1. Large Molecules are the Hallmark of Life
Chapter 5
Large Molecules are the Hallmark of Life
Figure 5.1 Why do scientists study the structures of macromolecules?

2. 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)

3. Outline
5.1 Polymers
5.2 Carbohydrates
5.3 Lipids
5.4 Proteins
5.5 Nucleic acids

4. 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

5. The Synthesis and Breakdown of Polymers
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

6. Dehydration removes a water molecule, forming a new bond H2O1 2 3 H HO H
Short polymer
(oligomer)
Unlinked monomer
Dehydration removes a water
molecule, forming a new bond
H2O
Figure 5.2 The synthesis and breakdown of polymers HO 1 2 3 4 H
Longer polymer
(a)
Dehydration reaction in the synthesis of a polymer

7. HO 1 2 3 4 H H2O HO 1 2 3 H HO H (b) Hydrolysis adds a water
molecule, breaking a bond
H2O
Figure 5.2 The synthesis and breakdown of polymers HO 1 2 3 H HO H
(b)
Hydrolysis of a polymer

8. 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 H HO

9. 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 H HO

10. 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

11. Glucose (C6H12O6) is the most common monosaccharide
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

12. Some Examples Trioses (C3H6O3) Pentoses (C5H10O5) Hexoses (C6H12O6)
Aldoses
Glyceraldehyde
Figure 5.3 The structure and classification of some monosaccharides
Ribose
Glucose
Galactose

13. Trioses (C3H6O3) Pentoses (C5H10O5) Hexoses (C6H12O6) Ketoses
Dihydroxyacetone
Figure 5.3 The structure and classification of some monosaccharides
Ribulose
Fructose

14. Though often drawn as linear skeletons, in aqueous solutions many sugars form rings

15. (a) Linear and ring forms of glucose
Figure 5.4 Linear and ring forms of glucose
(b) Abbreviated ring structure

16. A disaccharide is formed when a dehydration reaction joins two monosaccharides
This covalent bond is called a glycosidic linkage or glycoside bond

17. Dehydration reaction in the synthesis of maltose- a disaccharide
1–4
glycosidic
linkage
Maltose
Glucose
Glucose
Dehydration reaction in the synthesis of maltose- a disaccharide
Note position of glycoside bond
Figure 5.5 Examples of disaccharide synthesis

18. Note position of glycoside bond
1–2
glycosidic
linkage
Glucose
Fructose
Sucrose
Figure 5.5 Examples of disaccharide synthesis
Dehydration reaction in the synthesis of sucrose-a different disaccharide
Note position of glycoside bond

19. 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

20. 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

21. 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

22. (a) Starch: a plant polysaccharide
Chloroplast
Starch
Mitochondria
Glycogen granules
0.5 µm
1 µm
Figure 5.6 Storage polysaccharides of plants and animals
Amylose
Glycogen
Amylopectin
(a) Starch: a plant polysaccharide
(b) Glycogen: an animal polysaccharide

23. 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 ()

24. (a) Alpha and Beta glucose ring structures
alpha Glucose
beta Glucose
Figure 5.7 Starch and cellulose structures
(a) Alpha and Beta glucose ring structures

25. Note the different shapes
(b) Starch: 1–4 linkage of alpha glucose monomers
Figure 5.7 Starch and cellulose structures
(c) Cellulose: 1–4 linkage of beta glucose monomers
Note the different shapes

26. 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

27. Cell walls Cellulose microfibrils in a plant cell wall Microfibril
molecules
Figure 5.8 The arrangement of cellulose in plant cell walls
Glucose
monomer

28. Starch is not a good component for structures
Figure 5.9 Cellulose-digesting prokaryotes are found in grazing animals such as this cow

29. Some microbes use enzymes to digest cellulose
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

30. Use examples of complex carbohydrates to explain how the shape of a molecule can affect its biological function.

31. Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods
Chitin also provides structural support for the cell walls of many fungi
The structure
of the chitin
monomer.
Chitin forms the
exoskeleton of
arthropods.
Chitin is used to make
a strong and flexible
surgical thread.

32. 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 have nonpolar covalent bonds
The most biologically important lipids are fats, phospholipids, and steroids

33. 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

34. Fatty acid (palmitic acid) Glycerol
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
Figure 5.11 The synthesis and structure of a fat, or triacylglycerol

35. Fat molecule (triglyceride or triacylglycerol)
Ester bond
Figure 5.11 The synthesis and structure of a fat, or triacylglycerol

36. 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

37. 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

38. Structural formula of a saturated fat molecule Stearic acid, a
Figure 5.12 Examples of saturated and unsaturated fats and fatty acids
Stearic acid, a
saturated fatty
acid
Saturated fat

39. unsaturated fatty acid Oleic acid, an cis double Unsaturated fat
Structural formula
of an unsaturated
fat molecule
Oleic acid, an
unsaturated
fatty acid
Figure 5.12 Examples of saturated and unsaturated fats and fatty acids
cis double
bond causes
bending
Unsaturated fat

40. 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

41. 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

42. 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.

43. Other molecules can be attached to the phosphate to increase polarity
Choline
Hydrophilic head
Phosphate
Glycerol
Fatty acids
Hydrophobic tails
Hydrophilic
head
Figure 5.13 The structure of a phospholipid
Hydrophobic
tails
(a)
Structural formula
(b)
Space-filling model
(c)
Phospholipid symbol
Other molecules can be attached to the phosphate to increase polarity

44. In an aqueous environment, phospholipids will arrange with the heads close to water

45. 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
For the Cell Biology Video Space Filling Model of Cholesterol, go to Animation and Video Files.
For the Cell Biology Video Stick Model of Cholesterol, go to Animation and Video Files.

46. Figure 5.15 Cholesterol, a steroid