Lecture 2 Chemistry of Life

Living organisms share eight core features:

• Order — life is cell-based,complex and organised → cells → tissues → organs.

• Energy processing — autotrophs capture sunlight; heterotrophs consume food.

• Sensitivity/response to stimuli — organisms detect and react to environmental changes.

• Reproduction — passing genetic information to offspring.

• Growth & development — regulated by genetic programs.

• Homeostasis — maintaining stable internal conditions despite external change.

• Adaptation — traits that improve survival.

• Evolution — populations change over generations (e.g., MRSA antibiotic resistance).

Life is grouped into:

• 3 Domains: Bacteria, Archaea, Eukarya

• 6-8 Kingdoms: Bacteria, Archaea, Protists, Plants, Fungi, Animals

Elements Essential for Life

Major Element, minor element, trace element

Most of the human body is made from C, H, N, O (≈96%).
Minor but important ions include Ca, P, K, S, Na, Cl, Mg.
Trace elements (<0.01%) are also required for biochemical reactions.

Molecule = 2+ atoms bound by a chemical bond

Compound = 2+ different atoms bound

Carbon is uniquely suited for life because it is:

• Versatile — forms 4 covalent bonds, bonds with itself and other elements in different ways.

• Stable — “sweet spot” for forming strong but reactive bonds.

• Diverse — forms chains, rings, and complex polymers.
  All major biomolecules (carbs, lipids, proteins, nucleic acids) have carbon skeletons.

Molecular interactions with water:

hydrophobic/hydrophilic, amphipathic= a mixture (when a molecule has both polar and non‑polar regions, water interacts differently with each part)

C > neutral, non-polar/hydrophobic

O,N,P > polar/ hydrophilic, partly/fully charged

H>neutral

Emergent properties of water:

• Cohesion behaviour → cohesion leads to high surface tension

  • plants: water evaporates from leaves > capillary action

• Temperature moderation → high specific heat, high heat of vaporisation

• Water is less dense as solid than as a liquid

  • ice floats on water

  • H bonds keep molecules far enough apart to make ice about 10% less dense

• water is a versatile solvent due to its polarity

  • ideal for dissolving many substances

  • versatility facilitates trasnport of nutrients, waste, chemical change within organisms

carbohydrate types :

Types:

• Monosaccharides — simple sugars (glucose, fructose).

• Disaccharides — two sugars joined by glycosidic bonds (formed via a dehydration synthesis reaction, loss of H2O) (e.g., lactose).

• Polysaccharides:

  • Storage: starch (amylose(unbranched) + amylopectin(branched)), glycogen(more extensively branched than starch)

  • Structure: cellulose (plants), chitin (exoskeleton of arthropods 節肢動物), peptidoglycan (complex polymers found in bacteria cell walls, important targets for antibiotics ).

Made from C, H, O, produced by photosynthesis.

Lipids types and functions

Mostly C, H, O; hydrophobic, non-polar > made up of carboxylic acid group and a long hydrocarbon tail

• increased saturation (more hydrogen atoms, fewer double bonds) > increase melting temp

Types & Functions:

1. Energy stores

• Triacylglycerols

1. Signalling molecules

• Steroids

1. Protection and waterproofing

• Waxes

1. Membrane structure (with lipid bilayer)

• Phospholipids (glycerol + 2 fatty acids + phosphate.)> mostly hydrophobic but with polar end(amphipatic)

• Glycolipids

• Cholesterol

Saturated fats = no double bonds → higher melting point.
Unsaturated fats = double bonds → more fluid.

Nucleic Acids (DNA & RNA)

Made of nucleotides, each containing:

• Phosphate group (negatively charged)

• Sugar (ribose or deoxyribose)

• Nitrogenous base (A, G, C, T, U)

Nucleotides also function in energy storage (ATP), signal transduction, and metabolic regulation, immune function

difference between DNA and RNA

DNA vs RNA:

• DNA: deoxyribose, bases A–T–C–G

• RNA: ribose, bases A–U–C–G

Amino Acids

• building block of protein

• 20 common amino acids share:

  • Amino group (NH₃⁺)

  • Carboxyl group (COO⁻)

  • α‑carbon

  • R‑group (determines chemical properties)

  Categories:

  • Nonpolar (hydrophobic)

  • Polar (hydrophilic)

  • Charged (acidic/basic)

  each have a name, a 3-letter abbreviation, a 1-letter abbreviation