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Chapter 2

The Chemical Basis of Life

Essentials of Biology

SEVENTH EDITION

Sylvia S. Mader Michael Windelspecht

Because learning changes everything.®

2.1 Atoms and Atomic Bonds

Matter

Refers to anything that takes up space and has mass

Can exist as a solid, liquid, or gas

Composed of elements

Element—substance that cannot be broken down into another substance by ordinary chemical means

Only 92 naturally occurring elements

Four elements make up about 96% of the body weight of most living organisms—carbon, hydrogen, oxygen, and nitrogen

Where Do Elements Come from?

Normal chemical reactions do not produce elements.

The majority of heavier elements such as iron are produced when star explode as a supernova

Supernovas scatter heavier elements into space which then become parts of planets

The iron in blood was formed from the explosion of stars

“After all, what nobler thought can one cherish than that the universe lives within us all?” Neil deGrasse Tyson

Figure 2.1 Elements in Living Organisms

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(photo): PNC/Getty Images

Atomic Structure

Atomic structure:

Atomic theory states that elements consist of atoms.

Atomic symbol—name of the atom or element

H for hydrogen or Na for sodium

Subatomic elements

Neutrons—no electrical charge, found in nucleus

Protons—positive charge, found in nucleus

Electrons—negative charge, found outside of nucleus moving in orbitals

Mass number is equal to sum of protons and neutrons—electrons have about zero mass.

Atomic weight changes with gravity

Figure 2.2 Two Models of Helium (H e)

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Atomic Number

Atomic number:

All atoms of an element have this same number of protons.

Also gives number of electrons if an atom is electrically neutral

Periodic table

Elements’ chemical and physical characteristics recur in a predictable manner.

Atoms are arranged in periods (rows) and groups (columns).

Figure 2.3 A Portion of the Periodic Table

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Isotopes

Isotopes:

Atoms of the same element that differ in the number of neutrons

Isotopes have the same number of protons but a different number of neutrons (different mass numbers).

Unstable and may decay, emitting radiation

Radioactive isotope behavior is essentially the same as a stable isotope of same element.

Can be used as tracer—PET scan

Can cause damage to cells, leading to cancer

Can be used to sterilize medical equipment

Figure 2.4 PET Scan

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Science History Images/Alamy Stock Photo

Figure 2.5 High Levels of Radiation

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(a): Photo12/Ann Ronan Picture Library/Alamy Stock Photo; (b): Phonkrit Ninchak/Shutterstock

Arrangement of Electrons in an Atom 1

Arrangement of electrons in an atom:

Electrons are constantly moving.

Useful to construct models of atoms with energy levels or electron shells

Each shell contains a certain number of electrons.

For atoms up through number 20

Two electrons fill first shell

Eight electrons fill each additional shell

Arrangement of Electrons in an Atom 2

Arrangement of electrons in an atom, continued

Octet rule for valence shell

Valence shell—outermost shell

If an atom has more than two shells, the outer shell is most stable with eight electrons

Atoms can give up, accept, or share electrons to have eight.

Chemical properties of atoms are largely determined by the arrangement of their electrons.

Figure 2.6 Atoms of Six Important Elements

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Types of Chemical Bonds 1

Types of chemical bonds:

Molecule—group of atoms bonded together

O2, H2O, C6H12O6, N2

Compound—molecule containing atoms of more than one element

H2O, C6H12O6

Two types of bonds

Ionic—attraction between opposite charges

Covalent—sharing electrons to complete outer shell

Types of Chemical Bonds 2

Ionic bonding

Forms when two atoms are held together by the attraction between opposite charges

Sodium has one electron in valence shell.

Usually gives up an electron

Chlorine has seven electrons in valence shell.

Usually accepts an electron from another atom

Ions—charged atoms

Sodium has one more proton than electrons: now

Chlorine has one more electron than protons: now

Ionic compounds often called salts.

Figure 2.7 Formation of Sodium Chloride

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(b, photo): PM Images/Photodisc/Getty Images

Types of Chemical Bonds: Covalent

Covalent bonding

Two atoms share electrons

Two hydrogen atoms can share electrons to fill their outer shell—orbitals overlap.

Structural formula—uses straight lines H-H

One line indicates one pair of shared electrons.

Molecular formula—simply shows number of atoms involved H2

Double Covalent Bonds

Double covalent bonding:

Two atoms share four electrons

Double bonds are stronger than single bonds

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Figure 2.8 Shapes of Covalently Bonded Molecules

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Chemical Formulas and Reactions 1

Chemical formulas and reactions:

Reactants—molecules that participate in reactions

Shown to the left of the arrow

Products—molecules formed by reactions

Shown to the right of the arrow

Chemical Formulas and Reactions 2

Equation is balanced if the same number of each type of atom occurs on both sides of the arrow.

An overall equation for photosynthesis

Molecular formula for glucose

2.2 Water’s Importance to Life

Life began in water.

Single most important molecule on Earth

All organisms are 70–90% water

Water has unique properties that make it a life-supporting substance.

Properties stem from the structure of the molecule.

Water’s Importance to Life: Structure

The structure of water

Polar covalent bond

Atoms do not share electrons equally.

Oxygen is more electronegative than hydrogen.

Electrons spend more time around the oxygen nucleus than the hydrogen nuclei.

Oxygen end becomes slightly negative/hydrogens become slightly positive—NOT an ionic bond or ions

Hydrogen bond—slightly positive hydrogen of one water molecule attracted to the slightly negative oxygen in another water molecule

Figure 2.9 The Structure of Water

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(photo): annedehaas/E+/Getty Images

Properties of Water: Overview

Properties of water

Solvency

Cohesion and adhesion

High surface tension

High heat capacity

High heat of vaporization

Varying density

Properties of Water: Solvency

Water is a solvent.

Due to polarity and H-bonding, water dissolves many substances

Hydrophilic—molecules attracted to water

Hydrophobic—molecules not attracted to water

Water causes NaCl to dissociate

The salt NaCl dissociates in water.

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Properties of Water: Cohesion and Adhesion

Cohesion

Ability of water molecules to cling to each other due to hydrogen bonding

Adhesion

Ability of water molecules to cling to other polar surfaces

Allows water to be excellent transport system both inside and outside of living organisms

Contributes to water transport in plants

Figure 2.10 Cohesion and Adhesion of Water Molecules

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(tree): Paul Davies/Alamy Stock Photo; (man): Asiaselects/Getty Images

Properties of Water: High Surface Tension

Water has a high surface tension.

Water molecules at the surface cling more tightly to each other than to the air above.

Mainly due to hydrogen bonding

Jan Miko/Shutterstock

Properties of Water: Heat Capacity and Heat of Vaporization

Water has a high heat capacity.

The many hydrogen bonds linking water molecules allow water to absorb heat without greatly changing its temperature.

Temperature of water rises and falls slowly.

Heat of vaporization

Takes a great deal of energy to break H bonds for evaporation.

Heat is dispelled as water evaporates.

Figure 2.11 Heat Capacity and Heat of Vaporization

(a): Jill Braaten/McGraw Hill; (b): Cultura Creative RF/Alamy Stock Photo

Properties of Water: Varying Density

Ice is less dense than water.

Unlike other substances, water expands as it freezes.

Ice floats rather than sinks.

It makes life possible in water.

Ice acts as an insulator.

Jeff Vanuga/Getty Images

Figure 2.12 Properties of Ice

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2.3 Acids and Bases

Water dissociates into an equal number of hydrogen ions

and hydroxide ions

Acidic Solutions (High H raised to the plus power Concentration)

Lemon juice, vinegar, and coffee

Acids release hydrogen ions

or take up hydroxide ions

Basic Solutions (Low H raised to the plus power Concentration)

Milk of magnesia and ammonia

Either take up hydrogen ions

or release hydroxide ions

pH and pH Scale

pH and the pH scale:

Mathematical way to indicate the number of hydrogen ions in solution

pH scale ranges from 0 to 14

pH below 7 is acidic—more

pH above 7 is basic—more

pH of 7 is neutral—

Figure 2.13 The pH Scale

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Buffers and pH

Buffers and pH:

Chemical or combination of chemicals that keeps pH within normal limits

Resists pH change by taking up excess

pH of blood is about 7.35–7.45 and is maintained by buffer

Diseases such as congestive heart failure and diabetes can result in acidosis (body can’t buffer extra

ions); left

untreated death can result.

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Figure 2.1 Elements in Living Organisms - Text Alternative

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The pie chart is divided into eight sections and marked clockwise as follows: oxygen (O), 65%; carbon (C), 18%; hydrogen (H), 10%; nitrogen (N), 3%; calcium (Ca), 1.5%; phosphorus (P), 1.1%; lesser elements, including sulfur, 0.8%; and trace elements, 0.6%. The pie chart overlaps a photo of a female football player kicking the ball.

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Figure 2.2 Two Models of Helium (H e) - Text Alternative

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The first model (a) shows the nucleus containing two protons and two neutrons. To the left of the nucleus, a negatively charged cloud is formed by the electrons. The second model (b) shows the nucleus containing two protons and two neutrons. Two electrons are spinning around the nucleus in a circular path.

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Figure 2.3 A Portion of the Periodic Table - Text Alternative

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For each element, the atomic number is written at the top, atomic symbol at the center, and atomic mass at the bottom. The horizontal rows represent periods and the vertical columns represent groups. The periodic table infers the following data. Element 1: atomic symbol, H; atomic number, 1; atomic mass, 1.008; period number, 1; group number, 1. Element 2: atomic symbol, He; atomic number, 2; atomic mass, 4.003; period number, 1; group number, 8. Element 3: atomic symbol, Li; atomic number, 3; atomic mass, 6.941; period number, 2; group number, 1. Element 4: atomic symbol, Be; atomic number, 4; atomic mass, 9.012; period number, 2; group number, 2. Element 5: atomic symbol, B; atomic number, 5; atomic mass, 10.81; period number, 2; group number, 3. Element 6: atomic symbol, C; atomic number, 6; atomic mass, 12.01; period number, 2; group number, 4. Element 7: atomic symbol, N; atomic number, 7; atomic mass, 14.01; period number, 2; group number, 5. Element 8: atomic symbol, O; atomic number, 8; atomic mass, 16.00; period number, 2; group number, 6. Element 9: atomic symbol, F; atomic number, 9; atomic mass, 19.00; period number, 2; group number, 7. Element 10: atomic symbol, Ne; atomic number, 10; atomic mass, 20.18; period number, 2; group number, 8. Element 11: atomic symbol, Na; atomic number, 11; atomic mass, 22.99; period number, 3; group number, 1. Element 12: atomic symbol, Mg; atomic number, 12; atomic mass, 24.31; period number, 3; group number, 2. Element 13: atomic symbol, Al; atomic number, 13; atomic mass, 26.98; period number, 3; group number, 3. Element 14: atomic symbol, Si; atomic number, 14; atomic mass, 28.09; period number, 3; group number, 4. Element 15: atomic symbol, P; atomic number, 15; atomic mass, 30.97; period number, 3; group number, 5. Element 16: atomic symbol, S; atomic number, 16; atomic mass, 32.07; period number, 3; group number, 6. Element 17: atomic symbol, Cl; atomic number, 17; atomic mass, 35.45; period number, 3; group number, 7. Element 18: atomic symbol, Ar; atomic number, 18; atomic mass, 39.95; period number, 3; group number, 8. Element 19: atomic symbol, K; atomic number, 19; atomic mass, 39.10; period number, 4; group number, 1. Element 20: atomic symbol, Ca; atomic number, 20; atomic mass, 40.08; period number, 4; group number, 2. Element 31: atomic symbol, Ga; atomic number, 31; atomic mass, 69.72; period number, 4; group number, 3. Element 32: atomic symbol, Ge; atomic number, 32; atomic mass, 72.59; period number, 4; group number, 4. Element 33: atomic symbol, As; atomic number, 33; atomic mass, 74.92; period number, 4; group number, 5. Element 34: atomic symbol, Se; atomic number, 34; atomic mass, 78.96; period number, 4; group number, 6. Element 35: atomic symbol, Br; atomic number, 35; atomic mass, 79.90; period number, 4; group number, 7. Element 36: atomic symbol, Kr; atomic number, 36; atomic mass, 83.60; period number, 4; group number, 8.

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Figure 2.4 PET Scan - Text Alternative

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Left scan shows brain with yellow regions with tints of red at the top and green region with tints of blue at the bottom. There is a proper outline of the brain in blue color.

Right scan shows a brain with diminished colored regions. There is a proper outline of the brain and majority of the region is black.

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Figure 2.5 High Levels of Radiation - Text Alternative

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a. A satellite view shows a massive tsunami in a nuclear power plant causing a huge explosion of radioactive radiation in the environment.

b. A close-up shows a UV radiation machine treating several brown eggs.

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Figure 2.6 Atoms of Six Important Elements - Text Alternative

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The first diagram representing hydrogen atom shows a nucleus at the center with an electron revolving in the electron shell around it. The hydrogen atom, H has atomic number 1 and atomic mass 1. The second diagram representing carbon atom shows a nucleus at the center with two electron shells around it. The inner shell has two electrons and the outer shell (valence) has four electrons. The carbon atom, C has atomic number 6 and atomic mass 12. The third diagram representing nitrogen atom shows a nucleus at the center with two electron shells around it. The inner shell has two electrons and the outer shell has five electrons. The nitrogen atom, N has atomic number 7 and atomic mass 14. The fourth diagram representing oxygen atom shows a nucleus at the center with two electron shells around it. The inner shell has two electrons and the outer shell has six electrons. The oxygen atom, O has atomic number 8 and atomic mass 16. The fifth diagram representing phosphorus shows a nucleus at the center with three electrons around it. The first shell has two electrons, the second shell has eight electrons, and the third shell has five electrons. The phosphorus atom, P has atomic number 15 and atomic mass 31. The sixth diagram representing sulfur shows a nucleus at the center with three electrons around it. The first shell has two electrons, the second shell has eight electrons, and the third shell has six electrons. The sulfur atom, S has atomic number 16 and atomic mass 32.

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Figure 2.7 Formation of Sodium Chloride - Text Alternative

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The illustration labeled a, shows four atomic structures. The top two atomic structures show sodium atom (Na) and chlorine atom (Cl). A sodium atom, with nucleus has two electrons in the innermost shell, eight electrons in the middle shell, and one electron in the valence shell. The Chlorine atom with nucleus, has two electrons in the innermost shell, eight electrons in the middle shell, and seven electrons in the valence shell. The Sodium atom transfers the electron of its valence shell to chlorine atom. Now, the outer shells of both atoms are complete and sodium and chloride ions are formed. The sodium ion with one positive charge and chloride ions with one negative charge are shown as two bottom atomic structures. The reaction of positively charged sodium ion and negatively charged chloride results in the formation of Sodium Chloride.

The illustration labeled b from left shows the following:

Close-up of a hand sprinkling table salt on fries.

A three-dimensional lattice structure of sodium chloride shows purple sodium ions and green chloride ions.

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Double Covalent Bonds - Text Alternative

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Oxygen gas is formed by two oxygen atoms bonded by a double covalent bond. An atom of oxygen has 6 electrons in its outer valence shell thus two more would make it more stable. Two atoms of oxygen achieve stability by sharing two pairs of electrons in a double covalent bond.

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Figure 2.8 Shapes of Covalently Bonded Molecules - Text Alternative

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An electron model of methane shows a carbon atom surrounded by four hydrogen atoms. The inner shell of the carbon atom has two electrons. The outer shell of the carbon atom is shared with the four hydrogen atoms and hence, the outer shell has eight electrons. A structural model (b) of methane shows a carbon atom, C single bonded to four hydrogen atoms, H. The ball-and-stick model of methane shows a carbon atom bonded to four hydrogen atoms using a covalent bond. It forms a tetrahedron. The angle made by the two adjacent hydrogen atoms is 109°C The space-filling model (d) of methane shows a carbon atom represented as a black sphere partially overlapping four hydrogen atoms represented as white spheres.

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Figure 2.9 The Structure of Water - Text Alternative

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The first illustration (a) shows a space-filling model of a water molecule in which the oxygen atom (red sphere) partially overlaps two hydrogen atoms (white spheres) forming a V-shaped structure. Oxygen is slightly negative and hydrogens are slightly positive. The second illustration (b) shows a space-filling model involving five water molecules. The oxygen atom of the first water molecule shares hydrogen bond with any one of the hydrogen atoms of the second and third water molecules. The two hydrogen atoms of the first water molecule share a