The eight signs of life

Edexcel expects you to know the characteristics of living organisms. There are eight life processes — remember them with MRS GREN:

The eight in full: Movement, Respiration, Sensitivity, Control (of internal conditions), Growth, Reproduction, Excretion, Nutrition. Some people write it "MRS C GREN" to fit in control.

What each process means

• Movement — an action that changes position or place (even plants move parts, e.g. towards light).
• Respiration — chemical reactions that release energy from nutrient molecules.
• Sensitivity — detecting and responding to changes in the surroundings (stimuli).
• Control — keeping internal conditions steady (homeostasis).
• Growth — a permanent increase in size and dry mass.
• Reproduction — making more of the same kind of organism.
• Excretion — removing the toxic waste products of metabolism (and excess substances).
• Nutrition — taking in materials for energy, growth and development.

Watch out: excretion (getting rid of metabolic waste, e.g. carbon dioxide and urea) is not the same as egestion (removing undigested food). Edexcel wants excretion for the waste of chemical reactions.

Name that process

In your own words

Living or not?

Tap the option that says whether each thing is a living organism or not living.

A car moves and burns fuel. Why isn't it alive?

Why isn't a mushroom a plant?

Five groups + viruses

Edexcel groups living organisms into five groups, each with tell-tale features — and you must also know about viruses:

Plants

Plants are multicellular organisms that make their own food by photosynthesis. Their features:

• Contain chloroplasts and can photosynthesise.
• Have cell walls made of cellulose.
• Store carbohydrates as starch or sucrose.

Examples: flowering plants such as a cereal (e.g. maize) and a herbaceous legume (e.g. peas or beans).

Examples to remember: Edexcel names flowering plants: a cereal (maize) and a herbaceous legume (peas or beans).

Animals

Animals are multicellular and cannot make their own food — they feed on other organisms. Their features:

• No cell walls and no chloroplasts — they cannot photosynthesise.
• Usually have some kind of nervous coordination and can move their whole body.
• Often store carbohydrate as glycogen.

Examples: mammals (e.g. humans) and insects (e.g. housefly, mosquito).

Plant or animal cell?

Fungi

Fungi cannot photosynthesise. Most are made of thread-like hyphae that form a network called a mycelium; some (like yeast) are single cells.

• Feed by saprotrophic nutrition: they release digestive enzymes onto food (dead matter) outside the body, then absorb the soluble products.
• Store carbohydrate as glycogen.

Examples: Mucor (a mould with the typical hyphae-and-mycelium structure) and yeast (a single-celled fungus).

Fungi are NOT plants

It is tempting to file a mushroom with plants — but fungi are their own group.

Common mistake: "Fungi are plants that grow in the dark." No. Fungi have no chloroplasts and cannot photosynthesise. Instead they feed saprotrophically — digesting dead material externally and absorbing it. Their walls are made of chitin, not cellulose.

Explain saprotrophic feeding

Why isn't a mushroom a plant?

Bacteria

Bacteria are microscopic, single-celled organisms. They are prokaryotes — the key point is that they have no nucleus.

• Have a cell wall, cell membrane, cytoplasm and plasmids.
• Genetic material is a single loop of circular DNA — not held in a nucleus.
• Most feed off other living or dead organisms; some can photosynthesise.

Examples: Lactobacillus bulgaricus (a rod used to make yoghurt from milk) and Pneumococcus (a sphere that can cause pneumonia).

Bacteria have NO nucleus

Common mistake: drawing a bacterium with a nucleus like an animal cell. Bacteria are prokaryotic — their DNA floats free in the cytoplasm as a single circular loop, plus small rings called plasmids. There is no membrane-bound nucleus and no mitochondria or chloroplasts.

By contrast, plants, animals, fungi and protoctists are all eukaryotes: their DNA is enclosed in a nucleus. This is the single biggest divide among the five groups.

Spot the bacterium

Is a virus alive?

Protoctists

Protoctists are microscopic, mostly single-celled organisms. They are eukaryotes (they do have a nucleus). They are a mixed bag: some are animal-like, some plant-like.

• Some, like Amoeba, have features like an animal cell (live in pond water, move and feed).
• Some, like Chlorella, have chloroplasts and are more like a plant cell.
• A few are important pathogens — e.g. Plasmodium, which causes malaria.

Examples: Amoeba (pond-water, animal-like), Chlorella (plant-like, has chloroplasts) and Plasmodium (the malaria parasite).

Name the organism

Viruses

Viruses are not cells and are usually regarded as non-living. They are much smaller than bacteria.

• Structure: a piece of genetic material (DNA or RNA) inside a protein coat. No cytoplasm, no nucleus, no organelles.
• They can only reproduce inside a living host cell — this is why many biologists call them non-living.

Examples: Tobacco mosaic virus (discolours tobacco leaves), influenza virus (the "flu") and HIV (which causes AIDS).

Viruses are non-living & need a host

Common mistake: "A virus is a tiny bacterium." No. A virus is not a cell — it has no cytoplasm, no ribosomes and cannot respire or feed. It shows none of the MRS GREN processes on its own.

Because a virus cannot reproduce by itself, it must invade a living host cell and take over the cell's machinery to make copies of itself. That inability to carry out life processes independently is why viruses are treated as non-living.

Why "non-living"?

What is a pathogen?

A pathogen is a microorganism that causes disease.

Pathogens can come from several of the groups you've just met:

• Viruses — e.g. influenza, HIV, tobacco mosaic virus.
• Bacteria — e.g. Pneumococcus (pneumonia).
• Fungi — some cause diseases such as athlete's foot.
• Protoctists — e.g. Plasmodium (malaria).

Key definition to memorise: a pathogen is a microorganism that causes disease — a fungus, bacterium, protoctist or virus. Not every microorganism is a pathogen — many bacteria and fungi are harmless or even useful.

Is a virus alive?

Eukaryote or prokaryote?

Tap an organism, then tap the box it belongs in. (Remember: viruses are neither — but here we sort only the cellular groups.)

Match organism to its group

Tap an organism on the left, then tap its group on the right.

Know your examples

The facts to know

You've covered the key ideas in Edexcel 4BI1 Section 1 — the Nature & Variety of Living Organisms. Press Finish to see your score.