Table Of ContentMolecules of Life
Molecules of Life
A child’s building blocks are relatively simple structures. When they
come together, however, they can form magnificent structures.
Figure 1 shows an elaborate dragon made up of small Lego® blocks.
Organisms are built in much the same way. Despite their complexity,
organisms are made of relatively simple building blocks. These
building blocks start with the elements hydrogen, carbon, oxygen,
nitrogen, and phosphorus. How do these elements come together to
form complex organisms?
Types of Macromolecules
Macromolecules are molecules that
are made by organisms and are
essential for performing life
functions. They range in size and
perform specific functions in and
among cells. Their function is often
determined by their structure. If the
structure is disrupted, the macromolecule can no longer function
properly.
There are four main types of large, natural macromolecules called
biomolecules: carbohydrates, lipids, proteins, and nucleic acids. These
biomolecules are composed of different elements that combine
together in different ratios. Each class of biomolecule has a unique
arrangement of elements.
Elements Present in Biomolecules
Biomolecule Carbohydrate Lipid Protein Nucleic Acid
Elements C, H, O C, H, O
C, H, O, N, S C, H, O, N, P
Present (1:2:1 ratio) (very little O)
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Molecules of Life
Formation of Macromolecules
Macromolecules are made of monomers. A monomer is a small molecule that can be combined
chemically with other monomers to form larger molecules. Monomers are made up of relatively simple
elements. The most abundant elements in biological monomers are carbon, hydrogen, and oxygen. A
polymer is a group of monomers linked to form a much larger molecule. The prefix mono- means
“one,” and poly- means “many.” Think of monomers as the building blocks and polymers as the final
product. The process of making a polymer is called polymerization.
When a macromolecule is built, monomers link together by way of strong covalent bonds. Each time
two monomers are linked, a water molecule is released. This process is called dehydration
synthesis and can be seen in figure 2 below. This process involves the removal of a water molecule
when two monomers covalently bond to create a polymer. Additional monomers can be added to form
a polymer through the same process.
When a water molecule is added to a polymer, it breaks apart the polymer during a process called
hydrolysis. Digestion uses hydrolysis to break food molecules down into simpler molecules that can
be absorbed into the bloodstream. The process can be seen in figure 3.
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Molecules of Life
Carbohydrates
Carbohydrates are oftentimes collectively referred to as sugars. A
carbohydrate is better defined as a biomolecule composed entirely
of carbon, hydrogen, and oxygen in a 1:2:1 ratio. You may think of
sugar negatively because we oftentimes think of sugar in candy and
other junk food. However, carbohydrates are important energy
molecules in our cells. Often, runners or other athletes will consume
extra carbohydrates before a big race or game to give them more
energy to make it through. In the human diet, carbohydrates are
found in flour, fruits, grains, pasta, and starchy vegetables.
Carbohydrates also play a number of important structural and signaling
roles in the cell membranes and cell walls of living things. They form part
of the molecular backbone of nucleic acids, and they are critical for
maintaining life.
The monomer of a carbohydrate is a glucose molecule, such as the one
pictured in figure 4. One glucose molecule contains 6 carbons, 12 hydrogens, and 6 oxygens. Notice
that the combination of elements is still in the 1:2:1 ratio. A glucose molecule is known as a
monosaccharide because it is composed of only one sugar molecule. This glucose molecule is an
important nutrient molecule to living things and is a primary source of matter in ecosystems.
Autotrophs produce glucose through the process of photosynthesis by rearranging the elements in
CO and H O and using the energy from the sun.
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Other important monosaccharides include fructose from fruit and
galactose from dairy sources, such as milk. When two monosaccharides
combine, they form a disaccharide. Lactose, which is found in milk, is the
combination of glucose and galactose. Table sugar, called sucrose, is also
a disaccharide formed by the combination of glucose and fructose. The
combination of glucose and fructose to form sucrose by dehydration
synthesis can be seen below in figure 5.
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Molecules of Life
When organisms eat larger sugar molecules, such as lactose or table sugar, they are broken down
into their monomers in the digestive system by hydrolysis and then absorbed into the bloodstream.
The simple sugars can then be transferred to cells around the body that will convert the sugar into the
necessary energy to fuel cellular processes within the body through the process of cellular
respiration.
Sugar molecules can form large polymers using dehydration synthesis when needing to store sugar
for energy later on. Animals store excess sugars, such as glycogen, which are made of glucose
molecules linked together. Muscle cells contain large reservoirs of glycogen since energy is
constantly needed to contract your muscles during movement. Plants store sugars as starch
primarily in the seeds and bulbs. Root vegetables, such as potatoes, are high in carbohydrates and
are known as a starchy vegetable. Another carbohydrate polymer is cellulose which forms the rigid
cell walls of plants and gives the plant necessary structure and support.
Lipids
If you notice from Table 1, lipids are actually made of the same molecules as carbohydrates, but in a
different ratio. The structure of lipids are primarily long chains of carbon and hydrogen with very little
oxygen and are called fatty acids. Fatty acids are the building blocks of fats which are common type
of lipid formed from the combination of fatty acids and glycerol. The structure of one of these fatty
acid molecules is shown in figure 6 below. The function of these fats is to store energy and can be
found in fish, eggs, and oil.
When there is an abundance of carbohydrates and an excess of calories in the diet, your body will
convert the excess carbohydrates into large energy-storing fat molecules. However, when
carbohydrates are scarce, the body will break down the large lipid molecules to be used to produce
the necessary energy for the cell.
Lipids are also the main structural component of the cell membranes of all organisms. Lipids are
nonpolar, which makes them hydrophobic, or water-repellent. They do not dissolve in water. Cell
membranes, as shown in figure 7, are made up of a phospholipid bilayer. Since the lipid tails of a
phospholipid molecule are hydrophobic, they face each other, forming a barrier around the cell.
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Molecules of Life
Proteins
Amino acids are the building blocks, or monomers, of proteins. If you look again at Table 1, you can
see that these amino acids are composed of the same carbon, hydrogen, and oxygen atoms.
However, all proteins also contain nitrogen and some contain sulfur.
There are 20 amino acids that make up the proteins found in living things. All amino acids contain
the same basic chain made of carbon, hydrogen, oxygen, and nitrogen. The side chains are what
makes one amino acid different from another. Some of the amino acids can be made in the body,
while others have to be obtained by eating other protein sources. The basic amino acid structure is
shown in figure 8.
Amino acids are linked together by covalent bonds called peptide
bonds which only form between amino acids. The reaction to form
a peptide bond is a dehydration synthesis reaction which will
remove a water molecule when each peptide bond is formed.
Amino acids are linked together to form a polypeptide chain. Inside
the cell, an organelle called the ribosome is responsible for linking
together amino acids to form the polypeptide chain. When a chain
contains more than 50 amino acids arranged in a biologically
functional way, it is called a protein. There are thousands of
different proteins in living things.
Proteins are essential biomolecules to all cells. They give a cell its structure, communicate
information, synthesize molecules, transport molecules, and make up enzymes, which are
molecules that speed up the chemical reactions necessary for life.
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Molecules of Life
Nucleic Acids
Nucleotides, the monomers of nucleic acids, are small molecules made of a sugar
(monosaccharide), one or more phosphate groups, and a nitrogenous base. The elements that make
up a nucleotide are carbon, hydrogen, oxygen, nitrogen, and phosphorous. If you look closely at the
structure of a nucleotide in figure 9, you can see the sugar monomer sitting in the middle of the
structure.
Nucleotides are the building blocks of nucleic acids, including DNA
(deoxyribonucleic acid) and RNA (ribonucleic acid). DNA has four
nucleotides–guanine, adenine, thymine, and cytosine. In RNA, uracil
replaces thymine as a nucleotide. The sugar between the two molecules
is also slightly different. The nucleotides form together to create either a
double-stranded DNA molecule or single-stranded RNA molecule.
Nucleic acids are essential for storing and using genetic information.
DNA and RNA work together to create proteins and are vital in
maintaining our genetic code.
Importance of Sugar
As you can see, a sugar molecule contains the elements that are in common between all
biomolecules. It is important to realize that sugar molecules are the primary source of energy for all
living things. They produce the necessary cellular energy in the form of ATP through cellular
respiration. Some of the ATP will also be used to drive reactions between sugar molecules and other
elements obtained from your diet, to reform into other carbon-based compounds when needed.
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Molecules of Life
So how does your body respond when you have few carbohydrates in your system? Your cells are
continually working and need a constant supply of energy, but if there is no sugar available, do your
cells shut down? The answer is no. Since the lipids and proteins are composed of elements similar
to glucose, the body is able to process these to produce energy. The actual chemical reactions are
different, but the overall result is that the cell has the energy needed to perform all required
processes. This will be discussed further in Cellular Energy.
The biomolecules are vital to an organism's existence. Sugar, containing carbon, hydrogen. and
oxygen, can be used to provide energy for our cells. Sugar is a building block for nucleic acids and
can be assembled into larger molecules such as DNA. Sugar can also be used to help build proteins
or be converted to lipids. The chemical reactions that take place allow for the recombination of
elements to provide an organism with the necessary molecules of life.
Advanced Topics
Water is all around us. It covers almost 66% of Earth and is found in oceans, lakes, and streams.
Water cycles through the environment in the form of precipitation and water vapor. Water can be
accessed from a faucet or in bottles. It is one of our most valuable resources. But why? What makes
water so special?
Water has unique properties that are not found in other molecules. Water
is a polar molecule. Polar molecules possess regions that are slightly
positive or slightly negative due to an unequal sharing of electrons in a
covalent bond. Figure 10 shows the structure of a water molecule. Water
is called the universal solvent because it can dissolve most substances.
This unique quality comes from the polarity of water.
This polarity gives water its unique shape with the oxygen having a
slightly negative charge and each hydrogen having a slightly
positive charge. These slightly charged ends allow each molecule of water to be attracted to other
molecules of water. The attraction between the two water molecules creates a hydrogen bond. The
hydrogen bonds give water some unique characteristics.
The attraction of water molecules to each other is a cohesive
property. The cohesion is based on the hydrogen bonds and creates
surface tension. This surface tension can be seen when insects are
able to rest on the surface of water. It is also seen as water forms
droplets as shown in figure 11. As water molecules come close to
other water molecules, they will be attracted to each other, forming
larger drops. You can see this happen by putting two small drops of
water on a sheet protector then using a toothpick move one drop
closer to the other and observe what happens.
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Molecules of Life
Water is not only attracted to other water molecules, but it is
attracted to other substances as well. This gives water an
adhesive characteristic. Adhesion allows water to have capillary
action. This is how water is able to move up small tubes, such as
the stems of plants. It also causes the water to move up the
sugar cubes as shown in figure 12.
Water has a comparatively high specific heat, due to the
hydrogen bonds. This means that it takes more energy to break
those bonds than most other substances. This particular
characteristic is vital to maintaining temperatures in living
organisms.
When a substance becomes a solid, molecules generally get closer together. However, in water the
opposite actually occurs. Figure 13 shows how the molecules of water are arranged as a gas, liquid,
and solid. This property changes the density of water when it is a solid. As water molecules freeze,
they move into a set crystalline pattern that actually causes the density of ice to be less than that of
liquid water. This allows ice to float. As water freezes on lakes, the ice that forms on top creates a
layer of insulation to maintain the temperature of the water below, thereby supporting the organisms
that live in the water.
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