The Molecules of Life: Structure and Functions

Contributed by:
Sharp Tutor
We will discuss the complex biological molecules which form the base of life. A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development.
1. Chapter 5
The Structure and Function of
Large Biological Molecules
2. Overview: The Molecules of Life
• All living things are made up of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic
• Within cells, small organic molecules are joined together to form larger molecules
• Macromolecules are large molecules composed of thousands of covalently connected atoms
• Molecular structure and function are inseparable
• Macromolecules are polymers, built from monomers
• A polymer is a long molecule consisting of many similar building blocks
• These small building-block molecules are called monomers
• Three of the four classes of life’s organic molecules are polymers:
– Carbohydrates
– Proteins
– Nucleic acids
3. Concept 5.2: Carbohydrates serve as fuel and
building material
• Carbohydrates include sugars and the
polymers of sugars
• The simplest carbohydrates are
monosaccharides, or single sugars
• Carbohydrate macromolecules are
polysaccharides, polymers composed of many
sugar building blocks
4. • Monosaccharides have molecular formulas that are usually multiples
of CH2O
• Glucose (C6H12O6) is the most common monosaccharide
• Monosaccharides are classified by
– The location of the carbonyl group (as aldose or ketose)
– The number of carbons in the carbon skeleton
• A disaccharide is formed when a dehydration reaction joins two
• This covalent bond is called a glycosidic linkage
• Polysaccharides, the polymers of sugars, have storage and structural roles
• The structure and function of a polysaccharide are determined by its sugar monomers
and the positions of glycosidic linkages
5. Storage Polysaccharides
• Starch, a storage polysaccharide of plants,
consists entirely of glucose monomers
• Plants store surplus starch as granules within
chloroplasts and other plastids
6. Structural Polysaccharides
• The polysaccharide cellulose is a major component of the tough wall
of plant cells
• Like starch, cellulose is a polymer of glucose, but the glycosidic
linkages differ
• The difference is based on two ring forms for glucose: alpha () and
beta ()
• Polymers with  glucose are helical
• Polymers with  glucose are straight
• In straight structures, H atoms on one strand can bond with OH
groups on other strands
• Parallel cellulose molecules held together this way are grouped
into microfibrils, which form strong building materials for plants
7. • Enzymes that digest starch by hydrolyzing 
linkages can’t hydrolyze  linkages in cellulose
• Cellulose in human food passes through the
digestive tract as insoluble fiber
• Some microbes use enzymes to digest
• Many herbivores, from cows to termites, have
symbiotic relationships with these microbes
8. Concept 5.3: Lipids are a diverse group of
hydrophobic molecules
• Lipids are the one class of large biological
molecules that do not form polymers
• The unifying feature of lipids is having little or
no affinity for water
• Lipids are hydrophobic becausethey consist
mostly of hydrocarbons, which form nonpolar
covalent bonds
• The most biologically important lipids are fats,
phospholipids, and steroids
9. Fats
• Fats are constructed from two types of smaller molecules: glycerol and fatty
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each
• A fatty acid consists of a carboxyl group attached to a long carbon skeleton
• Fats separate from water because water molecules form hydrogen bonds with each other
and exclude the fats
• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a
triacylglycerol, or triglyceride
• , Fatty acids vary in length (number of carbons) and in the number and locations of double
• Saturated fatty acids have the maximum number of hydrogen atoms possible and no
double bonds
• Unsaturated fatty acids have one or more double bond
10. • Fats made from saturated fatty acids are called saturated fats, and are solid at room
• Most animal fats are saturated
• Fats made from unsaturated fatty acids are called unsaturated fats or oils, and are liquid
at room temperature
• Plant fats and fish fats are usually unsaturated
• A diet rich in saturated fats may contribute to cardiovascular disease through plaque
• Hydrogenation is the process of converting unsaturated fats to saturated fats by adding
• Hydrogenating vegetable oils also creates unsaturated fats with trans double bonds
• These trans fats may contribute more than saturated fats to cardiovascular disease
11. • In a phospholipid, two fatty acids and a phosphate group are attached to
• The two fatty acid tails are hydrophobic, but the phosphate group and its
attachments form a hydrophilic head
• When phospholipids are added to water, they self-assemble into a bilayer, with
the hydrophobic tails pointing toward the interior
• The structure of phospholipids results in a bilayer arrangement found in cell
• Phospholipids are the major component of all cell membranes
• Steroids are lipids characterized by a carbon skeleton consisting of four fused
• Cholesterol, an important steroid, is a component in animal cell membranes
• Although cholesterol is essential in animals, high levels in the blood may
contribute to cardiovascular disease
12. Concept 5.4: Proteins have many structures,
resulting in a wide range of functions
• Proteins account for more than 50% of the dry mass of most cells
• Protein functions include structural support, storage, transport, cellular communications, movement,
and defense against foreign substances
• Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions
• Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the
processes of life
• Polypeptides are polymers built from the same set of 20 amino acids
• A protein consists of one or more polypeptides
Amino Acid Monomers
• Amino acids are organic molecules with carboxyl and amino groups
• Amino acids differ in their properties due to differing side chains, called R groups
13. Amino Acid Polymers
• Amino acids are linked by peptide bonds
• A polypeptide is a polymer of amino acids
• Polypeptides range in length from a few to
more than a thousand monomers
• Each polypeptide has a unique linear
sequence of amino acids
14. Four Levels of Protein Structure
• The primary structure of a protein is its unique
sequence of amino acids
• Secondary structure, found in most proteins,
consists of coils and folds in the polypeptide
• Tertiary structure is determined by interactions
among various side chains (R groups)
• Quaternary structure results when a protein
consists of multiple polypeptide chains
15. • Primary structure, the sequence of amino acids in a protein, is like the order of letters in
a long word
• Primary structure is determined by inherited genetic information
• The coils and folds of secondary structure result from hydrogen bonds between
repeating constituents of the polypeptide backbone
• Typical secondary structures are a coil called an  helix and a folded structure called a 
pleated sheet
• Tertiary structure is determined by interactions between R groups, rather than
interactions between backbone constituents
• These interactions between R groups include hydrogen bonds, ionic bonds,
hydrophobic interactions, and van der Waals interactions
• Strong covalent bonds called disulfide bridges may reinforce the protein’s structure
• Quaternary structure results when two or more polypeptide chains form one
• Collagen is a fibrous protein consisting of three polypeptides coiled like a rope
• Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta
16. What Determines Protein Structure?
• In addition to primary structure, physical and chemical conditions can affect
• Alterations in pH, salt concentration, temperature, or other environmental
factors can cause a protein to unravel
• This loss of a protein’s native structure is called denaturation
• A denatured protein is biologically inactive
• It is hard to predict a protein’s structure from its primary structure
• Most proteins probably go through several states on their way to a stable
• Chaperonins are protein molecules that assist the proper folding of other
• It is hard to predict a protein’s structure from its primary structure
• Most proteins probably go through several states on their way to a stable structure
• Chaperonins are protein molecules that assist the proper folding of other proteins