Molecular and Cellular Biology
Molecular and Cellular Biology
This domain tests cell structure and function, membrane transport, energy transformation, and the flow of genetic information. Expect questions that compare prokaryotic and eukaryotic cells, predict the direction of osmosis, trace ATP production, or identify where transcription and translation occur.
Core rules and concepts
- Prokaryotes lack membrane-bound organelles and a nucleus; they have 70S ribosomes. Eukaryotes have a nucleus, membrane-bound organelles, and 80S ribosomes; mitochondria and chloroplasts contain 70S-like ribosomes.
- The plasma membrane is a phospholipid bilayer with embedded proteins. Small nonpolar molecules diffuse freely; ions and polar molecules need channels or carriers. Net water movement follows water potential, moving from higher to lower water potential.
- Endosymbiotic theory explains mitochondria and chloroplasts: double membranes, circular DNA, and bacteria-like ribosomes.
- Glycolysis occurs in the cytosol; the Krebs cycle and oxidative phosphorylation occur in mitochondria. Photosynthesis has light reactions in thylakoid membranes and the Calvin cycle in the stroma.
- DNA replication is semiconservative; transcription makes RNA; translation at ribosomes uses mRNA codons and tRNA anticodons. In eukaryotes, transcription is nuclear and translation is cytoplasmic; in prokaryotes, they can be coupled.
Worked example
A plant cell with solute potential −0.6 MPa and pressure potential 0.2 MPa has water potential −0.4 MPa. If placed in a solution with water potential −0.2 MPa, water moves from the solution into the cell, because water moves from higher water potential (−0.2) to lower water potential (−0.4). The cell becomes turgid. If the external solution were −0.4 MPa, net movement would stop at equilibrium.
Traps
Saying water moves toward higher solute concentration without noting water potential; reversing hypertonic and hypotonic effects; placing translation in the nucleus; assuming prokaryotes have no cytoskeleton; confusing the Calvin cycle with the light reactions; treating ATP yield as an exact number rather than an estimate.