Biological Macromolecules

Carbohydrates

  • Overview: Carbohydrates are one of the four major macromolecule classes; they provide energy storage, cell surface markers, and signaling roles. They are built from monomers called monosaccharides and can be organized into disaccharides and polysaccharides.

  • Building blocks and basic classifications:

    • Monomer: monosaccharide (one sugar)

    • Disaccharide: two sugars linked together

    • Trisaccharide: three sugars

    • Polysaccharide: 3+ sugar units (long chain)

    • Carbon-count classifications:

    • Triose: 3 C's

    • Pentose: 5 C's

    • Hexose: 6 C's

    • Carbonyl-location classifications:

    • Ketose: ketone as carbonyl within the molecule

    • Aldose: aldehyde at the end of the molecule

  • Ring vs linear forms and isomerism:

    • In aqueous solutions, sugars exist mainly in a ring form rather than linear

    • Isomers exist, including Alpha and Beta configurations for glucose

  • Disaccharides (two monosaccharides joined by a glycosidic linkage via a dehydration reaction):

    • Linkages described by the numbers of the involved carbons and the anomeric configuration

    • Examples:

    • Maltose: glucose + glucose, linkage ext{(α, α)} ext{-}1 ext{-}4 glycosidic linkage

    • Sucrose: glucose + fructose, linkage ext{(α, β)} ext{-}1 ext{-}4 linkage (note: transcription shows a mixed notation)

    • Lactose: glucose + galactose, linkage ext{(β, β)} ext{-}1 ext{-}4 linkage

    • Common examples are presented in the transcript with their linkages

  • Polysaccharides: long polymers used for energy storage or structural support

    • Starch (α-1,4 linkage): plant storage form of glucose

    • Amylose: unbranched

    • Amylopectin: branched

    • Glycogen (α-1,4 with α-1,6 branches): main animal storage form of glucose; highly branched

    • Cellulose (β-1,4 linkage): plant structural polymer in cell walls; not digested by most animals; arranged in parallel fibrils; not helical or branched

    • Chitin (β-1,4 linkage): structural polymer of N-containing glucose; found in fungal cell walls and insect exoskeletons; not helical or branched; can be digested by organisms with chitinase

  • Digestibility and differences:

    • Alpha glucose-based polysaccharides (starch, glycogen) are generally digestible by many animals

    • Beta glucose-based polysaccharides (cellulose, chitin) are not digestible by many animals unless enzymes like chitinase are present

  • Summary points to remember:

    • Polysaccharides differ in whether they are branched (glycogen, amylopectin) or unbranched (amylose, cellulose chains)

    • The ring form of glucose predominates in solution and contributes to isomerism (alpha vs beta)

Lipids

  • General properties:

    • Not true polymers but macromolecules

    • Hydrophobic; little to no solubility in water

  • Major types:

    • Triglycerides (fats)

    • Phospholipids

    • Steroids (sterols, e.g., cholesterol)

  • Triglycerides (fats): structure and function

    • Composed of: 1 glycerol backbone + 3 fatty acid tails

    • Used primarily for high-energy storage

  • Saturated vs. unsaturated fatty acids:

    • Saturated: all carbons saturated with hydrogens; no C=C double bonds

    • Unsaturated: contain C=C double bonds; higher energy and higher melting points generally

    • Hydrogenation converts unsaturated fats to saturated fats, lowering energy content and changing texture (e.g., Crisco)

    • Cis double bonds create kinks in the hydrocarbon chains and impact melting point

  • Phospholipids: amphipathic structure and membrane formation

    • Glycerol backbone with a phosphate group on the third carbon

    • Two fatty acid tails (hydrophobic)

    • Forms lipid bilayers in aqueous environments with hydrophobic tails inward and hydrophilic heads facing water

  • Steroids (Sterols):

    • Carbon skeleton of four fused rings

    • Cholesterol is the main animal steroid

    • Important as a membrane component and as a precursor to steroid hormones (e.g., estrogen, progesterone, testosterone, cortisol)

    • Small changes in side chains lead to large biological differences (example: estrogen vs. testosterone)

    • Individual variability can influence responses to stress hormones and metabolic outcomes

Proteins

  • Overview:

    • Most diverse macromolecule; >50% of dry cell weight

    • Polypeptides are polymers of amino acids (monomers)

    • A protein is one or more polypeptides folded into a functional conformation

    • Key relationship: conformation (shape) determines function

  • Amino acids: the building blocks

    • 20 naturally occurring amino acids

    • Each has:

    • an amino group (−NH₂)

    • a carboxyl group (−COOH)

    • a unique side chain (R-group)

    • At neutral pH, amino and carboxyl groups are ionized, giving positive and negative charges at the ends of the chain

    • Grouped by side-chain properties:

    • Non-polar (hydrophobic)

    • Polar (hydrophilic)

    • Charged (acidic/basic)

  • Peptide bonds and protein sequence:

    • Amino acids are linked by peptide bonds via a dehydration reaction

    • Each chain has an N-terminus (exposed amino group) and a C-terminus (exposed carboxyl group)

  • Protein structure levels:

    • Primary (1o): amino acid sequence

    • Secondary (2o): local folding into alpha-helices and beta-sheets; stabilized by hydrogen bonds between backbone atoms

    • Tertiary (3o): overall 3D shape; driven by side chain interactions (disulfide bridges, hydrophobic interactions, van der Waals, etc.)

    • Quaternary (4o): assembly of multiple polypeptides into a functional protein (e.g., hemoglobin)

    • Not all proteins reach quaternary structure; some exist only in tertiary form

  • Protein folding and stability:

    • Folding is guided by the primary sequence

    • Some proteins fold spontaneously; others require chaperonins to assist folding

    • Denaturation: extreme conditions (temperature, pH, salt, organic solvents) can unfold proteins, often abolishing activity; denaturation can be reversible or irreversible

  • Protein functions:

    • Enzymes: catalyze chemical reactions; often named for their function

    • Structural proteins: provide support and mechanical properties (e.g., collagen, elastin, keratin)

    • Storage proteins: store amino acids or energy for later use (e.g., ovalbumin in egg white, casein in milk)

    • Transport proteins: carry molecules (e.g., hemoglobin for O₂, proton pumps)

    • Hormonal and receptor proteins: signaling (e.g., insulin and its receptor)

    • Contractile/motor proteins: mediate movement (actin, myosin in muscles)

    • Immune defense proteins: antibodies

  • Enzymes (highlights):

    • Enzymes are often named for their function (e.g., sucrase breaks down sucrose)

    • Active site brings substrates and co-factors together to catalyze reactions

    • Enzymes act as catalysts, increasing reaction rate without being consumed

Nucleic Acids

  • Overview:

    • Acidic polymers of nucleotides found in the nucleus

    • Two main types: DNA and RNA

    • DNA stores genetic information; RNA expresses it

    • Central dogma: DNA
      ightarrow RNA
      ightarrow ext{protein}

  • Nucleotides:

    • Three parts: a five-carbon sugar, a nitrogenous base (A, C, G, T, U), and 1–3 phosphate groups

    • Nucleotides polymerize via phosphodiester linkages

    • Enzymes:

    • DNA polymerase synthesizes DNA

    • RNA polymerase synthesizes RNA

  • Structure of DNA:

    • 1) 5′-3′ connection: nucleotides connect through phosphodiester bonds; the α-phosphate on the 5′ C links to the 3′ C of the next sugar

    • 2) Double helix: two polymers run in opposite directions (anti-parallel) forming a twisted ladder

    • Sides: sugars + phosphate groups

    • Rungs: nitrogenous bases, paired by hydrogen bonds

    • 3) Base pairing:

    • A with T form two H-bonds

    • G with C form three H-bonds

    • Orientation: strands are anti-parallel, with one running 5′ to 3′ and the other 3′ to 5′

    • Chargaff’s rules enable prediction of complementary sequences

    • Example: 5′-A-C-C-G-T-G-A-3′ pairs with 3′-T-G-G-C-A-C-T-5′ on the complementary strand

Notes on cross-topic connections and implications:

  • Structure-function relationships are central across macromolecules: shape determines function in proteins; sequence determines folding and interactions in nucleic acids; bond types and linkages determine properties in carbohydrates and lipids.

  • Energy considerations: polysaccharides (starch, glycogen) provide readily mobilizable energy; fats store more energy per gram; proteins have diverse roles beyond energy storage (enzymes, signaling, structure).

  • Membrane biology hinges on amphipathic lipids (phospholipids) forming bilayers, with cholesterol modulating membrane fluidity in cholesterol-rich animal membranes.

  • Evolutionary and physiological relevance:

    • Beta-glucose polymers (cellulose, chitin) show how structural needs select for beta linkages that change digestibility and rigidity.

    • Hormones derived from steroids exemplify how small structural changes (e.g., in side chains) result in diverse physiological effects across organisms.

  • Practical implications:

    • Denaturation of proteins highlights sensitivity to environmental changes and underscores protein stability in industrial and physiological contexts.

    • Enzyme active sites and substrate specificity underpin drug design and metabolic regulation.

  • Key terminology for quick recall:

    • Monosaccharide, disaccharide, polysaccharide

    • Glycosidic linkage, dehydration synthesis

    • Alpha (
      \alpha\) and Beta (
      \beta\) isomers

    • Ester linkage, phosphodiester linkage

    • Peptide bond, N-terminus, C-terminus

    • Primary, secondary, tertiary, quaternary structures

    • Anti-parallel arrangement in DNA

    • Chargaff’s rules: A pairs with T, G pairs with C (with proper base-pairing counts)

End of notes