Cambridge International AS and A Level Biology 1: Cell structure

Cell Structure is Cambridge International AS and A Level Biology 9700 Topic 1. It establishes microscope concepts, image calculations, eukaryotic organelles, bacterial cell structure and the non-cellular organisation of viruses. Detailed slide preparation, calibration and drawing technique belong to the separate Biology practical hub; this theory note owns the structures, functions and interpretation needed to explain the evidence.

1. Magnification, actual size and resolution

Magnification describes how many times larger an image is than the specimen. Use magnification = image size divided by actual size, with both sizes converted to the same unit. Rearrange rather than memorising three disconnected formulas.

One millimetre is 1000 micrometres, and one micrometre is 1000 nanometres. Unit conversion errors can change an answer by factors of 1000, so write the conversion before substitution. A scale bar remains valid if an image is resized with it, whereas a printed magnification label may not.

Resolution is the ability to distinguish two close points as separate. Greater magnification without greater resolution produces a larger blurred image, called empty magnification. Electron microscopes achieve greater resolution than light microscopes because their electron wavelength is shorter.

Scanning electron microscopy mainly reveals detailed surface form and can produce a three-dimensional appearance. Transmission electron microscopy reveals internal ultrastructure in thin sections. Electron micrographs are not natural-colour images.

2. Interpreting cells from images

Biological interpretation combines visible evidence with known structure. Identify a plant cell from features such as a cellulose wall, chloroplasts or a large permanent vacuole, but remember that not every plant cell contains chloroplasts. Typical animal cells lack those structures.

Plan diagrams show the arrangement and relative proportions of tissues without individual cell detail. High-power cell drawings record selected cells and visible structures. Practical conventions and eyepiece calibration are developed in the companion practical note, while this theory page focuses on what the observed structures mean.

3. The endomembrane and protein-processing system

The cell-surface membrane controls exchange and communication. The nuclear envelope separates nuclear contents from cytoplasm and contains pores; the nucleolus makes components used in ribosome assembly.

Rough endoplasmic reticulum carries ribosomes and forms or transports proteins entering the endomembrane system. Smooth endoplasmic reticulum lacks ribosomes and is associated with lipid-related synthesis and other cell-specific functions. The Golgi body modifies, sorts and packages molecules into vesicles. Lysosomes contain hydrolytic enzymes within a membrane compartment.