Ch 3 Carbon and the Molecular Diversity of Life

Overview: Carbon Compounds and Life
•	Aside from water, living organisms consist mostly of carbon-based compounds
•	Carbon is unparalleled in its ability to form large, complex, and diverse molecules
•	A compound containing carbon is said to be an organic compound
•	Critically important molecules of all living things fall into four main classes
•	Carbohydrates
•	Lipids
•	Proteins
•	Nucleic acids
•	The first three of these can form huge molecules called macromolecules

Concept 1.1: Carbon atoms can form diverse molecules by bonding to four other atoms

The Formation of Bonds with Carbon
•	With four valence electrons, carbon can form four covalent bonds with a variety of atoms
•	This ability makes large, complex molecules possible
•	In molecules with multiple carbons, each carbon bonded to four other atoms has a tetrahedral shape
•	However, when two carbon atoms are joined by a double bond, the atoms joined to the carbons are in the same plane as the carbons
•	Figure 3.2
•	The electron configuration of carbon gives it covalent compatibility with many different elements
•	The valences of carbon and its most frequent partners (hydrogen, oxygen, and nitrogen) are the “building code” that governs the architecture of living molecules

Molecular Diversity Arising from Variation in Carbon Skeletons 
•	Carbon chains form the skeletons of most organic molecules
•	Carbon chains vary in length and shape
•	Figure 3.4
•	Hydrocarbons are organic molecules consisting of only carbon and hydrogen
•	Many organic molecules, such as fats, have hydrocarbon components
•	Hydrocarbons can undergo reactions that release a large amount of energy

The Chemical Groups Most Important to Life
•	Functional groups are the components of organic molecules that are most commonly involved in chemical reactions
•	The number and arrangement of functional groups give each molecule its unique properties
•	Figure 3.5

ATP: An Important Source of Energy for Cellular Processes
•	One organic phosphate molecule, adenosine triphosphate (ATP), is the primary energy-transferring molecule in the cell 
•	ATP consists of an organic molecule called adenosine attached to a string of three phosphate groups

Concept 3.2: Macromolecules are polymers, built from monomers
•	A polymer is a long molecule consisting of many similar building blocks 
•	These small building-block molecules are called monomers
•	Some molecules that serve as monomers also have other functions of their own

The Synthesis and Breakdown of Polymers
•	Cells make and break down polymers by the same process
•	A dehydration reaction occurs when two monomers bond together through the loss of a water molecule
•	Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction
•	These processes are facilitated by enzymes, which speed up chemical reactions
•	Figure 3.6

Concept 3.3: Carbohydrates serve as fuel and building material
•	Carbohydrates include sugars and the polymers of sugars
•	The simplest carbohydrates are monosaccharides, or simple sugars
•	Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks 

Sugars
•	Monosaccharides have molecular formulas that are usually multiples of CH2O
•	Glucose (C6H12O6) is the most common monosaccharide
•	Monosaccharides serve as a major fuel for cells and as raw material for building molecules
•	Figure 3.7
•	A disaccharide is formed when a dehydration reaction joins two monosaccharides 
•	This covalent bond is called a glycosidic linkage

Storage Polysaccharides
•	Starch, a storage polysaccharide of plants, consists entirely of glucose monomers
•	Plants store surplus starch as granules
•	Glycogen is a storage polysaccharide in animals
•	Humans and other vertebrates store glycogen mainly in liver and muscle cells

Structural Polysaccharides
•	The polysaccharide cellulose is a major component of the tough wall of plant cells
•	In straight structures (cellulose), H atoms on one strand can form hydrogen bonds with OH groups on other strands
•	Parallel cellulose molecules held together this way are grouped into microfibrils, which form strong building materials for plants

Concept 3.4: Lipids are a diverse group of hydrophobic molecules
•	Lipids do not form true polymers
•	Lipids are hydrophobic because they consist mostly 
of hydrocarbons, which form nonpolar covalent bonds
•	The most biologically important lipids are fats, phospholipids, and steroids

Fats
•	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
•	Held together by an Ester Linkage
•	Figure 3.12
•	Fats separate from water because water molecules hydrogen-bond to each other and exclude the fats
•	In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride
•	Figure 3.12b

•	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

Fat Functions
•	The major function of fats is energy storage
•	Insulation
•	Protection of Organs
•	Waterproof covering (Phospholipids)
•	Hormones (Steroids)

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 are major constituents of cell membranes
•	Figure 3.14cd

Steroids
•	Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
•	Different steroids are distinguished by the specific chemical group attached. Below is Cholesterol 

Concept 3.5: Proteins include a diversity of structures, resulting in a wide range of functions
•	Proteins account for more than 50% of the dry mass of most cells
•	Protein functions include
•	 defense, 
•	storage, 
•	transport,
•	 cellular communication,
•	 movement, and 
•	structural support

Structure of a Protein
•	A protein is a biologically functional molecule that consists of one or more polypeptides

Protein Structure and Function
•	A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape

•	The sequence of amino acids, determined genetically, leads to a protein’s three-dimensional structure
•	A protein’s structure determines its function

4 Levels of Protein Structure
•	The primary structure of a protein is its unique sequence of amino acids
•	Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain
•	Tertiary structure is determined by interactions among various side chains (R groups)
•	Quaternary structure results from interactions between multiple polypeptide chains
•	Figure 3.21

What Determines Protein Structure?
•	In addition to primary structure, physical and chemical conditions can affect structure
•	Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
•	This loss of a protein’s native structure is called denaturation
•	A denatured protein is biologically inactive
•	Figure 3.23-3

Protein Folding in the Cell
•	It is hard to predict a protein’s structure from its primary structure
•	Most proteins probably go through several intermediate structures on their way to their final, stable shape
•	Scientists use X-ray crystallography to determine 
3-D protein structure based on diffractions of an 
X-ray beam by atoms of the crystalized molecule

Concept 3.6: Nucleic acids store, transmit, and help express hereditary information
•	The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene
•	Genes are made of DNA, a nucleic acid made of monomers called nucleotides

The Roles of Nucleic Acids
•	There are two types of nucleic acids
•	Deoxyribonucleic acid (DNA)
•	Ribonucleic acid (RNA)
•	DNA provides directions for its own replication
•	DNA directs synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis
•	Figure 3.25-3

The Components of Nucleic Acids
•	Nucleic acids are polymers called polynucleotides
•	Each polynucleotide is made of monomers called nucleotides
•	Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups
•	The portion of a nucleotide without the phosphate group is called a nucleoside
•	Figure 3.26a

The Structures of DNA and RNA Molecules
•	RNA molecules usually exist as single polypeptide chains 
•	DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a?
•	In the DNA double helix, the two backbones run in opposite 5→ 3 directions from each other, an arrangement referred to as antiparallel
•	One DNA molecule includes many genes (hereditary information
•	Figure 3.26