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Cytology · Year 1 · Medical University of Sofia

10

Cell. Prokaryotic and eukaryotic cells

Free notes for topic 10 of the Cytology syllabus, open without an account. Written by a senior student against the syllabus question and checked line by line by a second student before publishing. How content is made

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Summary

note

The cell is the basic structural and functional unit of all known living organisms, and the smallest unit of life.

There are two kinds. A prokaryote has no nucleus. Its DNA is circular and sits free in a region called the nucleoid, it has a cell wall, it is always unicellular, and it measures 0.1 to 5 μm. Bacteria and archaea are the examples.

A eukaryote has a membrane-enclosed nucleus, linear double-helix DNA, a full set of membranous and non-membranous organelles and a cytoskeleton, and measures 10 to 100 μm. Animals, humans and plants are eukaryotes.

The single most useful contrast: a prokaryote has no membranous cytoplasmic structures at all. Every organelle a eukaryote has that is made of membrane is simply absent.

1. The cell and the levels of organisation

note

Cells are at the centre of the organisation of the human body:

  • They are the basic structural and functional unit of all known living organisms.
  • They are the smallest unit of life, the building block of life.

The levels of organisation run:

cell → tissue → organ → organ system → organism

2. Prokaryotes

note

Prokaryotes are single-celled organisms, so they are always unicellular.

  • They have no nucleus. Instead they have a nucleoid, which holds circular DNA.
  • They have no membranous cytoplasmic structures.
  • They have a cell wall.
  • The examples are bacteria and archaea.
  • Their size is 0.1 to 5 μm.
Diagram of a prokaryotic cell labelled with plasma membrane, cell wall, capsule, pili, nucleoid DNA, plasmid, ribosome, flagellum and cytoplasm
Diagram of a prokaryotic cell labelled with plasma membrane, cell wall, capsule, pili, nucleoid DNA, plasmid, ribosome, flagellum and cytoplasm

3. Eukaryotes

note

Eukaryotes have a cell membrane and a membrane-enclosed nucleus surrounded by cytoplasm, which is the fluid containing a system of membranous and non-membranous organelles, and a cytoskeleton.

  • Their DNA is linear, a double helix.
  • They are usually multicellular, sometimes unicellular.
  • Animals, humans and plants are eukaryotes.
  • Their size is 10 to 100 μm.
Diagram of a eukaryotic animal cell labelled with pinocytotic vesicle, lysosome, Golgi vesicles, rough endoplasmic reticulum, smooth endoplasmic reticulum, cell membrane, mitochondrion, Golgi apparatus, nucleolus, nucleus, centrioles, microtubules, cytoplasm and ribosome
Diagram of a eukaryotic animal cell labelled with pinocytotic vesicle, lysosome, Golgi vesicles, rough endoplasmic reticulum, smooth endoplasmic reticulum, cell membrane, mitochondrion, Golgi apparatus, nucleolus, nucleus, centrioles, microtubules, cytoplasm and ribosome

4. The two compared

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This is the table to be able to reproduce.

StructureEukaryotic cellProkaryotic cell
NucleusPresentAbsent
MitochondrionPresentAbsent
Golgi apparatusPresentAbsent
Nucleus and nucleolusPresentNucleoid
MicrotubulesPresentAbsent
CytoplasmPresentPresent
RibosomePresentPresent
LysosomePresentAbsent
Endoplasmic reticulum, rough and smoothPresentAbsent
Plasma membranePresentPresent, with no steroid
FlagellumIn some cells, such as the spermatozoonPresent
PiliAbsentPresent
Size10 to 100 μm0.1 to 5 μm

Three things are shared: the cytoplasm, the ribosome and the plasma membrane. One belongs to the prokaryote alone: the pili. A flagellum occurs in many prokaryotes and in some eukaryotic cells, such as the spermatozoon, where it is built differently, from microtubules. Everything else on the list is a eukaryotic organelle the prokaryote does without.

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