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Chapter 3
The Molecules of Life
Biological molecules
Carbon is a crucial element in biological molecules
Carbon can share electrons in as many as four covalent bonds
Since two or more carbons can bond together, it is possible to construct an endless
variety of carbon skeletons
Functional Groups
Some combinations of atoms act together as a functional group
Often, the functional group imparts an electrical charge or polarity onto molecules
affecting their chemical properties
Chemical reactions
So chemical reactions are all about breaking existing covalent bonds and forming new
ones
It’s about rearranging atoms in molecules
Many of our key biological molecules are large molecules (macromolecules) composed of
multiple smaller subunits
Four categories of macromolecules are
carbohydrates
proteins
nucleic acids
lipids
Macromolecules are polymers, made by connecting many smaller monomers
A dehydration reaction
links two monomers together
removes a molecule of water
Organisms also have to break down macromolecules
During digestion, macromolecules are broken down to monomers via chemical
reactions to make the monomers available to our cells
Hydrolysis breaks the bonds between monomers by adding a molecule of water
Carbohydrates
Carbohydrates are our primary energy source for cellular functions
Carbs range from simple (monosaccharides) through disaccharides to complex carbs
Glucose and fructose are isomers of each other - sharing the same chemical formula
but having different structures
Monosaccharides, particularly glucose, are the main fuels for cellular work
In water, many monosaccharides form rings
Disaccharides form when two monosaccharides are bonded together via a dehydration
reaction
Polysaccharides
are complex carbohydrates
are made of long chains of sugars - polymers of monosaccharides
Starch is a familiar example of a polysaccharide
It is used by plant cells to store energy
Potatoes and grains
Glycogen is used by animal cells to store energy
It is hydrolyzed to release glucose when we need energy
Cellulose is the most abundant organic compound on Earth
It makes up the walls of plant cells
It cannot be chemically broken by any enzyme produced by animals
Lipids
Lipids have several functions, including energy storage, components of biological
membranes ...
and insulation
All lipids are hydrophobic
Triglycerides contain three fatty acid tails
Fatty acid chains, and therefore the fat molecules, can be either saturated (C’s
bonded to the maximum number of H’s) or unsaturated (some C’s sharing double
bonds)
Most animal fats
have a high proportion of saturated fatty acids
can easily stack making them solid at room temperature
contribute to atherosclerosis, in which lipid-containing plaques accumulate along the
inside walls of blood vessels, decreasing blood flow
Most plant and fish fats
have a high proportion of unsaturated fatty acids
are usually liquid at room temperature
Good fat ... bad fat?
Hydrogenation
adds hydrogen atoms
converts unsaturated fats to saturated fats
produces trans fats, a type of unsaturated fat that is particularly bad for
cardiovascular health
Steroids are also hydrophobic
Proteins
Proteins
are polymers of amino acids
account for >50% of the dry weight of most cells
are extremely versatile molecules
All proteins are composed of the same 20 amino acids
All amino acids share a common basic chemical structure
The 20 amino acids differ in their side chain or R group
So it’s the properties of the R group that determine the properties of the amino acid
Some amino acids are hydrophobic; some are hydrophilic
Others have unique properties
Cells join amino acids together using dehydration reactions
The linkage between two amino acids is a peptide bond, eventually forming long
chains of amino acids called polypeptides
A functional protein consists of one or more polypeptide chain(s) precisely twisted, folded,
and coiled into a particular 3-dimensional shape
The 3-D shape of proteins is crucial to their function
Changes to the 3-D shape can drastically affect the functionality of the protein
The 3-D shape is dependent upon the order of the amino acids in the polypeptide
chain
Levels of protein 3-D structure
Primary is based on the sequence of amino acids in the chain
Secondary is based on H-bonding between amino acids in the chain
Tertiary is based on other interactions (hydrophobic and hydrophilic interactions,
covalent, and non-covalent bonds)
Quaternary is based on the various interactions between two or more polypeptides
to give a functional protein
Because the 3-D shape of a protein is crucial to its function, even slight changes in the
sequence can drastically affect protein function
Single amino acid changes can lead to changes in protein structure that can cause
disease
Misfolded proteins are associated with many diseases, including some severe
nervous system disorders
Some treatments can cause proteins to lose their 3-D shape
The change can be reversible or irreversible
Nucleic Acids
Nucleic acids are genetic molecules
They store and transfer genetic information
The monomers (nucleotides) can also be important energy carriers
A gene is a unit of inheritance encoded in a specific stretch of DNA that dictates the amino
acid sequence of a polypeptide
Each nucleotide consists of three parts
a 5-C sugar (ribose or deoxyribose)
a phosphate functional group
a nitrogen-containing base
There are 4 nitrogen-containing bases in DNA
A molecule of DNA is double-stranded, consisting of two polynucleotide strands coiled
around each other to form a double helix
In contrast to DNA, RNA
incorporates the sugar ribose rather than deoxyribose
has the similar base uracil (U) rather than thymine
is usually present in a single-stranded form