Cell structure: active recall

It contains most of the cell’s DNA, associated with proteins in chromosomes, and separates nuclear processes from the cytoplasm. Nuclear pores allow controlled exchange. The nucleolus is involved in ribosomal RNA production and ribosome-subunit assembly; it is not the site where complete proteins are translated.

Ribosomes translate the information in mRNA into a polypeptide. Both prokaryotic and eukaryotic cells contain them, though their sizes and details differ. Some eukaryotic ribosomes are free in the cytoplasm and others associate with rough endoplasmic reticulum.

Ribosomes associated with rough ER make proteins that can enter its membrane or lumen. These proteins can be processed and transported in vesicles to the Golgi apparatus. The ribosome performs translation; the ER supports entry, processing and transport.

It modifies and sorts material arriving in vesicles, then packages it into vesicles for destinations such as membranes, secretion or lysosomes. Vesicle movement and fusion connect the compartments. The Golgi does not read DNA and translate the amino-acid sequence itself.

Hydrolytic enzymes can break down material such as damaged cell components or engulfed particles. A membrane-bound compartment provides conditions suited to their activity and separates them from much of the cytoplasm. The enzymes are proteins produced through the cell’s protein-making systems.

The inner membrane contains electron-transfer machinery and ATP synthase, and its folds increase available area. The matrix contains enzymes for stages such as the link reaction and Krebs cycle. Compartments on either side of the inner membrane allow a proton gradient to be maintained.

Thylakoid membranes contain pigments and machinery for light-dependent reactions and a proton gradient. The stroma contains enzymes for the Calvin cycle. Chloroplasts also contain DNA and ribosomes, but they still depend on many proteins encoded by nuclear genes.

The cellulose-containing wall provides structural support outside the membrane. The membrane controls exchange through its lipid bilayer and proteins. The wall and membrane are separate structures; a wall does not replace the membrane’s transport functions.

Bacteria lack a membrane-bound nucleus and the usual eukaryotic membrane-bound organelles. Their main DNA is often circular and may be accompanied by plasmids. They have ribosomes and a cell-surface membrane. Many have a peptidoglycan wall, which differs from a plant cellulose wall.

A virus is acellular: its genetic material is enclosed in a protein capsid and sometimes a membrane envelope. It lacks the independent cellular machinery needed for processes such as translation and relies on a host cell for replication. Being microscopic does not make it a bacterium.

Magnification describes the ratio of image size to actual size. Resolution concerns whether nearby features can be distinguished as separate. Enlarging an image without additional resolved detail increases its size but does not reveal structures that the original imaging method could not separate.

An observation drawing records the visible structures, proportions and relationships in that specimen. Adding unseen structures creates evidence that the image did not provide. Labels and a scale can support interpretation while keeping observation separate from background knowledge.