Chapter 3

Biology and Society: Got Lactose?

            Lactose is the main sugar found in milk.

            Lactose intolerance is the inability to properly digest lactose.

           Instead of lactose being broken down and absorbed in the small intestine,

           lactose is broken down by bacteria in the large intestine, producing gas and discomfort.

            Lactose intolerance can be addressed by

           avoiding foods with lactose or

           consuming lactase pills along with food.

ORGANIC COMPOUNDS

           A cell is mostly water.

           The rest of the cell consists mainly of carbon-based molecules.

           Carbon forms large, complex, and diverse molecules necessary for lifes functions.

           Organic compounds are carbon-based molecules.

Carbon Chemistry

           Carbon is a versatile atom.

           It has four electrons in an outer shell that holds eight electrons.

           Carbon can share its electrons with other atoms to form up to four covalent bonds.

Carbon Chemistry

           Carbon can use its bonds to

           attach to other carbons and

           form an endless diversity of carbon skeletons varying in size and branching pattern.

           The simplest organic compounds are hydrocarbons, which contain only carbon and hydrogen atoms.

           The simplest hydrocarbon is methane, a single carbon atom bonded to four hydrogen atoms.

           Larger hydrocarbons form fuels for engines.

           Hydrocarbons of fat molecules are important fuels for our bodies.

           Each type of organic molecule has a unique three-dimensional shape.

           The shapes of organic molecules relate to their functions.

           The unique properties of an organic compound depend on

      its carbon skeleton and

      the atoms attached to the skeleton.

           The groups of atoms that usually participate in chemical reactions are called functional groups. Two common examples are

           hydroxyl groups (-OH) and

           carboxyl groups (-COOH).

           Many biological molecules have two or more functional groups.

Giant Molecules from Smaller Building Blocks

           On a molecular scale, many of lifes molecules are gigantic, earning the name macromolecules.

           Three categories of macromolecules are

             carbohydrates,

             proteins, and

             nucleic acids.

           Most macromolecules are polymers.

           Polymers are made by stringing together many smaller molecules called monomers.

           A dehydration reaction

           links two monomers together and

           removes a molecule of water.

LARGE BIOLOGICAL MOLECULES

           There are four categories of large biological molecules:

           carbohydrates,

           lipids,

           proteins, and

           nucleic acids.

Carbohydrates

           Carbohydrates include sugars and polymers of sugar. They include

           small sugar molecules in energy drinks and

           long starch molecules in spaghetti and French fries.

Carbohydrates

           In animals, carbohydrates are

           a primary source of dietary energy and

           raw material for manufacturing other kinds of organic compounds.

           In plants, carbohydrates serve as a building material for much of the plant body.

Monosaccharides

           Monosaccharides are

           simple sugars that cannot be broken down by hydrolysis into smaller sugars and

           the monomers of carbohydrates.

           Common examples are

           glucose in sports drinks and

           fructose found in fruit.

Monosaccharides

           Both glucose and fructose are found in honey.

           Glucose and fructose are isomers, molecules that have the same molecular formula but different structures.

           Monosaccharides are the main fuels for cellular work.

           In water, many monosaccharides form rings.

Disaccharides

           A disaccharide is

           a double sugar,

           constructed from two monosaccharides, and

           formed by a dehydration reaction.

           Disaccharides include

           lactose in milk,

           maltose in beer, malted milk shakes, and malted milk ball candy, and

           sucrose in table sugar.

           Sucrose is

           the main carbohydrate in plant sap and

           rarely used as a sweetener in processed foods in the United States.

           High-fructose corn syrup is made by a commercial process that converts

           natural glucose in corn syrup to

           much sweeter fructose.

           The United States is one of the worlds leading markets for sweeteners.

           The average American consumes

           about 45 kg of sugar (about 100 lb) per year,

           mainly as sucrose and high-fructose corn syrup.

Polysaccharides

           Polysaccharides are

           complex carbohydrates

           made of long chains of sugar units—polymers of monosaccharides.

           Starch

           is a familiar example of a polysaccharide,

           is used by plant cells to store energy, and

           consists of long strings of glucose monomers.

           Potatoes and grains are major sources of starch in our diet.

           Glycogen is

           used by animals cells to store energy and

           converted to glucose when it is needed.

           Cellulose

           is the most abundant organic compound on Earth,

           forms cable-like fibrils in the walls that enclose plant cells, and

           cannot be broken apart by most animals.

           Monosaccharides and disaccharides dissolve readily in water.

           Cellulose does not dissolve in water.

           Almost all carbohydrates are hydrophilic, or water-loving, adhering water to their surface.

Lipids

             Lipids are

           neither macromolecules nor polymers and

           hydrophobic, unable to mix with water.

Fats

           A typical fat, or triglyceride, consists of

           a glycerol molecule,

           joined with three fatty acid molecules,

           via a dehydration reaction.

           Fats perform essential functions in the human body including

           energy storage,

           cushioning, and

           insulation.

           If the carbon skeleton of a fatty acid

           has fewer than the maximum number of hydrogens, it is unsaturated;

           if it has the maximum number of hydrogens,
it is saturated.

           A saturated fat has

           no double bonds and

           all three of its fatty acids saturated.

           Most animal fats

           have a high proportion of saturated fatty acids,

           can easily stack, tending to be solid at room temperature, and

           contribute to atherosclerosis, in which lipid-containing plaques build up along the inside walls of blood vessels.

           Most plant and fish oils tend to be

           high in unsaturated fatty acids and

           liquid at room temperature.

           Hydrogenation

           adds hydrogen,

           converts unsaturated fats to saturated fats,

           makes liquid fats solid at room temperature, and

           creates trans fat, a type of unsaturated fat that is particularly bad for your health.

Steroids

           Steroids are very different from fats in structure and function.

           The carbon skeleton is bent to form four fused rings.

           Steroids vary in the functional groups attached to this set of rings, and these chemical variations affect their function.

Steroids

           Cholesterol is

           a key component of cell membranes and

           the base steroid from which your body produces other steroids, such as estrogen and testosterone.

           Synthetic anabolic steroids

           are variants of testosterone,

           mimic some of its effects,

           can cause serious physical and mental problems,

           may be prescribed to treat diseases such as cancer and AIDS, and

           are abused by athletes to enhance performance.

           Most athletic organizations now ban the use of anabolic steroids because of their

           health hazards and

           unfairness, by providing an artificial advantage.

Proteins

           Proteins

           are polymers constructed from amino acid monomers,

           account for more than 50% of the dry weight of most cells,

           perform most of the tasks required for life, and

           form enzymes, chemicals that change the rate
of a chemical reaction without being changed
in the process.

The Monomers of Proteins: Amino Acids

           All proteins are macromolecules constructed from a common set of 20 kinds of amino acids.

           Each amino acid consists of a central carbon atom bonded to four covalent partners.

           Three of those attachment groups are common to all amino acids:

           a carboxyl group (-COOH),

           an amino group (-NH2), and

           a hydrogen atom.

Proteins as Polymers

           Cells link amino acids together

           by dehydration reactions,

           forming peptide bonds, and

           creating long chains of amino acids called polypeptides.

           Your body has tens of thousands of different kinds of protein.

           Proteins differ in their arrangement of amino acids.

           The specific sequence of amino acids in a protein is its primary structure.

           A slight change in the primary structure of a protein affects its ability to function.

           The substitution of one amino acid for another in hemoglobin causes sickle-cell disease, an inherited blood disorder.

Protein Shape

           A functional protein consists of

           one or more polypeptide chains,

           precisely twisted, folded, and coiled into a molecule of unique shape.

Protein Shape

           Proteins consisting of one polypeptide have three levels of structure.

           Proteins consisting of more than one polypeptide chain have a fourth level, quaternary structure.

           A proteins three-dimensional shape

           typically recognizes and binds to another molecule and

           enables the protein to carry out its specific function in a cell.

What Determines Protein Shape?

           A proteins shape is sensitive to the surrounding environment.

           An unfavorable change in temperature and/or pH can cause denaturation of a protein, in which it unravels and loses its shape.

           High fevers (above 104F) in humans can cause some proteins to denature.

           Misfolded proteins are associated with

           Alzheimers disease,

           mad cow disease, and

           Parkinsons disease.

Nucleic Acids

           Nucleic acids are macromolecules that

           store information,

           provide the directions for building proteins, and

           include DNA and RNA.

           DNA resides in cells in long fibers called chromosomes.

           A gene is a specific stretch of DNA that programs the amino acid sequence of a polypeptide.

           The chemical code of DNA must be translated from nucleic acid language to protein language.

            Nucleic acids are polymers made from monomers called nucleotides.

            Each nucleotide has three parts:

             a five-carbon sugar,

             a phosphate group, and

             a nitrogen-containing base.

           Each DNA nucleotide has one of four possible nitrogenous bases:

           adenine (A),

           guanine (G),

           thymine (T), or

           cytosine (C).

            Dehydration reactions

           link nucleotide monomers into long chains called polynucleotides,

           form covalent bonds between the sugar of one nucleotide and the phosphate of the next, and

           form a sugar-phosphate backbone.

            Nitrogenous bases hang off the sugar-phosphate backbone.

           RNA, ribonucleic acid, is different from DNA.

           RNA uses the sugar ribose and the base uracil (U) instead of thymine (T).

           RNA is usually single-stranded, but DNA usually exists as a double helix.

The Process of Science: Does Lactose Intolerance Have a Genetic Basis?

           Observation: Most lactose-intolerant people have a normal version of the lactase gene.

           Question: What is the genetic basis for lactose intolerance?

           Hypothesis: Lactose-intolerant people have a mutation but not within the lactase gene.

           Prediction: A mutation would be found near the lactase gene.

           Experiment: Genes of 196 lactose-intolerant people were examined.

           Results: Researchers found a 100% correlation between lactose intolerance and a nucleotide at a site approximately 14,000 nucleotides away from the lactase gene.

Evolution Connection: The Evolution of Lactose Intolerance in Humans

           Most people are lactose-intolerant as adults.

           Lactose intolerance is found in

           80% of African Americans and Native Americans,

           90% of Asian Americans, but

           only 10% of Americans of northern European descent.

           Lactose tolerance appears to have evolved in northern European cultures that relied upon dairy products.

           Ethnic groups in East Africa that rely upon dairy products are also lactose tolerant but due to different mutations.