Cells and Life Processes
What you must understand
All living things are built from cells, and the cell is the smallest unit that can carry out the seven life processes: nutrition, respiration, excretion, sensitivity, movement, growth and reproduction. Know the difference between plant and animal cells. Both have a cell membrane, cytoplasm and a nucleus. Plant cells also have a cellulose cell wall, a large permanent vacuole and chloroplasts. Remember that chloroplasts are only in green parts and that the cell wall is non-living and fully permeable, unlike the selectively permeable membrane.
Learn the functions of the main organelles: nucleus (controls activity, holds genetic material), cytoplasm (where reactions occur), membrane (controls what enters and leaves), mitochondria (site of aerobic respiration), ribosomes (protein synthesis), vacuole (stores sap, keeps the cell firm) and chloroplasts (photosynthesis).
Traps examiners set
- Saying the cell wall controls entry of substances. It does not; the membrane does.
- Confusing the vacuole with a storage sac that is small and many in plant cells. Plant cells usually have one large vacuole.
- Forgetting that red blood cells lose their nucleus and that root hair cells have no chloroplasts.
- Mixing up magnification and size. Magnification = image size ÷ actual size, and both must be in the same units.
Worked example
A cell is drawn 40 mm long. Its real length is 0.2 mm. Magnification = 40 ÷ 0.2 = ×200.
Rule of thumb: convert to the same unit first, then divide image by actual. If the answer is less than 1, you have divided the wrong way round.
Specialised cells
Link structure to function: a sperm cell has a tail and many mitochondria for swimming; a nerve cell is long to carry impulses; a root hair cell is long and thin to increase surface area for absorption.