Essential Chemistry for Biology

1. Essential Chemistry for Biology Chapter 2 Part II

2. Water’s Life-Supporting Properties • The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties: - Water’s cohesive nature - Water’s ability to moderate temperature - Biological significance of floating ice - Versatility of water as a solvent

3. The Cohesion of Water • Water molecules stick together as a result of hydrogen bonding • H bonds between water molecules last for only a few trillionths of a second • At any instant many of the molecules are H-bonded to others - this is cohesion and is much stronger in water than other liquids - cohesion is vital for water transport in plants

4. Cohesion and water transport in plants • The evaporation of water from leaves pulls water upward from the roots through tubes in the trunk of the tree • Due to cohesion, the pulling force is relayed through the tubes all the way down to the roots • As a result, water rises against the force of gravity Fig 2.12

5. Surface Tension • The measure of how difficult it is to stretch or break the surface of a liquid • H bonds give water an unusually high surface tension Fig 2.13

6. How Water Moderates Temperature • Due to H bonding, water has a strong resistance to temperature change • Heat and temperature are related, but different: - Heat is the amount of energy associated with the movement of the atoms and molecules in a body of matter - Temperature measures the intensity of heat as the average speed of molecules rather than the total amount of heat energy in a body of matter • Heating water: 1st heat energy disrupts H bonds then 2nd water molecules jostle around faster - temperature goes up after molecules speed up

7. Heat and Temperature • Because heat has to first break H bonds before water temperature rises: - water can absorb and store large amounts of heat - change in temperature will be just a few degrees • Conversely when water cools, H bonds form, a process that releases heat - water can release relatively large amounts of heat to the surroundings while water temperature drops only slightly

8. Water Moderates Temperatures • Earth’s giant water supply causes temperatures to stay within limits that permit life - stores huge amount of heat from the sun when warm - gives off heat to warm the air during cold conditions • Evaporative cooling removes heat from the Earth and from organisms - when a substance changes physical state from a liquid to a gas, the surface of the liquid remaining behind cools down - molecules with the greatest energy vaporize first

9. Sweating A mechanism of evaporative cooling Fig 2.14

10. The Biological Significance of Ice Floating • When most liquids get cold, their molecules move closer together  colder  freezes  solid • But when water molecules get cold, they move apart, forming ice - a chunk of ice has fewer molecules than an equal volume of liquid water • The density of ice is lower than liquid water. - ice floats because it is less dense than liquid water around it - H bonds in solid ice last longer than in liquid water • Since ice floats, ponds, lakes, and even the oceans do not freeze solid - Marine life could not survive if bodies of water froze solid

11. Why ice floats Compare the tightly packed molecules in liquid water with the spaciously arranged molecules in the ice crystal. The more stable H-bonding in ice holds the molecules apart, resulting in ice being less dense than liquid water. The expansion of water as it freezes can crack boulders when water is in crevices. Fig 2.15

12. Water as the Solvent of Life • A solution is a liquid consisting of two or more substances evenly mixed - The dissolving agent is called the solvent - The dissolved substance is called the solute • When water is the solvent, the result is an aqueous solution - fluids of organisms are aqueous solutions - it is the solvent inside all cells, in blood, and in plant sap • Water is the solvent for many chemical reactions - dissolve ionic salts and many polar molecules

13. Na+ and Cl- at the crystal surface have affinities for different parts of the H2O molecules. Positive Na+ attract electrically negative oxygen regions (red) of the H2O molecules. The negative Cl- attract the positively charged H+ regions (gray) of water; result H2O molecules surround the ions dissolving the crystal. Fig 2.16

14. Acids, Bases, and pH • In organisms most water molecules are intact - some break apart into hydrogen ions (H+) and hydroxide ions (OH-) - a balance of these 2 ions is critical for proper functioning of chemical processes within organisms • Acid - a chemical compound that releases (donates) H+ ions to solutions • Base - a compound that accepts H+ ions and removes them from solution • To describe the acidity of a solution, we use the pH scale

15. pH Scale • A measure of the H+ concentration in a solution - ranges from 0 (most acidic) to 10 (most basic) - each unit is a 10fold change in H+ concentration • pH 7 = neutral • < pH 7 = acidic • > pH 7 = basic Figure 2.17

16. Buffers • A slight change in pH can be harmful to an organism • Biological fluids contains buffers, substances that harmful changes in pH - they accept H+ ions when they are in excess - they donate H+ ions when they are depleted • Buffering is not foolproof - acid precipitation

17. Effects of Acid Precipitation Acid fog and rain contributed to the death of many fir trees in this forest in the Czech Republic. Acid precipitation results from water in the atmosphere reacting with pollutants as car, factories, and power plant exhausts. Fig 2.18

18. Evolution Connection: Earth Before Life • Chemical reactions and physical processes on the early Earth created an environment that made life possible • Earth began as a cold world about 4.5 billion years ago when gravity drew together dust and ice orbiting a young sun about 4.5 billion years ago • The planet eventually melted from heat produced by: - Compaction - Radioactive decay - Impact of meteorites

19. Molten material sorted into layers of varying density - Most of the iron and nickel sank to the center and formed a dense core - Less dense material became concentrated in a layer called the mantle, which surrounds the core - And the least dense material solidified to form a thin crust • Present continents, including North America, are attached to plates of crust that float on the flexible mantle

20. First atmosphere was probably composed of hot hydrogen gas - gas escaped because Earth’s gravity was not strong enough - a new atmosphere was formed from the gases belched from volcanoes. - scientists have speculated that the second early atmosphere consisted mostly of water vapor, CO, CO2, N2, CH4, and NH3 • The first seas formed from torrential rains that began when Earth had cooled enough for water in the atmosphere to condense • In addition to an atmosphere very different from the one we know, lightning, volcanic activity, UV radiation were much more intense when Earth was young - in such a seemingly inhospitable world, life began about 3.5–4.0 billion years ago.

21. The first atmosphere, which was probably composed mostly of hot hydrogen gas (H2), escaped. The gravity of Earth was not strong enough to hold such small molecules. Volcanoes belched gases that formed a new atmosphere (Figure 2.19). Based on analysis of gases vented by modern volcanoes, scientists have speculated that the second early atmosphere consisted mostly of water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), methane (CH4), and ammonia (NH3). The first seas formed from torrential rains that began when Earth had cooled enough for water in the atmosphere to condense. In addition to an atmosphere very different from the one we know, lightning, volcanic activity, and ultraviolet radiation were much more intense when Earth was young. In such a seemingly inhospitable world, life began about 3.5–4.0 billion years ago.

22. Figure 2.19