A Level Biology: Cell Structure — Revision Notes

Condensed for the final weeks. For the full explanation, use the Cell Structure study guide.

Prokaryotic vs eukaryotic

Feature	Prokaryotic	Eukaryotic
Nucleus	Absent — DNA free in cytoplasm	Present, membrane-bound
DNA	Circular, naked	Linear, associated with histones
Ribosomes	70S	80S (70S in mitochondria and chloroplasts)
Membrane-bound organelles	None	Many
Cell wall	Peptidoglycan (murein)	Cellulose (plants), chitin (fungi), or absent
Size	1–5 μm	10–100 μm
Plasmids	Often present	Absent

Organelles and functions

Organelle	Function
Nucleus	Contains DNA; controls the cell; nucleolus makes ribosomes
Rough ER	Ribosomes on surface — synthesises and transports proteins
Smooth ER	Synthesises lipids and steroids
Golgi apparatus	Modifies, packages and sorts proteins; forms lysosomes
Mitochondria	Aerobic respiration — ATP synthesis; cristae increase surface area
Chloroplasts	Photosynthesis; thylakoids stacked into grana
Lysosomes	Contain hydrolytic enzymes; digest worn organelles and pathogens
Ribosomes	Protein synthesis
Centrioles	Form the spindle in animal cell division

Free vs bound ribosomes: free ribosomes in the cytoplasm make proteins used within the cell itself; ribosomes bound to the rough ER make proteins destined for secretion or for use in membranes, since the rough ER processes and packages the protein for export. The location of the ribosome, not the ribosome itself, decides the protein’s destination.

Trace a secreted protein and you have most of the topic: DNA in the nucleus → mRNA leaves through a nuclear pore → ribosome on the rough ER synthesises the protein → vesicle buds off → Golgi modifies and packages it → secretory vesicle → fuses with the cell surface membrane → exocytosis. That sequence is examined nearly every series.

Microscopy

	Light	Transmission EM	Scanning EM
Resolution	~200 nm	~0.5 nm	~3–10 nm
Magnification	×1,500	×500,000+	×100,000
Specimen	Living or dead	Dead only, in vacuum	Dead only
Image	Colour possible	2-D, internal	3-D, surface

Resolution is limited by the wavelength of the radiation used. Electrons have a far shorter wavelength than light, which is why electron microscopes resolve so much more. Two objects closer than half the wavelength cannot be distinguished.

Magnification and resolution are different quantities. Magnifying beyond the resolving limit produces a bigger blurred image, not more detail.

magnification = image size / actual size
1 mm = 1000 um       1 um = 1000 nm

Always convert both measurements to the same unit before dividing — this is where most calculation marks are lost.

Worked example. A micrograph shows a cell’s image measuring 45 mm across at ×3,000 magnification. Actual size = image size ÷ magnification = 45 mm ÷ 3,000 = 0.015 mm = 15 μm. Working in one unit throughout, and converting only at the very end, avoids the mid-calculation errors that lose most marks here.

Viruses

Viruses are acellular — they have no cytoplasm, no ribosomes and no metabolism of their own. A virus consists of genetic material (DNA or RNA) enclosed in a protein coat (capsid); some also have a lipid envelope derived from the host membrane. Viruses can only replicate by infecting a host cell and using its ribosomes and enzymes — outside a host they are inert.

Plant vs animal cells

Feature	Plant cell	Animal cell
Cell wall	Present — cellulose	Absent
Chloroplasts	Present (photosynthetic cells)	Absent
Vacuole	Large, permanent, central, sap-filled	Small, temporary, if present
Shape	Fixed, regular	Often irregular
Centrioles	Usually absent	Present

Eyepiece graticule and stage micrometre

The eyepiece graticule is a scale fitted inside the eyepiece with arbitrary units. It is calibrated against a stage micrometre (a slide with a scale of known length, e.g. divisions of 10 μm) for each objective lens, since the value of one graticule division changes with magnification. Once calibrated, the graticule measures specimens directly and the stage micrometre is removed. Units: 1 mm = 1000 μm = 1,000,000 nm.

Exam traps

• Saying prokaryotes have “no organelles” — they have ribosomes.
• Confusing magnification with resolution.
• Failing to convert units in a magnification calculation.
• Saying rough ER makes proteins and modifies them — modification is the Golgi’s job.
• Forgetting that EM specimens cannot be living.
• Naming a single “cell wall material” for all organisms — a prokaryotic wall is peptidoglycan (murein), a fungal wall is chitin, and a plant wall is cellulose; the three share a structural role but differ entirely in composition.
• Assuming any ribosome makes a protein for secretion — only ribosomes bound to the rough ER do; free ribosomes make proteins the cell uses itself.

Self-test

1. Give four differences between prokaryotic and eukaryotic cells.
2. Trace the path of a protein from gene to secretion.
3. Why do electron microscopes have higher resolution than light microscopes?
4. Why are viruses described as acellular, and how do they replicate?
5. Give two structural differences between a plant cell and an animal cell.
6. What determines whether a ribosome is free or bound to the rough ER, and does that change what the ribosome itself is made of?

Answers: 1. No nucleus; circular naked DNA rather than linear DNA with histones; 70S rather than 80S ribosomes; no membrane-bound organelles (also: smaller, peptidoglycan wall, plasmids). 2. Transcription in the nucleus → mRNA through a nuclear pore → translation on a rough ER ribosome → vesicle to the Golgi → modification and packaging → secretory vesicle → exocytosis. 3. Electrons have a much shorter wavelength than visible light, and resolution is limited by wavelength. 4. Viruses have no cytoplasm, ribosomes or metabolism of their own, so they cannot carry out life processes independently; they replicate only by infecting a host cell and using its ribosomes and enzymes. 5. Any two: a plant cell has a cellulose cell wall, an animal cell does not; a plant cell may contain chloroplasts, an animal cell never does; a plant cell has a large permanent vacuole, an animal cell’s vacuoles (if present) are small; an animal cell has centrioles, a plant cell usually does not. 6. Whether it is attached to the rough ER or free in the cytoplasm, which determines the destination of the protein it makes (secretion/membrane versus use within the cell) — not the ribosome’s own structure, which is identical either way.