Biology

Biological macromolecules

Carbohydrates, lipids, proteins, and nucleic acids — structure and role sketches.

Basics

Carbohydrates

Monosaccharides such as glucose link into di- and polysaccharides. Starch and glycogen store energy; cellulose scaffolds plant walls. α versus β glycosidic bonds decide digestibility. Blood-group antigens are sugar chains too. Energy density is lower than lipids, but carbs dissolve and burn quickly.

Lipids

Fatty acids and triglycerides are long-term fuel stores. Phospholipids point heads to water and tails inward to build membranes. Steroids such as cholesterol tune fluidity and hormone scaffolds. Unsaturation lowers melting point. The word fat hides several structures.

Proteins

Amino acids join by peptide bonds. Primary is sequence; secondary is α helix and β sheet; tertiary is the fold; quaternary is multiple chains. Enzymes fit substrates in an active site. Denaturation by heat or pH unfolds and kills function. Shared sequence often means evolutionary kinship.

Nucleic acids

DNA is a double helix with A–T and G–C pairs. RNA is usually single-stranded and uses U instead of T. The classroom slogan for information flow is DNA→RNA→protein. ATP is a nucleic cousin and the cell’s energy coin. Sequence alone does not reveal expression, folding, or cellular context.

Formulas

Peptide bond (condensation)

−COOH + H₂N− → −CO−NH− + H₂O

The ribosome joins them and releases one water.

ATP hydrolysis (sketch)

ATP + H₂O → ADP + Pᵢ

A common drive for cellular work. Real ΔG depends on concentrations.

Symbols

Pᵢ inorganic phosphate

DNA melting trend

T_m ↑ as %G·C ↑

G·C has three hydrogen bonds, so it holds tighter.

Symbols

T_m temperature at half-denaturation

Key table

Essential amino acids	humans cannot make enough; diet must supply them
Trans fats	from partial hydrogenation; often harsh on membranes and metabolism
Chirality	protein amino acids are nearly all L; sugars are mostly D