CHAPTER 2. Life s Chemical Basis

Transcription

1 CHAPTER 2 Life s Chemical Basis

2 The Chemistry of Life We are made up of elements. Atoms of one kind make up an element. Atoms are the smallest unit of an element still maintaing the element s properties. 25 of 92 naturally occurring elements are essential for life. Carbon, oxygen, hydrogen and nitrogen make up 96% of living matter.

3 Human Composition The most common elements in the human body are oxygen, carbon and hydrogen. Oxygen 61.0% Carbon 23.0 Hydrogen 10.0 Nitrogen 2.6 Calcium 1.4 Phosphorus 1.1 Potassium 0.2 Sulfur 0.2 Trace elements are required in minute quantities. ex. iodine essential for thyroid

4 Atoms Made of three subatomic units: protons- positive charge (p + ), in nucleus neutrons-no charge (n 0 ), in nucleus electrons- negative charge (e-), cloud around nucleus Atomic number = # of protons ex. carbon s number is 6 Mass number = protons + neutrons (isotopes vary)

5 Orbitals Atoms are reactive if there are unpaired electrons in one or more orbitals. Columns in periodic table behave similarly, same number of electrons in outer valence shell First orbital can hold 2 electrons, second can hold 8

6 Different Representations Three ways of showing the hydrogen atom

7 Isotopes Isotopes have the same number of protons but differ in the number of neutrons. Carbon has three isotopes: carbon-12 (99% of carbon in nature), carbon-14 (most of remaining 1%), and carbon-13, which all behave identically in biological reactions. Unstable, or radioactive isotopes decay spontaneously, giving off particles and energy and can transform to a different element if they give a proton. Ex. carbon-14 decays to nitrogen

8 Radioisotopes Radioactive isotopes can be used in biology to date fossils, as tracers in metabolism, and to monitor cancer growth. They release radiation which can be harmful to cells, but is thought to be safe in small amounts. The type and amount absorbed matters.

9 Tracers Radioisotopes can be substituted for stable molecules in biological pathways and then detected in the organism by radioactivity detecting devices. Melvin Calvin used carbon-14 to trace photosynthesis. The PET scanner is used in medicine to study metabolism--the rate at which cells take in glucose with a radioisotope attached, scanner forms image of body tissues. Some radioisotopes used to kill cancer cells.

10 PET Scanner

11 Bonding Electrons can move to outer orbitals when they gain energy such as plant pigments absorbing energy from the sun. Electrons drop to inner orbitals when they emit energy. Atoms with vacancies in outer shells will give up, acquire, or share electrons. Oxygen, carbon, hydrogen, nitrogen, and calcium are

12 Terms Molecule- 2 or more atoms joined in a chemical bond Compound- a molecule with 2 or more different elements in proportions that never vary, ex. H 2 O, table salt NaCl Mixture- 2 or more substances intermingle without bonding, proportions may vary, ex. sugar water

13 Major Bonds Ionic bonds- ions (atoms that gain or lose electrons take on a negative or positive charge, respectively) of opposite charges attract and stay close to each other, ex. NaCl Covalent bonds- atoms share electrons in hybrid orbital spanning both atomic nuclei if both atoms need an electron, this is stable and strong, and atoms can share one, two, or three electrons

14 More about covalent... If both atoms share the electrons equally, the molecule is nonpolar, there is no difference in the charges at the ends of molecule, ex. H 2, O 2, N 2 If electrons are not shared equally, the molecule is polar, and the atom of one end of the molecule has more protons in its nucleus so it has a greater pull on the electrons, giving that end a slight negative charge, ex. H 2 O

15 Types of Bonds Hydrogen bonds- this is a weak attraction between covalently bound hydrogen atoms and an electronegative (polar) atom in a different molecule or in a different region of the same molecule in large molecules that fold back on themselves, ex. DNA the bonds form and break easily, but together can stabilize a molecule ex. liquid water

16 Ionic Bond, NaCl sodium atom electron transfer chlorine atom sodium ion chlorine ion

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18 Covalent Bond, Nonpolar Hydrogen H 2

19 Polar Covalent Bond slight negative charge slight positive charge slight positive charge water (H 2 O) H O H

20 Hydrogen Bond water molecules, DNA

21 Properties of Water POLARITY oxygen end slight neg. charge, hydrogen ends slight positive attracts ions and other polar molecules, creating hydrogen bonds makes water cohesive, a universal solvent, and have a high specific heat and heat of vaporization

22 Universal Solvent hydrophilic: molecules attracted to water, ex. sugar, ionic and polar molecules hydrophobic: molecules repelled by water, ex. oil, lipids in cell membrane, nonpolar molecules

23 Solvent Properties ions, polar molecules easily dissolve in water water clusters around solute forming sphere of hydration important in fluids for life (blood, sap)

24 spheres of hydration

25 Temperature Stabilizer- High Specific Heat cells release a lot of metabolic heat heat makes hydrogen bonds vibrate more, disrupting bonds, and some bonds break and water molecules escape water can absorb a lot of heat energy before it rises in temperature and vice versa

26 High Heat of Vaporization as water heats, molecular motion increases, some water molecules escape into the air, cooling the surface = evaporation prevents overheating, moderates Earth s climate

27 Aquatic Ecosystems hydrogen bonds are locked into position in ice, making ice less dense than liquid water ice can float on a lake, insulating the water and life below

28 liquid ice Text

29 Cohesion Property water resists rupturing when stretched or under tension hydrogen bonds create surface tension as plants transpire, cohesion of hydrogen bonds pulls replacement water molecules into cells animation

30 Acids and Bases water molecules split into H+ and OH- ions ph scale measures the concentration of hydrogen ions in solutions higher H+ = lower ph (less OH-) lower H+ = higher ph (more OH-)

31 Acids and Bases one unit increase on ph scale= 10 times less H+ one unit decrease on ph scale = 10 times more H+ lower ph= acids, donate or release H+ when dissolved in water, ex. lemon juice, gastric juice higher ph= bases, accept or combine with H+ when dissolved in water, ex. sea water, egg white

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33 Acids and Bases weak acids do not really want to donate, ex. carbonic acid, H2CO3 strong acids do

34 Salts any compound that dissolves easily in water and releases ions other than H+ and OHacid + base = salt: HCl + NaOH NaCl + H2O Na, K, Ca ions important for life

35 Buffers salt and weak acid working together to maintain homeostasis system to respond to slight changes in ph so cell processes are not disrupted blood has carbonic acid and bicarbonate