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CHAPTER 2 : Basic components of living systems
Prokaryotic and eukaryotic cells
Prokaryotic cells
-Prokaryotic organisms are always unicellular, relatively simple structure
-DNA is not contained within a nucleus
-Have some organelles, they’re not membrane-bound
DNA
-Structure of DNA is contained within prokaryotes is fundamentally the same as in
eukaryotes but it’s packaged differently
-Prokaryotes generally have one molecule of DNA, a chromosome, which is
supercoiled to make it more compact
-Genes on the chromosome are often grouped into operons, number of genes are
switched off or on at the same time
Ribosomes
-Prokaryotic cells have smaller ribosomes compared to eukaryotic cells
-Relative size is determined by the rate at which they settle, or form a sediment in a
solution
-Larger eukaryotic organisms are designated 80S and smaller prokaryotic
ribosomes, 70S
-They’re both important for protein synthesis, although larger 80S ribosomes are
involved in formation of more complex proteins
Cell wall
-Prokaryotic cells have a cell wall made from peptidoglycan, also known as murenin,
it’s a complex polymer formed from amino acids and sugars
Flagella
-Flagella of prokaryotes is thinner than the equivalent structure of eukaryotes and
don’t have the 9+2 arrangement
-Energy to rotate the filament that forms the flagellum is supplied from the process of
chemiosmosis, not from ATP as in eukaryotic cells
-Basal body attaches the filament comprising the flagellum to the cell-surface
membrane of a bacterium
-Molecular motor causes hook to rotate giving the filament a whip like movement,
propelling the cells

A comparison with eukaryotic cells
-Eukaryotic cells are more complex than prokaryotic cells
-Their DNA is in a nucleus and exists as multiple chromosomes, which are
supercoiled, each wraps around a number of proteins (histones), forming a complex
for efficient packing, called chromatin
-Chromatin coils and condenses to form chromosomes
-Eukaryotic genes are generally switched on and off individually
The similarities and differences between prokaryotic and eukaryotic cells
Feature

Prokaryotic

Eukaryotic

Nucleus

Not present

Present

DNA

Circular

Linear

Extra chromosomal DNA

Circular DNA called
plasmids

Only present in certain
organelles such as
chloroplast and
mitochondria

Organelles

Non membrane-bound

Both membrane-bound
and non membranebound

Cell wall

Peptidoglycan

Chitin in fungi, cellulose in
plants, not present in
animals

Ribosomes

Smaller, 70S

Larger, 80S

Cytoskeleton

Present

Present, more complex

Reproduction

Binary fission

Asexual or sexual

Cell type

Unicellular

Unicellular and
multicellular

Cell-surface membrane

Present

Present

DNA organisation

Protein folds and
condense DNA

Associated with proteins
called histones



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