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
Storage Polysaccharides
• Starch – plants store starch – Glucose polymer
• Glycogen
– Animals store glycogen (glucose polymer) – Humans in liver and muscle cells
(b) Glycogen: an animal polysaccharide
Starch
Glycogen Amylose
Chloroplast
(a) Starch: a plant polysaccharide
Amylopectin
Mitochondria Glycogen granules
0.5 µm
1 µm
Amylose - unbranched Amylopectin - branched
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
The structure of the chitin monomer.
(a)
(b)
(c)
Chitin forms the exoskeleton of arthropods.
Chitin is used to make a strong and flexible surgical thread.
Cicada exoskeleton
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 Liquid at room T
(plant, fish oils)
(a)
• 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
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
A ribbon model of lysozyme (a) (b) A space-filling model of lysozyme
Groove Groove
• 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
25
20
15
10
5 1
• Secondary structure = coils and folds – α helix and β pleated sheet – H-bonds
β pleated sheet
α helix
Example: spider silk Strong as steel Stretchy
• Tertiary structure determined by interactions between amino acids
Tertiary structure
• Quaternary structure two or more polypeptide chains may form one macromolecule
• ex. hemoglobin
α Chains
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)
(c) Nucleoside components: nitrogenous bases
Purines
Guanine (G) Adenine (A)
Cytosine (C) Thymine (T, in DNA)
Uracil (U, in RNA)
Nitrogenous bases Pyrimidines
Ribose (in RNA) Deoxyribose (in DNA)
Sugars
(c) Nucleoside components: sugars
•Nucleotides contain sugar
•DNA deoxyribose
•RNA ribose (ribonucleic acid)
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%