Cell Biology and Biomolecules
Cell structure
The exam tests prokaryotic versus eukaryotic organisation, plant cell organelles, membrane structure, and the chemical nature and tests of biomolecules. Enzyme action and factors affecting it are also tested.
Core rules
- Cell theory states that cells are the structural and functional units of life and arise from pre-existing cells.
- Plant cells have a cellulose wall, middle lamella of calcium pectate, plasmodesmata and a large vacuole bounded by the tonoplast.
- Mitochondria and chloroplasts are semi-autonomous, with 70S ribosomes and circular DNA; cytoplasmic ribosomes are 80S.
- The plasma membrane follows the fluid mosaic model: a phospholipid bilayer with embedded proteins.
- Carbohydrates include monosaccharides, disaccharides and polysaccharides; proteins are amino acid polymers; lipids are hydrophobic; nucleic acids are DNA and RNA.
- Enzymes are protein biocatalysts with active sites; they lower activation energy and are not consumed. Cofactors include coenzymes and prosthetic groups.
Worked example
A solution turns Benedict's reagent brick red but gives no blue-black colour with iodine. Reasoning: Benedict's detects reducing sugar, so a reducing sugar is present; iodine detects starch, so starch is absent or below detection. Sucrose is non-reducing and gives a negative Benedict's test unless hydrolysed first.
Traps
Starch and sucrose are not reducing sugars. Enzymes are not used up in reactions. A coenzyme is loosely bound; a prosthetic group is tightly bound. Do not say 70S ribosomes occur only in prokaryotes; they also occur in mitochondria and chloroplasts.