Life, Domains and the Cell
General Biology · Section 1.1 · 17 study cards
Properties of life, levels of organisation, the three domains, prokaryotic vs eukaryotic cells, and why cells stay small.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Build the hierarchy first
Nearly every question in this unit is really asking where something sits in a hierarchy, either the levels of organisation or the taxonomic ranks. Learn both ladders cold, in order, before anything else. Then attach each new term to a rung. Emergent properties only make sense once you can say which level they emerge at.
Sort cells by compartments, not by size
When you compare prokaryotic and eukaryotic cells, resist listing size and complexity. The operational test is the presence of internal membrane-bound compartments, above all the nucleus. Everything else follows: a nucleus implies separated transcription and translation, which implies processing steps in between.
Handle scaling questions arithmetically
- Write down the shape and the linear dimension.
- Compute surface area and volume separately with the formula for that shape.
- Divide surface area by volume and keep the units, µm² over µm³, giving per-µm.
- State the biological consequence: a lower ratio means slower exchange relative to demand.
For a cube the shortcut is 6/s, and for a sphere it is 3/r. Knowing that the ratio is inversely proportional to linear size lets you answer qualitative versions instantly.
Keep microscopy claims separate
Magnification, resolution and contrast are three independent things. Ask which one the question is testing. If the question involves seeing a live process, only light microscopy will do. If it involves seeing anything smaller than about 0.2 µm, only electron microscopy will do.
Use the domains to anchor evolution
The three-domain scheme is not just a list to memorise. It encodes a claim: Archaea and Eukarya share a more recent common ancestor with each other than either does with Bacteria. Any question about archaeal biochemistry becomes easier if you hold that claim in mind.
Definitions and theorems
Worked example
A spherical bacterium has a radius of 1 µm. A spherical eukaryotic cell has a radius of 10 µm. Calculate the surface-area-to-volume ratio of each and explain, in terms of nutrient uptake, why the eukaryotic cell needs internal membranes while the bacterium does not.
For a sphere, surface area is 4πr2 and volume is (4/3)πr3, so the ratio simplifies to 3/r.
Bacterium: 3/1 = 3 per µm. Eukaryotic cell: 3/10 = 0.3 per µm.
The bacterium therefore has ten times as much plasma membrane per unit of cytoplasm as the eukaryotic cell.
Nutrients enter across the plasma membrane and must then reach the cell interior by diffusion, whose time cost rises with the square of the distance travelled.
In the bacterium every point of cytoplasm is within about 1 µm of the membrane, so plasma-membrane uptake alone supplies the whole cell.
In the eukaryotic cell the interior is up to 10 µm from the surface and the membrane area per unit volume is far smaller, so metabolic reactions are instead housed on extensive internal membranes, which restores membrane surface without increasing the outer boundary.
Conclusion: the ratio falls as 3/r, and compartmentalisation is the eukaryotic solution to that geometric constraint.
Common mistakes
- Confusing magnification with resolution. Students say an electron microscope is better because it magnifies more. It is better because it resolves more, thanks to the short wavelength of electrons. Magnification without matching resolution just enlarges a blur, and exam questions are usually written to catch exactly this substitution.
- Treating homeostasis as constancy. Writing that homeostasis means the internal environment does not change loses marks. It means deviations are detected and corrected, so values oscillate within a narrow range, and the process consumes energy.
- Saying prokaryotes have no organelles at all. They have ribosomes, which are organelles in the broad sense. The accurate statement is that prokaryotes lack membrane-bound organelles and a true nucleus. Always include the qualifier.
- Getting the domain relationships backwards. Because Archaea and Bacteria look alike under a microscope, students group them as each other's closest relatives. Archaea are in fact more closely related to Eukarya. Shared cell architecture here reflects retained ancestral features, not recent common ancestry.
- Mishandling the scaling arithmetic. Doubling the radius is often written as doubling the surface-area-to-volume ratio. It halves it. Compute area and volume separately every time rather than reasoning from a remembered direction.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 17 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.