Cell Biology
What you must understand
A cell is the structural and functional unit of life. You should be able to distinguish prokaryotic cells (no true nucleus, no membrane-bound organelles, circular DNA, 70S ribosomes) from eukaryotic cells (true nucleus, organelles, linear chromosomes). Know the organelles and their functions: nucleus (DNA storage and transcription), mitochondrion (aerobic respiration, ATP production), chloroplast (photosynthesis, contains chlorophyll), ribosome (protein synthesis), rough and smooth endoplasmic reticulum (protein transport; lipid synthesis), Golgi apparatus (modification and packaging of proteins), lysosome (hydrolysis of macromolecules), vacuole (storage and turgor), and cell membrane (selective permeability). Understand the fluid-mosaic model of the membrane and the difference between diffusion, osmosis, facilitated diffusion and active transport. Know the stages of mitosis (prophase, metaphase, anaphase, telophase) and the difference between mitosis and meiosis.
Traps that appear in questions
- Saying plant cells have no mitochondria - they do; they respire too.
- Confusing osmosis (water across a semi-permeable membrane) with diffusion of solutes.
- Claiming prokaryotes have no ribosomes - they have 70S ribosomes.
- Mixing up which organelle produces ATP (mitochondrion) and which uses it for protein synthesis (ribosome).
- Forgetting that the cell wall is not selectively permeable.
Worked example
A plant cell placed in a hypertonic solution loses water and the vacuole shrinks; the membrane pulls away from the wall (plasmolysis). In a hypotonic solution, water enters, the vacuole swells, and turgor pressure rises. Rule of thumb: water moves toward the more concentrated solution (lower water potential).