Biology 102 Fall 2001
R. Brundage
Lecture 3
Carbon Compounds in Cells I
Figure 2.12 Examples of Electron Distributions in Atoms
B.Electrons and the Bonding Behavior of Atoms
1.A chemical bond is a union between atoms formed when they give up, gain, or
share electrons.
2.Whether one atoms will bond with another depends on the number and
arrangement of its electrons.
Figure 2.12 Electron Distributions Continued
C.From Atoms to Molecules
1.A molecule is a bonded unit of two or more (same or different) atoms.
2.Compounds are substances in which two or more different elements are
combined in fixed proportions.
3.A mixture contains two or more elements in intermingled proportions
that can vary.
III.Important Bonds in Biological Molecules
Figure 2.13 Ionization
A.Ion Formation and Ionic Bonding
1.When an atom loses or gains one or more electrons, it becomes positively or
negatively charged—an ion.
2.In an ionic bond, (+) and (–) ions are linked by mutual attraction of opposite
charges, for example, NaCl.
Figure 2.13 Salt Crystal
Figure 2.14 Orderly Patterns
B.Covalent Bonding
1.A covalent bond holds together two atoms that share one or more pairs of
electrons.
2.In a nonpolar covalent bond, atoms share electrons equally.
3.In a polar covalent bond, because atoms share the electron unequally,
there is slight difference in charge between the two poles of the
bond; water is an example.
Figure 2.15 Hydrogen Bonding
C.Hydrogen Bonding
1.In a hydrogen bond, an atom or a molecule interacts weakly with a
hydrogen atom already taking part in a polar covalent bond.
2.These bonds impart structure to liquid water and stabilize nucleic acids
and other large molecules.
IV.Properties of Water
Figure 2.16 The Structure of Water
A.Polarity of the Water Molecule
1.Because of the electron arrangements in the water molecule,
a polarity results that allows water to form hydrogen bonds
with one another and other polar substances.
2.Polar substances are hydrophilic (water-loving); nonpolar ones are
hydrophobic (water-dreading) and are repelled by water.
B.Water’s Temperature-Stabilizing Effects
1.Water tends to stabilize temperature because it can absorb
considerable heat before its temperature changes.
2.This is an important property in evaporative and freezing processes.
C.Water’s Cohesion
1.Hydrogen bonding of water molecules provides cohesion
(capacity to resist rupturing).
2.Cohesion imparts surface tension and helps pull water through plants for
example.
Figure 2.18 Water as a Solvent
D.Water’s Solvent Properties
1.Water is a great solvent because ions and polar molecules (solutes)
dissolve in it.
2.The solvent properties of water are greatest with respect to polar
molecules because "spheres of hydration" are formed around
the solute molecules.
V.Acids, Bases, and Buffers
A.The pH Scale
1.pH is a measure of the H+ concentration in a solution;
the greater the H+ the lower the pH scale.
2.The scale extends from 0 (acidic) to 7 (neutral) to 14 (basic).
3.The interior of living cells is near pH = 7.
Figure 2.19 The pH of Common Solutions
B.Acids and Bases
1.A substance that releases hydrogen ions (H+) in solution is
an acid; for example, HCl.
2.Substances that release ions such as OH– (hydroxide ions)
that can combine with hydrogen ions are called bases.
C.Buffers Resist Shifts in pH
1.A buffer system is a partnership between a weak acid and the
base that forms when it dissolves in water.
2.Buffer molecules combine with, or release, H+ to prevent
drastic changes in pH.
3.Carbonic acid is one of the body’s major buffers.
D.Salts
1.A salt is an ionic compound formed when an acid reacts with a base; example:
NaOH + HCl ——> NaCl + H2O.
2.Many salts dissolve into ions that serve key functions in cells; nerve function,
for example, is dependent on ions of sodium, potassium, and calcium.
Carbon in its many forms permeates the entire world of life.
A.Coniferous trees are the premier producers of the great northern forests.
1.They take in large amounts of carbon dioxide during photosynthesis.
2.Carbon dioxide levels in the atmosphere decline during the warm months.
3.An indication that the earth may be getting warmer, earlier is the shift
toward breaking dormancy earlier in the growing season.
I.Properties of Organic Compounds
A.An organic compound consists of carbon and one or more additional elements,
covalently bonded to one another.
B.Effects of Carbon’s Bonding Behavior
1.Oxygen, hydrogen, and carbon are the most abundant elements
in living matter.
2.Much of the H and O are linked as water.
3.Carbon can share pairs of electrons with as many as four other
atoms to form organic molecules of several configurations.
4.A carbon atom can rotate freely around a single covalent bond.
5.A double covalent bond restricts rotation.
6.Such interactions help give rise to the three-dimensional shapes
and functions of biological molecules.
C.Hydrocarbons and Functional Groups
1.In hydrocarbons, only hydrogen atoms are attached to the carbon
backbone; these molecules are quite stable.
2.Functional groups are atoms or groups of atoms covalently bonded to a
carbon backbone; they convey distinct properties, such as solubility and
chemical reactivity, to the complete molecule.
II.How Cells Use Organic Compounds
A.Five Classes of Reactions
1.Enzymes are a special class of proteins that mediate five categories of
reactions:
a.functional-group transfer from one molecule
to another,
b.electron transfer —stripped from one molecule
and given to another,
c.rearrangement of internal bonds converts one
type of organic molecule to another,
d.condensation of two molecules into one,
e.cleavage of one molecule into two.
2.In a condensation reaction, one molecule is stripped of its H+,
another is stripped of its OH–; then the two molecule
fragments join to form a new compound and the H+ and
OH– form water.
3.Hydrolysis is the reverse: one molecule is split by the addition
of H+ and OH– (from water) to the components.
B.The Molecules of Life
1.These molecules are used as an energy source or as
building blocks for the synthesis of macromolecules:
polysaccharides, lipids, proteins, and nucleic acids.
2.They include simple sugars, fatty acids, amino acids, and
nucleotides, all of which have duties of their own as
well as the ability to form larger macromolecules.
III.Carbohydrates
A.The Simple Sugars
1.Monosaccharides—one sugar unit—are the simplest carbohydrates.
2.They are characterized by solubility in water, sweet taste, and
several —OH groups.
3.Ribose and deoxyribose (five-carbon backbones) are building blocks
for nucleic acids.
4.Glucose and fructose (six-carbon backbones) are used in assembling
larger carbohydrates.
5.Other important molecules derived from sugar monomers include
glycerol and vitamin C.
B.Short-Chain Carbohydrates
1.An oligosaccharide is a short chain of two or more sugar monomers.
2.Disaccharides—two sugar units—are the simplest.
a.Lactose (glucose + galactose) is present in milk.
b.Sucrose (glucose + fructose) is a transport form
of sugar used by plants and harvested by
humans for use in food.
c.Maltose (two glucose units) is present in germinating
seeds.
3.Oligosaccharides with three or more sugar monomers are attached
as short side chains to proteins where they participate in
membrane function.
C.Complex Carbohydrates
1.A polysaccharide is a straight or branched chain of hundreds or thousands
of sugar monomers.
2.Starch is a plant storage form of energy, arranged as unbranched coiled
chains, easily hydrolyzed to glucose units.
3.Cellulose is a fiberlike structural material—tough, insoluble—used
in plant cell walls.
4.Glycogen is a highly-branched chain used by animals to store energy
in muscles and liver.
5.Chitin is a specialized polysaccharide with nitrogen attached to the glucose
units; it is used as a structural material in arthropod exoskeletons
and fungal cell walls.
IV.Lipids
A.Lipids are greasy or oily compounds with little tendency to dissolve in water.
1.They can be broken down by hydrolysis reactions.
2.They function in energy storage, membrane structure, and coatings.
B.Fatty Acids
1.A fatty acid is a long chain of mostly carbon and hydrogen atoms
with a —COOH group at one end.
2.When they are part of complex lipids, the fatty acids resemble long,
flexible tails.
a.Unsaturated fats are liquids (oils) at room temperature
because one or more double bonds between the
carbons in the fatty acids permits "kinks" in the tails.
b.Saturated fats (triglycerides) have only single C—C bonds
in their fatty acid tails and are solids at room temperature.
C.Triglycerides (Neutral Fats)
1.These are formed by the attachment of one (mono-), two (di-),
or three (tri-) fatty acids to a glycerol.
2.They are a rich source of energy, yielding more than twice the energy per
weight basis as carbohydrates.
D.Phospholipids
1.They are formed by attachment of two fatty acids plus a phosphate group
to a glycerol.
2.They are the main structural material of membranes where they arrange in
bilayers.
E.Sterols and Their Derivatives
1.Sterols have a backbone of four carbon rings but no fatty acid tails.
2.Cholesterol is a component of cell membranes in animals and can be
modified to form sex hormones (testosterone and estrogen)
and vitamin D.
F.Waxes
1.They are formed by attachment of long-chain fatty acids to long-chain
alcohols or carbon rings.
2.They serve as coatings for plant parts and as animal coverings.