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Chapter 2Basic

Chemistry and Cells

All Matter Is Composed of Chemical Elements

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2.1 Six elements are basic to life Six Elements of Life

Carbon – C Hydrogen – H Nitrogen – N Oxygen – O Phosphorus – P Sulfur – S

Element: substance that cannot be degraded by chemical means into a substance having different properties

Every element has a name and an atomic symbol Atoms: the smallest particles that retain the

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Figure 2.1 Elements that make up the Earth’s crust and its organisms

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2.2 Atoms contain subatomic particles

Two main parts of an atom Nucleus

Center of the atom where almost all the mass is located

Electron Shells Area surrounding the nucleus where the

electrons are located

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Subatomic Particles

Protons Positively charged particles located in the nucleus Protons have mass

Electrons Negatively charged particles located in the electron

cloud Electrons have so little mass that they are not

considered when determining the mass of an atom Neutrons

Neutrally charged (no charge) particles located in the nucleus

The mass of a neutron is equal to that of a proton

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Figure 2.2A The stippled area shows the probable location of electrons

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Figure 2.2B The shells in this atomic model represent the average location of electrons

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Figure 2.2C Atomic model of a carbon atom

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Isotopes

Atoms of a single element that differ in their number of neutrons Isotopes have the same number of protons,

but different atomic masses

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

2.3 Radioactive isotopes have many medical uses

Low Levels Small amounts of radiation can be used as a

tracer allowing doctors to ‘see’ inside of the body

High Levels Lots of radiation will damage DNA and kill

cells Can also be used to kill pathogens and treat

disease, like cancer

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Atoms React with One Another to Form Molecules

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2.4 After atoms react, they have a completed outer shell

Atoms are most stable when the outer shell has eight electrons Atoms exchange electrons in order to have a

complete outer shell or valence shell

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Figure 2.4A A portion of the periodic table of the elements

Figure 2.4B Models of the six elements that are predominant in living things

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Compounds and Molecules

Compound – two or more different elements bonded together

Molecule – the smallest part of a compound that still has the properties of that compound

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2.5 An ionic bond occurs when electrons are transferred

Ion - an atom or molecule that has either lost or gained electrons

Losing an electron results in a net positive Gaining an electron results in a net negative

charge

Ions with opposite charges attract and form bonds known as ionic bonds

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Figure 2.5 Formation of sodium chloride (table salt)

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2.6 A covalent bond occurs when electrons are shared

When two atoms do not have completely full outer shells, they may share electrons so that each has a full outer shell

Example: Hydrogen (H2)

Bond Notation Single covalent bonds are written as H-H Double covalent bonds are written as O=O Triple covalent bonds are written as N≡N

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Figure 2.6A Electron models and formulas representing covalently bonded molecules

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Figure 2.6B Other types of molecular molecules—in this case, for methane (CH4)

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Chemical Reactions

The exchange of electrons between atoms create chemical reactions

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2.7 A covalent bond can be nonpolar or polar

Nonpolar covalent bond – when the sharing of electrons between atoms is fairly equal Electronegativity – the attraction of an atom for the

electrons in a covalent bond

Polar covalent bond – unequal sharing of electrons

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Figure 2.7 Three models of water

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2.8 A hydrogen bond can occur between polar molecules

Polar bonds have charges on either end and often attract to each other

When a hydrogen is bonded to an electronegative atom, it becomes electropositive

This causes it to be attracted to electronegative parts of the same or other molecules

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Figure 2.8 Hydrogen bonding between water molecules

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The Properties of Water Benefit Life

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2.9 Water molecules stick together: Cohesion

The tendency of water molecules to cling together Due to the four hydrogen bonds water has

Cohesion contributes to water transport in plants Adhesion of water to the walls of the vessels also

helps prevent the water column from breaking apart

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Figure 2.9 Water as a transport medium in trees

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2.10 Water warms up and cools down slowly

Calorie – the amount of heat energy needed to raise the temperature of 1 g of water 1°C

Water has a high heat of vaporization because so many hydrogen bonds must be broken

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2.11 Water dissolves other polar substances

A solution contains dissolved substances called solutes that which is dissolved

Water’s hydrogen bonds help it dissolve other polar molecules

Hydrophilic (water-loving) molecules form hydrogen bonds with water

Hydrophobic (water-fearing) molecules do not form hydrogen bonds with water

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Figure 2.11 An ionic salt dissolves in water

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2.12 Frozen water is less dense than liquid water

When water freezes, the crystal lattice expands With a greater volume and the same mass, the

density decreases and the solid, ice, floats in the denser liquid, water

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Figure 2.12A Ice is less dense than liquid water

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Living Things Require a Narrow pH Range

2.13 Acids and bases affect living things

When water ionizes it releases equal amounts of hydrogen ions (H+) and hydroxide ions (OH-)

Some substances release more or fewer or each Acids - Excess hydrogen ions Bases - Excess hydroxide ions

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Figure 2.13A Dissociation of water molecules

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Figure 2.13B Acids cause H+ to increase

Figure 2.13C Bases cause OH- to increase

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2.14 The pH scale measures acidity and basicity

The pH scale goes from 0 to 14, 7 is neutral

The greater the H+ concentration, the lower the pH and the lower the hydrogen ion concentration, the higher the pH

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Figure 2.14 The pH scale

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2.15 Buffers help keep the pH of body fluids relatively constant

Buffer system: the combination of a weak acid and the base that forms when the acid dissolves in water

By binding or releasing H+’s, it maintains a constant concentration of free H+’s in the solution

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

2.16 Acid deposition has many harmful effects

Air pollution, particularly from burning fossil fuels, acidifies rainwater Leaches toxic aluminum and creates methyl mercury

in lakes Destroys leaves, stopping photosynthesis and making

trees susceptible to disease Increases respiratory disease and erodes buildings

and monuments

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Connecting the Concepts: Chapter 2

Cells are made up of chemicals and therefore we must understand chemistry Cells are composed of carbon, hydrogen, nitrogen,

oxygen, phosphorus, and sulfur Carbon forms long chains of carbon atoms

Water is a polar molecules and can dissolve other polar substances Water’s cohesiveness, tendency to change

temperature slowly, ability to expand as it freezes are due to hydrogen bonding

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