Levels of Organisation of Life and Cell Chemistry
Life is organised hierarchically: molecule → organelle → cell → tissue → organ → organ system → organism → population → species → biogeocoenosis (ecosystem) → biosphere. Each level has its own laws, and exam questions often ask which level a given object belongs to. A mitochondrion is an organelle; a leaf is an organ; a herd of deer is a population; a forest is a biogeocoenosis.
The cell is the elementary structural and functional unit of life. Its chemical composition falls into inorganic substances (water, mineral salts) and organic substances (proteins, lipids, carbohydrates, nucleic acids, ATP). Water is the most abundant compound and the universal solvent; mineral ions maintain osmotic pressure and buffer systems. Proteins are built from amino acids and perform enzymatic, structural, transport and protective roles. Carbohydrates are the main energy source and structural material (cellulose in plant walls). Lipids store energy and form membranes. Nucleic acids store and transmit hereditary information: DNA is double-stranded and contains deoxyribose, RNA is usually single-stranded and contains ribose; DNA holds A–T and G–C pairs.
Traps
Questions mix up the monomer and the polymer (glucose vs starch, amino acid vs protein, nucleotide vs nucleic acid) and the roles of DNA versus RNA. Remember that ATP is a universal energy carrier, not a storage polymer.
Worked example
"Which substance is a polymer?" Options: glucose, amino acid, starch, water. Starch is a polymer of glucose; glucose and the amino acid are monomers; water is inorganic. Answer: starch.