The Structure and Function of
Large Biological Molecules
Chapter 5
The Molecules of Life
• Living things made up of 4 classes of large
biological molecules (macromolecules) :
1. Carbohydrates
2. Lipids
3. Proteins
4. Nucleic acids
• Molecular structure and function are linked
• Unique, emergent properties
Macromolecules are polymers, built
from monomers
• polymer -long molecule of many building blocks
• monomers - single unit
Sucrose
The Diversity of Polymers
• Each cell has thousands of different
macromolecules
– built from monomers
• Macromolecules vary among cells, among
species, and between species
Carbohydrates serve as fuel and
building material
• Carbohydrates = sugars and sugar polymers
– Monosaccharides = single sugars
• Ex. glucose (C6H12O6)
• major fuel for cells
• raw material for building molecules
• Sugars often form rings (in aqueous solution)
(a) Linear and ring forms
(b) Abbreviated ring structure
• Disaccharide = two sugars
• Ex. lactose, sucrose, maltose
Polysaccharides
• Polysaccharides - polymers of sugars = starch
– storage and structural roles
• The structure and function of a polysaccharide are determined by its sugar
monomers and the positions of glycosidic linkages
Storage Polysaccharides
• Starch
– plants store starch
– Glucose polymer
• Glycogen
– Animals store glycogen (glucose polymer)
– Humans in liver and muscle cells
Chloroplast
Mitochondria Glycogen granules
Starch
0.5 µm
1 µm
Glycogen
Amylose
Amylopectin
(a) Starch: a plant polysaccharide
Amylose - unbranched
Amylopectin - branched
(b) Glycogen: an animal polysaccharide
Glycogen is more branched than
starch
Structural Polysaccharides
• Cellulose =component of tough wall of plant cells
•
polymer of glucose (glycosidic linkages differ from starch)
•
The difference is based on two ring forms for glucose:
• Enzymes digest cellulose in some animals
– Cows, termites, have symbiotic relationships with
microbes that digest cellulose
• In humans, cellulose is indigestible fiber
Mastigophoran, anaerobic, methane
• Chitin in the exoskeleton of arthropods and in
fungi
(a) The structure
of the chitin
monomer.
Cicada exoskeleton
(b) Chitin forms the
exoskeleton of
arthropods.
(c) Chitin is used to make
a strong and flexible
surgical thread.
Lipids are hydrophobic
• Lipids - fats, phospholipids, steroids
Triglyceride
= 3 fatty acids
joined
glycerol

Saturated fats
maximum number of
H possible (no double
bonds)
 Solid at room T
(animal fats)


Unsaturated fats
one or more double
bonds
(a)
 Liquid at room T
(plant, fish oils)

• Coronary artery disease associated with diet rich
in saturated fats
• Hydrogenation
– process of converting unsaturated fats to
saturated fats by adding hydrogen
– Extends shelf life, prevents oil separation
– Ex. margarine, peanut butter
•
•
•
•
The good news:
Fats store energy (adipose cells)
Cell membranes need lipid
Lipid cushions and insulates
Steroids
• Steroids –
– Ex. estrogen, testosterone
• Cholesterol
– Steroid in animal cell membranes
– Synthesized in the liver
Proteins
• Proteins = more than 50% of dry mass of cells
• Protein functions
– structural support –collagen
– pigment - melanin
– transport - hemoglobin
– cellular communications
– movement
– defense against foreign substances-antibodies
• Enzymes
– All are proteins
–
–
–
–
–
catalyst speeds up chemical reactions
reusable
specific to each reaction
essential to life
Heat or chemicals may denature
– animation
Polypeptides
• Polypeptides
– polymers built from set of 20 amino acid building
blocks
– may be a few or thousands long
• protein
– one or more polypeptides
– has a function
Peptide
Protein
Protein Structure and Function
• proteins consists of one or more polypeptides
twisted, folded, and coiled into unique shape
Groove
Groove
(a) A ribbon model of lysozyme
(b)
A space-filling model of lysozyme
• sequence of aa determines a 3D structure
• structure determines function
Antibody protein
Protein from flu virus
Four Levels of Protein Structure
• Primary structure =unique sequence of amino
acids
1
5
10
15
20
25
• Secondary structure = coils and folds
–  helix and  pleated sheet
– H-bonds
β pleated sheet
Example: spider silk
Strong as steel
Stretchy
α helix
• Tertiary structure determined by interactions
between amino acids
–
–
–
–
hydrogen bonds
ionic bonds
hydrophobic interactions
disulfide bridges (covalent bonds)
Tertiary structure
• Quaternary structure two or more polypeptide
chains may form one macromolecule
• ex. hemoglobin
α Chains
• activity
β Chains
Hemoglobin
A patient with sickle cell disease
Denaturation of proteins
• Denaturation
– Loss of protein structure  biologically inactive
– pH, heat, chemicals
The Roles of Nucleic Acids
Deoxyribonucleic acid (DNA)
replicates prior to cell division
contains codes for proteins (genes)
Nucleic acids hold a code
• Gene
– unit of inheritance
– code for protein primary structure
– composed of DNA
The Structure of Nucleic Acids
• Nucleotides
G,A,T,C building blocks
(monomers)
– Pyrimidines (cytosine,
thymine, and uracil)
– Purines (adenine and
guanine)
Nitrogenous bases
Pyrimidines
Cytosine (C)
Uracil (U, in RNA)
Thymine (T, in DNA)
Purines
Adenine (A)
Guanine (G)
(c) Nucleoside components: nitrogenous bases
•Nucleotides contain sugar
•DNA deoxyribose
•RNA ribose (ribonucleic acid)
Sugars
Deoxyribose (in DNA)
(c) Nucleoside components: sugars
Ribose (in RNA)
DNA Polymers
Sugar phosphate backbone
The DNA Double Helix
• A DNA molecule has 2 strands that form double helix
• hydrogen bonds between:
– adenine (A) thymine (T)
– guanine (G) cytosine (C)
• DNA replication
– Before a cell divides
DNA, Proteins and Evolution
• DNA is inherited
– Cell to cell
– Parent to offspring
• Closely related species more similar in DNA sequence than
more distantly related species
– Human/human 99.1 %
– Human/chimp 98.5%
• Molecular biology used to assess evolutionary relatedness