Variety of Living Organisms

Explained

Sorting organisms, starting with the nucleus

Classification questions look intimidating when they describe an organism you have never heard of. They become straightforward once you realise you are being asked to work through a short sequence of yes or no questions, and the first one is always the same.

The first question: is there a nucleus?

Organisms whose DNA is enclosed in a nucleus are eukaryotic. Plants, animals, fungi and protoctists all are.

Bacteria are prokaryotic. They have no nucleus, so their genetic material lies loose in the cytoplasm as a circular chromosome, often with smaller rings called plasmids alongside it. If a description mentions a circular chromosome or plasmids and no nucleus, it is a bacterium, and you need go no further.

Then separate the eukaryotes

• Plants are multicellular, have cell walls made of cellulose, and have chloroplasts in the cells that photosynthesise. They store carbohydrate as starch or sucrose. Examples in the specification are the maize plant and the pea plant.
• Animals are multicellular with no cell wall and no chloroplasts. They store carbohydrate as glycogen and usually have nervous coordination.
• Fungi cannot photosynthesise. Most are multicellular with a body made of threads called hyphae, forming a network called a mycelium, and their walls contain chitin. Yeast is the exception you must know, because it is single celled.
• Protoctists are microscopic and usually single celled. Some are plant like, such as Chlorella, which has chloroplasts. Some are animal like, such as Amoeba. Plasmodium, which causes malaria, is a pathogenic protoctist.

How fungi feed

Fungi feed saprotrophically. They secrete enzymes onto the food outside their bodies, the enzymes digest it there, and the soluble products are then absorbed. This is extracellular digestion, and the word saprotrophic is the one the mark scheme wants.

Bacteria vary. Most feed on other organisms living or dead, but a few, such as Lactobacillus, are used industrially, and some photosynthesise.

Working through an unfamiliar example

A question might describe a single celled organism with a cell wall, no nucleus, and a circular loop of DNA. Nucleus first: none, so it is prokaryotic, so it is a bacterium. The cell wall is not the deciding feature, because plants and fungi have walls too.

Another might describe a multicellular organism with a cell wall and no chloroplasts that absorbs nutrients from dead material. It has a nucleus, so it is eukaryotic. No chloroplasts rules out a plant. A cell wall rules out an animal. Feeding on dead material by absorption makes it a fungus.

Say which feature rules out each group as you go. Naming the group without giving the evidence usually earns only part of the credit, because the question is testing the reasoning rather than the label.