1. The Chemical Context of Life
Chapter 6
The Chemical Context of Life

2. Overview: A Chemical Connection to Biology
Biology is a multidisciplinary science
Living organisms are subject to basic laws of physics and chemistry
Example: Ants - Species Myrmelachista
Kill non-host trees by injecting formic acid into their leaves
They maintain an environment that their colony can survive and thrive
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3. Life can be organized into a different structural levels.
Organism
Organ System
Tissues
Cellular
Molecular
Atomic
At each level additional distinct properties appear.
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4. Matter consists Elements & Compounds
Organisms are composed of matter
Matter is anything that takes up space and has mass
Matter is made up of elements and compounds
ELEMENTS & COMPOUNDS
Element: a substance that cannot be broken down into other substances by chemical reactions.
Compound: is a substance consisting of two or more elements in a fixed ratio
A compound has characteristics different from those of its elements
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6. E.g. Table Salt Sodium Chlorine Sodium chloride
Figure 2.3 The emergent properties of a compound
Sodium
Chlorine
Sodium
chloride

7. Essential Elements of Life
About 25 of the 92 elements are essential to life
96% of living matter
Carbon
Hydrogen
Oxygen
Nitrogen
Remaining 4%
Phosphorus, Sulfur, Calcium, Potassium, Others
Trace elements are those required by an organism in minute quantities
Iodine & Iron
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8. (a) Nitrogen deficiency (b) Iodine deficiency
Fig. 2-4
Figure 2.4 The effects of essential-element deficiencies
(a) Nitrogen deficiency
(b) Iodine deficiency

9. Concept 2.2: Structure of An Atom
Each element consists of unique atoms
Atom: Smallest unit of matter that still retains the properties of an element
Atoms are composed
Neutrons (no electrical charge)
Protons (positive charge)
Electrons (negative charge)
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10. Cloud of negative charge (2 electrons) Nucleus Electrons
Atom
Charge
Properties
Proton
+ve
Stays the same in an element & determines element
Neutron
Neutral
Varies in an element
Electron
-ve
Determine chemical properties & reactivity of an element
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11. Atomic Number and Atomic Mass
Atoms of the various elements differ in number of subatomic particles
Atomic number: Number of protons in its nucleus
Mass number: Sum of protons plus neutrons in the nucleus
Atomic mass: The atom’s total mass, can be approximated by the mass number
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12. Isotopes All atoms of an element have the:
Same number of protons
But may differ in number of neutrons
Isotopes: Are two atoms of an element that differ in number of neutrons
Radioactive Isotopes: Decay spontaneously, giving off particles and energy
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13. Some applications of radioactive isotopes in biological research are:
Dating fossils
Tracing atoms through metabolic processes
Diagnosing medical disorders
PET Scan – Detects location of intense chemical activity
Pt. injected with glucose labeled with radioactive carbon
Particles collide with electrons from chemical reactions in the body
The PET detects these hot spots”
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14. Chemical Bonds
Interactions between usually result in atoms staying close together, held by attractions called chemical bonds
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15. Covalent Bonds
A covalent bond is the sharing of a pair of electrons by two atoms
Covalent bonds can form between atoms of the same element or atoms of different elements
In a covalent bond, the shared electrons count as part of each atom’s valence shell
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16. Hydrogen atoms (2 H) Hydrogen molecule (H2)
Fig. 2-11
Hydrogen
atoms (2 H)
Figure 2.11 Formation of a covalent bond
Hydrogen
molecule (H2)

17. A compound is a combination of two or more different elements
A molecule consists of two or more atoms held together by covalent bonds
A compound is a combination of two or more different elements
A single covalent bond, or single bond, is the sharing of one pair of valence electrons
A double covalent bond, or double bond, is the sharing of two pairs of valence electrons
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18. Ionic Bonds
Atoms sometimes steal electrons from their bonding partners
An example is the transfer of an electron from sodium to chlorine
After the transfer of an electron, both atoms have charges
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19. Ionic Bonds A charged atom (or molecule) is called an ion
Those that gain electrons become negatively charged
Those that lose electrons are positively charged Na Cl
Sodium atom
Chlorine atom
Na+
Sodium ion
(a cation)
Cl–
Chloride ion
(an anion)
Sodium chloride (NaCl)
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20. A Cation is a positively charged ion
An Anion is a negatively charged ion
An Ionic bond is an electrical attraction between an anion and a cation
Compounds formed by ionic bonds are called ionic compounds, or salts
Ionic Bonds are weak
Salts, such as sodium chloride (table salt), are often found in nature as crystals
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21. Fig. 2-15
Na+
Cl–
Figure 2.15 A sodium chloride crystal

22. Hydrogen Bonds
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
In living cells, the electronegative partners are usually oxygen or nitrogen atoms
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23.   + Water (H2O) + Hydrogen bond   Ammonia (NH3) + + +
Fig. 2-16
  +
Water (H2O) +
Hydrogen bond
 
Ammonia (NH3)
Figure 2.16 A hydrogen bond + + +

24. Van der Waals Interactions
If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative charge
Van der Waals interactions are attractions between molecules that are close together as a result of these charges
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25. Collectively, such interactions can be strong, as between molecules of a gecko’s toe hairs and a wall surface
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26. You should now be able to:
Identify the four major elements
Distinguish between the following pairs of terms: neutron and proton, atomic number and mass number, atomic weight and mass number
Distinguish between and discuss the biological importance of the following: nonpolar covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions
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