Eukaryotic Cells and Specialisation

A eukaryotic nucleus is surrounded by an envelope and contains chromosomes made from linear DNA associated with proteins. Nuclear pores permit regulated exchange, including export of RNA. The nucleolus is involved in producing components of ribosomes. Ribosomes translate mRNA into polypeptides; they are not membrane-bound organelles.

Mitochondria have an outer membrane and a folded inner membrane forming cristae, with a matrix inside. These compartments support stages of aerobic respiration. Chloroplasts in plants and algae have thylakoid membranes and a surrounding stroma that support different photosynthetic reactions. Folded or stacked membranes increase the area available for particular reaction systems, rather than simply giving a cell more space.

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Rough endoplasmic reticulum has associated ribosomes and is involved in synthesis and processing of proteins destined for secretion or membranes. Transport vesicles carry material to the Golgi apparatus, which modifies, sorts and packages products. Secretory vesicles can fuse with the cell-surface membrane to release their contents.

Smooth endoplasmic reticulum lacks attached ribosomes and participates in lipid synthesis and other metabolic functions. Lysosomes contain hydrolytic enzymes in a membrane-bound compartment, allowing controlled digestion of material. The cell-surface membrane regulates exchange and communication; it is a dynamic phospholipid-and-protein structure, not a rigid wall.

Plant cells commonly have a cellulose wall and a large permanent vacuole bounded by a membrane. Vacuolar cell sap and water entry contribute to turgor; the wall resists expansion and provides support. Fungi also have cell walls, but their walls are not simply plant cellulose walls. Not every plant cell contains chloroplasts: many root cells are not photosynthetic.

Specialised cells combine organelles in proportions appropriate to function. A protein-secreting cell may have extensive rough ER, Golgi apparatus and mitochondria. Similar specialised cells form tissues; tissues work together in organs; organs cooperate in systems. Apply this hierarchy to a named example and avoid assuming every eukaryotic cell has the same visible structures.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.