Cells are the basic structural and functional units of living organisms, and new cells arise from existing cells. Prokaryotes lack a membrane-bound nucleus; eukaryotes have a nucleus and membrane-bound organelles. Both have DNA, cytoplasm, ribosomes and a cell membrane. Plants and fungi have cell walls, but their wall composition differs; animal cells lack a cell wall.
The nucleus contains nuclear DNA; the nucleolus helps assemble ribosomal components. Ribosomes synthesise proteins. Rough endoplasmic reticulum and Golgi process and route many secreted proteins, while smooth ER has lipid-related roles. Lysosomes digest materials, vacuoles store substances, and the cytoskeleton supports organisation and movement. Mitochondria support aerobic respiration; chloroplasts carry out photosynthesis.
Example 1
A cell has ribosomes and circular DNA but no nucleus. Classify it.
These features support a prokaryotic cell. Ribosomes occur in both major cell types, so the absence of a nucleus is decisive here.
Example 2
Trace a typical exported protein after translation begins.
It enters the rough ER pathway, is carried in vesicles to the Golgi, then reaches the cell membrane in a secretory vesicle.
Watch out: They have structures including ribosomes and a chromosome, but lack a membrane-bound nucleus.