Bài giảng The chemical basis of life

Chapter 2 
The Chemical Basis of Life 
Nature's Chemical Language 
Chemicals play an important role in all organisms 
The rattlebox moth provides a good example of chemicals used in mating and defense 
ELEMENTS, ATOMS, AND MOLECULES 
2.1 Living organisms are composed of about 25 chemical elements 
Elements are the basic chemical units that cannot be broken apart by typical chemical processes 
There are 92 naturally occurring elements 
25 are required by living organisms 
4 make up 96.3 of the human body 
CONNECTION 
2.2 Trace elements are common additives to food and water 
Trace elements are essential in minute quantities for proper biological functioning 
Example: iodine is a trace element that prevents goiter 
Many foods are fortified with trace elements and vitamins (which consist of two or more elements) 
2.3 Elements can combine to form compounds 
Compounds contain two or more elements in a fixed ratio 
Different arrangements of the atoms of elements determine the unique properties of each compound 
The smallest unit of an element is an atom 
LE 2-3 
Sodium 
Chlorine 
Sodium Chloride 
2.4 Atoms consist of protons, neutrons, and electrons 
Subatomic particles 
Protons and neutrons occupy the central region (nucleus) of an atom 
A proton has a single positive charge 
A neutron is electrically neutral 
Electrons surround the nucleus 
An electron has a single negative charge 
LE 2-4a 
2 
2 
2 
Protons 
Neutrons 
Electrons 
Helium atom 
Mass 
number = 4 
6 
6 
6 
Protons 
Neutrons 
Electrons 
Carbon atom 
Mass 
number = 12 
Electron 
cloud 
Nucleus 
2e – 
6e – 
Differences in Elements 
All the atoms of a particular element have the same number of protons 
The number of protons-the atomic number-defines the element's unique properties 
An atom's mass number (atomic mass) is the sum of its protons and neutrons 
Isotopes 
Isotopes are atoms with the same atomic number but different atomic masses (different number of neutrons) 
CONNECTION 
2.5 Radioactive isotopes can help or harm us 
Radioactive isotopes are useful as tracers to study the fate of elements and molecules in living systems 
Radioactive tracer isotopes are often used in combination with sophisticated imaging instruments for medical diagnosis 
Uncontrolled exposure to radioactive material can harm living organisms 
2.6 Electron arrangement determines the chemical properties of an atom 
Electrons in an atom are arranged in electron shells, which may contain different numbers of electrons 
The chemical reactivity of an atom depends on the number of electrons in the outer shell 
Atoms whose outer shells are not full share or transfer electrons to other atoms, forming molecules 
Two major types of chemical bonds between atoms form compounds 
Ionic bonds 
Covalent bonds 
2.7 Ionic bonds are attractions between ions of opposite charge 
An ion is a charged atom that has lost or gained electrons in its outer shell 
A positively charged ion ( cation ) is an atom that has lost an electron 
A negatively charged ion (anion) is an atom that has gained an electron 
An electrical attraction between ions with opposite charges results in an ionic bond 
Example: sodium chloride (table salt) results from an ionic bond between sodium and chlorine 
Animation: Ionic Bonds 
LE 2-7a-1 
Transfer of 
electron 
Na 
Sodium atom 
Cl 
Chlorine atom 
LE 2-7a-2 
Na + 
Sodium ion 
Cl - 
Chloride ion 
Sodium chloride ( NaCl ) 
LE 2-7b 
Na + 
Cl - 
2.8 Covalent bonds join atoms into molecules through electron sharing 
Covalently bonded atoms share one or more pairs of outer shell electrons, forming a molecule 
In a double bond, two pairs of electrons are shared 
Covalent bonds can be represented in various ways 
Animation: Covalent Bonds 
2.9 Unequal electron sharing creates polar molecules 
A molecule whose covalently bonded atoms share electrons equally is nonpolar 
A molecule whose covalently bonded atoms share electrons unequally is polar 
One part of the molecule is slightly positive, and one part is slightly negative 
LE 2-9 
A water molecule 
2.10 Hydrogen bonds are weak bonds important in the chemistry of life 
The attraction between slightly positive regions and slightly negative regions creates hydrogen bonds 
Hydrogen bonding occurs in many biologically important compounds 
Water 
DNA 
Proteins 
LE 2-10 
Hydrogen bond 
Animation: Water Structure 
WATER'S LIFE-SUPPORTING PROPERTIES 
2.11 Hydrogen bonds make liquid water cohesive 
Cohesion is the tendency of molecules to stick together 
Surface tension results from the cohesion of water molecules 
Animation: Water Transport 
2.12 Water's hydrogen bonds moderate temperature 
When water is heated, the heat energy is absorbed, disrupting hydrogen bonds 
The water stores a large amount of heat while warming only a few degrees 
When water is cooled, heat energy is released as hydrogen bonds are formed 
The temperature of the water is lowered slowly 
Water also moderates temperature by evaporative cooling 
The surface cools as the hottest molecules leave 
2.13 Ice is less dense than liquid water 
Hydrogen bonds in ice create a stable, three-dimensional structure 
Ice is less dense than water, because it has fewer molecules in the same volume 
LE 2-13 
Hydrogen bond 
Ice 
Hydrogen bonds are stable 
Liquid water 
Hydrogen bonds 
constantly break and re-form 
2.14 Water is the solvent of life 
A solution is a homogeneous mixture of a liquid solvent and one or more dissolved solutes 
Because water is a polar molecule, it readily forms solutions with many other polar and ionic compounds 
A solution in which water is the solvent is an aqueous solution 
LE 2-14 
Ion in 
solution 
Salt 
crystal 
2.15 The chemistry of life is sensitive to acidic and basic conditions 
A compound that releases H+ ions in solution is an acid 
A compound that accepts H+ ions in solution is a base 
Acidity is measured on the pH scale from 0 (most acidic) to 14 (most basic) 
The pH of most cells is kept close to 7 (neutral) by buffers that resist pH change 
LE 2-15 
Acidic solution 
OH  
H + 
H + 
H + 
H + 
OH  
H + 
H + 
H + 
OH  
OH  
OH  
OH  
OH  
H + 
H + 
H + 
H + 
H + 
H + 
Increasingly ACIDIC 
(Higher concentration of H + ) 
Neutral solution 
OH  
OH  
OH  
OH  
OH  
OH  
H + 
H + 
Basic solution 
NEUTRAL 
[ H     OH - ] 
pH scale 
Lemon juice, gastric juice 
Grapefruit juice, soft drink 
Tomato juice 
Human urine 
Pure water 
Human blood 
Seawater 
Milk of magnesia 
Household ammonia 
Household bleach 
Oven cleaner 
Increasingly BASIC 
(Lower concentration of H + ) 
CONNECTION 
2.16 Acid precipitation threatens the environment 
Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids 
Some ecosystems and structures are threatened by acid precipitation 
CHEMICAL REACTIONS 
2.17 Chemical reactions change the composition of matter 
In a chemical reaction, reactants interact, leading to products 
Atoms are rearranged, but the number of atoms stays constant on both sides of the equation 
LE 2-17a 
+ 
2 H 2 
O 2 
2 H 2 O 
+ 
Living cells carry out thousands of chemical reactions that rearrange matter in significant ways 
LE 2-17b 
Beta-carotene 
Vitamin A 
(2 molecules)