Cells, Organisms and Life Processes
Cells as the unit of life
Every organism is built from cells. You must know the shared features of plant and animal cells (cell membrane, cytoplasm, nucleus, mitochondria, ribosomes) and the structures found only in plant cells (cellulose cell wall, chloroplasts, a large permanent vacuole). Learn the function of each part, not just its name: mitochondria release energy by respiration, chloroplasts absorb light for photosynthesis, the vacuole stores cell sap and keeps the cell firm.
Levels of organisation
Cells group into tissues, tissues into organs, organs into organ systems, and systems into the whole organism. Be ready to place an example at each level, such as muscle cell, muscle tissue, heart, circulatory system.
Life processes
All living things carry out the seven life processes: nutrition, respiration, excretion, sensitivity, movement, growth and reproduction. A common trap is confusing respiration with breathing; respiration is a chemical reaction releasing energy in cells, while breathing is the movement of air into and out of the lungs. Another trap is calling a virus a living cell, or assuming all cells have a nucleus (bacterial cells do not).
Worked example
A question asks why a root hair cell has a long, thin extension. Reason from structure to function: the extension increases surface area, so more water and mineral ions can be absorbed by osmosis and active transport. Always link a named structure to the job it performs.
Magnification and observation
Use the rule of thumb: magnification = image size divided by actual size. Keep units consistent, and convert millimetres to micrometres by multiplying by 1000. When describing a microscope image, state the cell type and give evidence, such as a visible cell wall showing it is a plant cell.