Introduction to Biological Macromolecules and Cell Composition

Macromolecules and Cellular Composition

  • Cells consist of approximately 70%70\% water and 30%30\% chemicals/macromolecules.

  • Chemical composition of a cell:

    • Lipids / Phospholipids: 2%2\%

    • Nucleic Acids: 7%7\%

    • Proteins: 15%15\%

    • Carbohydrates / Polysaccharides: 2%2\%

  • Monomers are single building block units that covalently link to form polymers:

    • Sugars build Polysaccharides / Carbohydrates.

    • Fatty acids build Lipids / Fats.

    • Amino acids build Proteins / Polypeptides.

    • Nucleotides build Nucleic Acids.

Proteins and Amino Acids

  • Proteins comprise over 50%50\% of a cell's dry mass.

  • Functions include cellular movement, catalysis (enzymes), structural support, storage, transport, cellular communication, and immunological defense (antibodies).

  • Structure and Traits:

    • Polymers formed from 2020 standard amino acids encoded by the genetic code; 99 are essential amino acids that must be ingested.

    • Amino acids contain a central structure with a carboxyl group, an amino group, and a variable R group (side chain).

    • R groups dictate chemical properties (hydrophobic, hydrophilic, acidic/negatively charged, or basic/positively charged).

Nucleic Acids: DNA and RNA

  • Nucleic acids store genetic information and direct protein production via genes.

  • Each human cell contains approximately 2 to 3 meters2\text{ to }3\,\text{meters} of DNA wrapped around histone proteins inside the nucleus, or in circular plasmids within prokaryotes.

  • Nucleotide Structure: Pentose sugar, phosphate group (forming a sugar-phosphate backbone), and a nitrogenous base.

    • Purines: Adenine (A), Guanine (G).

    • Pyrimidines: Cytosine (C), Thymine (T), Uracil (U).

  • Base Pairing in DNA Double Helix:

    • Adenine pairs with Thymine via 22 hydrogen bonds.

    • Guanine pairs with Cytosine via 33 hydrogen bonds.

  • Central Dogma: DNA is transcribed into RNA, which is translated into protein.

  • DNA vs. RNA Differences:

    • Sugar: Deoxyribose (DNA) vs. Ribose (RNA).

    • Bases: A, G, C, T (DNA) vs. A, G, C, U (RNA).

    • Strand Structure: Double-stranded helix, self-replicating (DNA) vs. Single-stranded, non-self-replicating (RNA).

    • Location: Nucleus, mitochondria, chloroplasts (DNA) vs. Nucleus, cytoplasm, ribosomes (RNA).

Lipids

  • Hydrophobic, non-polar hydrocarbon molecules that do not form polymers.

  • Major Types:

    • Steroids: Cell membrane components and signaling molecules (e.g., estrogen, testosterone).

    • Triglycerides: Energy storage in fat tissue.

    • Phospholipids: Structural components of plasma and organelle membranes.

  • Phospholipid Structure: Hydrophilic phosphate head and two hydrophobic hydrocarbon fatty acid tails forming a lipid bilayer.

  • Fatty Acid Types:

    • Saturated: Maximum hydrogen content, no double bonds, straight structure, solid at room temperature (animal-derived).

    • Unsaturated: Carbon-carbon double bonds cause kinks, liquid at room temperature (plant-derived).

    • Membrane Fluidity: The ratio of saturated to unsaturated lipids adjusts to maintain plasma membrane integrity across extreme environmental temperatures.

Carbohydrates

  • Sugar polymers utilized for cellular energy, storage, and structure.

  • Classification:

    • Monosaccharides: Single sugar units (carbon, hydrogen, oxygen in hexose or pentose rings); glucose is the preferred cellular energy source.

    • Disaccharides: Two linked monosaccharides (e.g., maltose, lactose).

    • Polysaccharides: Three or more linked monosaccharides.

  • Polysaccharide Functions:

    • Storage: Starch (plants, stored in starch granules) and Glycogen (animals, stored in liver and muscle tissue).

    • Structure: Cellulose (plant cell walls, cross-linked microfibrils) and Chitin (amine-modified polysaccharide in fungal cell walls, arthropod exoskeletons, and surgical threads).