Unit 1.2 – Elements of Life

Essential Elements of Life (CHNOPS)

  • Carbon, hydrogen, oxygen, and nitrogen make up ~96% of living matter.

    • Carbon (C) – backbone of all organic molecules; forms 4 covalent bonds

    • Hydrogen (H) – part of water and organic compounds

    • Oxygen (O) – part of water; involved in cellular respiration

    • Nitrogen (N) – found in proteins and nucleic acids

    • Phosphorus (P) – part of DNA, RNA, ATP, phospholipids

    • Sulfur (S) – found in some amino acids; forms disulfide bridges

Carbon: The Backbone of Life

Why Carbon Is Special

  • Has 4 valence electrons → can form up to 4 covalent bonds

  • Can form:

    • Single, double, or triple bonds

    • Long chains

    • Branched structures

    • Rings

  • Bond type and position affect molecule shape and function

Organic Chemistry

  • Organic compounds contain carbon and hydrogen

  • All four biological macromolecules are carbon-based

  • Earth life is carbon-based life

Carbon Skeletons & Hydrocarbons

Carbon Skeletons

  • Carbon atoms bond to other carbons → chains

  • Skeletons vary by:

    • Length

    • Branching

    • Double bond location

    • Presence of rings

  • Skeletons form the framework of organic molecules

Hydrocarbons

  • Organic molecules containing only carbon and hydrogen

  • Nonpolar and hydrophobic

  • Serve as simple frameworks for more complex molecules

Functional Groups

Definition

  • Functional groups are specific chemical groups attached to a carbon skeleton

  • They determine a molecule’s:

    • Polarity

    • Solubility

    • Acidity/basicity

    • Hydrophilic or hydrophobic behavior

    • Chemical reactivity

Major Functional Groups (AP Focus)

Functional Group

Structure

Properties / Function

Hydroxyl

–OH

Polar, forms hydrogen bonds

Carbonyl

C=O

Found in aldehydes & ketones

Carboxyl

–COOH

Acidic, donates H⁺

Amino

–NH₂

Basic, accepts H⁺

Sulfhydryl

–SH

Forms disulfide bridges in proteins

Phosphate

–PO₄³⁻

Stores/transfers energy (ATP), charged

Methyl

–CH₃

Nonpolar, affects gene expression

Macromolecules of Life

The Four Biological Macromolecules

  1. Carbohydrates

  2. Lipids

  3. Proteins

  4. Nucleic Acids

Carbohydrates

  • Elements: C, H, O (≈ 1:2:1 ratio)

  • Monomers: Monosaccharides (e.g., glucose)

  • Functions:

    • Quick energy

    • Structural support (cellulose)

Lipids

  • Elements: C, H, O (sometimes P)

  • Not true polymers

  • Types:

    • Triglycerides (fats)

    • Phospholipids (cell membranes)

    • Steroids (hormones)

  • Functions:

    • Long-term energy storage

    • Insulation

    • Cell membranes

Proteins

Structure

  • Polymers of amino acids

  • Linked by peptide bonds

  • Elements: C, H, O, N (sometimes S)

Protein Shape & Function

  • Structure determines function

  • Levels of structure:

    • Primary – amino acid sequence

    • Secondary – hydrogen bonds (α-helices, β-sheets)

    • Tertiary – R-group interactions (ionic, hydrophobic, disulfide bridges)

    • Quaternary – multiple polypeptide chains

  • Shape creates active sites and binding sites

Denaturation

  • Loss of 3D structure → loss of function

  • Caused by:

    • High temperature

    • Extreme pH

    • Salts, solvents, heavy metals

  • Peptide bonds are not broken

Enzymes (Proteins That Catalyze Reactions)

What Enzymes Do

  • Speed up biochemical reactions

  • Lower activation energy

  • Allow reactions to occur at biological temperatures

Substrate & Active Site

  • Substrate = reactant that binds enzyme

  • Active site is specific to the substrate

  • Specificity due to shape and chemical environment

    • Lock-and-key / induced fit models

Enzyme Catalytic Cycle

  1. Substrate binds → enzyme–substrate complex

  2. Transition state stabilized

  3. Products formed

  4. Products released; enzyme reused

Enzyme Regulation

  • Optimal conditions: normal temperature & pH

  • Least effective when:

    • Extreme pH

    • High heat (denaturation)

    • Low substrate concentration

    • Inhibitors present

Inhibitors

  • Competitive inhibitor: binds active site

  • Noncompetitive inhibitor: binds allosteric site, changes enzyme shape

Nucleic Acids

Structure

  • Polymers of nucleotides

  • Each nucleotide contains:

    • Sugar

    • Phosphate group

    • Nitrogenous base

  • Elements: C, H, O, N, P

Function

  • DNA: stores genetic information

  • RNA: transmits instructions for protein synthesis

Chemical Reactions in Biology

Dehydration vs. Hydrolysis

  • Dehydration (condensation):

    • Joins monomers

    • Removes H₂O

  • Hydrolysis:

    • Breaks polymers

    • Adds H₂O

Energy & Matter in Living Systems

  • Living systems require:

    • Constant energy input

    • Exchange of matter

  • Energy is stored in chemical bonds

  • Law of conservation of energy applies to life

  • Matter from environment builds macromolecules

Unit 1.2 Glossary – Elements of Life

Activation Energy:
    Minimum energy required to start a chemical reaction.

Active Site:
    Specific region of an enzyme where the substrate binds.

Amino Group:
    Functional group (–NH₂) that acts as a base by accepting H⁺.

Amino Acid:
    Monomer of proteins; contains an amino group, carboxyl group, hydrogen, and variable R group.

Carbon Skeleton:
    Chain of carbon atoms forming the backbone of an organic molecule.

Carbohydrate:
    Macromolecule made of sugars; used for energy and structural support.

Carboxyl Group:
    Functional group (–COOH) that acts as an acid by donating H⁺.

Catalyst:
    Substance that speeds up a chemical reaction without being consumed.

CHNOPS:
    Six elements essential to life: carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur.

Competitive Inhibitor:
    Molecule that competes with substrate for an enzyme’s active site.

Denaturation:
    Loss of a protein’s shape and function due to environmental changes.

Dehydration Reaction (Condensation):
    Reaction that forms a polymer by removing water.

Disulfide Bridge:
    Covalent bond between sulfur atoms that stabilises protein structure.

Enzyme:
    Protein that catalyzes biochemical reactions by lowering activation energy.

Functional Group:
    Specific group of atoms that determines a molecule’s properties and reactivity.

Hydrocarbon:
    Organic molecule consisting only of carbon and hydrogen; nonpolar.

Hydrolysis:
    Reaction that breaks polymers into monomers by adding water.

Induced Fit:
    Model describing how an enzyme changes shape to bind substrate more tightly.

Inhibitor:
    Molecule that reduces enzyme activity.

Lipid:
    Hydrophobic macromolecule used for energy storage, membranes, and signaling.

Macromolecule:
    Large biological molecule formed from smaller subunits.

Methyl Group:
    Functional group (–CH₃) that affects gene expression and molecular interactions.

Monomer:
    Small building block molecule of a polymer.

Noncompetitive Inhibitor:
    Inhibitor that binds outside the active site and alters enzyme shape.

Nucleic Acid:
    Macromolecule (DNA or RNA) that stores and transmits genetic information.

Nucleotide:
    Monomer of nucleic acids; made of sugar, phosphate, and nitrogenous base.

Organic Compound:
    Carbon-based molecule containing carbon–hydrogen bonds.

Phosphate Group:
    Functional group (–PO₄³⁻) involved in energy transfer and nucleic acids.

Polymer:
    Large molecule made of repeating monomers.

Primary Structure:
    Sequence of amino acids in a protein.

Protein:
    Macromolecule made of amino acids; functions include catalysis, structure, and signaling.

Quaternary Structure:
    Protein structure formed from multiple polypeptide chains.

Secondary Structure:
    Local protein folding stabilized by hydrogen bonds.

Substrate:
    Reactant that binds to an enzyme.

Sulfhydryl Group:
    Functional group (–SH) involved in protein folding.

Tertiary Structure:
    Overall 3D shape of a protein due to R-group interactions.