1. The Chemical Context of Life Chapter 2

2. Life depends on Chemistry Life is organized into a hierarchy of structural levels. emergent properties appear at each successive level

3. Matter Organisms are composed of matter Matter is anything that takes up space and has mass. Matter consists of chemical elements in pure form and in combinations called compounds

4. Element Pure substance made up of one kind of atom Cannot be broken down into other substances by chemical means Shown by a short hand symbol Either a capital letter only  C, N, H, O Or a capital and lower case letter  Ca, Na, Mg

5. Chemical Compound A compound is a substance consisting of two or more elements in a fixed ratio. Physical and chemical properties usually very different from those of the elements from which they are formed

6. Chemical Compound Table salt (sodium chloride or NaCl) is a compound with equal numbers of chlorine and sodium atoms.

7. Pure sodium  metal, chlorine  gas, combination forms an edible compound.

8. Life Requires 25 Elements More than 100 known, about 25 found in living organisms Key Element is Carbon 4 make up 96% C  carbon N  nitrogen O  oxygen H  hydrogen Most of the remaining 4%  Phosphorus, Potassium, Sulfur (CHNOPS) Calcium  inorganic catalyst Trace elements  <0.01% but are essential Ex: 0.15mg Iodine/day  Thyroid function

9. Chemical Properties Based on Structure of Atoms Atom is the smallest unit of matter that still retains the properties of an element.  Atoms  composed subatomic particles. Neutrons and Protons, are packed together to form a dense core, the atomic nucleus, at the center of an atom. Electrons form a cloud around the nucleus.

10. Atoms Protons  positively charged Neutrons  no charge Electron  (-), are in constant motion Atomic Number  # of Protons

11. Atomic mass  # of protons + # of neutrons Carbon  6P +6N=12 A neutron and a proton  almost identical in mass,  1.7 x 10 -24 gram per particle. The dalton, is used to measure the mass subatomic particles, atoms or molecules Mass of a neutron or a proton = 1 dalton. The mass of an electron is about 1/200th that of a neutron or proton. The contribution of electrons when determining the total mass of an atom is ignored

12. Attraction between (+) Proton & (-) Electron keep the electrons in the vicinity of the nucleus.

13. Isotopes A given element have the same number of protons, they may differ in the number of neutrons Two atoms of the same element that differ in the number of neutrons are called isotopes In nature, an element occurs as a mixture of isotopes  99% of carbon atoms have 6 neutrons ( 12 C).  Most of the remaining 1% of carbon atoms have 7 neutrons ( 13 C) while the rarest isotope, with 8 neutrons is 14 C.

14. Radioactive Isotopes The nuclei are unstable and decay spontaneously, giving off particles and energy In its decay, an neutron is converted to a proton and electron.  This converts 14 C to 14 N, changing the identity of that atom.

15. Radioactive isotopes have many applications in biological research.  Radioactive decay rates can be used to date fossils.  Radioactive isotopes can be used to trace atoms in metabolism.

16. Electrons and Reactivity Electrons in the highest energy level of an atom determines how the atom reacts 1st / lowest energy level – can hold 2 electrons 2nd / highest energy level – can hold 8 electrons, when there is a partially filled energy level the atoms become chemically reactive, tend to react with other atoms

17. Bonding Properties Effect of electrons  chemical behavior of an atom depends on its electron arrangement  depends on the number of electrons in its outermost shell, the valence shell How does this atom behave?

18. Bonding properties Effect of electrons  chemical behavior of an atom depends on number of electrons in its outermost shell How does this atom behave?

19. Elements & their valence shells Elements in the same row have the same number of shells

20. Elements & their valence shells Elements in the same column have the same valence & similar chemical properties

21. Elements & their valence shells Moving from left to right, each element has a sequential addition of electrons (and protons)

22. Chemical reactivity Atoms tend to  Complete a partially filled outer (valence) electron shell or  Empty a partially filled outer (valence) electron shell  This tendency drives chemical reactions

23. Ionic bonds-Transfer of an electron Forms + & - ions  + = cation  – = anion Weak bond

24. Covalent bonds Two atoms need an electron Share a pair of electrons Strong bond  both atoms holding onto the electrons Forms molecules example:  water = takes energy to separate

25. Double covalent bonds Two atoms can share more than one pair of electrons  double bonds (2 pairs of electrons)  triple bonds (3 pairs of electrons) Very strong bonds

26. Multiple covalent bonds 1 atom can form covalent bonds with two or more other atoms  forms larger molecules  ex. carbon

27. Polar covalent bonds Pair of electrons not shared equally by 2 atoms Water = O + H oxygen has stronger “attraction” for the shared electrons than hydrogen oxygen has higher electronegativity

28. Polar Covalent Bonds 2 hydrogens in the water molecule form an angle Water molecule is polar  oxygen end is –  hydrogen end is + Leads to many interesting properties of water….

29. Hydrogen bonds Positive H atom in 1 water molecule is attracted to negative O in another Can occur wherever an -OH exists in a larger molecule Weak bonds

30. Van der Waals interactions Nonpolar covalent bonds can have partially negative and positive regions. Because electrons are constantly in motion, there can be periods when they accumulate by chance in one area of a molecule. This creates ever-changing regions of negative and positive charge within a molecule. Molecules or atoms in close proximity can be attracted by these fleeting charge differences, creating van der Waals interactions. While individual bonds (ionic, hydrogen, van der Waals) are weak, collectively they have strength

31. Weak Bonds H- Bonds  bond to electroneg elements,lasts only 1/109 sec, strength is 1/20 of a H/C covalent bond Van der Waals  large org. mols. Close to each other have transient +/- zones  temp interactions Ionic  protiens have =?- charges at diff points  holds shape  Ex: sickle cell anemia  1 neg charge disappears  deformed, can only bind w/1 oxygen instead of 4  low flow of oxy

32. Form fits Function Living systems are very delicate  never have ionic bonding  use polar covalent and non-polar covalent bonds The shape of a molecule is related to its biological function

33. Molecules with similar shapes can interact in similar ways  For example, morphine, heroin, and other opiate drugs are similar enough in shape that they can bind to the same receptors as natural signal molecules, called endorphins.  Binding to the receptors produces euphoria and relieves pain. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 2.19

34. Chemical Reactions Bonds are made or broken

35. Chemical Equilibrium The rate of formation of products is the same as the rate of breakdown of products (formation of reactants)  products and reactants are continually being formed, but there is no net change in the concentrations of reactants and products.  the concentrations of reactants and products are typically not equal, but their concentrations have stabilized.