1bVariety of living organisms

Eukaryotic organisms have cells with a nucleus and membrane-bound organelles. Four groups are on the spec: plants, animals, fungi and protoctists. This note covers the first two.

Plants

• Multicellular organisms.
• Their cells contain chloroplasts and are able to carry out photosynthesis.
• Their cells have cellulose cell walls.
• They store carbohydrates as starch or sucrose.

Examples you must be able to name — flowering plants:

Note the detail: not every plant cell has chloroplasts. A root hair cell has none, because there is no light underground — but the plant as a whole is able to photosynthesise, and that is what the definition is about.

Animals

• Multicellular organisms.
• Their cells do not contain chloroplasts and are not able to carry out photosynthesis.
• They have no cell walls.
• They usually have nervous co-ordination and are able to move from one place to another.
• They often store carbohydrate as glycogen.

Examples you must be able to name:

Side-by-side

Common mistake: writing that plants "have a cell wall" without saying what it is made of. The mark is for cellulose — and it separates plants from fungi, whose walls are chitin, and from bacteria, whose walls are neither. Always name the material.

Also be precise about storage: plants store starch or sucrose, animals store glycogen. Do not write "glucose" — glucose is the soluble sugar that gets converted into the store.

Examiner tip. Read every word of the question stem. If it asks for features that plants and animals have in common, chloroplasts and cell walls score nothing — you need 'multicellular' and 'eukaryotic/cells contain a nucleus'. If it asks how they differ, give paired statements (plants have cellulose cell walls, animals have no cell walls), not a single one-sided fact.

Fungi

• Not able to carry out photosynthesis (they have no chloroplasts).
• The body is usually organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei.
• Some examples are single-celled.
• Their cells have walls made of chitin.
• They feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic products — this is called saprotrophic nutrition.
• They may store carbohydrate as glycogen.

Examples you must be able to name:

Saprotrophic nutrition in three steps

1. Enzymes are secreted out of the hyphae onto the food (digestion is extracellular — outside the body).
2. The large insoluble molecules are digested into small soluble products.
3. The soluble organic products are absorbed back into the hyphae.

Common mistake: saying fungi "eat" or "take in" food and then digest it. That is animal nutrition. The whole point of saprotrophic nutrition is that digestion happens outside the organism first, then the products are absorbed. Use the words secrete, extracellular, absorb.

Protoctists

• Microscopic single-celled organisms.
• Some have features like an animal cell; others have chloroplasts and are more like plants.

Examples you must be able to name:

Protoctists are the "leftovers" group: single-celled eukaryotes that are not plants, animals or fungi. That is why the group contains both plant-like and animal-like members.

Quick comparison

Examiner tip. Candidates lose marks by confusing or misnaming structures. Learn the exact words: hyphae are the individual threads, the mycelium is the whole network they form. Writing 'mycelium' where 'hyphae' is wanted (or spelling it 'hypha' when you mean plural) can cost you. Likewise, learn the italicised names — Mucor, Chlorella, Amoeba, Plasmodium — and which is which.

Bacteria are prokaryotic — the defining feature is that they lack a nucleus.

Common features of bacteria

• Microscopic single-celled organisms.
• They have a cell wall, cell membrane, cytoplasm and plasmids.
• They lack a nucleus but contain a circular chromosome of DNA.
• Some bacteria can carry out photosynthesis, but most feed off other living or dead organisms.

A plasmid is a small extra circle of DNA, separate from the main circular chromosome. Bacteria can have several.

Common mistake: writing that bacteria "have no DNA" or "have no genetic material". They have plenty — a circular chromosome of DNA, plus plasmids. What they lack is a nucleus to keep it in. The DNA lies free in the cytoplasm.

Examples you must be able to name

Learn the shape and the role for each — questions often give you one and ask for the other.

Prokaryotic vs eukaryotic

Nutrition

Do not write that all bacteria are decomposers or all are disease-causing. The spec position is: some can photosynthesise, but most feed off other living or dead organisms. Both halves matter.

Examiner tip. Examiners report candidates muddling bacteria with enzymes and with the effects of temperature — writing that high temperature 'denatures bacteria' or 'kills enzymes'. Keep the verbs straight: high temperature kills bacteria (they are living organisms) and denatures enzymes (they are proteins, not alive). Swapping the verbs loses the mark even when the idea is right.

What is a pathogen?

A pathogen is an organism (or particle) that causes disease.

Pathogens may include fungi, bacteria, protoctists or viruses — all four groups. Learn one example from each so you are never stuck:

Viruses

Viruses are not living organisms. They are small particles, smaller than bacteria.

• They are parasitic and can reproduce only inside living cells.
• They infect every type of living organism — plants, animals, fungi, protoctists and bacteria.
• They have a wide variety of shapes and sizes.
• They have no cellular structure, but have a protein coat and contain one type of nucleic acid — either DNA or RNA (never both).

Common mistake — flagged directly by examiners: candidates write that a virus "does not contain a nucleus" or "has no organelles". Both statements are true, but they earn nothing: they describe a virus as if it were a cell with parts missing. A virus has no cellular structure at all — just genetic material inside a protein coat. Say that instead.

Named viruses

For tobacco mosaic virus, the mechanism is examinable: it prevents chloroplasts forming, so the leaves lose their green colour in a mosaic pattern. Less chlorophyll means less photosynthesis and poorer growth.

Why viruses are not classed as living

• No cellular structure.
• Cannot reproduce independently — only inside a living host cell.
• No cytoplasm, no organelles, no metabolism of their own.

Virus vs bacterium

Examiner tip. Watch the command word. 'Describe the features of a virus' wants the list — protein coat, one type of nucleic acid, no cellular structure, smaller than bacteria. 'Explain why a virus is not considered a living organism' wants reasons linked to those features — e.g. it cannot reproduce except inside a living host cell. Examiners repeatedly report describe/explain being confused; if the question says explain, every point needs a 'because'.