Chapter 2 · the partially positive H atoms of two water molecules . 14 Polarity of water •...

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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