BIOL 600 Lecture Notes - Lecture 6: Glycosidic Bond, Asymmetric Carbon, Aldose
26 May 2018
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AP Bio Chapter 5 The Structure and Function of Macromolecules
Lecture Outline
Overview: The Molecules of Life
Within cells, small organic molecules are joined together to form larger molecules.
These large macromolecules may consist of thousands of covalently bonded atoms and
weigh more than 100,000 daltons.
The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic
acids.
Concept 5.1 Most macromolecules are polymers, built from monomers
Three of the four classes of macromolecules—carbohydrates, proteins, and nucleic
acids—form chainlike molecules called polymers.
o A polymer is a long molecule consisting of many similar or identical building
blocks linked by covalent bonds.
o The repeated units are small molecules called monomers.
o Some of the molecules that serve as monomers have other functions of their own.
The chemical mechanisms that cells use to make and break polymers are similar for all
classes of macromolecules.
Monomers are connected by covalent bonds that form through the loss of a water
molecule. This reaction is called a condensation reaction or dehydration reaction.
o When a bond forms between two monomers, each monomer contributes part of
the water molecule that is lost. One monomer provides a hydroxyl group (—OH),
while the other provides a hydrogen (—H).
o Cells invest energy to carry out dehydration reactions.
o The process is aided by enzymes.
The covalent bonds connecting monomers in a polymer are disassembled by hydrolysis, a
reaction that is effectively the reverse of dehydration.
o In hydrolysis, bonds are broken by the addition of water molecules. A hydrogen
atom attaches to one monomer, and a hydroxyl group attaches to the adjacent
monomer.
o Our food is taken in as organic polymers that are too large for our cells to absorb.
Within the digestive tract, various enzymes direct hydrolysis of specific polymers.
The resulting monomers are absorbed by the cells lining the gut and transported to
the bloodstream for distribution to body cells.
o The body cells then use dehydration reaction to assemble the monomers into new
polymers that carry out functions specific to the particular cell type.
An immense variety of polymers can be built from a small set of monomers.
Each cell has thousands of different kinds of macromolecules.
o These molecules vary among cells of the same individual. They vary more among
unrelated individuals of a species, and even more between species.
This diversity comes from various combinations of the 40–50 common monomers and
some others that occur rarely.
o These monomers can be connected in a great many combinations, just as the 26
letters in the alphabet can be used to create a great diversity of words.
Concept 5.2 Carbohydrates serve as fuel and building material
Carbohydrates include sugars and their polymers.
The simplest carbohydrates are monosaccharides, or simple sugars.
Disaccharides, or double sugars, consist of two monosaccharides joined by a
condensation reaction.
Polysaccharides are polymers of many monosaccharides.
Sugars, the smallest carbohydrates, serve as fuel and a source of carbon.
Monosaccharides generally have molecular formulas that are some multiple of the unit
CH2O.
o For example, glucose has the formula C6H12O6.
Monosaccharides have a carbonyl group (>C=O) and multiple hydroxyl groups (—OH).
o Depending on the location of the carbonyl group, the sugar is an aldose or a
ketose.
o Most names for sugars end in -ose.
o Glucose, an aldose, and fructose, a ketose, are structural isomers.
Monosaccharides are also classified by the number of carbons in the carbon skeleton.
o Glucose and other six-carbon sugars are hexoses.
o Five-carbon backbones are pentoses; three-carbon sugars are trioses.
Monosaccharides may also exist as enantiomers.
o For example, glucose and galactose, both six-carbon aldoses, differ in the spatial
arrangement of their parts around asymmetrical carbons.
Monosaccharides, particularly glucose, are a major fuel for cellular work.
They also function as the raw material for the synthesis of other monomers, such as
amino acids and fatty acids.
While often drawn as a linear skeleton, monosaccharides in aqueous solutions form rings.
Two monosaccharides can join with a glycosidic linkage to form a disaccharide via
dehydration.
o Maltose, malt sugar, is formed by joining two glucose molecules.
o Sucrose, table sugar, is formed by joining glucose and fructose. Sucrose is the
major transport form of sugars in plants.
o Lactose, milk sugar, is formed by joining glucose and galactose.
Polysaccharides, the polymers of sugars, have storage and structural roles.
Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by
glycosidic linkages.
Some polysaccharides serve for storage and are hydrolyzed as sugars are needed.
Other polysaccharides serve as building materials for the cell or the whole organism.
Starch is a storage polysaccharide composed entirely of glucose monomers.
o Most of these monomers are joined by 1–4 linkages (number 1 carbon to number
4 carbon) between the glucose molecules.
o The simplest form of starch, amylose, is unbranched and forms a helix.
o Branched forms such as amylopectin are more complex.
Plants store surplus glucose as starch granules within plastids, including chloroplasts, and
withdraw it as needed for energy or carbon.
o Animals that feed on plants, especially parts rich in starch, have digestive
enzymes that can hydrolyze starch to glucose.
Animals store glucose in a polysaccharide called glycogen.
o Glycogen is highly branched like amylopectin.
o Humans and other vertebrates store a day’s supply of glycogen in the liver and
muscles.
Cellulose is a major component of the tough wall of plant cells.
o Plants produce almost one hundred billion tons of cellulose per year. It is the most
abundant organic compound on Earth.
Like starch, cellulose is a polymer of glucose. However, the glycosidic linkages in these
two polymers differ.
o The difference is based on the fact that there are actually two slightly different
ring structures for glucose.
o These two ring forms differ in whether the hydroxyl group attached to the number
1 carbon is fixed above (beta glucose) or below (alpha glucose) the plane of the
ring.
Starch is a polysaccharide of alpha glucose monomers.
Cellulose is a polysaccharide of beta glucose monomers, making every other glucose
monomer upside down with respect to its neighbors.
The differing glycosidic links in starch and cellulose give the two molecules distinct
three-dimensional shapes.
o While polymers built with alpha glucose form helical structures, polymers built
with beta glucose form straight structures.
o The straight structures built with beta glucose allow H atoms on one strand to
form hydrogen bonds with OH groups on other strands.
o In plant cell walls, parallel cellulose molecules held together in this way are
grouped into units called microfibrils, which form strong building materials for
plants (and for humans, as lumber).
The enzymes that digest starch by hydrolyzing its alpha linkages cannot hydrolyze the
beta linkages in cellulose.
o Cellulose in human food passes through the digestive tract and is eliminated in
feces as “insoluble fiber.”
o As it travels through the digestive tract, cellulose abrades the intestinal walls and
stimulates the secretion of mucus, aiding in the passage of food.
Some microbes can digest cellulose to its glucose monomers through the use of cellulase
enzymes.
Many eukaryotic herbivores, from cows to termites, have symbiotic relationships with
cellulolytic microbes, providing the microbe and the host animal access to a rich source
of energy.
Document Summary
Ap bio chapter 5 the structure and function of macromolecules. Within cells, small organic molecules are joined together to form larger molecules. These large macromolecules may consist of thousands of covalently bonded atoms and weigh more than 100,000 daltons. The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Concept 5. 1 most macromolecules are polymers, built from monomers. The chemical mechanisms that cells use to make and break polymers are similar for all classes of macromolecules. Monomers are connected by covalent bonds that form through the loss of a water molecule. This reaction is called a condensation reaction or dehydration reaction: when a bond forms between two monomers, each monomer contributes part of the water molecule that is lost. One monomer provides a hydroxyl group ( oh), while the other provides a hydrogen ( h): cells invest energy to carry out dehydration reactions, the process is aided by enzymes.
