CHAPTER 3CHAPTER 3

The 4 Macromolecules of Life

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 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.

• Each type of organic molecule has a unique 3D shape that defines its function in an organism.

– The molecules of your body recognize one another based on their shapes.

• A compound’s properties depend on both the shape and 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 (or condensation) reactions.

• Organisms also have to break down macromolecules.

– Cells do this by a process called hydrolysis.

Figure 3.6a

Figure 3.6b

• There are four categories of large molecules in cells:

– Carbohydrates

– Lipids

– Proteins

– Nucleic acids

Carbohydrates

• Carbohydrates include:

– Small sugar molecules in soft drinks

– Long starch molecules in pasta and potatoes

Monosaccharides

• Monosaccharides are simple sugars.

– Glucose is found in sports drinks.

– Fructose is found in fruit.

• Honey contains both glucose and fructose.

• The monosaccharides glucose and fructose are isomers.

– They have the same formula, but their atoms are arranged differently.

• Monosaccharides are the main fuel that cells use for cellular work.

Disaccharides

• A disaccharide is a double sugar.

– It is constructed from two monosaccharides.

• Disaccharides are joined through a dehydration reaction. (Condensation)

Disaccharides

• Lactose is another type of disaccharide.

– Some people have trouble digesting lactose, a condition called lactose intolerance.

• 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.

• Simple sugars and double sugars dissolve readily in water.

– They are hydrophilic, or “water-loving.”

Polysaccharides

• Complex carbohydrates are called polysaccharides.

– They are long chains of sugar units.

– They are polymers of monosaccharides.

Polysaccharides

• 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.

• There are four categories of large molecules in cells:

– Carbohydrates

– Lipids

– Proteins

– Nucleic acids

Lipids

• Lipids are hydrophobic.

– They do not mix with water.

– Examples: fats and steroids

Fats

Fats

• Dietary fat consists largely of the molecule triglyceride.

– Triglyceride is a combination of glycerol and three fatty acids.

• Fats perform essential functions in the human body:

– Energy storage

– Cushioning

– Insulation

• Unsaturated fatty acids = double bond– Have less than the maximum number of hydrogens bonded to the carbons.

• Saturated fatty acids = single bond– Have the maximum number of hydrogens bonded to the carbons.

Figure 3.15b

• 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 (or high in unsaturated) fatty acids.

– Example: corn oil

• Not all fats are unhealthy. MUST have some fat.

Steroids

• 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.

• There are four categories of large molecules in cells:

– Carbohydrates

– Lipids

– Proteins

– Nucleic acids

Proteins

• A protein is a polymer constructed from amino acid monomers.

• Proteins perform most of the tasks the body needs to function.

Structural Proteins

Storage Proteins

Contractile Proteins

Transport Proteins

Defensive Proteins

Receptor Proteins

Enzymes

Hormonal Proteins

Sensory Proteins

Gene Regulatory Proteins

Figure 3.19

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

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

• 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.

Protein Shape

• Proteins have four levels of structure.

– Transformers

• Plastic, paint, metal, rubber = primary

• Wheels, door, engine = secondary

• Car = tertiary

• Megadude = quartinary

• DEVASTATOR!

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.

• There are four categories of large molecules in cells:

– Carbohydrates

– Lipids

– Proteins

– Nucleic acids

Nucleic Acids

• 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

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.

• Nucleic acids are polymers of nucleotides.

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

Figure 3.28a

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)

– Generally single stranded

Figure 3.29

Evolution Connection:DNA and Proteins as Evolutionary Tape Measures

Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Structures in cells are made of macromolecules

Cell Membrane = PhosphoLIPID bilayer

Also called the plasma membrane

Phospholipid bilayer

-Hydrophilic head

-Hydrophobic tails

-Proteins

All can rotate and move in the membrane

-Fluid Mosaic Model

-Extracellular Matrix (ECM)

holds cells together, protection, support

Figure 5.11  Primary functions of membrane proteins. 

• Attachment to the cytoskeleton and extracellular matrix. Elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and fixes the location of certain membrane proteins. Proteins that adhere to the fibers of the extracellular matrix can coordinate extracellular and intracellular changes.

• Cell signaling. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a change in the protein that relays the message to the inside of the cell.

• Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out the sequential steps of a metabolic pathway.

• Transport. A protein that spans the membrane may provide a channel across the membrane that is selective for a particular solute.

• Intercellular joining. Membrane proteins of adjacent cells may be hooked together to form various kinds of junctions. Cell-cell recognition. Some proteins with short chains of sugars serve as identification tags that are specifically recognized by other cells.