Life s Chemical Basis

Transcription

1 Life s Chemical Basis

2 Life s Chemical Basis Ø Atoms and Elements Ø Why Electrons Matter Ø Atomic Bonds Ø Water molecule properties Ø Hydrogen Power (ph)

3 Matter & Elements Ø Matter is anything that occupies space and has mass Ø Matter is composed of chemical elements Ø An element is a substance that cannot be broken down into other substances

4 Elements and Atoms Ø Each element consists of one kind of atoms Ø An atom is the smallest unit of matter that still retains the properties of an element Ø The three subatomic particles are: Protons are positively charged Neutrons are electrically neutral Electrons are negatively charged

5 Terminology/ Elements Ø Atomic Number: The number of protons in an atom atomic number element symbol mass number Ø Mass Number: The sum of the numbers of protons and neutrons in the nucleus

6 Periodic Table of Elements atomic number element symbol mass number elemental substance element name carbon

7 Elements Required for Life Ø O, C, H, N make up about 96% of the human body weight Ø Ca, P, K, S, Na, Cl, Mg make up most of the rest Ø Trace elements make less than 0.01% of human body weight Required in minute amounts but they are crucial!

8 Isotopes Isotopes of an element have the same number of protons but different numbers of neutrons (different mass numbers)

9 Isotopes Stable isotope: enough binding energy to hold the nucleus together permanently 12 C and 13 C Unstable isotope, Radioactive isotopes, Radioisotopes Isotope with an unstable nucleus/ 14 C the nucleus decays spontaneously, giving off particles and energy

10 nucleus of 14 C, with 6 protons, 8 neutrons nucleus of 14 N, with 7 protons, 7 neutrons

11 Ø Under natural conditions, elements occur as a mixture of isotopes Ø Different isotopes of an element behave identically in chemical reactions Ø Living cells cannot distinguish between isotopes of the same element Ø Isotopes have fixed half-life/ radioisotopes decay at a predictable rate into predictable products 14 C 14 N

12 Biological Applications of Radioactive Isotopes Basic Research/ radioactive tracers/ detectable component/can be tracked after delivery into the body or system Archaeological Dating/ based on decay rate of radioisotopes (half life) Medical Treatments and Diagnosis

13 β-amyloid/ PiB

14 Why do Atoms Interact? Ø Of the three subatomic particles, only electrons are directly involved in chemical activity

15 Arrangement of electrons around the nucleus Ø Electrons occupy different orbitals (volumes of space around an atom s nucleus) Ø Orbitals are filled from lower to higher energy Ø The farther an electron is from the nucleus, the greater its energy shell model

16 one proton first shell hydrogen (H) one electron helium (He) second shell carbon (C) oxygen (O) neon (Ne) third shell sodium (Na) chlorine (Cl) argon (Ar)

17 Vacancies/ Can the outer shell hold more electrons? Ø No vacancies: an atom s outermost shell is filled with electrons Ø Most stable state Ø Vacancy: an atom s outermost shell has room for other electrons Ø Chemically active; atoms interact with one another Ø Example: oxygen atom has six electrons in its outer (second) shell, which can hold eight Oxygen (O) Neon (Ne)

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19 Chemical Bonds An atom can get rid of vacancies by participating in chemical bonds with other atoms Atoms with incomplete outer shells share, donate or receive electrons Chemical bond is the attractive force that keeps atoms together in a molecule

20 Ø Atoms with an unequal number of protons and electrons are called ions Ø Carry a net (overall) charge electron loss Sodium atom 11p + 11e - charge: 0 Sodium ion 11p + 10e - charge: +1

21 Compounds Ø A compound is a substance consisting of two or more different elements in a fixed ratio Ø Compounds are more common than pure elements Sodium + Chlorine Ø Sodium chloride, table salt, is a common compound of equal parts of sodium (Na) and chlorine (Cl) Sodium Chloride

22 Types of chemical bonds Ø Ionic Bonds Ø Covalent Bonds Ø Hydrogen Bonds

23 Ionic Bonds Ø An ion is an atom or group of atoms with an electrical charge resulting from gain or loss of electrons Ø Two ions with opposite charges attract each other Ø When the attraction holds the ions together ionic bond

24 Example of ionic bond: Sodium Chloride Transfer of electron Sodium atom Chlorine atom Na + Sodium ion Cl - Chloride ion Sodium chloride (NaCl)

25 Ions retain their respective charges when participating in an ionic bond Electronegativity measure of the ability of an atom to pull electrons away from other atoms Polarity separation of charge into positive and negative regions

26 Covalent Bonds Ø Two atoms with incomplete outer shells share one or more electrons

27 Types of Covalent Bonds Non-polar: Ø Bond between atoms with the same electronegativity Polar: Ø Bond between atoms of different electronegativity Methane Water

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30 Hydrogen Bonds Ø Hydrogen, as part of a polar covalent bond, has a partial positive charge Ø Hydrogen atoms that are covalently bonded in a molecule can be attracted to atoms with slight negative charges in other molecules such as oxygen and nitrogen

31 Ø Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond Ø H-bonds help define the properties of water, create the shape of proteins, hold DNA strands together, etc

32 Water s Life-supporting Properties Ø 70% of Earth is covered with water Ø All living organisms require water more than any other substance Ø Most cells are surrounded by water, and cells themselves are about 70-95% water

33 Polar Molecule Hydrogen Bonds

34 Universal Solvent Solution/ homogeneous mixture Solvent + Solute Aqueous solution hydrophilic ( water-loving ) molecules/ polar hydrophobic ( water-hating ) molecules/ non-polar

35 Ø Water is an effective solvent because of its polarity Ø When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules, a hydration shell Universal Solvent

36 Non ionic compounds such as sugar dissolve as water molecules surround them and form hydrogen bonds with their polar regions Universal Solvent

37 Cohesion Ø The tendency of molecules of the same kind to stick together Hydrogen bonds make liquid water cohesive Surface tension Ø Cohesion in water is much stronger than other liquids

38 Water stabilizes temperature Ø Because of hydrogen bonds between water molecules, it takes more heat to raise the temperature of water compared with other liquids (Temperature: measure of molecular motion) Ø Below 0 C (32 F), water molecules become locked in the bonding pattern of ice (lattice pattern) Ø Sheets of ice that form on the surface of ponds, lakes, and streams insulate the water Ø Protects aquatic organisms during cold winters

39 Ice is less dense than liquid water Ice floats in liquid water because hydrogen bonds in ice are more ordered, making ice less dense

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41 Ø ph: measure of the number of hydrogen ions in a fluid Ø Base: substance that accepts hydrogen ions in water Ø ph above 7 Ø Acid: substance that releases hydrogen ions in water Ø ph below 7

42 Ø Most biological systems can function properly only within a narrow range of ph Ø The fluids inside cells stay within a consistent range of ph because they are buffered Ø Buffer: set of chemicals that can keep the ph of a solution stable by alternately donating and accepting ions that contribute to ph

43 14 13 oven cleaner bleach 11 household ammonia 10 9 Tums baking soda blood, tears pure water milk corn urine, tea, typical rain bananas vinegar acid rain tomatoes, wine cola lemon juice 1 gastric fluid 0

44 HCl is a strong acid (the gastric fluid) Carbonic acid is a weak acid: CO 2 + H 2 O = H 2 CO 3 H 2 CO 3 H + + HCO 3 - H 2 CO 3 Carbonic Acid Bicarbonate

45 Changes in Acidity can Have Environmental Consequences Ø Acid rain (ph < 5.2)

46 Ø Ocean acidification CO 2 + H 2 O H 2 CO 3