Prokaryotic and eukaryotic cells

MDCAT Biology · Biology · Cell structure and function. 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

A prokaryotic cell (bacteria, cyanobacteria) has no true nucleus and no membrane-bound organelles. Its DNA is a single circular molecule lying free in the cytoplasm in a region called the nucleoid. A eukaryotic cell (animals, plants, fungi, protists) has a nucleus with an envelope and organelles such as mitochondria, ER and Golgi apparatus.

Plant and animal cells are both eukaryotic, but plant cells have a cellulose cell wall, plastids and a large central vacuole, while animal cells have centrioles.

Cells are studied with microscopes. Resolution, the ability to show two close points as separate, matters more than magnification. Electron microscopes resolve far more detail than light microscopes.

Rules to remember

Prokaryote: 70S ribosomes, wall of peptidoglycan (murein), circular DNA without histones, no mitochondria (respiratory enzymes on the plasma membrane), divides by binary fission
Eukaryote: 80S ribosomes in the cytoplasm, linear DNA with histones in a nucleus, membrane-bound organelles, divides by mitosis or meiosis
Typical size: prokaryotic cell about 1 to 10 μm; eukaryotic cell about 10 to 100 μm
Plant cell only: cellulose wall, chloroplasts and other plastids, large central vacuole. Animal cell only: centrioles; stores glycogen, not starch
magnification = image size ÷ actual size; 1 mm = 1000 μm; 1 μm = 1000 nm
Light microscope: lower resolution, can view living cells. Electron microscope: resolution to below 1 nm, specimen dead and in a vacuum; TEM shows internal detail, SEM shows 3-D surfaces
Structure fits function: sperm has many mitochondria for movement; red blood cell has no nucleus, more room for haemoglobin; root hair has a large surface area

Common mistake

Saying prokaryotes have no ribosomes or no DNA. They have both; what they lack is a nuclear envelope and membrane-bound organelles.

Worked example

A mitochondrion is 2 μm long. How long will it appear, in mm, in an electron micrograph with a magnification of × 25 000?

image size = actual size × magnification.
2 μm × 25 000 = 50 000 μm.
Convert to mm: 50 000 ÷ 1000 = 50.

Answer: The image is 50 mm long.

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