Cells as the basic units of living organisms

Biology AS · Cell structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Eukaryotic cells (plants, animals) have a nucleus and other organelles surrounded by membranes. Each organelle has its own job, and you must link structure to function. A useful pathway: proteins are made on ribosomes of the rough endoplasmic reticulum, modified and packaged by the Golgi body, then carried in vesicles to the cell surface membrane. All energy-requiring processes use ATP from respiration.

Prokaryotic cells (bacteria) are much smaller and simpler. They have no nucleus and no organelles surrounded by double membranes.

Viruses are not cells at all. They are a nucleic acid core inside a protein coat.

Rules to remember

Common mistake

Saying that bacteria have no ribosomes or no DNA. They have both: the ribosomes are smaller (70S) and the DNA is circular and not inside a nucleus.

Worked example

An electron micrograph shows a cell with a cell wall, a nuclear envelope and mitochondria, but no chloroplasts. A student says it is a bacterium. Is the student right?

  1. A nuclear envelope and mitochondria are organelles with double membranes.
  2. Bacteria have no organelles surrounded by double membranes, so this cell is eukaryotic.
  3. A cell wall with a nucleus fits a plant cell. Some plant cells, such as root cells, have no chloroplasts.

Answer: No. It is a eukaryotic cell, most likely a plant cell that does not photosynthesise, such as a root cell.

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