Chapter 2 The Chemical Context of Life Campbell and Reece. Biology 6th edition

Chemical Elements and Compounds □Matter consists of chemical elements in pure form and in combinations called compounds. □Organisms are composed of matter □Matter is anything that takes up space and has mass □rocks, metals, oils, gases, plants, animals

Chemical Elements and Compounds □A substance that cannot be broken down to other substances by chemical reactions is an element. □92 elements occur in nature (gold, copper, oxygen, etc)

Chemical Elements and Compounds □A compound is a substance consisting of two or more elements combined in a fixed ratio □Sodium chloride NaCl. (Na is a metal, Cl is a poisonous gas, together they are table salt.) □A compound has characteristics beyond those of its combined elements.

Chemical Elements and Compounds □Life Requires about 25 chemical elements □96% of living matter is C, O, H, N □Most of the remaining 4% is P, S, Ca, K □Trace elements-required by organisms in only small amounts □Fe-required by all life □I- essential ingredient of a hormone released by thyroid. Daily intake of only 0.15mg

Atoms and Molecules □Atomic structure determines the behavior of an element □An atom is the smallest unit of matter □Made up of subatomic particles: Neutrons (0), protons (+), electrons (-) □Neutrons and protons are in the atom’s nucleus □Electrons orbit the atom’s nucleus

Atoms and Molecules □Isotopes □All atoms of a given element have the same number of protons, but can differ in the number of neutrons □C 12 and C 14 are isotopes. Both have 6 protons but have a different number of neutrons

Atoms and Molecules □Atoms combine by chemical bonding to form molecules □Chemical bonds- atoms sharing or transferring electrons resulting in atoms staying close together □Covalent bonds- sharing of a pair of valence electrons (the outer electrons) by two atoms □H 2, O 2,H 2 O, CH 4

Atoms and Molecules □Ionic Bonds- a cation and an anion attract each other □Cation is a positively charged ion (a charged atom) □Anion is a negatively charged ion □Ionic bonds create ionic compounds or salts

Atoms and Molecules Covalent Bond Ionic Bond

Weak chemical bonds play important roles in the chemistry of life □In living organisms, most of the strongest chemical bonds are covalent ones, which link atoms to form a cell’s molecules. □When two molecules in the cell make contact, they may adhere temporarily by types of chemical bonds that are much weaker than covalent bonds □The advantage of weak bonding is that the contact between the molecules can be brief; molecules come together, interact, then separate.

Weak chemical bonds play important roles in the chemistry of life □Hydrogen Bonds □A type of weak chemical bond important to the chemistry of life. □Hydrogen bonds form when hydrogen atoms covalently bond to an electronegative atom. □In living organisms, the electronegative partners are usually oxygen or nitrogen atoms

Weak chemical bonds play important roles in the chemistry of life Oxygen Hydrogen The oxygen of one water molecule will hydrogen bond to the hydrogen of another molecule. This is what allows water to be cohesive (adhere to itself)

Weak chemical bonds play important roles in the chemistry of life □Van der Waals Interactions □Because electrons are constantly in motion, they are not always symmetrically distributed in the molecule; at any instant, they may accumulate by chance in one part of a molecule or another. □The resulting ever-changing “hot spots” of positive and negative charge that enable all atoms and molecules to stick to one another. □These van der waals interactions are weak and occur only when atoms and molecules are very close together

Weak chemical bonds play important roles in the chemistry of life

Chemical reactions make and break chemical bonds □The making and breaking of chemical bonds, leading to changes in the composition of matter, are called chemical reactions. □Photosynthesis, which takes place within the cells of green plant tissues, is a particularly important example of chemical reactions rearranging matter.

Chemical reactions make and break chemical bonds □This is the chemical equation that summarizes the process of photosynthesis 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 □The raw materials of photosynthesis are carbon dioxide, which is taken from the air, and water, which is absorbed from the soil. Within the plants, sunlight powers the conversion of these ingredients to glucose (a form of sugar) and the by-product oxygen □In the end, there has been a rearrangement of molecules to produce sugar.

Chemical reactions make and break chemical bonds □One of the factors affecting the rate of reaction (how often reactions occur), is the concentration of reactants. □The points at which the concentrations of reactants offsets the concentrations of products is called chemical equilibrium □This is called dynamic equilibrium; reactions are still going on, but with no net effect of the concentrations of reactants and products □Equilibrium does not mean that the concentrations are equal, but that their concentrations have stabilized