Cells as the Basis of Life
What you must understand
Cells are the functional units of life, and their internal structure reflects function. Know the difference between prokaryotic cells (no membrane-bound organelles, circular DNA, 70S ribosomes) and eukaryotic cells (membrane-bound organelles, linear chromosomes, 80S ribosomes). Be able to relate organelle structure to function: mitochondria for aerobic respiration, chloroplasts for photosynthesis, rough endoplasmic reticulum and ribosomes for protein synthesis and transport, Golgi apparatus for modification and packaging, lysosomes for hydrolysis, and the fluid mosaic model of the cell membrane.
Understand surface area to volume ratio: as a cell grows, volume increases faster than surface area, so exchange becomes inefficient. This explains why cells are small and why adaptations such as microvilli, root hairs and flattened shapes exist. Also know the transport processes - diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis - and which require energy.
Traps in questions
- Osmosis questions often ask for the direction of net water movement, not solute movement. Water moves from low solute concentration to high solute concentration across a semi-permeable membrane.
- Do not say a cell "wants" or "tries" to do something; use passive/active language and concentration gradients.
- Distinguish magnification from resolution when interpreting microscopy questions.
- Diffusion is passive; facilitated diffusion is also passive but uses a carrier protein - students often wrongly label it active.
Rule of thumb
For any transport question, ask two things: Is the substance moving down or against its concentration gradient? and Is a membrane protein or ATP involved? Down the gradient with no ATP means passive; against the gradient with ATP means active.
Worked example
A plant cell with water potential −800 kPa is placed in a solution of −400 kPa. Water moves into the cell because the external solution has higher water potential. The cell becomes turgid, and pressure potential rises until it balances the osmotic gradient.