chapter 2 · the partially positive h atoms of two water molecules . 14 polarity of water •...
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Biological Atoms & Molecules
Chapter 2
In this section…
• Atoms – Atomic number – Atomic mass – Names of parts – Biologically important
atoms • Molecules
– Types of bonds
• Water – Polarity – Hydrogen bonding – Special physical
characteristics – Solvent
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Nature of Atoms
• Matter has mass and occupies space • All matter is composed of atoms
• Understanding the structure of atoms is critical to understanding the nature of biological molecules
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Atomic Structure
• Atoms are composed of: – Protons
• Positively charged particles • Located in the nucleus
– Neutrons • Neutral particles • Located in the nucleus
– Electrons • Negatively charged particles • Found in orbitals surrounding the nucleus
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Periodic Table of the Elements
• Living organisms are made up of mostly 12 elements:
• Four elements make up 96.3% of human body weight – Carbon, hydrogen, oxygen,
nitrogen • Organic molecules contain
primarily CHON • Some trace elements are very
important
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Chemical Bonds
• Molecule – two or more atoms held together in a stable association
• Atoms are held together in molecules by chemical bonds 1. Ionic – electrons gained or lost 2. Covalent – electrons shared 3. Hydrogen – special case
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Ionic bonds
• Formed by the attraction of oppositely charged ions
• Gain or loss of electrons forms ions – Na atom loses an electron to become Na+
– Cl atom gains an electron to become Cl–
– Opposite charges attract so that Na+ and Cl– remain associated as an ionic compound
v Electrical attraction of water molecules can disrupt forces holding ions together
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Covalent bonds
• Form when atoms share 2 or more electrons
• Results in no net charge • Strength of covalent bond depends on the
number of shared electrons
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Water
• Life is inextricably tied to water • Single most outstanding chemical property
of water is its ability to form hydrogen bonds – Weak chemical associations that form
between the partially negative O atoms and the partially positive H atoms of two water molecules
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Polarity of water
• Within a water molecule, the bonds between oxygen and hydrogen are highly polar – O is much more electronegative than H
• Partial electrical charges develop – Oxygen is partially negative (δ–)
– Hydrogen is partially positive (δ+)
– Opposites attract!
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METHANE MOLECULE MOVIE
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Hydrogen bonds • Attraction between partial charges in water
(or when H is bonded to certain other atoms) produces hydrogen bonds
• Each individual bond is weak • Cumulative effects are enormous • Responsible for many of water’s important
physical properties
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HYDROGEN BONDS MOVIE
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Critical Properties of Water
1. Cohesion & Adhesion
2. Moderation of temperature
3. Solvent of life!
Cohesion
• Polarity of water allows water molecules to be attracted to one another
• Water likes water
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Cohesion of water molecules
Fig. 3.4 Fig. 3.4
Adhesion
• Capillary Action • Meniscus
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