CHAPTER 3 The Molecules of Life

Biology and Society: Does Thanksgiving Dinner Make You Sleepy? After finishing a huge Thanksgiving dinner, Many people feel especially lethargic and a few even doze off. Many people think that turkey makes you sleepy. Is there a biological basis to this claim? Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Turkey meat is high in tryptophan. Tryptophan is a molecule that is converted in your body to serotonin, which promotes sleep.

Figure 3.1

However, there is little evidence That a turkey dinner encourages sleep more than any other meal.

Organic Molecules A cell is mostly water. The rest of the cell consists mostly of carbon-based molecules. Organic chemistry is the study of carbon compounds.

Carbon is a versatile atom. Carbon Chemistry Carbon is a versatile atom. It has four electrons in an outer shell that holds eight. Carbon can share its electrons with other atoms to form up to four covalent bonds.

Carbon can use its bonds to Attach to other carbons. Form an endless diversity of carbon skeletons.

Figure 3.2

The simplest organic compounds are hydrocarbons. These are organic molecules containing only carbon and hydrogen atoms. The simplest hydrocarbon is methane.

Figure 3.3

The hydrocarbons of fat molecules provide energy for our bodies. Larger hydrocarbons Are the main molecules in the gasoline we burn in our cars. The hydrocarbons of fat molecules provide energy for our bodies.

Figure 3.4

Each type of organic molecule has a unique three-dimensional shape that defines its function in an organism. The molecules of your body recognize one another based on their shapes.

The unique properties of an organic compound depend not only on its carbon skeleton but also on the atoms attached to the skeleton. These atoms are called functional groups.

Figure 3.5

Giant Molecules from Smaller Building Blocks On a molecular scale, many of life’s molecules are gigantic. Biologists call them macromolecules. Examples: DNA, carbohydrates

Most macromolecules are polymers. Polymers are made by stringing together many smaller molecules called monomers. Cells link monomers by dehydration reactions.

Figure 3.6a

Organisms also have to break down macromolecules. Cells do this by a process called hydrolysis.

Figure 3.6b

There are four categories of large molecules in cells: Biological Molecules There are four categories of large molecules in cells: Carbohydrates Lipids Proteins Nucleic acids

Carbohydrates include: Small sugar molecules in soft drinks Long starch molecules in pasta and potatoes

Monosaccharides are simple sugars. Glucose is found in sports drinks. Fructose is found in fruit. Honey contains both glucose and fructose.

Figure 3.7

The monosaccharides glucose and fructose are isomers. They have the same formula, but their atoms are arranged differently. Isomers L-Dopa

Figure 3.8

In aqueous solutions, monosaccharides form rings. Monosaccharides are the main fuel that cells use for cellular work.

Figure 3.9

A disaccharide is a double sugar. Disaccharides A disaccharide is a double sugar. It is constructed from two monosaccharides. Disaccharides are joined through a dehydration reaction. Disaccharides

Figure 3.10

Lactose is another type of disaccharide. Some people have trouble digesting lactose, a condition called lactose intolerance.

Figure 3.11

The most common disaccharide is sucrose, common table sugar. It consists of a glucose linked to a fructose. Sucrose is extracted from sugar cane and the roots of sugar beets.

The United States is one of the world’s leading markets for sweeteners. The average American consumes about 64 kg of sugar per year.

Figure 3.12

Complex carbohydrates are called polysaccharides. They are long chains of sugar units. They are polymers of monosaccharides. Polysaccharides

Figure 3.13

One familiar example of a polysaccharide is starch. Plant cells store starch for energy. Potatoes and grains are major sources of starch in the human diet.

Animals store excess sugar in the form of a polysaccharide called glycogen. Glycogen is similar in structure to starch.

Cellulose is the most abundant organic compound on Earth. It forms cable-like fibrils in the tough walls that enclose plants. It is a major component of wood. It is also known as dietary fiber.

Most animals cannot derive nutrition from fiber. Grazing animals survive on a diet of cellulose because they have prokaryotes in their digestive tracts that can break down cellulose.

Figure 3.14

Simple sugars and double sugars dissolve readily in water. They are hydrophilic, or “water-loving.”

In recent years, “low-carb diets” have become popular. But consumers need to be wary of products boasting that they are “low-carb” because they can sometimes be unhealthy.

Lipids are hydrophobic. They do not mix with water. Examples: fats and steroids Fats

Dietary fat consists largely of the molecule triglyceride. Fats Dietary fat consists largely of the molecule triglyceride. Triglyceride is a combination of glycerol and three fatty acids.

Figure 3.15a

Fats perform essential functions in the human body: Energy storage Cushioning Insulation

Unsaturated fatty acids Have less than the maximum number of hydrogens bonded to the carbons. Saturated fatty acids Have the maximum number of hydrogens bonded to the carbons.

Figure 3.15b

Most plant oils tend to be low in saturated fatty acids. Most animal fats have a high proportion of saturated fatty acids, which can be unhealthy. Example: butter Most plant oils tend to be low in saturated fatty acids. Example: corn oil

Not all fats are unhealthy. Some fats perform important functions in the body and are essential to a healthy diet.

Figure 3.16

Steroids are very different from fats in structure and function. The carbon skeleton is bent to form four fused rings. Cholesterol is the “base steroid” from which your body produces other steroids. Example: sex hormones

Figure 3.17

Some athletes use anabolic steroids to build up their muscles quickly. However, these substances can pose serious health risks.

Figure 3.18

Figure 3.19

Each amino acid consists of The Monomers: Amino Acids All proteins are 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. A side group that is variable among all 20.

Figure 3.20

Cells link amino acids together by dehydration reactions. Proteins as Polymers Cells link amino acids together by dehydration reactions. The resulting bond between them is called a peptide bond.

Figure 3.21

Your body has tens of thousands of different kinds of protein. The arrangement of amino acids makes each one different.

Primary structure The specific sequence of amino acids in a protein

Figure 3.22

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.

Figure 3.23

Figure 3.24

What Determines Protein Structure? A protein’s shape is sensitive to the surrounding environment. Unfavorable temperature and pH changes can cause a protein to unravel and lose its shape. This is called denaturation.

Nucleic acids are information storage molecules. They provide the directions for building proteins.

There are two types of nucleic acids: DNA, deoxyribonucleic acid RNA, ribonucleic acid

The genetic instructions in DNA Must be translated from “nucleic acid language” to “protein language.”

Figure 3.25

Nucleic acids are polymers of nucleotides.

Figure 3.26

Each DNA nucleotide has one of the following bases: Adenine (A) Guanine (G) Thymine (T) Cytosine (C)

Figure 3.27

Nucleotide monomers are linked into long chains. These chains are called polynucleotides, or DNA strands. A sugar-phosphate backbone joins them together. DNA and RNA Structure

Figure 3.28a

Two strands of DNA join together to form a double helix.

Figure 3.28b

RNA, ribonucleic acid, is different from DNA. Its sugar has an extra OH group. It has the base uracil (U) instead of thymine (T).

Figure 3.29

Evolution Connection: DNA and Proteins as Evolutionary Tape Measures Evolutionary relationships between organisms can be assessed. Molecular genealogy extends to relationships between species. Biologists use molecular analysis of DNA and protein sequences for testing evolutionary hypotheses. Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Figure 3.30