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Spec Mapping — OCR H420 Module 2.1.1 — Cell structure, content statements covering the ultrastructure of eukaryotic organelles, specifically the nucleus, nucleolus, and rough and smooth endoplasmic reticulum (refer to the official OCR H420 specification document for exact wording). This lesson is the foundation for the protein-production-and-secretion synthesis later in the course, and is examined synoptically with module 6.1 (cellular control / gene expression) on Paper 3.
Understanding the detailed ultrastructure of eukaryotic organelles is at the heart of OCR module 2.1.1. This lesson examines the nucleus, the nucleolus, the nuclear envelope, and the endoplasmic reticulum (both rough and smooth), explaining how their structures are suited to their functions. Because A-Level depth demands precise terminology, you should learn the molecular-level features summarised here.
The systematic dissection of the eukaryotic cell into discrete organelles is one of the central achievements of twentieth-century biology, and its key practitioners are still named in modern exam questions. George Palade at the Rockefeller Institute developed cell-fractionation techniques in the 1950s and traced the secretory pathway from the rough endoplasmic reticulum through the Golgi to the cell surface — work for which he shared the 1974 Nobel Prize. Christian de Duve discovered lysosomes through differential centrifugation and identified them as the cell's digestive compartment. The endosymbiotic theory of Lynn Margulis (1967) re-interpreted the double-membraned mitochondria and chloroplasts as engulfed prokaryotes — a paradigm shift that explains the 70S ribosomes and circular DNA those organelles still carry.
Key Definitions:
- Ultrastructure — the detailed three-dimensional organisation of a cell as revealed by electron microscopy.
- Organelle — a discrete subcellular structure with a specific function.
- Eukaryotic cell — a cell possessing a membrane-bound nucleus and membrane-bound organelles.
The diagram makes one crucial point explicit: the outer membrane of the nuclear envelope is physically continuous with the rough endoplasmic reticulum (visible as the studded membrane to the right). This is why mRNA emerging through a nuclear pore can begin translation on a ribosome that is already docked on the RER within micrometres of its exit point — the spatial economy is exquisite.
The nucleus is usually the largest organelle in a eukaryotic cell, typically 5–10 µm in diameter, and is easily visible under a light microscope. It contains almost all of the cell's genetic material in the form of chromatin — a complex of DNA wound around histone proteins.
The nucleus is enclosed by a double membrane known as the nuclear envelope:
Nuclear pores are complex structures composed of around 30 different nucleoporin proteins, collectively forming the nuclear pore complex (NPC). They regulate the passage of molecules between the nucleus and cytoplasm:
Inside the nucleus, DNA is associated with histone proteins to form chromatin. During interphase, chromatin exists in two forms:
At cell division, chromatin condenses into the visible rod-like structures known as chromosomes. Humans have 46 chromosomes (23 pairs) per somatic cell. Each chromosome consists of a single, linear DNA molecule wrapped around nucleosomes (histone octamers).
Within the nucleus, a densely staining region known as the nucleolus is visible under both light and electron microscopy. A typical cell has one or two nucleoli, each approximately 1–3 µm across.
Key Point: Cells that secrete large amounts of protein (e.g., pancreatic acinar cells, plasma cells) have prominent nucleoli because they must produce huge numbers of ribosomes.
The endoplasmic reticulum is an extensive, continuous network of flattened membrane-bound sacs and tubules called cisternae. The lumen (interior) of the ER is distinct from the cytosol. There are two types of ER, distinguishable in electron micrographs by the presence or absence of ribosomes.
| Feature | Rough ER | Smooth ER |
|---|---|---|
| Ribosomes on cytosolic face? | Yes (80S) | No |
| Shape under TEM | Flattened stacked cisternae | Interconnected tubules |
| Main biosynthetic role | Protein synthesis (secretory, membrane, lysosomal) | Lipid and steroid synthesis |
| Folding / quality control role | Yes (chaperones, disulfide isomerase) | No |
| Glycosylation? | Yes (N-linked, asparagine) | No |
| Detoxification role? | No | Yes (cytochrome P450 enzymes) |
| Calcium storage? | No | Yes (sarcoplasmic reticulum in muscle) |
| Continuity with outer nuclear membrane? | Direct | Indirect (via RER) |
| Cells with abundant examples | Plasma cells, pancreatic acinar, hepatocytes | Liver hepatocytes, Leydig cells, muscle |
The two systems share the same membrane chemistry and continuous lumen, but their cytosolic surface architecture (ribosome-studded vs smooth) and luminal enzyme content (chaperones/oxidoreductases vs lipid-biosynthesis enzymes) differ markedly. The ratio of RER to SER in a cell is therefore a reliable indicator of whether the cell's primary output is protein (high RER:SER) or lipid/steroid/detoxification work (high SER:RER).
By the end of this lesson you should be able to describe not just that the nuclear pore is selective, but how selective transport is achieved — a distinction that separates A from A* answers. The nuclear pore complex (NPC) does not behave like a sieve with a fixed cut-off; instead, it operates a signal-and-carrier system that is the eukaryotic solution to keeping transcription and translation in separate compartments.
Molecules below about 40 kDa can diffuse passively through the central channel of the pore, but every larger cargo requires an address label. Proteins destined for the nucleus (histones, transcription factors, DNA and RNA polymerases) carry a short nuclear localisation signal (NLS) — a stretch of basic amino acids (lysine and arginine). Cargo carrying an NLS is recognised in the cytoplasm by a carrier protein called importin, which chaperones it through the pore. Conversely, molecules destined for export (mRNA, tRNA, ribosomal subunits) carry a nuclear export signal (NES) recognised by exportin. The interior of the NPC is filled with disordered nucleoporin filaments rich in phenylalanine–glycine (FG) repeats; importin and exportin transiently bind these FG repeats and "hop" through the meshwork, while cargo without a carrier cannot.
Directionality — the reason cargo does not simply drift back out — is imposed by a concentration gradient of the small GTPase Ran. Ran-GTP is abundant inside the nucleus (because the enzyme that loads GTP onto Ran, RanGEF, is tethered to chromatin) and scarce in the cytoplasm (where RanGAP triggers hydrolysis to Ran-GDP). When importin arrives in the nucleus, Ran-GTP binds it and forces it to release its cargo; when exportin reaches the cytoplasm, hydrolysis of Ran-GTP to Ran-GDP forces it to release its cargo. The energy for the whole cycle comes from GTP hydrolysis, making nucleocytoplasmic transport an active, energy-requiring process — which is why it halts when a cell's ATP/GTP supply collapses.
Exam Tip: If a 6- or 9-mark question asks you to explain how the nucleus controls what enters and leaves, the discriminating points are: (i) passive diffusion of small molecules; (ii) signal sequences (NLS/NES) on large cargo; (iii) carrier proteins (importins/exportins); and (iv) the Ran-GTP gradient giving directionality and requiring energy. Most candidates get (i) and (ii); the top band reaches (iii) and (iv).
A recurring A-Level theme — and a favourite synoptic exam angle — is that the nucleus, ER, Golgi, vesicles, lysosomes and plasma membrane are not independent compartments but a single, dynamically connected endomembrane system through which membrane and its cargo flow. Membrane made at the ER moves as vesicles to the Golgi, then to the plasma membrane; membrane is retrieved from the surface by endocytosis; and the whole network is maintained by a balance of forward (anterograde) and return (retrograde) traffic.
Two conservation principles are worth internalising because they let you reason about any secretory cell:
Membrane is conserved and recycled. When a secretory vesicle fuses with the plasma membrane (exocytosis), its membrane is added to the cell surface. If the cell is not to grow indefinitely, an equal area must be retrieved by endocytosis and returned to the ER/Golgi. This is why the SER, which manufactures phospholipid, is essential even in a protein-secreting cell — it replaces the lipid consumed in forming vesicles.
Membrane keeps its "sidedness". The face of the ER membrane that faces the lumen becomes the face of the vesicle that faces the lumen, which becomes the outer (extracellular) face of the plasma membrane after fusion. This topological rule explains why the glycosylation added to a protein's surface in the ER/Golgi lumen ends up on the outside of the cell — the sugar coat (glycocalyx) faces outward because that lumenal face is turned inside-out during exocytosis. This connects directly to the fluid-mosaic membrane you meet in the membranes module.
Understanding the system as one connected whole, rather than as a list of separate organelles, is exactly the "synthesis" move that lifts an extended answer into the top band: instead of describing each organelle in isolation, a strong candidate traces a single protein or a single patch of membrane through the entire pathway.
The outer nuclear membrane is physically continuous with the RER. This direct continuity is significant because:
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