Characteristics & Variety of Living Organisms

What makes something "alive"?

Biologists describe living organisms by a shared set of life processes. Anything that is truly alive carries out all of them, even if some are hard to see. A handy way to remember the seven characteristics is the phrase MRS GREN.

Key terms Excretion is removing waste made inside the body by chemical reactions (such as carbon dioxide and urea). It is not the same as egestion, which is removing undigested food.

Sensitivity is also called control or response to stimuli; it lets organisms react to light, chemicals, temperature and more.

Exam tip A common question asks why a particular feature alone does not prove something is alive. The safe answer: living things carry out all seven life processes. A car moves and "uses fuel", but it does not grow, reproduce, excrete or respond, so it is not alive.

The main groups of living organisms

Living organisms are sorted into broad groups. The first big division is between eukaryotes (cells with a true nucleus) and prokaryotes (cells without one). Viruses sit outside this scheme because they are not cells at all.

Plants

Plants are multicellular and their cells contain chloroplasts, so they photosynthesise. This is autotrophic nutrition: they make their own food from carbon dioxide and water using light energy. Plant cells have a cell wall made of cellulose and often store carbohydrate as sucrose or starch.

Examples you should know:

Flowering plants, including cereals such as maize (a crop grown for grain), and herbaceous legumes such as the pea plant.

Key terms Autotroph — an organism that makes its own organic food using an energy source (light in photosynthesis). Plants and many protoctists (like Chlorella) are autotrophs.

Animals

Animals are multicellular and feed by heterotrophic nutrition — they take in ready-made organic food from other organisms. Their cells have no cell wall and no chloroplasts. They usually store carbohydrate as glycogen, and most have nervous coordination, so they react quickly to their surroundings, and can move their whole body.

Examples you should know:

Mammals, such as humans.
Insects, such as houseflies and mosquitoes.

Fungi

Fungi are usually multicellular, although yeast is a single-celled exception. Most fungi are made of thread-like structures called hyphae, and a network of hyphae forms a mycelium. Their cells have a cell wall (made of chitin, not cellulose) and they cannot photosynthesise.

Fungi feed by saprotrophic nutrition: they secrete digestive enzymes onto dead or decaying material outside their body, then absorb the digested nutrients. Some store carbohydrate as glycogen.

Examples you should know:

Mucor, a mould with the typical hyphae-and-mycelium body.
Yeast, a single-celled fungus used in baking and brewing.

Key terms Saprotroph — an organism that feeds on dead material by extracellular digestion (releasing enzymes outside the body, then absorbing the products). Hyphae — fine threads making up a fungus. Mycelium — the whole mesh of hyphae.

Bacteria

Bacteria are single-celled and microscopic. They are prokaryotes: they have no nucleus. Instead, their genetic material is a single loop of circular DNA floating in the cytoplasm, often with smaller rings of DNA called plasmids. They have a cell wall (not made of cellulose) and a cell membrane, but no mitochondria.

Some bacteria photosynthesise; many feed on other living or dead organisms.

Examples you should know:

Lactobacillus, a rod-shaped bacterium used to turn milk into yoghurt.
Pneumococcus, a spherical bacterium that can cause pneumonia.

Protoctists

Protoctists are a mixed "leftover" group of mostly single-celled, microscopic organisms whose cells do have a nucleus (they are eukaryotes). Some are more like plant cells (with chloroplasts), others more like animal cells.

Examples you should know:

Amoeba, a pond organism with an animal-like cell that changes shape to move and feed.
Chlorella, which has chloroplasts and photosynthesises like a plant.
Plasmodium, the parasite that causes malaria.

Viruses

Viruses are not cells and are not made of cells. They are far smaller than bacteria and have a very simple structure: a piece of genetic material (DNA or RNA) surrounded by a protein coat. They have no cytoplasm, no nucleus and no machinery of their own.

Because of this, a virus can only reproduce inside a living host cell, where it hijacks the host's machinery to make copies of itself. This is why all viruses are parasites.

Examples you should know:

Tobacco mosaic virus (TMV), which damages the leaves of tobacco plants.
Influenza virus, which causes flu.
HIV, which causes AIDS in humans.

Watch out Whether viruses are "alive" is debated, but for the exam: viruses are not cells, they do not carry out all seven life processes on their own, and they reproduce only inside a host. Don't write that a virus has a nucleus or cytoplasm — it has neither.

Pathogens

A pathogen is any microorganism that causes disease. Pathogens are found in several of the groups above:

Key terms Pathogen — a microorganism (a virus, bacterium, fungus or protoctist) that causes disease in its host.

Real world Knowing a pathogen's group matters for treatment. Antibiotics kill bacteria but have no effect on viruses, which is why they don't cure flu or HIV. Malaria, caused by a protoctist, needs different antimalarial drugs again.

Exam tip Learn one named example for each group — maize/pea (plants), mammals/insects (animals), Mucor/yeast (fungi), Lactobacillus/Pneumococcus (bacteria), Amoeba/Chlorella/Plasmodium (protoctists), TMV/influenza/HIV (viruses). Naming the correct example often earns an easy mark.