3.3—Eukaryotic cell ultrastructure

Eukaryotic cells contain membrane-bound organelles whose structures support distinct functions. The nucleus stores chromatin, mitochondria make ATP in aerobic respiration, ribosomes translate mRNA, and the endoplasmic reticulum, Golgi, lysosomes and centrioles have different specialised roles.

The rough ER carries ribosomes and processes proteins; smooth ER lacks ribosomes and is involved in lipid and steroid production. The Golgi modifies and packages cargo, lysosomes digest material, and centrioles organise spindle fibres in animal-cell division.

A secreted protein follows a route from nucleus (transcription) to ribosome/RER (translation and folding), then in vesicles to the Golgi for modification and onward to the cell surface. A free ribosome instead makes a protein that remains in the cytoplasm.

Ribosomes are not membrane-bound, and plant and animal cells are both eukaryotic. Do not identify an organelle from a label alone; connect its visible structure to its function.