Identify eukaryotic and prokaryotic cells and describe where their genetic material is found.
BEEZYREVISOR
GCSE Biology Revision
Learn it. Recall it. Revise it.
GCSE Biology revision
Eukaryotes and prokaryotes
Cell structure
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Compare the organisation and relative size of eukaryotic and prokaryotic cells.
Apply knowledge of cell structure, scale and size to distinguish between eukaryotic and prokaryotic cells from unfamiliar information.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
The Five Kingdoms and Eukaryotes vs Prokaryotes
- Almost all living organisms belong to one of the five kingdoms: animals, plants, fungi, protoctists, and bacteria
- Animals, plants, fungi, and protoctists are all eukaryotes, meaning their cells contain a nucleus where DNA is stored as chromosomes
- Bacteria are prokaryotes, meaning they lack a nucleus and their DNA floats freely within the cell
- Prokaryotic cells (bacteria) are 10–100 times smaller than eukaryotic cells
Viruses: Not Living Organisms
- Viruses are not classified as living organisms and therefore do not belong to any of the five kingdoms
- Viruses are tiny particles — roughly 10–100 times smaller than prokaryotic cells — and are not made of cells
- Their basic structure consists of a protein coat surrounding genetic material, which can be either DNA or RNA
- All viruses are considered pathogens because they always cause harm to the host organism they infect
- Viruses can only reproduce by infecting living cells (of animals, plants, fungi, protoctists, or bacteria), making them parasites
- Examples of viruses include the influenza virus, tobacco mosaic virus, HIV, and the
- SARS-CoV-2 virus that causes COVID-19
Animals: Multicellular Heterotrophs
- Animals are multicellular organisms, meaning each individual is made up of many cells rather than just one
- Animals are heterotrophs, meaning they must obtain energy by consuming other organisms such as plants or animals
- There are an estimated 5–10 million species of animals on Earth, ranging from humans to insects to fish
- Most animals reproduce sexually
Plants: Multicellular Autotrophs
- Plants are multicellular organisms made up of many different types of cells
- Unlike animals, plants are autotrophs, meaning they produce their own energy from sunlight through photosynthesis
- There are approximately 300,000 species of plants, including trees, flowers, and aquatic plants
Fungi: Saprotrophic Heterotrophs
- Fungi can be either multicellular (eg mushrooms and moulds) or unicellular (eg yeast), making them unique among the kingdoms
- Fungi cannot photosynthesise and are classified as heterotrophs, obtaining energy from other organisms
- Most fungi feed by saprotrophic nutrition, secreting digestive enzymes onto food outside their body and then absorbing the broken-down nutrients
- Multicellular fungi have a body called a mycelium, which is made up of many thread-like structures called hyphae
- Some fungi are pathogens and can cause disease in humans; for example, the fungus that causes athlete's foot
Protoctists: Mostly Unicellular and Varied
- Protoctists (also called protists or protoctista) are nearly all unicellular organisms, though there is great variety between species
- Some protoctists, such as Chlorella and Euglena, contain chloroplasts and can photosynthesise, making them similar to plant cells
- Other protoctists, such as Amoeba, behave more like animal cells and must consume other organisms to obtain energy
- A small number of protoctists are pathogens; for example, Plasmodium is the organism responsible for causing malaria
Bacteria: Prokaryotic and Widespread
- Bacteria are single-celled prokaryotic organisms found almost everywhere, including on human skin and in the intestines
- Although some species of bacteria can photosynthesise, none of them possess chloroplasts
- Scientists believe there are more species of bacteria than all other kingdoms combined
- Some bacteria are pathogens — for example, Salmonella can cause food poisoning — but many are harmless or even beneficial to humans
- Beneficial bacteria in the human intestines help with the digestion of food
Comparing Nutrition: Autotrophs vs Heterotrophs
- Autotrophs (such as plants and some protoctists) produce their own food using energy from sunlight via photosynthesis
- Heterotrophs (such as animals and fungi) must obtain energy by consuming or breaking down other organisms
- Saprotrophic nutrition, used by most fungi, involves external digestion — enzymes are secreted outside the body to break down food before absorption
Key Differences: Cell Structure Summary
- Eukaryotic cells (found in animals, plants, fungi, and protoctists) contain a nucleus housing DNA in the form of chromosomes
- Prokaryotic cells (bacteria) lack a nucleus, with DNA found loose in the cytoplasm, and are significantly smaller than eukaryotic cells
- Viruses are not cells at all — they are particles consisting only of a protein coat and genetic material (DNA or RNA)
Pathogens Across the Groups
- A pathogen is any organism or particle that can cause disease in a host
- Some protoctists are pathogens, such as Plasmodium, which causes malaria
- Some bacteria are pathogens, such as Salmonella, which causes food poisoning
- Some fungi are pathogens; for example, the fungus responsible for athlete's foot
- All viruses are considered pathogens because they always cause harm when replicating inside a host's cells