Unit 1.2: Cell
Structure and
Organisation
Part 1: Cell Theory, Eukaryotic Cells
and Microscopy
1. Cell theory
Key definition
Cell theory states that: all living organisms are made of one or more cells, the cell is the basic unit of
structure and function, new cells come from existing cells, and cells can become specialised.
Unicellular vs multicellular
Feature Unicellular organism Multicellular organism
Made of One cell Many cells
Examples Bacteria, Amoeba, some fungi and Humans, other animals, plants
algae
How functions are The single cell does everything: Cells become specialised (cell
carried out respiration, nutrition, excretion, differentiation) so different cells do
growth, reproduction, sensitivity different jobs
Examples of Not applicable Red blood cells (transport O2), ciliated
specialised cells cells (move mucus), root hair cells
(absorb water/ions), palisade cells
(photosynthesis)
, 2. Eukaryotic cells
Eukaryotic cells have a nucleus surrounded by a nuclear envelope, membrane-bound
organelles, linear chromosomes and 80S ribosomes. Animal, plant, fungal and protist
cells are all eukaryotic.
An organelle is a specialised structure inside a cell that carries out a particular function.
Organelles in eukaryotic cells
Nucleus
• Contains most of the cell's DNA, surrounded by a double membrane, the nuclear
envelope.
• Controls cell activity, provides the DNA template for transcription and allows DNA
replication before division.
• DNA is linked to histone proteins. Uncondensed DNA is chromatin; before division
it condenses into visible chromosomes.
Watch out! Marks get lost here
• Do not write 'the nucleus makes proteins'. It contains the DNA that provides the instructions
for protein synthesis. Protein synthesis (translation) happens at ribosomes.
Nuclear envelope and nuclear pores
• The nuclear envelope is a double membrane that separates the nucleus from the
cytoplasm and protects the DNA.
• Nuclear pores are gaps in the envelope. They let mRNA and ribosomal subunits
leave, and let proteins and nucleotides enter.
Nucleolus
• A dense region inside the nucleus.
• Makes ribosomal RNA (rRNA), combines it with protein, and produces ribosomal
subunits, which leave through nuclear pores and form ribosomes.
Watch out! Marks get lost here
• Do not say 'the nucleolus stores DNA'. Say it produces rRNA and assembles ribosomal
subunits.
Ribosomes
• Made of rRNA and protein. The site of translation, where amino acids join to form
a polypeptide.
• Free in the cytoplasm, or attached to the RER.
Structure and
Organisation
Part 1: Cell Theory, Eukaryotic Cells
and Microscopy
1. Cell theory
Key definition
Cell theory states that: all living organisms are made of one or more cells, the cell is the basic unit of
structure and function, new cells come from existing cells, and cells can become specialised.
Unicellular vs multicellular
Feature Unicellular organism Multicellular organism
Made of One cell Many cells
Examples Bacteria, Amoeba, some fungi and Humans, other animals, plants
algae
How functions are The single cell does everything: Cells become specialised (cell
carried out respiration, nutrition, excretion, differentiation) so different cells do
growth, reproduction, sensitivity different jobs
Examples of Not applicable Red blood cells (transport O2), ciliated
specialised cells cells (move mucus), root hair cells
(absorb water/ions), palisade cells
(photosynthesis)
, 2. Eukaryotic cells
Eukaryotic cells have a nucleus surrounded by a nuclear envelope, membrane-bound
organelles, linear chromosomes and 80S ribosomes. Animal, plant, fungal and protist
cells are all eukaryotic.
An organelle is a specialised structure inside a cell that carries out a particular function.
Organelles in eukaryotic cells
Nucleus
• Contains most of the cell's DNA, surrounded by a double membrane, the nuclear
envelope.
• Controls cell activity, provides the DNA template for transcription and allows DNA
replication before division.
• DNA is linked to histone proteins. Uncondensed DNA is chromatin; before division
it condenses into visible chromosomes.
Watch out! Marks get lost here
• Do not write 'the nucleus makes proteins'. It contains the DNA that provides the instructions
for protein synthesis. Protein synthesis (translation) happens at ribosomes.
Nuclear envelope and nuclear pores
• The nuclear envelope is a double membrane that separates the nucleus from the
cytoplasm and protects the DNA.
• Nuclear pores are gaps in the envelope. They let mRNA and ribosomal subunits
leave, and let proteins and nucleotides enter.
Nucleolus
• A dense region inside the nucleus.
• Makes ribosomal RNA (rRNA), combines it with protein, and produces ribosomal
subunits, which leave through nuclear pores and form ribosomes.
Watch out! Marks get lost here
• Do not say 'the nucleolus stores DNA'. Say it produces rRNA and assembles ribosomal
subunits.
Ribosomes
• Made of rRNA and protein. The site of translation, where amino acids join to form
a polypeptide.
• Free in the cytoplasm, or attached to the RER.
