bowel movements and helps make the stools large and soft. A diet rich
in fiber can prevent a painful intestinal disorder called diverticulosis.
Hard impacted stools can sometimes cause the walls of the colon to
form blind outpockets called diverticula which can periodically inflame.
So what makes cellulose different from starch? Isn’t it made of
glucose? Well it is but the glucose monomers are organized in an
interesting fashion. The orientation of the glucose molecules
alternates. So if the first one is right side up, the next one is upside
down and then the next is right side up and the next one is upside
down. Apparently this is a tricky arrangement for an enzyme to break.
Chitin is similar in structure to cellulose. The only difference is that is
has some nitrogen-containing side chains stuck to it. Chitin is strong,
lightweight, and waterproof. Arthropods build their exoskeletons out of
chitin. The cell walls of fungi are made out of it too.
14. What are the major functions of carbohydrates?
Carbohydrates supply quick and short-term energy. Carbohydrates are
a structural material. Plants, fungi, bacteria, and arthropods build their
bodies with it. Carbohydrates are the raw material to make proteins,
lipids, and nucleic acids. For example, plants make glucose first and
then all their other organic compounds are built from that.
Principal Carbohydrates
Subtypes
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
Examples
Glucose
Fructose
Galactose
Maltose
Sucrose
Lactose
Raffinose
Stachyose
Starch (Amylose)
Glycogen
Cellulose
Chitin