The Role of macromolecules
Carbohydrates are molecules containing carbon atoms flanked by hydrogen atoms and hydroxyl
groups (H—C—OH). They have two major biochemical roles:
•Carbohydrates are a source of energy that can be released in a form usable by body
•Carbohydrates serve as carbon skeletons that can be rearranged to form new molecules
that are essential for biological structures and functions.
Some carbohydrates are relatively small, with molecular weights of less than 100. Others are true
macromolecules, with molecular weights in the hundreds of thousands.
There are four categories of biologically important carbohydrates, which we will discuss in turn:
•Monosaccharide’s (mono, “one”; saccharide, “sugar”), such as glucose, ribose, and
fructose, are simple sugars. They are the monomers from which the larger carbohydrates are
•Disaccharides (di, “two”) consist of two monosaccharides linked together by covalent
•Oligosaccharides (oligo, “several”) are made up of several (3–20) monosaccharides.
•Polysaccharides (poly, “many”), such as starch, glycogen, and cellulose, are large
polymers composed of hundreds or thousands of monosaccharides.
The general formula for carbohydrates, CH2O, gives the relative proportions of carbon, hydrogen,
and oxygen in a monosaccharide (i.e., the proportions of these atoms are 1:2:1). In disaccharides,
oligosaccharides, and polysaccharides, these proportions differ slightly from the general formula
because two hydrogens and an oxygen are lost during each of the condensation reactions that
Different monosaccharides contain different numbers of carbons. Most of the monosaccharides found in
living systems belong to the d series of optical isomers. (Recall also that only l-amino acids occur in
proteins—there is amazing specificity in biology!) Some monosaccharides are structural isomers, with the
same kinds and numbers of atoms, but in different arrangements. For example, the hexoses (hex, “six”), a
group of structural isomers, all have the formula C6H12O6. Included among the hexoses are glucose,
fructose (so named because it was first found in fruits), mannose, and galactose
Pentoses (pente, “five”) are five-carbon sugars. Two pentoses are of particular biological importance:
ribose and deoxyribose form part of the backbones of the nucleic acids RNA and DNA, respectively .
These two pentoses are not isomers; rather, one oxygen atom is missing from carbon 2 in deoxyribose
(de-, “absent”). The absence of this oxygen atom is an important distinction between RNA and DNA.
Optical isomers: Two isomers that are mirror images of each other.
Monosaccharide’s Are Simple Sugars Monosaccharide’s are made up of varying numbers of
carbons. Some hexoses are structural isomers that have the same kind and number of atoms, but
the atoms are arranged differently. Fructose, for example, is a hexose, but forms a five-membered
ring like the pentoses.
Glycosidic linkages bond monosaccharides
Polysaccharides, disaccharides and oligosaccharides are constructed from monosaccharides.
Glycosidic linkages (only about disaccharides)
The disaccharide maltose contains two glucose molecules, but it is not the only disaccharide that
can be made from two glucoses. When glucose molecules form a glycosidic linkage, the linkage
will be one of two types, or , depending on whether the molecule that bonds its carbon 1 is -
D-glucose or -D-glucose. An linkage with carbon 4 of a second glucose molecule produces
maltose, whereas a linkage gives cellobiose. Maltose and cellobiose are structural isomers,
both having the formula C12H22O11. However, they are different compounds with different
properties. They undergo different chemical reactions and are recognized by different enzymes.
For example, maltose can be hydrolyzed into its monosaccharides in the human body, whereas
cellobiose cannot. Certain microorganisms have the chemistry needed to break down cellobiose.
Oligosaccharides contain several monosaccharides bound by glycosidic linkages at various sites.
Many oligosaccharides have additional functional groups, which give them special properties.
Oligosaccharides are often covalently bonded to proteins and lipids on the outer cell surface,
where they serve as recognition signals. The different human blood groups (such as the ABO
blood types) get their specificity from oligosaccharide chains.
Polysaccharides store enrgy and provide structural materials.
Polysaccharides are giant polymers of monosaccharides connected by glycosidic linkages:
•Starch is a polysaccharide of glucose with -glycosidic linkages.
•Glycogen is a highly branched polysaccharide of glucose.
•Cellulose is also a polysaccharide of glucose, but its individual monosaccharides are
connected by -glycosidic linkages.
Starch comprises a family of giant molecules of broadly similar structure. While all starches are
large polymers of glucose with linkages. The different starches can be distinguished by the
amount of branching that occurs at carbons 1 and 6 . Some plant starches are unbranched, such as
plant amylose; others are moderately branched, such as plant amylopectin. Starch readily binds
water, and when that water is removed, unbranched starch tends to form hydrogen bonds between
the polysaccharide chains, which then aggregate.
Bread becomes hard and stale because when it dries out, the polysaccharide chains in starch
aggregate. Adding water and gentle heat separates the chains and the bread becomes softer.
Glycogen stores glucose in animal livers and muscles. Starch and glycogen serve as energy
storage compounds for plants and animals, respectively. Both of these polysaccharides are readily
hydrolyzed into glucose monomers, which in turn can be further degraded to liberate their stored
energy. But if it is glucose that is actually needed for fuel, why must it be stored as a polymer?
The reason is that 1,000 glucose molecules would exert 1,000 times the osmotic pressure of a
single glycogen molecule, causing water to enter the cells . If it were not for polysaccharides,
many organisms would expend a lot of time and energy expelling excess water from their cells.
Cellulose is the predominant component of plant cell walls, and is by far the most abundant
organic compound on Earth. Starch can be easily degraded by the actions of chemicals or
enzymes. Cellulose, however, is chemically more stable because of its -glycosidic linkages .
Thus starch is a good storage medium that can be easily broken down to supply glucose for
energy-producing reactions, while cellulose is an excellent structural material that can withstand
harsh environmental conditions without changing.
Roles of lipids include:
There are several different types of lipids, and they play a number of roles in living organisms:
•Fats and oils store energy.
•Phospholipids play important structural roles in cell membranes.
•The carotenoids help plants capture light energy.
•Steroids and modified fatty acids play regulatory roles as hormones and vitamins.
•The fat in animal bodies serves as thermal insulation.
•A lipid coating around nerves provides electrical insulation.
•Oil or wax on the surfaces of skin, fur, and feathers repels water.
Chemically, fats and oils are triglycerides, also known as simple lipids. Triglycerides that are
solid at room temperature (20°C) are called fats; those that are liquid at room temperature
are called oils. Triglycerides are composed of two types of building blocks: fatty acids and
glycerol. Glycerol is a small molecule with three hydroxyl (—OH) groups (thus it is an
alcohol). A fatty acid is made up of a long nonpolar hydrocarbon chain and a polar carboxyl
group (—COOH). A triglyceride contains three fatty acid molecules and one molecule of
glycerol. The carboxyl group of a fatty acid can bond with the hydroxyl group of glycerol,
resulting in a covalent bond called an ester linkage and water
Carbohydrates are molecules containing carbon atoms flanked by hydrogen atoms and hydroxyl groups (h c oh). Some carbohydrates are relatively small, with molecular weights of less than 100. Others are true macromolecules, with molecular weights in the hundreds of thousands. There are four categories of biologically important carbohydrates, which we will discuss in turn: monosaccharide"s (mono, one ; saccharide, sugar ), such as glucose, ribose, and fructose, are simple sugars. The general formula for carbohydrates, ch2o, gives the relative proportions of carbon, hydrogen, and oxygen in a monosaccharide (i. e. , the proportions of these atoms are 1:2:1). In disaccharides, oligosaccharides, and polysaccharides, these proportions differ slightly from the general formula because two hydrogens and an oxygen are lost during each of the condensation reactions that form them. Most of the monosaccharides found in living systems belong to the d series of optical isomers. (recall also that only l-amino acids occur in proteins there is amazing specificity in biology!)