Cell structure and microscopy: organelles, magnification and the maths examiners love

Describe the structure, then the function

Organelle questions are marked in pairs: one mark for a structural feature and one for what that feature allows the cell to do. If you write 'mitochondria make energy' you score nothing — energy is not made, ATP is synthesised, and no structure has been described.

Build every answer as 'structure, therefore function'. Mitochondria have a folded inner membrane (cristae) giving a large surface area for the electron transfer chain, and a matrix containing enzymes for the Krebs cycle, so aerobic respiration produces ATP. Chloroplasts have stacked thylakoids holding chlorophyll for the light-dependent reaction and a stroma with enzymes for the light-independent reaction.

The organelles that come up most

• Rough endoplasmic reticulum: ribosome-studded membranes that fold and transport proteins made for secretion.
• Golgi apparatus: modifies proteins, often adding carbohydrate, and packages them into vesicles — including lysosomes.
• Lysosome: a vesicle of hydrolytic enzymes (lysozymes) that digests worn organelles and ingested pathogens.
• Ribosome: 80S in eukaryotes, 70S in prokaryotes and in mitochondria and chloroplasts — a favourite comparison question.
• Cell wall: cellulose in plants, murein (peptidoglycan) in bacteria, chitin in fungi.

Prokaryotic versus eukaryotic

The safest comparison answer names three differences with both halves stated: prokaryotes have no nucleus, their DNA is circular and free in the cytoplasm, while eukaryotes have DNA that is linear and associated with histones inside a nuclear envelope; prokaryotes have 70S ribosomes against 80S; prokaryotes lack membrane-bound organelles such as mitochondria.

Add plasmids, capsules and flagella only if the question asks for structures that may be present — they are not found in every prokaryote, and examiners penalise answers that state them as universal.

Cell fractionation in order

• Homogenise tissue in a cold, isotonic, buffered solution: cold reduces enzyme activity, isotonic prevents osmotic damage to organelles, buffer keeps pH stable so proteins do not denature.
• Filter the homogenate to remove debris and whole cells.
• Ultracentrifuge at low speed, decant the supernatant, then respin faster each time.
• Pellets appear in order of density: nuclei, then chloroplasts (in plants), mitochondria, lysosomes, endoplasmic reticulum, finally ribosomes.

Magnification, resolution and the calculation

Magnification is how much larger the image is; resolution is the smallest distance at which two points are still distinguishable. A light microscope is limited by the wavelength of light to roughly 0.2 micrometres of resolution, so a higher magnification on a light microscope shows a bigger but not a clearer image. Electron microscopes use a much shorter electron wavelength, giving resolution near 0.1 nanometres for a TEM.

The formula is magnification = image size / actual size. Convert both measurements to the same unit before you divide: 1 mm = 1000 micrometres = 1 000 000 nanometres. Measure the scale bar with a ruler in millimetres, convert, and keep the answer to the same number of significant figures as the data.

TEM versus SEM

• TEM: electrons pass through a thin section, producing a high-resolution two-dimensional image of internal structure; specimens must be dead and in a vacuum.
• SEM: electrons bounce off the surface, giving a three-dimensional surface image at lower resolution.
• Artefacts are structures produced by preparation, not present in the living cell — always an available evaluation mark.