Characteristics of Living Organisms
The Nature and Variety of Living Organisms
Edexcel 4BI1 does not ask you to learn a "five kingdoms" scheme. Instead it groups organisms by their cell type:
(plants, animals, fungi and protoctists), (for example bacteria), and viruses (non-living particles)
Eukaryotes have cells with a true nucleus and other membrane-bound organelles. Prokaryotes have cells without a true nucleus, and bacteria are the prokaryotes you need to know. Viruses are listed separately because they are not made of cells and are not classed as living (see section 3).
Eukaryotes vs prokaryotes: the headline difference
| Eukaryotes | Prokaryotes | |
|---|---|---|
| Cell size | Larger (typically 10–100 μm) | Much smaller (typically 1–10 μm) |
| Nucleus | Yes, a true nucleus surrounded by a membrane | No; the DNA floats free in the cytoplasm |
| Genetic material | Long linear chromosomes inside the nucleus | A single circular chromosome in the cytoplasm, often with small extra rings of DNA called plasmids |
| Other membrane-bound organelles | Yes: mitochondria, chloroplasts, ER, Golgi etc. | None at all |
| Cell wall? | Some have one (plants, fungi); animals do not | Always; all bacteria have a cell wall made of peptidoglycan (not cellulose or chitin) |
| Single-celled / multicellular? | Can be either | Always single-celled |
Common exam question
Placing an organism in its group from its features
Question: Multiple choice: which group an organism with the stated features belongs to, what its cell wall is made of, or which features two groups share (1 mark).
Asked in 6 of the 23 papers. A chitin cell wall means a fungus: yeast, a single-celled organism with a nucleus and a chitin wall, is a fungus rather than a protoctist, and its wall is chitin, not cellulose. A single-celled organism with a nucleus and a chloroplast, such as Chlorella or phytoplankton, is a protoctist: it cannot be a plant (plants are multicellular) or a bacterium (bacteria have no nucleus).
For shared features, every cell has cytoplasm and a cell membrane, but a cell wall is not universal because animal cells lack one. Photosynthetic protoctists share chloroplasts and a nucleus with plants but not a chitin wall. Animals and fungi both have nuclei and store glycogen, but only animals have nervous coordination.
Animals
- Multicellular, built from many cells working together
- Cells have a nucleus and no cell wall
- Cells have no chloroplasts, so animals cannot photosynthesise
- Feed by eating other organisms (heterotrophic). They digest food in a gut and absorb the products
- Often store carbohydrate as glycogen in the liver and muscles
- Have nervous coordination to respond quickly to stimuli
- Can usually move from place to place (locomotion)
- Examples the spec names include an insect such as the housefly or the mosquito. Other animals include humans, dogs, fish and worms
Plants
- Multicellular
- Cells have a nucleus
- Cells have cell walls made of cellulose (a rigid carbohydrate)
- Cells contain chloroplasts, so plants are autotrophic and feed by photosynthesis
- Store carbohydrate as starch in roots and tubers, or as sucrose in fruit and storage tissue
- No nervous coordination; responses to stimuli are slower and chemical
- Are rooted and unable to relocate, although they can reorient themselves (for example, shoots bending towards light)
- Examples the spec names: a cereal such as maize, and a herbaceous legume such as peas or beans. Other plants include rose bushes, oak trees, grasses and ferns
Fungi
- Usually multicellular, but some (e.g. yeast) are single-celled
- Multicellular fungi are built from long thread-like cells called hyphae, which form a tangled network called the mycelium. Each hypha contains many nuclei stretched along its length
- Cells have a nucleus
- Cells have cell walls made of chitin (the same material as insect exoskeletons), not cellulose
- Cells contain no chloroplasts, so fungi cannot photosynthesise
- Feed by saprotrophic nutrition: they release digestive enzymes out onto the surrounding food (dead organic matter), the enzymes break the food down to small molecules outside the fungus, and the fungus then absorbs the digested molecules
- Some fungi are parasitic and feed on living organisms (e.g. athlete's foot fungus feeding on human skin)
- Often store carbohydrate as glycogen
- No nervous coordination
- Examples: mushrooms, mould (e.g. Mucor), yeast (used in baking and brewing)
Common exam question
Describing how fungi and decomposers feed
Question: Describe how a saprotrophic fungus such as a mould obtains its food, or how the bacteria and fungi in a food web obtain energy from dead organic matter (3 marks).
Asked in 2 of the 23 papers. The marks follow the stages of saprotrophic nutrition: the fungus secretes enzymes from its hyphae onto the dead or decaying material, the enzymes digest it outside the organism (extracellular digestion), and the fungus then absorbs the small soluble products. In one scheme "digestive enzymes" earns two of these marks at once, one for digestion and one for the enzymes. The decomposer version also credits a further point: the absorbed food is used in respiration to release energy. "It rots the food" is ignored, so say digested, decomposed or broken down.
Protoctists
- A very mixed bag. The protoctists are the "kingdom of leftovers", containing every eukaryote that is not clearly a plant, animal or fungus
- Mostly single-celled and microscopic, but some form colonies or chains of cells
- Cells have a nucleus
- Some protoctists are plant-like: they have cellulose cell walls and chloroplasts, and they photosynthesise. Example: Chlorella (a green alga)
- Some protoctists are animal-like: no cell walls, no chloroplasts, and they feed on other organisms. Example: Plasmodium (the malaria parasite)
- No nervous coordination
- Examples: Amoeba (single-celled animal-like), Paramecium (a swimming cilia-covered animal-like protoctist), Plasmodium (parasite that causes malaria), Chlorella (single-celled plant-like alga)
Prokaryotes
- Always single-celled and microscopic
- Cells have no nucleus. The DNA is a single circular chromosome sitting free in the cytoplasm, sometimes with smaller circular pieces called plasmids
- No membrane-bound organelles at all (no mitochondria, no chloroplasts, no nucleus)
- Have a cell wall (made of peptidoglycan, not cellulose or chitin), a cell membrane inside the wall, and cytoplasm
- Feed in many different ways:
- Some photosynthesise even though they have no chloroplasts. They use chlorophyll-like pigments dissolved in the cytoplasm instead
- Most are heterotrophs, taking nutrients from the bodies of other organisms (alive or dead)
- Those that break down dead organic matter are called saprobionts (or decomposers); they recycle the building blocks of life back into the soil
- The prokaryotes you need to know for this spec are the bacteria
- Examples of bacteria:
- Lactobacillus bulgaricus: rod-shaped; used to make yoghurt from milk
- Pneumococcus (Streptococcus pneumoniae): spherical; causes pneumonia
- Escherichia coli (E. coli): rod-shaped; lives harmlessly in our guts (some strains are harmful)
Common exam question
Completing a passage about fungi or bacteria
Question: Complete a passage about fungi, or about bacteria, by filling each blank with a suitable word or words (4–8 marks, one per blank).
Set in 2 of the 23 papers, both on Paper 1. The blanks want the bold terms from these notes. For fungi: the mycelium is made of thread-like hyphae, the cell wall is chitin, feeding is by secreting enzymes onto the food and absorbing the products, and this is saprotrophic nutrition (saprophytic is also accepted). For bacteria: they are single-celled, have a cell wall made of peptidoglycan, some carry out photosynthesis using the Sun's energy while others are decomposers (or saprotrophs) feeding on dead matter, the genetic material is a circular chromosome (nucleoid is accepted), the small circles of DNA are plasmids, Lactobacillus is the bacterium used to make yoghurt (Streptococcus also scores), and disease-causing bacteria such as Pneumococcus are pathogens.