Macromolecules

Overview: The Molecules of Life

• Within cells, small organic molecules are joined together to form larger molecules.
• These large macromolecules may consist of thousands of covalently bonded atoms and weigh more than 100,000 daltons.
• The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

Concept 5.1 Most macromolecules are polymers, built from monomers

• Three of the four classes of macromolecules—carbohydrates, proteins, and nucleic acids—form chainlike molecules called polymers.

  • A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds.
  • The repeated units are small molecules called monomers.
  • Some of the molecules that serve as monomers have other functions of their own.

• The chemical mechanisms that cells use to make and break polymers are similar for all classes of macromolecules.

• Monomers are connected by covalent bonds that form through the loss of a water molecule. This reaction is called a condensation reaction or dehydration reaction.

  • When a bond forms between two monomers, each monomer contributes part of the water molecule that is lost. One monomer provides a hydroxyl group (—OH), while the other provides a hydrogen (—H).
  • Cells invest energy to carry out dehydration reactions.
  • The process is aided by enzymes.

• The covalent bonds connecting monomers in a polymer are disassembled by hydrolysis, a reaction that is effectively the reverse of dehydration.

  • In hydrolysis, bonds are broken by the addition of water molecules. A hydrogen atom attaches to one monomer, and a hydroxyl group attaches to the adjacent monomer.
  • Our food is taken in as organic polymers that are too large for our cells to absorb. Within the digestive tract, various enzymes direct hydrolysis of specific polymers. The resulting monomers are absorbed by the cells lining the gut and transported to the bloodstream for distribution to body cells.
  • The body cells then use dehydration reaction to assemble the monomers into new polymers that carry out functions specific to the particular cell type.

  An immense variety of polymers can be built from a small set of monomers.

• Each cell has thousands of different kinds of macromolecules.

  • These molecules vary among cells of the same individual. They vary more among unrelated individuals of a species, and even more between species.

• This diversity comes from various combinations of the 40–50 common monomers and some others that occur rarely.

  • These monomers can be connected in a great many combinations, just as the 26 letters in the alphabet can be used to create a great diversity of words.

Concept 5.2 Carbohydrates serve as fuel and building material

• Carbohydrates include sugars and their polymers.

• The simplest carbohydrates are monosaccharides, or simple sugars.

• Disaccharides, or double sugars, consist of two monosaccharides joined by a condensation reaction.

• Polysaccharides are polymers of many monosaccharides.

  Sugars, the smallest carbohydrates, serve as fuel and a source of carbon.

• Monosaccharides generally have molecular formulas that are some multiple of the unit CH2O.

  • For example, glucose has the formula C6H12O6.

• Monosaccharides have a carbonyl group (>C=O) and multiple hydroxyl groups (—OH).

  • Depending on the location of the carbonyl group, the sugar is an aldose or a ketose.
  • Most names for sugars end in -ose.
  • Glucose, an aldose, and fructose, a ketose, are structural isomers.

• Monosaccharides are also classified by the number of carbons in the carbon skeleton.

  • Glucose and other six-carbon sugars are hexoses.
  • Five-carbon backbones are pentoses; three-carbon sugars are trioses.

• Monosaccharides may also exist as enantiomers.

  • For example, glucose and galactose, both six-carbon aldoses, differ in the spatial arrangement of their parts around asymmetrical carbons.

• Monosaccharides, particularly glucose, are a major fuel for cellular work.

• They also function as the raw material for the synthesis of other monomers, such as amino acids and fatty acids.

• While often drawn as a linear skeleton, monosaccharides in aqueous solutions form rings.

• Two monosaccharides can join with a glycosidic linkage to form a disaccharide via dehydration.

  • Maltose, malt sugar, is formed by joining two glucose molecules.
  • Sucrose, table sugar, is formed by joining glucose and fructose. Sucrose is the major transport form of sugars in plants.
  • Lactose, milk sugar, is formed by joining glucose and galactose.

  Polysaccharides, the polymers of sugars, have storage and structural roles.

• Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages.

• Some polysaccharides serve for storage and are hydrolyzed as sugars are needed.

• Other polysaccharides serve as building materials for the cell or the whole organism.

• Starch is a storage polysaccharide composed entirely of glucose monomers.

  • Most of these monomers are joined by 1–4 linkages (number 1 carbon to number 4 carbon) between the glucose molecules.
  • The simplest form of starch, amylose, is unbranched and forms a helix.
  • Branched forms such as amylopectin are more complex.

• Plants store surplus glucose as starch granules within plastids, including chloroplasts, and withdraw it as needed for energy or carbon.

  • Animals that feed on plants, especially parts rich in starch, have digestive enzymes that can hydrolyze starch to glucose.

• Animals store glucose in a polysaccharide called glycogen.

  • Glycogen is highly branched like amylopectin.
  • Humans and other vertebrates store a day’s supply of glycogen in the liver and muscles.

• Cellulose is a major component of the tough wall of plant cells.

  • Plants produce almost one hundred billion tons of cellulose per year. It is the most abundant organic compound on Earth.

• Like starch, cellulose is a polymer of glucose. However, the glycosidic linkages in these two polymers differ.

  • The difference is based on the fact that there are actually two slightly different ring structures for glucose.
  • These two ring forms differ in whether the hydroxyl group attached to the number 1 carbon is fixed above (beta glucose) or below (alpha glucose) the plane of the ring.

• Starch is a polysaccharide of alpha glucose monomers.

• Cellulose is a polysaccharide of beta glucose monomers, making every other glucose monomer upside down with respect to its neighbors.

• The differing glycosidic links in starch and cellulose give the two molecules distinct three-dimensional shapes.

  • While polymers built with alpha glucose form helical structures, polymers built with beta glucose form straight structures.
  • The straight structures built with beta glucose allow H atoms on one strand to form hydrogen bonds with OH groups on other strands.
  • In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils, which form strong building materials for plants (and for humans, as lumber).

• The enzymes that digest starch by hydrolyzing its alpha linkages cannot hydrolyze the beta linkages in cellulose.

  • Cellulose in human food passes through the digestive tract and is eliminated in feces as “insoluble fiber.”
  • As it travels through the digestive tract, cellulose abrades the intestinal walls and stimulates the secretion of mucus, aiding in the passage of food.

• Some microbes can digest cellulose to its glucose monomers through the use of cellulase enzymes.

• Many eukaryotic herbivores, from cows to termites, have symbiotic relationships with cellulolytic microbes, providing the microbe and the host animal access to a rich source of energy.

  • Some fungi can also digest cellulose.

• Another important structural polysaccharide is chitin, used in the exoskeletons of arthropods (including insects, spiders, and crustaceans).

  • Chitin is similar to cellulose, except that it contains a nitrogen-containing appendage on each glucose monomer.
  • Pure chitin is leathery but can be hardened by the addition of calcium carbonate.

• Chitin also provides structural support for the cell walls of many fungi.

Concept 5.3 Lipids are a diverse group of hydrophobic molecules

• Unlike other macromolecules, lipids do not form polymers.

• The unifying feature of lipids is that they all have little or no affinity for water.

• This is because they consist mostly of hydrocarbons, which form nonpolar covalent bonds.

• Lipids are highly diverse in form and function.

  Fats store large amounts of energy.

• Although fats are not strictly polymers, they are large molecules assembled from smaller molecules by dehydration reactions.

• A fat is constructed from two kinds 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, often 16 to 18 carbons long.
  • The many nonpolar C—H bonds in the long hydrocarbon skeleton make fats hydrophobic.
  • Fats separate from water because the water molecules hydrogen bond to one another and exclude the fats.

• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride.

• The three fatty acids in a fat can be the same or different.

• Fatty acids may vary in length (number of carbons) and in the number and locations of double bonds.

  • If the fatty acid has no carbon-carbon double bonds, then the molecule is a saturated fatty acid, saturated with hydrogens at every possible position.
  • If the fatty acid has one or more carbon-carbon double bonds formed by the removal of hydrogen atoms from the carbon skeleton, then the molecule is an unsaturated fatty acid.

• A saturated fatty acid is a straight chain, but an unsaturated fatty acid has a kink wherever there is a double bond.

• Fats made from saturated fatty acids are saturated fats.

  • Most animal fats are saturated.
  • Saturated fats are solid at room temperature.

• Fats made from unsaturated fatty acids are unsaturated fats.

  • Plant and fish fats are liquid at room temperature and are known as oils.
  • The kinks caused by the double bonds prevent the molecules from packing tightly enough to solidify at room temperature.
  • The phrase “hydrogenated vegetable oils” on food labels means that unsaturated fats have been synthetically converted to saturated fats by the addition of hydrogen.
  • Peanut butter and margarine are hydrogenated to prevent lipids from separating out as oil.
  • A diet rich in saturated fats may contribute to cardiovascular disease (atherosclerosis) through plaque deposits.
  • The process of hydrogenating vegetable oils produces saturated fats and also unsaturated fats with trans double bonds. These trans fat molecules contribute more than saturated fats to atherosclerosis.

• The major function of fats is energy storage.

  • A gram of fat stores more than twice as much energy as a gram of a polysaccharide such as starch.
  • Because plants are immobile, they can function with bulky energy storage in the form of starch. Plants use oils when dispersal and compact storage is important, as in seeds.
  • Animals must carry their energy stores with them and benefit from having a more compact fuel reservoir of fat.
  • Humans and other mammals store fats as long-term energy reserves in adipose cells that swell and shrink as fat is deposited or withdrawn from storage.

• Adipose tissue also functions to cushion vital organs, such as the kidneys.

• A layer of fat can also function as insulation.

  • This subcutaneous layer is especially thick in whales, seals, and most other marine mammals.

  Phospholipids are major components of cell membranes.

• Phospholipids have two fatty acids attached to glycerol and a phosphate group at the third position.

  • The phosphate group carries a negative charge.
  • Additional smaller groups may be attached to the phosphate group to form a variety of phospholipids.

• The interaction of phospholipids with water is complex.

  • The fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head.

• When phospholipids are added to water, they self-assemble into assemblages with the hydrophobic tails pointing toward the interior.

  • This type of structure is called a micelle.

• Phospholipids are arranged as a bilayer at the surface of a cell.

  • Again, the hydrophilic heads are on the outside of the bilayer, in contact with the aqueous solution, and the hydrophobic tails point toward the interior of the bilayer.
  • The phospholipid bilayer forms a barrier between the cell and the external environment.
  • Phospholipids are the major component of all cell membranes.

  Steroids include cholesterol and certain hormones.

• Steroids are lipids with a carbon skeleton consisting of four fused rings.

• Different steroids are created by varying functional groups attached to the rings.

• Cholesterol, an important steroid, is a component in animal cell membranes.

• Cholesterol is also the precursor from which all other steroids are synthesized.

  • Many of these other steroids are hormones, including the vertebrate sex hormones.

• While cholesterol is an essential molecule in animals, high levels of cholesterol in the blood may contribute to cardiovascular disease.

• Both saturated fats and trans fats exert their negative impact on health by affecting cholesterol levels.

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