Chapter 3
Biological Molecules

3.1. Why Is Carbon So Important in
Biological Molecules?
• All contain carbon
– Organic refers to molecules containing a carbon skeleton
– Inorganic refers to carbon dioxide and all molecules without carbon

3.1. Why Is Carbon So Important in
Biological Molecules?
• Each carbon can form up to four bonds
(single(2 electrons), double, or triple) and rings
• Carbon makes bonds mostly with H, N, and
O in living systems
• Biomolecules are large and contain
“functional groups” attached to the carbon backbone.
• Functional groups in organic molecules confer chemical reactivity and other characteristics

• Biomolecules are polymers (chains) of subunits called monomers
• Monomers are joined together through dehydration synthesis
– An H and an OH are removed, resulting in the loss of a water molecule (H
2
O)

• Polymers are broken apart through hydrolysis (“water cutting”)
– Water is broken into H and OH and used to break the bond between monomers

• All biological molecules fall into one of four categories
– Carbohydrates
– Lipids
– Proteins
– Nucleic Acids

• Composition
– Made of C, H, and O in the ratio of 1:2:1
( CH
2
O) n
• Types
– Simple or single sugars are monosaccharides
– Two linked monosaccharides are disaccharides
– Long chains of monosaccharides are polysaccharides

• Function:
– Energy source
– Structure
• Most are water- soluble due to the polar
OH functional groups

• Basic Structure
– Backbone of 3-7 carbon atoms
– Many –OH and –H functional groups
– Usually found in a ring form in cells
• Examples:
– Glucose (C
6
H
12
O
6
): the most common
– Fructose (found in corn syrup and fruits)
– Galactose (found in lactose)
– Ribose and deoxyribose (found in RNA and
DNA)

• Fate inside a cell
– Some broken down to free their chemical energy
– Some are linked together by dehydration synthesis

• Disaccharides are two-part sugars
– Sucrose (table sugar) = glucose + fructose
– Lactose (milk sugar) = glucose + galactose
– Maltose (malt sugar)= glucose + glucose

• Monosaccharides are linked together to form chains (polysaccharides)
• Storage polysaccharides:
– Polymers of Glucose:
• STARCH. Formed in roots and seeds
• GLYCOGEN.
Found in liver and muscles

• Structural polysaccharides
– Cellulose (polymer of glucose)
– Found in the cell walls of plants
• Indigestible for most animals due to orientation of bonds between glucoses

• Structural polysaccharides continued
– Chitin (polymer of modified glucose units)
• Found in the outer coverings of insects, crabs, and spiders
• Found in the cell walls of many fungi

• Oils, Fats, and Waxes
• Function
– Energy source ; storage
– Water-proofing (membranes)
– Hormones
• Contain Only Carbon, Hydrogen, and Oxygen
• Most are hydrophobic and water insoluble, due to long non-polar chains
• Types: fatty acids, triglycerides, phospholipids, steroids
• Main subunit:
– Fatty acid

• Made of one or more fatty acid subunits
• Formed by dehydration synthesis
– 3 fatty acids + glycerol
 triglyceride (95% of all lipids in the body)

• Fats and oils used for long-term energy storage
– high density of stored chemical energy

• Fat solidity is due to single or double carbon bonds
– Fats that are solid at room temperature are saturated (carbon chain has as many hydrogen atoms as possible, and mostly or all C-C bonds), e.g. beef fat

• Fats that are liquid at room temperature are unsaturated (fewer hydrogen atoms, many
C=C bonds), e.g. corn oil
• Unsaturated trans fats have been linked to heart disease

• Waxes are composed of long hydrocarbon chains and are strongly hydrophobic
• Waxes are highly saturated and solid at room temperature

• Waxes form waterproof coatings
– Leaves and stems of plants
– Fur in mammals
– Insect exoskeletons
• Used to build honeycomb structures

• Phospholipids : form plasma membranes around all cells
• Construction
– 2 fatty acids + glycerol + a short polar functional group

• Phospholipids have hydrophobic and hydrophilic portions
– Polar functional groups are water soluble
– Nonpolar fatty acid “tails” are water insoluble

• Steroids are composed of four carbon rings fused together
• Examples of steroids
– Cholesterol
• Found in membranes of animal cells
– Male and female sex hormones

• Proteins are formed from chains of amino acids (monomers; 20 different)
• The type, number and position of amino acids in a protein dictates its function

• All amino acids have similar structure
– All contain amino and carboxyl groups
– All have a variable “R” group
• Some R groups are hydrophobic
• Some are hydrophilic
• Cysteine R groups can form disulfide bridges

• Amino acids are joined to form chains by dehydration synthesis
– An amino group reacts with a carboxyl group, and water is lost

• Resultant covalent bond is a peptide bond
• Long chains of amino acids are known as polypeptides or just proteins

• Proteins exhibit up to four levels of structure
– Primary structure is the sequence of amino acids linked together in a protein
– Secondary structures are helices and pleated sheets
– Tertiary structure refers to complex foldings of the protein chain held together by disulfide bridges, hydrophobic/hydrophilic interactions, and other bonds
– Quaternary structure is found where multiple protein chains are linked together

R
C
O
R R
H
C
O
N C
C
N
O
R
C
H
C
N
R R
O
C
C
N
O
C
R H
C
N
O
C
C
R
N
O
C
H
C
N
O
C
C
R
N
O
C
C
N C
O
C
N
R
R
O
C C
N C
O
C
N
R
R
O
C C
N C C
N
O
R
Pleated sheet
R O
C C
N C C
N
R
O
R
C
R
C polypeptide hydrogen bond

s s s-s s-s s s keratin s-s s s

– Precise positioning of amino acid R groups leads to bonds that determine secondary and tertiary structure
– Disruption of these bonds leads to denatured proteins and loss of function

• Nucleotides are the monomers of nucleic acid chains
• Each nucleotide has three parts
– 5-carbon sugar (ribose or deoxyribose)
– phosphate group
– Nitrogen containing base

• A Phosphodiester bond between nucleotides to make a chain:
• Two types of nucleotides
– Ribonucleotides (A, G, C, and U ) found in RNA
– Deoxyribonucleotides (A, G, C, and T ) found in DNA

• Two types of polymers of nucleic acids
– DNA (deoxyribonucleic acid) found in chromosomes
• Carries genetic information needed for protein construction
– RNA (ribonucleic acid)
• Copies of DNA used directly in protein construction

• Each DNA molecule consists of two chains of nucleotides that form a double helix

Functions of Nucleic Acids
• DNA and RNA, the Molecules of Heredity, Are Nucleic
Acids
• Other Nucleotides Act as :
– Messengers: Intracellular Cyclic nucleotides (e.g. cyclic
AMP) carry chemical signals between molecule
– Energy carriers:
• Adenosine triphosphate (ATP) carries energy stored in bonds between phosphate groups
• NAD + and FAD carry electrons
– Enzyme assistants
• Coenzymes help enzymes promote and guide chemical reactions

O
O CH
2
O
N
HC
N
NH
2
C
C
N
C
N
CH
P
OH
H H
O
H
H
OH
HO
OH
P O
O
OH
P O
O
OH
P O
O
CH
2
O
H H
NH
2
HC
N
N
C
C
C
N
N
CH
H
H
OH OH
Cyclic adenosine monophosphate (cyclic AMP)
(intracellular communication)
Adenosine triphosphate (ATP)
(energy carrier)
Vitamin
HO
HO
O
P O
O
P O
O
N
O
HC
N
CH
2
C
C
NH
2
C
N
N
CH
H H H
H
OH OH
Coenzyme
(active in cellular metabolism)
A sampling of the diversity of nucleotides