Cytology: Chemical Composition and Cell Structure
What you must understand
Cells contain inorganic substances (water, mineral ions) and organic ones: proteins, carbohydrates, lipids, nucleic acids, ATP, vitamins. Water is the solvent and participant in hydrolysis; ions buffer and stabilize structures. Proteins are built from amino acids joined by peptide bonds; their function follows from primary structure folded into secondary, tertiary and quaternary levels, and denaturation destroys the higher levels while keeping the primary. Carbohydrates are energy and structural material (cellulose, chitin, glycogen, starch). Lipids form membranes and are the most energy-dense reserve. Nucleic acids store and transmit information.
Organelles: the nucleus stores DNA and directs synthesis; ribosomes assemble proteins; the endoplasmic reticulum (rough and smooth) transports and synthesizes; the Golgi apparatus modifies, packages and secretes; lysosomes digest; mitochondria perform aerobic respiration; plastids (chloroplasts, leucoplasts, chromoplasts) carry out photosynthesis or storage; the cell wall of plants (cellulose) and fungi (chitin) differs from the animal glycocalyx. Prokaryotes have no membrane-bound nucleus or organelles, only a nucleoid, ribosomes, and often a capsule, flagella, and plasmids.
Traps
- Do not attribute mitochondria to prokaryotes, or a nucleus to bacteria.
- The nucleolus is inside the nucleus and makes ribosomal RNA, not proteins.
- Smooth ER makes lipids, rough ER makes proteins for export - questions swap these.
- Cellulose is a plant wall; chitin is a fungal wall and arthropod exoskeleton.
Rule of thumb
Match each organelle to a single verb: ribosome - assemble, mitochondrion - release energy, lysosome - digest, Golgi - package, chloroplast - capture light.
Worked example: A cell with a cellulose wall, large vacuole, chloroplasts and no centrioles is a mature plant cell; absence of centrioles is typical of higher plants.