The Chemical Basis of Life

1
The Chemical Basis of Life

• Chapter 2

2
Matter

• Substance that has mass and takes up space
• Compose all living things
• Generally found in 1 of 3 states
• Composed of 1 or more elements

3
Elements

• 92 occur naturally on Earth
• 4 make up 96 of the human body (CHON)
• Composed of atoms

4
Atoms

• Smallest particles that retains properties of an
  element
• Made up of subatomic particles
• Protons () in nucleus
• Electrons (-) orbits nucleus
• Neutrons (no charge) in nucleus
• Protons and neutrons
• Mass of about 1
• Electrons
• Mass is negligible (1/2000)

5
Reading A Periodic Table

• Elements differ depending on the number of
  subatomic particles
• Atomic symbol
• 1st letter or 2 (usually)
• Atomic number
• Determined by number of protons
• Element specific
• Mass number
• Determined by number of protons neutrons

6
Isotopes

• Atoms with different number of neutrons
• Effects mass number how?
• Effects atomic number how?
• Can be stable or unstable (radioactive)
• Behave the same as respective element (electrons
  are key)
• Occur naturally as a mix in elements
• Living cells cant distinguish between them
• Applications
• Dating fossils
• Biological tracers
• Brain scanning
• Cancer treatments
• Dangers
• Radioactive atoms give off energy that destroys
  chemical bonds when they collide

7
Chemical Properties of Atoms

• Electrons are key
• Move in orbitals called shells
• Repel one another, but attracted to protons
• Electron shells
• Outermost determines chemical properties
• Closer to the nucleus lower energy and are
  filled first
• Holds up to 2 or 8 electrons
• Filled are unreactive
• Unfilled are reactive
• Number differs between atoms

8
Electron Shell Models
9
Chemical Bonds

• Hold 2 or more atoms together
• Complete outer shells
• By sharing, donating, or receiving electrons
• Form molecules (H2, I2, and O2) or compounds
  (H20, NaCl, C6H12O6)
• Demonstrates emergent properties
• 2 H (gas) O- (gas) H2O (water)
• Na (metal) Cl- (poisonous gas) NaCl (table
  salt)

10
Ionic Bond

• One atom loses electrons ? cation (charge?)
• Another atom gains these electrons ? anion
  (charge?)
• Charge difference attracts the two
• Very weak bond
• Table salt (NaCl)

11
Covalent Bond

• Atoms share outer pair or pairs of valence
  electrons
• Single, double, or triple covalent bond
• Non-polar covalent
• Electrons shared equally
• E.g. hydrogen gas/ H2/ H-H
• Polar covalent
• Electrons spend more time near most
  electronegative nucleus
• E.g. water/ H20
• electrons more attracted to O nucleus than to
  either H nuclei

12
Hydrogen Bond

• Positive charge on H attracts negative charge on
  another atom
• Individually weak, but often numerous strong
• Important to many biological compounds
• E.g. water
• Makes up 70 90 of all living things
• Bonds create unique properties

13
Properties of Water

• Solubility
• Solvent dissolving agent
• Solute substance that is dissolved
• Solution liquid mix of 2 substances
• Aqueous solution when water is solvent
• Polarity
• Like dissolves like
• Hydrophilic
• Sugar or salt and water
• Hydrophobic
• Oil and water

14
Properties of Water (cont.)

• Water movement
• Cohesion
• Creates surface tension
• Overfilling a glass, a belly flop, or beading
  of water
• Adhesion
• Water moves up plants against gravity or paper
  towels
• Temperature stabilization
• Molecule movt affects temp
• Hard to change H-bonds
• Increase ? ice formation
• Reduce ? evaporation
• Vary ? internal temp.

15
pH Scale

• Measures H of a solution (acidity)
• Change of 1 on scale means 10X change in H
  (logarithmic scale)
• Highest H Lowest H
• 0---------------------7-------------------14
• Acidic Neutral Basic

16
pH Scale (cont.)

• Acids
• Increase H
• pH less than 7
• Bases
• Decrease OH-
• pH greater than 7
• Neutrals
• Equal H and OH-
• pH of 7.0

17
Buffer Systems

• Minimize shifts in pH
• Can accept H when in excess and donate H when
  depleted
• Form water
• Common in biological fluids
• Human blood at 7.4, slight deviations can be
  deadly

18
Making and Breaking Bonds

• Chemical reactions are often reversible
• Chemical equilibrium forward and reverse
  reactions occurring at the same rate (no net
  change)
• Starting materials are reactants, ending are
  products
• of atoms are conserved
• 1st law of thermodynamics