Cells
What you must understand
Cells are the basic unit of life, bounded by a selectively permeable membrane described by the fluid mosaic model. The phospholipid bilayer creates a hydrophobic core; proteins, cholesterol, and glycoproteins modulate transport and signaling. Compare prokaryotic cells (no membrane-bound organelles, circular DNA, nucleoid) with eukaryotic cells (nucleus, membrane-bound organelles).
Organelle function is heavily tested. Mitochondria and chloroplasts perform energy transduction; the rough ER and ribosomes synthesize proteins; the Golgi modifies, sorts, and packages; lysosomes degrade; the cytoskeleton provides structure and transport. The endosymbiotic theory explains mitochondria and chloroplasts having their own DNA and double membranes.
Surface area to volume ratio constrains cell size: as a cell grows, volume rises faster than surface area, limiting diffusion. Compartmentalization solves this by concentrating reactions.
Traps
- Passive transport (diffusion, osmosis, facilitated diffusion) requires no ATP; active transport does. Facilitated diffusion uses proteins but is still passive.
- Osmosis moves water, not solute, toward higher solute concentration.
- A plant cell in a hypotonic solution becomes turgid, not lysed, because of the cell wall.
- Ribosomes are not membrane-bound organelles.
Rule of thumb
For any transport question, ask two things: does it require energy (ATP), and does it move with or against the concentration gradient? With gradient = passive; against = active.
Worked example
A cell is placed in a hypertonic solution. Water leaves the cell by osmosis, the cell shrinks (crenation in animal cells; plasmolysis in plant cells).