Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Magnification is how many times larger the image is than the real object: magnification = image size ÷ actual size, with both in the same unit (1 mm = 1000 μm, 1 μm = 1000 nm). Resolution is the smallest distance between two points that can still be seen as separate. It is limited by wavelength: a light microscope resolves about 200 nm, an electron microscope about 0.5 nm, so only the electron microscope shows organelle detail. To measure with a light microscope, calibrate the eyepiece graticule against a stage micrometer for each objective.
Increasing the magnification does not improve the resolution. Beyond about ×1500 a light microscope just gives a larger blur, because two points closer than 200 nm still merge. Define both terms separately: magnification is about size, resolution is about detail.
Convert before you divide. The most common slip is dividing a length in mm by a length in μm, which puts the answer out by a factor of 1000. Write both measurements in μm first, and check the answer is a sensible size for that structure.
All living organisms are made of cells, the basic units of life. A eukaryotic cell has a nucleus and many membrane-bound organelles, each with its own function. Proteins for secretion are made on 80S ribosomes on the rough ER, modified in the Golgi body and released by exocytosis. Mitochondria and chloroplasts have a double membrane, their own small circular DNA and 70S ribosomes. Plant cells add a cellulose cell wall, chloroplasts, a large permanent vacuole and plasmodesmata; animal cells have centrioles. Cells use ATP from respiration for every energy-requiring process.
Do not confuse cilia and microvilli. Cilia are longer, contain microtubules and move; microvilli are much smaller folds that only increase surface area. And ribosomes have no membrane: they are not membrane-bound organelles.
ATP does not “make” energy and is not a long-term store. It transfers energy from respiration to the processes that need it, and each molecule is recycled within seconds of being made.
A prokaryotic cell, such as a bacterium, is unicellular and usually 1 to 5 μm across. It has a peptidoglycan (murein) cell wall, circular DNA free in the cytoplasm with no histones, 70S ribosomes, and no organelles surrounded by double membranes: no nucleus, mitochondria or chloroplasts. A virus is non-cellular: a core of nucleic acid, either DNA or RNA, inside a protein capsid, and in some viruses an outer envelope of phospholipids taken from the host cell. Viruses can only reproduce inside a host cell.
Prokaryotes do have a cell surface membrane, cytoplasm and ribosomes: they lack only the membrane-bound organelles. And their cell wall is peptidoglycan, never cellulose. Writing “bacteria have no ribosomes” or “a cellulose wall” loses the mark.
Never describe a virus as a “cell” or as a type of bacterium. A virus is 10 to 100 times smaller than a bacterium, has no cytoplasm or ribosomes, and is not affected by antibiotics, which target bacterial structures such as the peptidoglycan wall and 70S ribosomes.