BIOL10008 · Foundations Of Biology: Life's Machinery
Cell Structure
This chapter assembles the biomolecules into the cell. It contrasts the two basic plans — the open-plan prokaryote (no nucleus, no membrane-bound organelles, circular DNA) versus the compartmentalised eukaryote (membrane-bound nucleus and organelles, linear DNA) — and asks the key question: why do compartments matter? It then tours the eukaryotic organelles as a factory with departments, each structure tuned to its job, and shows that the nuclear envelope, ER, Golgi, vesicles/lysosomes and plasma membrane form one continuous endomembrane system through which a protein travels from gene to destination. It closes with the geometry that explains why cells stay small and why inner membranes fold: the surface-area-to-volume ratio (SA:V = 6/L for a cube) shrinks as a cell grows, so cells either stay small or fold their membranes (cristae, thylakoids) to pack more reaction surface into the same space.
What this chapter covers
- 01Two cell plans: prokaryote vs eukaryote — and why compartments matter
- 02The eukaryotic organelles: nucleus, rough/smooth ER, Golgi, mitochondrion, lysosome
- 03Structure → function: the nucleus, chromatin and histones
- 04The endomembrane system as one connected shipping network
- 05The secretory pathway, gene to destination, in steps
- 06Surface-area-to-volume ratio (SA:V = 6/L) and folded inner membranes
Worked example: surface-area-to-volume ratio and why membranes fold
- +1(a) Set up the geometry: a cube of side L has surface area = 6L² and volume = L³, so SA:V = 6L² / L³ = 6/L.
- +1(b) Compute the ratios: L=1 → 6:1; L=2 → 24/8 = 3:1; L=4 → 96/64 = 1.5:1.
- +1(b) State the trend: as L increases, SA:V falls — volume grows faster than surface area, so a big cell has too little membrane to supply its large interior.
- +1(c) Apply to mitochondria: the membrane is where the reactions of oxidative phosphorylation happen, so more membrane area means more ATP capacity.
- +1(c) Explain the folding: folding the inner membrane into cristae packs far more surface area into the same volume — a way to beat the falling SA:V without making the organelle bigger.
Key terms
- Prokaryote vs eukaryote
- Prokaryotes have no nucleus or membrane-bound organelles and carry circular DNA in a nucleoid; eukaryotes wrap their (linear) DNA in a membrane-bound nucleus and carry membrane-bound organelles. Both share a genetic code, cytoplasm and a plasma membrane.
- Organelle
- A membrane-bound compartment specialised for one process. Compartmentation lets a eukaryote keep incompatible chemistry apart and concentrate reactants — division of labour inside a single cell.
- Endomembrane system
- One continuous, connected set of membranes — nuclear envelope, ER, Golgi, vesicles/lysosomes and plasma membrane — through which a protein moves enclosed, step by step, from gene to destination.
- Chromatin
- DNA wound around histone proteins into nucleosomes and coiled into chromosomes. This packs metres of DNA into a tiny nucleus and lets it be divided equally at mitosis.
- Surface-area-to-volume ratio (SA:V)
- Surface area divided by volume; for a cube it is 6/L. It shrinks as a cell grows, because volume rises faster than surface. Cells fight it by staying small and by folding membranes (cristae, thylakoids).
Cell Structure FAQ
What is the single most important difference between prokaryotes and eukaryotes?
Compartmentation. Eukaryotes enclose their DNA in a membrane-bound nucleus and run processes in membrane-bound organelles; prokaryotes do not. The exam rewards stating why that matters, not just listing it: separating reactions into compartments lets a cell keep incompatible chemistry apart and concentrate reactants — like having a separate kitchen and bathroom — which made larger, more complex cells possible.
How does a protein move through the endomembrane system?
It is built and folded on the rough ER, packaged into a vesicle that fuses with the cis Golgi, modified further as it moves to the trans Golgi, then shipped in a vesicle (e.g. as a lysosome) to its target. Because the membranes are connected, the protein never crosses open cytoplasm exposed — it is modified at each station. A flagship integrative question asks you to trace one protein from gene to function: nuclear pore → rough ER → cis/trans Golgi → vesicle/lysosome.
Why can't cells just grow bigger instead of dividing?
Because surface-area-to-volume ratio falls as size increases. The membrane is where nutrients enter and wastes leave, but volume (the demand) grows as L³ while surface (the supply) grows only as L². Past a certain size the surface cannot keep up, so cells either stay small (a reason they divide) or fold their membranes to add surface.
Why do mitochondria and chloroplasts have folded inner membranes?
To maximise membrane area for their membrane-bound energy reactions. Folding the inner membrane into cristae (mitochondria) or stacked thylakoids (chloroplasts) packs far more surface into the same volume — like crumpling a sheet of paper to fit more surface in your fist — so each organelle can do more oxidative phosphorylation or photophosphorylation.
Exam move
Anchor the chapter on three diagrams you can re-draw with labels: the prokaryote-vs-eukaryote comparison, the eukaryotic cell with its organelles, and the SA:V cube table. For each organelle, pair structure with function (folded cristae → more ATP surface; ribosome-studded rough ER → protein synthesis; stacked Golgi sacs → modify and ship). Be ready to trace one protein through the endomembrane system in order — that integrative question recurs. And practise the SA:V calculation both ways: compute 6/L for a given size, and explain why the falling ratio forces cells to stay small or fold their membranes.
