8. Macromolecules I

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BIOLOGICAL
MACROMOLECULES
Figure 3.1
Foods such as bread, fruit, and cheese are rich sources of biological macromolecules.
(credit: modification of work by Bengt Nyman)
Chapter Outline
3.1: Synthesis of Biological Macromolecules
3.2: Carbohydrates
3.3: Lipids
3.4: Proteins
3.5: Nucleic Acids
Introduction
Food provides the body with the nutrients it needs to survive. Many of these critical nutrients are
biological macromolecules, or large molecules, necessary for life. These macromolecules (polymers)
are built from different combinations of smaller organic molecules (monomers). What specific types of
biological macromolecules do living things require? How are these molecules formed? What functions
do they serve? In this chapter, these questions will be explored.
CHAPTER 3 | BIOLOGICAL MACROMOLECULES 73
3.1
|
Synthesis of Biological Macromolecules
By the end of this section, you will be able to:
•
Understand the synthesis of macromolecules
•
Explain dehydration (or condensation) and hydrolysis reactions
As you’ve learned,
��
are large molecules, necessary for life, that are built from
smaller organic molecules. There are four major classes of biological macromolecules (carbohydrates,
lipids, proteins, and nucleic acids); each is an important cell component and performs a wide array of
functions. Combined, these molecules make up the majority of a cell’s dry mass (recall that water makes
up the majority of its complete mass). Biological macromolecules are organic, meaning they contain
carbon. In addition, they may contain hydrogen, oxygen, nitrogen, and additional minor elements.
Dehydration Synthesis
Most macromolecules are made from single subunits, or building blocks, called
��
. The
monomers combine with each other using covalent bonds to form larger molecules known as
��
.
In doing so, monomers release water molecules as byproducts. This type of reaction is known as
��
, which means “to put together while losing water.”
Figure 3.2
In the dehydration synthesis reaction depicted above, two molecules of glucose are
linked together to form the dissacharide maltose. In the process, a water molecule is formed.
In a dehydration synthesis reaction (
��
), the hydrogen of one monomer combines with the
hydroxyl group of another monomer, releasing a molecule of water. At the same time, the monomers
share electrons and form covalent bonds. As additional monomers join, this chain of repeating monomers
forms a polymer. Different types of monomers can combine in many configurations, giving rise to a
diverse group of macromolecules. Even one kind of monomer can combine in a variety of ways to form
several different polymers: for example, glucose monomers are the constituents of starch, glycogen, and
cellulose.
Hydrolysis
Polymers are broken down into monomers in a process known as hydrolysis, which means “to split
water,” a reaction in which a water molecule is used during the breakdown (
��
). During these
reactions, the polymer is broken into two components: one part gains a hydrogen atom (H+) and the other
gains a hydroxyl molecule (OH–) from a split water molecule.
Figure 3.3
In the hydrolysis reaction shown here, the disaccharide maltose is broken down to form
two glucose monomers with the addition of a water molecule. Note that this reaction is the reverse
of the synthesis reaction shown in
Figure 3.2
.
Dehydration and
��
are catalyzed, or “sped up,” by specific enzymes; dehydration
reactions involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds
74 CHAPTER 3 | BIOLOGICAL MACROMOLECULES
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and release energy. These reactions are similar for most macromolecules, but each monomer and polymer
reaction is specific for its class. For example, in our bodies, food is hydrolyzed, or broken down,
into smaller molecules by catalytic enzymes in the digestive system. This allows for easy absorption
of nutrients by cells in the intestine. Each macromolecule is broken down by a specific enzyme. For
instance, carbohydrates are broken down by amylase, sucrase, lactase, or maltase. Proteins are broken
down by the enzymes pepsin and peptidase, and by hydrochloric acid. Lipids are broken down by lipases.
Breakdown of these macromolecules provides energy for cellular activities.
Visit
��
to see visual representations of dehydration
synthesis and hydrolysis.
3.2
|
Carbohydrates
By the end of this section, you will be able to:
•
Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
•
Explain the classifications of carbohydrates
•
List common monosaccharides, disaccharides, and polysaccharides
Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to
what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often
“carb-load” before important competitions to ensure that they have enough energy to compete at a high
level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural
sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a
simple sugar that is a component of
��
and an ingredient in many staple foods. Carbohydrates also
have other important functions in humans, animals, and plants.
Molecular Structures
��
can be represented by the stoichiometric formula (CH
2
O)
n
, where n is the number
of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in
carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the
components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are
classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides
��
(mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which
is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most
monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group
with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group
with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar,
they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons).
See
��
for an illustration of the monosaccharides.
CHAPTER 3 | BIOLOGICAL MACROMOLECULES 75
Figure 3.4
Monosaccharides are classified based on the position of their carbonyl group and the
number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end
of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses,
pentoses, and hexoses have three, five, and six carbon backbones, respectively.
The chemical formula for glucose is C
6
H
12
O
6
. In humans, glucose is an important source of energy.
During cellular respiration, energy is released from glucose, and that energy is used to help make
adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose
in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is
catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on
plants.
Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common
monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula
(C
6
H
12
O
6
), they differ structurally and chemically (and are known as isomers) because of the different
arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have
more than one asymmetric carbon (
��
).
76 CHAPTER 3 | BIOLOGICAL MACROMOLECULES
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3.2 Carbohydrate (OpenStax CNX)
3.3 Lipids (OpenStax CNX)

Learning Objectives

Name the four major types of organic molecules found in living organisms and tell what they all have in common.
Define the term carbohydrate and distinguish between a monosaccharide, a disaccharide, and a polysaccharide.
Describe the process of dehydration synthesis and hydrolysis, and explain why they are important in living organisms.
Define the term lipid and list some of the roles played by lipids in the cell.
Explain the difference between (a) a saturated and an unsaturated fatty acid, (b) a fat an an oil, (c) a phospholipid and a glycolipid, and (d) a steroid and a wax.

How are macromolecules assembled?

The common organic compounds of living organisms are carbohydrates, proteins, lipids, and nucleic acids. Each of these are macromolecules or polymers made of smaller subunits called monomers. The bonds between these subunits are formed by a process called dehydration synthesis. This process requires energy; a molecule of water is removed (dehydration) and a covalent bond is formed between the subunits. Because a new water molecule is formed, this is also referred to as condensation. The opposite where water and energy are used to break apart polymers into simpler monomers is called hydrolysis (hydro– water, lysis– to break or split).

Polymer formation. Dehydration reactions join monomers. Hydrolysis splits polymers.

Carbohydrates

Carbohydrates serve 2 major functions: energy and structure. As energy, they can be simple for fast utilization or complex for storage. Simple sugars are monomers called monosaccharides. These are readily taken into cells and used immediately for energy. The most important monosaccharide is glucose (C₆H₁₂O₆), since it is the preferred energy source for cells. The conversion of this chemical into cellular energy can be described by the equation below:

C₆H₁₂O₆ (s) + 6 O₂ (g) → 6 CO₂ (g) + 6 H₂O (l) + energy

Long polymers of carbohydrates are called polysaccharides and are not readily taken into cells for use as energy. These are used often for energy storage. Examples of energy storage molecules are: amylose or starch (plants) and glycogen (animals). Some polysaccharides are so long and complex that they are used for structure like cellulose in the cell walls of plants. Cellulose is very large and practically indigestible, making it unsuitable as a readily available energy source for cells.

Carbohydrates: Carbohydrates are composed of sugar units referred to as -saccharides.

Monosaccharides contain a carbonyl group. The carbonyl is a source of electrons (the double bond on the oxygen). These electrons can be donated (or lost and oxidized) to reduce another compound (that gains those electrons).

Glucose is the preferred carbohydrate of cells. In solution, it can change from a linear chain to a ring.

Monosaccharides are capable of isomerizing. This means they alternate in structure from a linear chain to a ring form in solution.

Structural Carbohydrates

In food, more complex carbohydrates are derived from larger polysaccharides. These larger carbohydrates are fairly insoluble in water. Dietary fiber is name given to indigestible materials in food most often derived from the complex carbohydrates from vegetable material. Some of this material serves the plants as a structural component of the cells and is completely insoluble. Cellulose is the major structural carbohydrate found in plant cell walls. Similarly, animals and fungi have structural carbohydrates that are composed of the indigestible compound called chitin. We will not be testing for these items.

Cellulose is a complex carbohydrate of glucose molecules. It is the major structural component of plant cell walls. It’s structural durability is enhanced by intramolecular hydrogen bonds.

Chitin is a structural carbohydrate found in animal shells or fungi cell walls. The polymer contains amide groups that differentiates it from other carbohydrates composed of glucose.

A cicada molting from it’s shell made of chitin.

Lipids

Lipids are the class of macromolecules that mostly serve as long-term energy storage. Additionally, they serve as signaling molecules, water sealant, structure and insulation. Lipids are insoluble in polar solvents such as water, and are soluble in nonpolar solvents such as ether and acetone.

Fats/Triglycerides

Fats or triglycerides are made of glycerol and three fatty acid chains. They form through 3 dehydration synthesis reactions between a hydroxyl of the glycerol and the carboxyl group of the fatty acid. Fatty acids tend to be chains of 16-18 carbons long.

Saturated versus Unsaturated fats

A saturated fatty acid. The molecule takes up little space in three dimensions. Many molecules can stack upon each other. Saturated fats are solid at room temperature.

A polyunsaturated fatty acid. A kink from the double bond increases the amount of three dimensional space that the molecule fills. Unsaturated fats tend to be liquid at room temperature.

A trans fatty acid. Despite an unsaturated bond, the molecule fills as much space as a saturated fatty acid and is solid at room temperature. Trans fats usually arise from artificial saturation techniques.

Butterfat is almost completely saturated. Notice how molecules can stack very closely.

Because butterfat can stack together very closely, it is dense and found as a solid at room temperature.

Olive oil is a triglyceride that contains the monounsaturated fatty acid, oleic acid. Credit: Alex_Ex [CC-BY-SA 3.0]

Phospholipids

The structure of phospholipids in a cell membrane. Credit: Superscience71421 [CC-BY-SA-4.0]

Phospholipids contain the phosphate group attached to a glycerol and two fatty acid chains. The phosphate is negatively charged and will interact with polar compounds, like a water environment. The fatty acid chains appear as lipid tails and hide from the water. The act of having both a polar side and a non-polar side is called amphiphilic. Phospholipids are the major component of cell membranes and serve a structural function.

Test your knowledge

http://www.visionlearning.com/en/library/Biology/2/Carbohydrates/61/quiz

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