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Every living thing, from an oak tree to you, is built from cells too small to see. A microscope lets you look inside them and measure them, and what you find explains how organisms stay alive.
Recommended for MYP 2 · About 4 lessons · Criteria A, B, C and D
By the end of this topic you should be able to:
You will use these skills. If any feel shaky, review them first.
A cell is a tiny, membrane-bound unit of life. All cells have a cell membrane, cytoplasm and genetic material; plant and animal cells keep that material in a nucleus. Plant cells also have a cell wall, usually a large vacuole, and, in green parts, chloroplasts.
Each structure does a job the cell needs to survive: the membrane controls what enters and leaves, mitochondria release energy, chloroplasts capture light for photosynthesis. Cells stay small because everything they need must cross their surface quickly.
Cells were first seen in the 1600s, when early microscopes revealed tiny compartments in cork and living “animalcules” in pond water. Better lenses, stains and, later, electron microscopes showed the organelles inside. Every observation so far supports cell theory: all living things are made of cells, and new cells come from existing cells.
Diseases start with changes in cells, so doctors examine cells to diagnose illnesses such as cancer. Understanding cells also underpins vaccines, IVF, and new technologies such as meat grown from cells.
Cells link to organ systems, photosynthesis and respiration, cell division and inheritance, and microorganisms. Magnification calculations use ratio, standard form and unit conversion from mathematics.
A light microscope uses two lenses. Total magnification = eyepiece magnification × objective magnification, so a ×10 eyepiece with a ×40 objective gives ×400.
Both sizes must be in the same unit before you divide. 1 mm = 1000 µm, so to change mm to µm, multiply by 1000.
Hospital scientists stain and examine tissue samples to confirm a diagnosis. Water-quality laboratories count microorganisms under microscopes to check drinking water is safe. Plant scientists compare leaf cells to breed crops that cope better with drought.
A cheek cell appears 12 mm wide in a drawing made at ×200. Calculate its real width in µm.
A pollen grain 25 µm across appears 50 mm across in a photograph. Calculate the magnification.
Quick questions on the ideas above. Try each one before using a hint.
Guided investigation · about 40 minutes · pairs
Safety. Never point a microscope mirror at the Sun, which can damage eyes. Carry the microscope with two hands, and lower the stage before changing to a higher-power objective so it does not crack the slide.
Results from one student are used in the Criterion C questions below. Then evaluate: why is it hard to measure the field of view accurately at ×400 with a millimetre ruler?
Harder problems in unfamiliar contexts. Plan before you calculate.
Five questions picked at random from the whole topic. Take a new set whenever you like.
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Tracked separately for each skill, on this device only.
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