Cell Biology and Biomolecules
What the domain covers
Cell biology and biomolecules tests cell structure and function, differences between prokaryotic and eukaryotic cells, organelles, membrane organisation, and the chemistry of carbohydrates, proteins, lipids, nucleic acids, vitamins, and enzymes. Expect questions on bonds, polymer structure, enzyme kinetics, and how structure relates to function.
Core rules and concepts
- Prokaryotes lack a membrane-bound nucleus and organelles; ribosomes are 70S. Eukaryotes have a nucleus, membrane-bound organelles, and 80S cytoplasmic ribosomes (70S in mitochondria and chloroplasts).
- Carbohydrates: glucose is a monosaccharide; starch and glycogen are storage homopolymers of α-glucose; cellulose is structural β-1,4-glucan.
- Proteins are polymers of amino acids joined by peptide bonds; primary to quaternary structure determines function.
- Lipids include triglycerides, phospholipids, and steroids; phospholipids form bilayers because of their amphipathic nature.
- Nucleic acids are nucleotides joined by phosphodiester bonds; DNA stores information, RNA participates in expression.
- Enzymes lower activation energy, are usually proteins, and show specificity, pH and temperature optima, and saturation kinetics.
Worked example
A polysaccharide from a plant cell wall is insoluble, gives glucose on hydrolysis, and is not digested by human amylase. It is cellulose. Reasoning: starch is α-linked and digestible; cellulose is β-1,4-linked, forms microfibrils, and human digestive enzymes cannot cleave β-1,4 linkages. The linkage, not the glucose monomer alone, explains the properties.
Traps
Do not confuse starch and cellulose just because both are glucose polymers. Do not say enzymes change the equilibrium constant; they speed approach to equilibrium. Do not place 80S ribosomes in bacteria. Do not call phospholipids hydrophobic only-they are amphipathic.