CELLS
CELL TYPES
Types of Cells
According to cell theory, a cell is the fundamental unit of life (nothing smaller than a cell is alive)
• All cells share four essential features: cell membrane, cytosolic fluid, DNA and ribosomes
Cells are classed into different groups based on the presence of key structural components
PROKARYOTE
Archaea
Bacteria
EUKARYOTE
Protista
Fungi
Animal
Plant
Prokaryotes
Prokaryotes are a simple class of cells that lack a nucleus (‘pro’ = before ; ‘karyon’ = nucleus)
• They also lack any compartmentalisation (do not possess membrane-bound organelles)
Prokaryotes are typically unicellular (some exceptions) and can exist in a variety of shapes
Coccus (round)
Bacillus (rod)
Spirilla (spiral)
Prokaryote Structure
Prokaryotic cells typically share a number of key characteristics:
• The genetic material (DNA) is found in a region called the nucleoid
• Prokaryotes may contain autonomous DNA molecules called plasmids
• Ribosomes (responsible for protein synthesis) are smaller in size (70S)
• They possess an external cell wall for stability (made of peptidoglycan)
Prokaryotic cells may also possess a number of other components:
• Hair-like extensions (pili) or whip-like projections (flagella) for movement
• An additional outer covering to prevent desiccation (slime capsule)
A2.2.5 Prokaryote cell structure
Prokaryote Drawing
Cytosol: Internal cell fluid
Ribosomes (70S): Protein synthesis
Cell Wall: Rigid outer coat
Nucleoid: Site containing DNA
Plasma Membrane: Cell boundary
Pilus: Attachment or conjugation
Flagellum: Movement
Plasmid: Autonomous DNA molecule
Slime Capsule (glycocalyx):
Outer coat to prevent desiccation
A2.2.11 Drawing and annotation based on electron micrographs
Prokaryote Micrographs
LIGHT MICROSCOPY
Bacterial cocci
ELECTRON MICROSCOPY (FALSE COLOUR)
Cell Wall (yellow)
Flagella (yellow)
A2.2.10 Cell types and cell structures viewed in light and electron micrographs
Pili (blue)
Bacteria vs Archaea
Prokaryotes can be classified into two domains:
Bacteria
Archaea
• Bacteria – most commonly encountered forms
Can be pathogenic
Not pathogenic
• Archaea – extremophiles (harsh environments)
Peptidoglycan
in cell wall
Peptidoglycan
not in cell wall
Archaea have certain features that are more in
Ester-linked
membrane lipids
Ether-linked
membrane lipids
DNA is naked
DNA is bound to
histone proteins *
Only one type of
RNA polymerase
Several types of
RNA polymerase *
Introns are rare
Can have introns *
common with eukaryotes (share recent ancestry)
BACTERIA
ARCHAEA
EUKARYA
* Feature shared with eukaryotic cells
Eukaryotes
Eukaryotes are a complex class of cells that possess a nucleus (‘eu’ = true ; ‘karyon’ = nucleus)
• They also demonstrate compartmentalisation (they possess membrane-bound organelles)
Eukaryotic cells are classified into four kingdoms based on certain common characteristics
Animal
Plant
Fungus
Protist
Cell Nucleus
The nucleus is the main distinguishing feature of eukaryotic cells
• It is a double-membrane structure with pores that stores DNA
The presence of a nucleus allows the eukaryote to separate the
processes of transcription (nucleus) and translation (cytoplasm)
• Transcription converts DNA instructions (genes) into mRNA
• Translation synthesises polypeptides from mRNA transcripts
Separating these processes allows for the post-transcriptional
modification of mRNA prior to translation by the ribosomes
B2.2.2
Cell Nucleus (false colour)
Advantage of the separation of the nucleus and cytoplasm into separate compartments
Compartmentalisation
Another defining feature of eukaryotes is compartmentalisation
• Eukaryotic cells possess several membrane-bound organelles
An organelle is a sub-cellular structure adapted to a specific role
• Membrane-bound organelles will have an internal chemistry
that is specific to its function (and different to the cytoplasm)
For example, lysosomes and phagocytic vesicles both contain
hydrolytic enzymes responsible for digesting cellular contents
• Containing the enzymes, prevents cell degradation (autophagy)
B2.2.3
Advantage of compartmentalization in the cytoplasm of cells
Compartmentalisation
Organelles
B2.2.4
Ribosome
Site of protein synthesis
Nucleus
Stores genetic material (DNA)
Endoplasmic Reticulum
Intracellular transport network
Golgi Complex
Exports secretory products
Mitochondria
Site of aerobic respiration
Peroxisome
Digests toxic metabolites
Chloroplast (Plants Only)
Site of photosynthesis
Sap Vacuole (Plants Only)
Internal fluid cavity
Organelles as discrete subunits of cells that are adapted to perform specific functions
Organelle Micrographs
Key:
1. Nucleus
2. ER network
1
2
3
4
5
6
3. Microvilli
4. Golgi Body
5. Mitochondria
6. Chloroplast
A2.2.10 Cell types and cell structures viewed in light and electron micrographs
Eukaryote Structure
Eukaryotic cells all possess a double-membrane bound nucleus
ANIMAL
and a compartmentalised cytoplasm with a variety of organelles
Some cells have specific organelles to carry out unique functions:
• Animal cells have lysosomes to break down cellular wastes and
centrosomes for cell division (these are specific to animal cells)
• Plant cells have a large central sap vacuole for storing excess
fluid and chloroplasts for photosynthesis (unique to plant cells)
Fungi do not have unique organelles and protists are very diverse
A2.2.6 Eukaryote cell structure
PLANT
Types of Eukaryotes
PLANT
Cell Wall
FUNGUS
ANIMAL
Yes (cellulose)
Yes (chitin)
No
Chloroplasts
Yes
No
No
Vacuoles
Yes
Yes
Small, temporary
Centrioles
No
No
Yes
Cilia / Flagella
No
No
Yes
Photosynthesis
Absorption
Ingestion
Nutrition
A2.2.8 Differences in eukaryotic cell structure between animals, fungi and plants
Animal Cell Drawing
Lysosome: Digests cell contents
Mitochondrion: ATP production
Rough ER: Transports proteins
Nucleolus: Ribosome assembly
Smooth ER: Transports lipids
Cytosol: Internal cell fluid
Nucleus: Stores DNA
Plasma Membrane: Cell boundary
Golgi Complex: Exports contents
Ribosome (80S): Protein synthesis
A2.2.11 Drawing and annotation based on electron micrographs
Plant Cell Drawing
Cellulose Cell Wall:
External barrier
Chloroplast:
Site of
photosynthesis
A2.2.11 Drawing and annotation based on electron micrographs
Sap Vacuole:
Stores internal fluid
(regulate pressure)
Eukaryote Micrographs
ANIMAL CELL
PLANT CELL
A2.2.10 Cell types and cell structures viewed in light and electron micrographs
Prokaryote vs Eukaryote
Prokaryotic and eukaryotic cells differ in a number
Prokaryotes
Eukaryotes
of key features that can be used for classification:
DNA is naked
Bound to histone
Circular
chromosome
Linear
chromosomes
No membranebound organelles
Has membranebound organelles
70S ribosomes
80S ribosomes
• DNA – composition and structure
D
• Organelles – types present
• Reproduction – methods and ploidy
• Additional features – cytoskeleton and size
O
Helpful Mnemonic:
R
Asexual only
(binary fission)
Sexual and
asexual methods
DORA
A
Simple
cytoskeleton
Complex
cytoskeleton
Topic Connections
Origin of Cells (AHL)
• Eukaryotes evolved from prokaryotic cells via endosymbiosis (A2.2.12)
Cell Division
• Animal and plant cells have different methods of cytokinesis (D2.1.1)
Translocation (AHL)
• Cell walls help to withstand potential pressure changes in cells (D2.3.10)
Climate Change
• Methanogenic prokaryotes (Archaea) promote methane release (D4.3.1)
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