Lecture Notes-09 Biology 1004

North Arkansas College

Topic: Chemical Basis of Life

 Covalent Bonds

1. Sharing of valence electrons in order to stabilize atoms completes octet or 2 for first shell.

2. Hydrogen has 1 proton and 1 electron (first energy level).

3. Two hydrogen molecules can share their one electron to complete energy shell for both to form H₂.

4. A number of elements do this to form diatomic molecules H₂, N₂, O₂, etc. Oxygen has 6 valence electrons. Two atoms of oxygen can share 2 pairs of electrons to form O₂.

5. Hydrogen and oxygen can share electrons to form water.

6. Carbon has 4 valence electrons. It can share electrons with 4 hydrogen molecules to make methane which is CH₄.

1. Two identical elements (2H; 2N) share electrons equally. They have same number of protons and electrons in each. Each nucleus has equal pull on shared electrons. This compound/atoms have no charge.
2. This is a non-polar covalent compound.
3. Some elements lose an electron to form a positively charged ion (cation). Sodium has 11 electrons and 1 valence electron. It loses 1 valence electron to form sodium and 1 ion (Na⁺¹).
4. Some atoms gain 1 or more electrons to form negatively charged ions (anion). Chlorine has 17 electrons with 7 valence electrons. It can gain one electron to form Cl^-1.
5. Ions combine to form ionic bonds or ionic compounds. Some examples are table salt (sodium chloride). Na⁺¹ + Cl^-1 - NaCl.
6. Polar covalent bonds occur when 2 different kinds of elements share electrons: 2H & oxygen - H₂O. The nucleus of one of the elements has more protons than the other atom (more positive charges), so it pulls the shared electron(s) of the other element closer to its nucleus. The second element acts as though it lost its electron(s) and take on a partial positive charge: e.g. H in H₂O. The oxygen acts as if it gained the H electron, and it takes on a partial negative charge. Water is an excellent solvent because it is polar.  (+ and – charges associated with it). Non-polar are hard to put in solution.

Covalent and Ionic Bonds   ©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter.   http://www.essentialcellbiology.com     Published by Garland Publishing, a member of the Taylor & Francis Group.

1. Weak chemical bond; break easily.
2. Occurs between partially charged positive atoms and partially charged negative atoms.
3. Water molecules react together to form surface tension.

1. All living organisms are composed largely of water. Cells have shape due to water (turgid).
2. The amount of water varies from 20% in bone to 80-85%. Brain is > 70% H₂O.
3. The average water content of human cells equals 70-75%.
4. Water is a universal solvent. Ionic compounds or those with a partial charge are soluble in water.
5. Water is polar (carries both positive and negative charge). Oxygen nucleus pulls harder on H electrons than does H nucleus. Hydrogen nucleus acts as if it has lost its electron and becomes partially positively charged. Oxygen acts as if it gained an electron and becomes partially negatively charged.

                        

1. Water has high specific heat (the heat required to raise the temperature of 1 gram of water 1 degree Celsius). This allows water to absorb large quantities of heat or cold without major changes in temperature. The temperature can remain fairly constant. In plants and animals, high water content allows regulation of body temperature. Earth’s surface is ~ ¾ water- helps maintain Earth’s water temperature. To convert 1 gram of water to vapor requires 540 calories. Biological/chemical reactions take place over narrow temperature range.
2. Cohesive/adhesive properties of water. Because of partial positive and negative charges, water molecules cling (cohesive) to create surface tension. The surface of water can have adhesive properties that allow it to react with other substances and which contribute to solubility. Adhesive property causes it to attract other substances.
3. Water has a maximum density at 4 degrees Celsius and the density does not change much during heating or cooling. Temperature changes cause it to expand without changing its density. This is why ice floats instead of sinking. This protects and insulates the life forms beneath the water. Most substances become denser as temperature increase/decrease, but water remains the same density over a wide increase/decrease in temperature.
4. Water ionizes. Water dissociates for form H+ ions and OH- ions (hydroxyl) (OH - H+ & OH-).
5. In nature, water reaches equilibrium to maintain pH close to neutral. Most cells grow best (optimum) at pH near neutral. An excess of H+ ions create an acid environment. An excess of OH- ions creates an alkaline environment.

1. Excess H+ ions-acid
2. Excess OH- ions-alkaline

11. Biological/chemical reactions occur over a narrow pH range. pH = level of H+ or OH- ions. Most biological reactions occur near neutral (6.5-7.5 pH).
12. Too much acid or too much base (alkaline) poisons reactions.

1. Acid rain (environmental problem), burning of fossil fuels (coal, petroleum, etc.) produce sulfur dioxide (SO₂) and/or nitrites (NO₂) that go into the atmosphere and combine to form sulfuric acid (H₂SO₄) or nitric acid (HNO₃). These acids fall to the ground as acid rain or acid fog. East coast; rain measures 2.5 pH. Down wind of Los Angeles; fog has pH 1. Poisons respiratory tract, kills trees, pollutes streams, etc.
2. To maintain pH near neutral, we must use a buffer. A buffer is a substance (chemical) that prevents drastic changes in pH. In humans, bicarbonate (HCO₃) acts as a buffer in blood to prevent acidosis or alkalosis. CO₂ & H₂O ^{(carbon dioxide)} - H₂CO₃ ^{(carbonic acid)} - H+ & HCO₃ ^{(bicarbonate)}
3. Many industries release SO₂ (sulfur dioxide) or NO₂ (nitrite) into the air, where they combine with water vapor in the air and form nitric acid (HNO₃) and sulfuric acid (H₂SO₄).

* Review for class discussion

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