Cells and Cell Processes
Cell structure and function
Every living organism is built from cells, and the exam expects you to link each structure to its job. Plant, animal and bacterial cells share a cell membrane, cytoplasm and genetic material, but differ in detail: plant cells have a cellulose cell wall, a large permanent vacuole and chloroplasts; animal cells have no wall or chloroplasts and only small temporary vacuoles; bacteria have no nucleus, so their DNA lies free in the cytoplasm. Know the functions of the nucleus (controls activity, holds chromosomes), mitochondria (aerobic respiration), ribosomes (protein synthesis), chloroplasts (photosynthesis) and the selectively permeable membrane.
Movement in and out of cells
Diffusion is net movement of particles from high to low concentration; osmosis is diffusion of water through a partially permeable membrane from dilute to concentrated solution; active transport moves substances against a concentration gradient using energy. A frequent trap is confusing osmosis with diffusion of solute, or forgetting that osmosis needs a membrane. Another trap is describing a plant cell as 'bursting' in pure water; the cell wall prevents this and the cell becomes turgid instead.
Enzymes and the cell
Enzymes are protein catalysts, specific to their substrate, denatured by high temperature or wrong pH. The lock-and-key idea explains specificity.
Worked example
A student places a potato strip in concentrated sugar solution. Water leaves the vacuole by osmosis, the cell loses turgor and the strip becomes flaccid and shorter. Rule of thumb: water always moves towards the more concentrated solution.
Practical traps
When using a microscope, always state magnification of the drawing, and remember that staining (iodine for starch, methylene blue for nuclei) makes structures visible.