Polymers, monomers, and bonding
Carbohydrates, proteins, and lipids are primary nutritional ingredients for humans. The
breakdown of nutrients (carbohydrates, proteins and lipids) in the intestines results in
small compounds (metabolites) that can pass through the wall of the intestines into the
blood. Complex carbohydrates and proteins are polymers that break up into a large
number of smaller similar compounds, the monomers. Lipids do not have monomer and
polymer forms. All complex compounds in an organism are produced within the organism
itself and are also specific to it.
Complex carbohydrates/polysaccharides
There are three main complex carbohydrates in nature. All three are polysaccharides (see
also section 3.2.).
forms of glucose, and glucose is considered the monomer of starch and cellulose. Even
though they are both complexes of glucose in plants, starch and cellulose have a different
shape and a different function. Glycogen is the glucose polymer in animals and humans.
Each of the three breaks down to become a large number of D-glucose molecules.
The glucose molecules in these polysaccharides are linked together by - or -glycosidic
linkages. Glycosidic bonds are special covalent bonds. The special nature of these bonds
determines much of the shape of the more complex compound, largely because these
linkages inhibit the rotation of specific molecules toward each other.
Starch, cellulose, and glycogen are homopolysaccharides because they contain only onetype of monomer, glucose. There are heteropolysaccharides which contain more than onemonomer, such as the peptidoglycans in bacterial cell walls. Mostly there are not morethan two different kinds of monomer in carbohydrate polymers. We will find thatcarbohydrates are less differentiated in their polymer structure than proteins (section2.1.2.).StarchStarch occurs as granules in plant cells. Itbreaks down to become a large numberof -D-glucose molecules. Starch has -glycosidic bonds to link glucosemolecules. The preferred conformation ofamylose, the simplest form of starch, is ahelix. The energy that holds together thehelical shape and the glycosidic linkagescomes free when it is broken down inplants or in the digestive tract of animalsand humans. Its role is to be a majorenergy source in the living world. Enzymesin plant, animal and human organisms caneasily break down the -glycosidic linkagesof the starch helix to yield glucose, andglucose can be broken down further toyield energy (section 3.3.). The -linkagebetween the glucose molecules in starchalso determines its function as an energystorage compound.
Fig. 2.1 Starch (from Campbell, 1999)
Fig. 2.2 Cellulose (from Campbell, 1999)
-D-glucose with -glycosidic linkages. The -glycosidic linkages of cellulose allow for
additional hydrogen bonding between linear polysaccharide chains. This results in a
strong planar shape which can neither be broken down easily in plants nor in the digestive
tract of humans and many animals. The typical -linkages of cellulose, and the possibility
of hydrogen bonding which this allows, make cellulose a structural carbohydrate in plants.
The cell wall around the cell membrane of plants consists mainly of cellulose, giving
plants their stability. Woody plants contain more cellulose.
Carbohydrates, proteins, and lipids are primary nutritional ingredients for humans. The
breakdown of nutrients (carbohydrates, proteins and lipids) in the intestines results in
small compounds (metabolites) that can pass through the wall of the intestines into the
blood. Complex carbohydrates and proteins are polymers that break up into a large
number of smaller similar compounds, the monomers. Lipids do not have monomer and
polymer forms. All complex compounds in an organism are produced within the organism
itself and are also specific to it.
Complex carbohydrates/polysaccharides
There are three main complex carbohydrates in nature. All three are polysaccharides (see
also section 3.2.).
Starch
and cellulose are both typical plant products. They are polymericforms of glucose, and glucose is considered the monomer of starch and cellulose. Even
though they are both complexes of glucose in plants, starch and cellulose have a different
shape and a different function. Glycogen is the glucose polymer in animals and humans.
Each of the three breaks down to become a large number of D-glucose molecules.
The glucose molecules in these polysaccharides are linked together by - or -glycosidic
linkages. Glycosidic bonds are special covalent bonds. The special nature of these bonds
determines much of the shape of the more complex compound, largely because these
linkages inhibit the rotation of specific molecules toward each other.
Starch, cellulose, and glycogen are homopolysaccharides because they contain only onetype of monomer, glucose. There are heteropolysaccharides which contain more than onemonomer, such as the peptidoglycans in bacterial cell walls. Mostly there are not morethan two different kinds of monomer in carbohydrate polymers. We will find thatcarbohydrates are less differentiated in their polymer structure than proteins (section2.1.2.).StarchStarch occurs as granules in plant cells. Itbreaks down to become a large numberof -D-glucose molecules. Starch has -glycosidic bonds to link glucosemolecules. The preferred conformation ofamylose, the simplest form of starch, is ahelix. The energy that holds together thehelical shape and the glycosidic linkagescomes free when it is broken down inplants or in the digestive tract of animalsand humans. Its role is to be a majorenergy source in the living world. Enzymesin plant, animal and human organisms caneasily break down the -glycosidic linkagesof the starch helix to yield glucose, andglucose can be broken down further toyield energy (section 3.3.). The -linkagebetween the glucose molecules in starchalso determines its function as an energystorage compound.
Cellulose
Cellulose is the main component of the cellwall of plants. Cellulose is formed fromFig. 2.1 Starch (from Campbell, 1999)Fig. 2.2 Cellulose (from Campbell, 1999)-D-glucose with -glycosidic linkages. The -glycosidic linkages of cellulose allow for
additional hydrogen bonding between linear polysaccharide chains. This results in a
strong planar shape which can neither be broken down easily in plants nor in the digestive
tract of humans and many animals. The typical -linkages of cellulose, and the possibility
of hydrogen bonding which this allows, make cellulose a structural carbohydrate in plants.
The cell wall around the cell membrane of plants consists mainly of cellulose, giving
plants their stability. Woody plants contain more cellulose.
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