Chapter 5 Large Molecules are the Hallmark of Life.
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Transcript of Chapter 5 Large Molecules are the Hallmark of Life.
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Chapter 5Large Molecules are the Hallmark of Life
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Overview: The Structure and Function of Large Biological Molecules
• Within cells, small organic molecules are joined together to form larger molecules called macromolecules.
• Macromolecules are large molecules composed of thousands of covalently connected atoms
• Proteins, nucleic acids, lipids (as aggregates) and complex carbohydrates are the four classes of macromolecules in biological systems.
• Molecular structure and function are inseparable (structure determines function; function is dependent on structure; function is an emergent property based on structure)
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Outline
• 5.1 Polymers
• 5.2 Carbohydrates
• 5.3 Lipids
• 5.4 Proteins
• 5.5 Nucleic acids
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Concept 5.1: Macromolecules are polymers, built from monomers
• A polymer is a long molecule consisting of many similar or identical building blocks
• These small building-block molecules are subunits called monomers
• Three of the four classes of life’s organic molecules are polymers:
– Carbohydrates
– Proteins
– Nucleic acids
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• A condensation reaction or more specifically a dehydration reaction occurs when two monomers bond together through the loss of a water molecule
• Enzymes are macromolecules that speed up the dehydration process (and others)
• Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction
The Synthesis and Breakdown of Polymers
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Dehydration removes a watermolecule, forming a new bond
Short polymer (oligomer)
Unlinked monomer
Longer polymer
Dehydration reaction in the synthesis of a polymer
HO
HO
HO
H2O
H
HH
4321
1 2 3
(a)
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Hydrolysis adds a watermolecule, breaking a bond
Hydrolysis of a polymer
HO
HO HO
H2O
H
H
H321
1 2 3 4
(b)
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The Diversity of Polymers
• Each cell has thousands of different kinds of macromolecules
• Macromolecules vary among cells of an organism, vary more within a species, and vary even more between species
• All living systems contain the same classes of polymers and macromolecules
• The exact identity of the large molecules is different from one cell to another
2 3 HOH
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The Advantages of Polymer Construction
• Mass production-efficiency
• A vast number of polymers can be built from different combinations of a small set of monomers-variety
2 3 HOH
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Concept 5.2: Carbohydrates serve as fuel and building material
• Carbohydrates are sugars-including individual sugars and the polymers of sugars
• The simplest carbohydrates are called simple sugars or monosaccharides
• Monosaccharides can link together via condensation reactions to form disaccharides, trisaccharides, oligosaccharides, etc.
• Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks
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Sugars
• 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
– The number of carbons in the carbon skeleton
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Some ExamplesA
ldo
ses
Glyceraldehyde
Ribose
Glucose Galactose
Hexoses (C6H12O6)Pentoses (C5H10O5)Trioses (C3H6O3)
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Ket
ose
s
Dihydroxyacetone
Ribulose
Fructose
Hexoses (C6H12O6)Pentoses (C5H10O5)Trioses (C3H6O3)
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• Though often drawn as linear skeletons, in aqueous solutions many sugars form rings
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(a) Linear and ring forms of glucose
(b) Abbreviated ring structure
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• A disaccharide is formed when a dehydration reaction joins two monosaccharides
• This covalent bond is called a glycosidic linkage or glycoside bond
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Maltose
GlucoseGlucose
(a) Dehydration reaction in the synthesis of maltose- a disaccharide
(b) Note position of glycoside bond
1–4glycosidic
linkage
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(a) Dehydration reaction in the synthesis of sucrose-a different disaccharide
(b) Note position of glycoside bond
Glucose Fructose
Sucrose
1–2glycosidic
linkage
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Polysaccharides
• 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
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Storage Polysaccharides
• Starch, a storage polysaccharide of plants, consists entirely of glucose monomers
• Plants store surplus starch as granules within chloroplasts and other plastids
• Alpha 1-4 and 1-6 bonds
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• Glycogen is a storage polysaccharide in animals
• Humans and other vertebrates store glycogen mainly in liver and muscle cells
• Alpha 1-4 and 1-6 bonds
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(b) Glycogen: an animal polysaccharide
Starch
GlycogenAmylose
Chloroplast
(a) Starch: a plant polysaccharide
Amylopectin
Mitochondria Glycogen granules
0.5 µm
1 µm
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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 ()
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(a) Alpha and Beta glucose ring structures
alpha Glucose beta Glucose
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(b) Starch: 1–4 linkage of alpha glucose monomers
(c) Cellulose: 1–4 linkage of beta glucose monomers
Note the different shapes
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• 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
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b Glucosemonomer
Cellulosemolecules
Microfibril
Cellulosemicrofibrilsin a plantcell wall
0.5 µm
10 µm
Cell walls
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Starch is not a good component for structures
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• 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 cellulose
Cows and other organisms have microbes in their gut to hydrolyze cellulose
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• Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods
• Chitin also provides structural support for the cell walls of many fungi
The structureof the chitinmonomer.
Chitin forms theexoskeleton ofarthropods.
Chitin is used to makea strong and flexiblesurgical thread.
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Sample question:
Which of the following is an example of a polysaccharideUsed for energy storage by plants?
(a)Glycogen(b)Chitin(c)Cellulose(d)Amylose(e)Lactose
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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 becausethey consist mostly of hydrocarbons, which have nonpolar covalent bonds
• The most biologically important lipids are fats, phospholipids, and steroids
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Fats (aka triglycerides)
• Fats are constructed from two types of smaller molecules: glycerol and fatty acids
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon
• A fatty acid consists of a carboxyl group attached to a long carbon skeleton
• Fatty acids are “tails”; glycerol is the backbone
• Lightweight energy storage
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Fatty acid(palmitic acid)
Fatty acid contains a hydrocarbon “tail” and a carboxylic acid “head” Dehydration reaction links a fatty acid and glycerol
The head attaches to glycerol” ester bond
Glycerol
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Fat molecule (triglyceride or triacylglycerol)
Ester bond
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• Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fats-the tails cannot interact with water so they try to associate with each other
• Hydrophobic interactions
• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride
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• Fatty acids vary in length (number of carbons) and in the number and locations of double bonds
• Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds
• Unsaturated fatty acids have one or more double bonds
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Saturated fat
Structuralformula of asaturated fatmolecule
Stearic acid, asaturated fattyacid
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Unsaturated fat
Structural formulaof an unsaturatedfat molecule
Oleic acid, anunsaturatedfatty acid
cis doublebond causesbending
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• Fats made from saturated fatty acids are called saturated fats, and are solid at room temperature
• 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
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• The major function of fats is energy storage-fatty acids can be broken down to release energy
• Humans and other mammals store their lipids in adipose cells; plants tend to store lipids in seeds
• Adipose tissue also cushions vital organs and insulates the body
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Phospholipids
• In a phospholipid, two fatty acids and a phosphate group are attached to glycerol
• The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head
Phospholipids have both hydrophobic and hydrophilic characteristics: amphipathic.
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(b) Space-filling model(a) (c)Structural formula Phospholipid symbol
Fatty acids
Hydrophilichead
Hydrophobictails
Choline
Phosphate
Glycerol
Hyd
rop
ho
bic
tai
lsH
ydro
ph
ilic
hea
d
Other molecules can be attached to the phosphate to increase polarity
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In an aqueous environment, phospholipids will arrange with the heads close to water
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Steroids
• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
• 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
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Note card question:
1) Explain the hydrophobic interactions in this diagram
2) Where are the hydrogen bonds in this diagram?