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Biology Concepts and Applications | 9e

Starr | Evers | Starr

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Chapter 2

Life’s Chemical Basis

A pure substance that

consists only of atoms

with the same number

of protons.

Elements

http://images-of-elements.com

Figure 2-1bc p24

Periodic Table of the Elements

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Atoms: Basic Building Blocks of All Matter

proton

neutron

electron

Atomic Number: # of protons

Mass Number: # of protons + # of neutrons

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Table 2.1

Isotopes: Different forms of an element with the same

number of protons but different numbers of neutrons

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Carbon (C): 18.5%

Oxygen (O):

65.0%

Calcium (Ca): 1.5%

Phosphorus (P): 1.0%

Potassium (K): 0.4%

Sulfur (S): 0.3%

Sodium (Na): 0.2%

Chlorine (Cl): 0.2%

Hydrogen (H):

9.5%

Magnesium (Mg): 0.1%

Trace elements: less than 0.01%

Boron (B) Chromium (Cr) Cobalt (Co) Copper (Cu) Fluorine (F) Iodine (I) Iron (Fe)

Manganese (Mn) Molybdenum (Mo) Selenium (Se) Silicon (Si) Tin (Sn) Vanadium (V) Zinc (Zn)

Nitrogen (N):

3.3%

4 elements

make up

96% of the

weight of

the body

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Figure 2.3

Trace elements:

Minute, but crucial!

Goiter due to lack of iodine

Atoms Interact via Chemical Bonding

Electron Capacity of Each Shell:

1st shell can hold 2 electrons

2nd shell can hold 8 electrons

3rd shell can hold 8 electrons

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Na Cl Na+ Cl

Na Sodium atom

Cl Chlorine atom

Na+

Sodium ion

Cl

Chloride ion

Sodium chloride (NaCl)

Ionic Bonds

Electron is transferred and a bond is created by

the electrical attraction of oppositely charged ions.

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Covalent Bonds

A bond in which two atoms share one or more

pairs of outer shell electrons.

H2O =

Water!

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Covalent Bond =

sharing electrons

(within water

molecules)

The Water Molecule

Slight negative charge

Slight positive charge

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Hydrogen Bonding

Hydrogen bond

Hydrogen Bond

= attraction

between + and –

sides of water

molecules

(Between water

molecules)

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Water's Special Properties

• Water is an excellent solvent

– When water dissolves salt ions, each ion becomes

surrounded by oppositely charged regions of water

molecules.

Chloride ion in solution

Sodium ion in solution

Cl

Cl

Na+

Na+

Salt crystal

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Water's Special Properties

• Water is an excellent solvent

– Substances that dissolve easily in water are

hydrophilic

• Solvent: A liquid in which other substances can

be dissolved. (example: water)

• Solute: A substance that can be dissolved in a

liquid. (example: salt)

• Solution: A mixture of a solute and solvent.

(example: salt water)

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Water's Special Properties

• Water has cohesion (stickiness)

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Water's Special Properties

• Water stabilizes temperature

• Water stabilizes temperature

• Earth’s giant water supply enables temperatures on the planet to stay within limits that permit life by

• storing a huge amount of heat from the sun during warm periods and

• giving off heat that warms the air during cold periods.

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Water's Special Properties

• Ice is less dense than liquid water!

© Cengage Learning 2015 Figure 2-9 p32

Why are hydrogen

ions important in

biological systems?