Bio 211 Intro Molecular and Cell Biology Molecules of Life I
Lecture 4 “Sugars and Fats”
Reading: Campbell Chap. 5 pp. 58-68
Last time, we looked at the special properties of carbon. Carbon chemistry is the
basis for many of the processes that take place in cells. In a series of chemical reactions,
large molecules such as sugars, fats, proteins and nucleic acids are synthesized in cells, or
broken down to derive energy. Today and in the next lecture we will look more closely at
the large molecules or “macromolecules” in cells.
Outline:
1. Macromolecules
2. Carbohydrates
3. Lipids
Macromolecules are large molecules containing many atoms. In cells,
macromolecules are synthesized by linking together small molecules into chains called
polymers. The building blocks or subunits of polymers are monomers.
To make a polymer, small molecules are linked together in a condensation or
dehydration reaction. In condensation, water is removed from two small molecules:
Fig. 5-2 a.
short polymer + monomer -----> longer polymer + H20
Water is removed by taking a hydroxyl (-OH) group from one molecule and a
hydrogen from another molecule (-H). To make a longer polymer, this reaction is
repeated over and over again. To make proteins, small building blocks called amino
acids are polymerized by dehydration into long chains.
Breakdown of a polymer occurs by hydrolysis. Hydrolysis is essentially the
opposite to dehydration. Bonds in the polymer are broken by adding atoms from a water
molecule to the bonds connecting the individual monomer units.
Fig. 5-2 b long polymer + H20 -----> shorter polymer + monomer
A hydrogen from water is added to one side monomer and a hydroxyl is added to
the adjacent monomer. Hydrolysis occurs when we digest the components of food into
smaller building blocks.
Synthesis of macromolecules from smaller building blocks enables a wide variety
of molecules to be made. Our cells contain a variety of different macromolecules to carry
out their functions. These are made from only about 40 or 50 types of small molecules.
Putting together the pieces in different ways enables a variety of large molecules each
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with different functions to be assembled. Organisms differ from one another in the
combination of large molecules that they assemble and use, but are highly similar to each
other in the building blocks used in making the macromolecules.
2. Carbohydrates are sugars and long polymers of sugars, such as starches and
cellulose. We will discuss three levels of organization of carbohydrates:
a. monosaccharides
b. disaccharides
c. polysaccharides
Monosaccharides are single sugar units and are also called “simple sugars”.
Disaccharides consist of two sugar units linked together; polysaccharides are long
polymers made of individual sugar units, usually of the monomer glucose.
Monosaccharides generally have molecular formulas that are some multiple of
CH20. Glucose (C6H12O6) is the most important simple sugar for life. Here are some
different formulas and structures used to represent glucose (Fig. 5.4):
C
C6H12O6
C
C
C
C
C
C
formula
C
O
C
C
C
linear
C
ring (structure in solution)
What are the key functional groups present in simple sugars? Remember in the
last lecture we identified several key types of groups that are typically attached to carbon
skeletons. Carbohydrates can be thought of as hydrocarbon chains to which functional
groups are attached. They usually have hydroxyl groups (-OH) on nearly every carbon
except one which may have a carbonyl group (-CHO).
In Figure 5.3, let’s look at some other simple sugars. What are the ways in which
these molecules are similar to one another? What are the ways in which they differ?
Similarities
built on carbon chain
contain -OH groups
contain a -CHO group
Differences
different # of carbons (common 3C- 6C)
aldoses vs. ketoses
different structure around certain C atom
(glucose vs. galactose)
different functions
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Examples of some of the different functions of these simple sugars in cells:
glyceraldehyde and dihydroxyacetone: glycolysis (breakdown of sugars for energy).
ribose: the sugar in RNA and ATP
glucose: source of nutrients, source of carbon for synthesis, storage in polysaccharides
Disaccharides consist of two monosaccharides joined together by a covalent bond,
called a glycosidic linkage.
Some examples (Fig. 5.5)
maltose = glucose + glucose; malt sugar (brewing)
lactose = glucose + galactose; milk sugar
sucrose = glucose + fructose; storage sugar in plants
Polysaccharides are long polymers made of many monosaccharide units, usually
glucose. They function in storage of sugars and in structures such as cell walls.
Storage polysaccharides include starch and glycogen. Fig. 5-6 if possible.
Starch: chains of alpha glucose in 1--> 4 linkages
amylose = unbranched
amylopectin = branched
found in plants
Glycogen: chains of alpha glucose in 1--> 4 linkages
highly branched
found in animals (liver and muscle)
Structural polysaccharides are chains of carbohydrates used to build coverings
such as cell walls and the exoskeletons of insects. Some examples include
Cellulose: long chains of beta glucose in 1--> 4 linkages (see Fig. 5.7)
form cell walls in plants
most abundant organic compound on earth
most organisms cannot break down
Chitin: long chains of an “amino sugar” (modified glucose molecule)
forms the exoskeletons of insects
forms cell walls of some fungi
3. Lipids consist of molecules such as fats and steroids which mostly share the
property of being hydrophobic (not liking water). Lipids are mostly hydrocarbons, long
chains consisting mostly of carbon and hydrogen with very little oxygen.
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a. fats
b. phospholipids
c. steroids
Fats are large molecules made from two types of building blocks, glycerol (a
polyalchohol) and fatty acids (long hydrocarbon chains of 16-18 C with a single
carboxylic acid group at one end). They are not polymers. Fig. 5.10 shows the structure
of a fat, also known as a triacylglycerol.
Fats are synthesized by the condensation of 3 fatty acids joined to one glycerol
molecule. The hydroxyl group of each glycerol is joined to the carboxylic acid end of
each fatty acid by an ester linkage.
Saturated fats contain many saturated fatty acids, those that lack double bonds in
their hydrocarbon skeleton. These are usually solids at room temperature and are
prevalent in foods such as lard, bacon grease and butter. Unsaturated fats contain some
unsaturated fatty acids, which are those with one or more double bonds. These usually
are liquids at room temperature and are in abundance in oils, such as corn oil or olive oil.
The role of fat is mostly in storage. Fats are more compact and have higher
potential energy stores than polysaccharides. Animals use fat storage over polysaccharide
storage.
Phospholipids are related to fats, but also contain phosphate and other functional
groups. They function mostly in membranes.
The structure of a phospholipid (phosphatidyl choline) can be seen in Fig. 5.12.
Phospholipids contain glycerol, two fatty acids, phosphate and usually some other
functional group (like choline) attached to the phosphate.
Phospholipids behave differently from fats in water. The fatty acid chains still
behave by avoiding water (shown as “hydrophobic tails”). The phosphate group and
attached groups are usually charged or polar and have an affinity for water (we say these
head groups are hydrophilic). Therefore, in water phospholipids form structures (micelles
or bilayers, see Fig. 5.13) in which the hydrophilic head group is associated with water
and the hydrophobic tails associate with each other. Cells are surrounded by membranes
which consist mainly of a bilayer of phospholipids.
Steroids are lipids consisting of four interconnected rings. Various steroids have
different functional groups attached to the set of rings. Examples of steroids include:
cholesterol: important for cell membranes, for synthesis of other steroids
sex hormones: including testosterone and estrogen which help determine
anatomical and physiological differences between the sexes (Fig. 4.8?).
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Summary: Today we looked at some of the types of large molecules crucial for cell
function and how they are constructed of smaller building blocks. We looked at the
various types of carbohydrates, including disaccharides and polysaccharides and the
simple sugars they are made of. We also briefly examined the types of lipids that are
common in cells, such as fats, phospholipids and steroids. Sugars and fats are major
contributors to processes the cell uses for deriving energy, so we will look at some of
these structures again in the coming weeks. In the meantime, we will spend the next
lecture studying the structure and function of two other major types of macromolecules in
cells, proteins and nucleic acids.
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