Chapter 2

The Nature of Life

Attributes of Living Organisms

• Composition and Structure: cell is the basic unit of life; prokarotes; eukaryotes; cytoplasm; DNA; plant cells have cell wall.

• Growth: increase in mass and volume and results from the production of new cells.

• Reproduction

• Response to Stimuli: Plants respond to wounding by forming a callose first then forms callus. Plants respond in different ways to environmental stimuli such heat, salinity, drought and flooding.

• Metabolism: All the biochemical reactions taking place in a living body. Cells have primary and secondary metabolism.

• Movement: cytoplasm movement is known as cyclosis or cytoplasmic streaming.

• Complexity of Organization: Cells of living organisms are made of biomolecules that are arranged in a variety of ways that differs from one species to another. This organization is genetically determined.

Chemical and Physical Basis of LifeA. The Elements: Units of Matter

• Matter is “stuff of the universe”

• Matter exists in three forms: solid, liquid and gas

• Matter has three characteristics: it occupies space, it has mass and is composed of elements (92 natural and 19 artificial)

• The atom is the smallest subdivision of an element. It is composed of a nucleus (protons + neutrons) and electrons.

• Atomic number = number of protons

• Atomic mass = number of protons and neurons

• Electrons exist in atomic orbitals around the nucleus

• Isotopes exist for atoms when the number of neutrons exceed the number of protons in the nucleus. The atom becomes unstable and splits to smaller parts releasing radioactivity.

B. Molecules: Combinations of Elements

• Compound is formed when two or more atoms unite in a definite ratio by chemical bonds.

• Molecule is the smallest independently existing particle of a compound or element.

• In living cells, chemical reactions between molecules are controlled by organic catalysts called enzymes.

• A water molecule has one side that has a slightly negative charge while the other side has a slightly positive charge (Polar Molecule).

• Due to its polarity, water molecules form a cohesive network and leads to the capillary movement of water in fine tubes (xylem of plants).

• The attraction between the hydrogen atoms of water and other negatively charged molecules causes adhesion (-ve cellulose in xylem tubes of plants).

C. Bonds and Ions

• Electrons can be lost, gained or shared between different atoms, forming bonds, so that the outermost orbital contains 8 electrons.

• Three type of chemical bonds are important in living organisms

- Covalent bonds form when two atoms share a pair of electrons between their outermost orbits. Sometimes more than two atoms are held together by covalent bonds. Every

pair of electrons shared is a single covalent bond (e.g. H2, CH4).

- Ionic bonds form when electrons are completely removed removed from one atom (+ve ion) and transferred to another atom (-ve ion) (e.g. NaCl -----> Na+ + Cl-).

- Hydrogen bonds form due to the attraction of a positively charged hydrogen atom in a polar molecule and a negatively charged atom (such as O or N) in another polar molecule. Hydrogen bonds are very abundant in biomolecules (DNA, proteins) and only have 7%-10% of the energy in a covalent bond.

D. Acids, Bases, and Salts

• Acids: release positively hydrogen (H+) ions when dissolved in water.

• Bases: release negatively charged hydroxyl (OH-) ions when dissolved in water.

• Salts: when an acid and a base are mixed, the H+ ions of the acid bond with the OH- ions of the base forming water and the remaining ions bond together forming a salt.

HCl + NaOH ------------------> H2O + NaCl

• pH is the hydrogen (H+) ions concentration of solution (ranges from 0 -14).

• Pure water has a pH of 7.0

• Acids have a pH below 7 and alkalines have a pH above 7.

E. Energy:

• definition: is the ability to do work.

• First Law of Thermodynamics: Energy is constant; it cannot be increased or diminished, but it can be converted from one form to another.

- Energy has different forms: chemical, electrical, heat and light.

• Second Law of Thermodynamic: When energy is converted from one form to another form, the amount of useful energy remaining is always less than before.

- Energy in motion is kinetic energy

- Energy is storage form is potential energy

• Electrons have varying amounts of potential energy. The closer to the nucleus the electron is the less potential energy it carries.

E. Chemical components of Protoplasm:

• Organic molecules contain C while inorganic molecules no C.

• Macromolecules- biological macromolecules are polymers

- polymers are composed of identical, smaller molecules called monomers

- bonding of molecules occurs by dehydration synthesis

- polymers broken down into monomers by reverse reaction called hydrolysis

• Four important classes of polymers

1. Carbohydrates: molecules composed of C, H, & O in a ratio (CH2O)n.

Examples: - Monosaccharides (glucose, fructose)

- Disacchardis (sucrose)

- Polysaccharides (starch, cellulose)

2. Lipids: fatty or oily substances that are insoluble in water

- Fats & Oils: saturated, unsaturated

- Waxes

- Phospholipids

3. Proteins, Polypeptides, and Amino Acids

- proteins are composed of amino acids

- amino acids: 20 essential amino acids, each contains an amino group NH2, carboxyl group COOH, and a side chain R which is different for each of the twenty different essential amino acids.

- polypeptides are two or more amino acids bonded together by a peptide bond

- structural organization of peptides (proteins)

- primary

- secondary

- tertiary

- quaternary

• Enzymes:

- are large complex proteins

- a special type of RNA can function as an enzyme (Ribozyme)

- all enzymes are organic catalysts that have lower energy of activation

- enzyme names end in -ase, e.g. maltase

4. Nucleic Acids:

- are complex polymers consisting of repeating subunits called nucleotides

- Types

- DNA (deoxyribonucleic acid)

- RNA (ribonucleic acid)