Q.Describe the structure of the following with the help of labelled diagrams.

(i) Nucleus: The nucleus is enclosed by a nuclear envelope of two parallel membranes with a perinuclear space between them; the outer membrane may bear ribosomes and be continuous with the endoplasmic reticulum. The envelope is interrupted by nuclear pores. Inside lies the nucleoplasm (nuclear matrix), which holds one or more spherical nucleoli and the chromatin — the network of nucleoprotein fibres made of DNA, histones, non-histone proteins and RNA. Labels to show: nuclear envelope (outer and inner membranes), perinuclear space, nuclear pore, nucleoplasm, nucleolus, chromatin/chromatin fibres, and ribosomes on the outer membrane. …

(i) Nucleus. The nucleus is the control centre of the eukaryotic cell. Under the electron microscope it is seen to be bounded by a nuclear envelope made of two parallel membranes, with a gap between them called the perinuclear space, about ten to fifty nanometres wide. This double membrane keeps the nuclear contents separate from the cytoplasm. Its outer membrane is usually continuous with the endoplasmic reticulum and even carries ribosomes on its surface. At many points the envelope is interrupted by minute nuclear pores, formed where the two membranes fuse; these act as passageways through which RNA and protein molecules move in both directions.

Within the envelope lies the nuclear matrix, or nucleoplasm, which contains two main things:

• One or more spherical bodies called nucleoli. A nucleolus is not membrane-bound, so its contents merge with the surrounding nucleoplasm; it is the site of active ribosomal RNA synthesis.
• The chromatin — in a non-dividing (interphase) nucleus this is a loose, elaborate network of nucleoprotein fibres, made up of DNA, histones, non-histone proteins and RNA. During cell division this material condenses into well-defined chromosomes.

Parts to label in the diagram: the nuclear envelope showing its outer and inner membranes, the perinuclear space, a nuclear pore, ribosomes on the outer membrane, the nucleoplasm, the nucleolus, and the chromatin (nucleoprotein) fibres.

(ii) Centrosome. The centrosome is the organelle that organises the microtubule structures of an animal cell, and it is built around a pair of small cylinders called centrioles. A centrosome usually contains two such centrioles, embedded in an amorphous material known as the pericentriolar material, and the two centrioles lie perpendicular to each other — set at right angles.

Each centriole has an internal pattern resembling a cartwheel:

• Its wall is made of nine evenly spaced peripheral fibrils built of the protein tubulin.
• Each peripheral fibril is a triplet — three tubules grouped together — and adjacent triplets are linked to one another.
• The central part of the inner (proximal) region is proteinaceous and is called the hub. …

Method: Answer each numbered sub-part as a self-contained mini-answer, in a fixed order

When a question numbers its parts (i),

(ii)..., treat each one as needing its own complete, standalone answer — structure, then function, then what to label in the diagram — rather than letting information from one part bleed into the other. This also makes the answer easy to mark part-by-part, which matters for multi-part, high-mark questions like this one.

For each structure, use the same three-layer description every time: the overall boundary/covering, what is inside it and how it is arranged, and any distinctive internal pattern that has its own name (the chromatin network for the nucleus; the cartwheel arrangement of triplet fibrils for the centrosome's centrioles). Naming the "9+0" pattern for centrioles explicitly — and, if you know it, contrasting it with the "9+2" pattern of cilia/flagella — …