Cell Biology: Structure, Membranes and Division
What you must understand
Know the organelles and their jobs: nucleus (DNA storage, transcription), mitochondrion (aerobic respiration, ATP), chloroplast (photosynthesis, only in plants/algae), ribosome (protein synthesis), rough and smooth ER (protein processing; lipid synthesis), Golgi (modification and sorting), lysosome (hydrolytic digestion), vacuole (storage, turgor), cytoskeleton (shape, movement). Compare plant and animal cells: plant cells have a cellulose wall, chloroplasts, and a large central vacuole; animal cells have centrioles and lysosomes but no wall or chloroplasts.
The plasma membrane is a phospholipid bilayer with embedded proteins - the fluid mosaic model. Transport: simple diffusion (down gradient, no energy), facilitated diffusion (down gradient via a protein), osmosis (water across a selectively permeable membrane), active transport (against gradient, needs ATP, e.g. the sodium–potassium pump). Cell division: mitosis produces genetically identical diploid cells for growth and repair; meiosis produces four genetically varied haploid cells for gametes.
Traps
Osmosis moves water, not solute. "Active transport" is defined by energy use and direction against the gradient, not by the molecule being large. Mitochondria are not the only site of ATP production (glycolysis is cytosolic). Chloroplasts are not "the plant's mitochondria" - they capture energy; mitochondria release it. In mitosis, DNA replicates before division, so chromosome number is conserved, not doubled.
Worked example
A cell is placed in a hypertonic solution. Water leaves by osmosis, the cell shrinks (plasmolyses if plant, crenates if animal). Rule: water always moves toward the side with the higher solute concentration, i.e. the lower water potential.