2aLevel of organisation

The five levels of organisation

Living organisms are built up in a hierarchy. Each level is made of the level below it:

organelles → cells → tissues → organs → organ systems → organism

You must be able to name the levels in order and give a definition and an example of each.

Working up the hierarchy — one worked example

• Organelle: the nucleus inside a muscle cell
• Cell: a muscle cell
• Tissue: muscle tissue — many muscle cells of the same type together
• Organ: the stomach — muscle tissue plus glandular tissue plus epithelial tissue
• System: the digestive system — stomach, small intestine, liver, pancreas
• Organism: the human

And a plant example

• Organelle: a chloroplast in a palisade cell
• Cell: a palisade cell
• Tissue: palisade mesophyll tissue
• Organ: the leaf — mesophyll, epidermis, xylem and phloem together
• System: the shoot system (leaves, stem, flowers)
• Organism: the plant

Common mistake: candidates regularly slip a level, e.g. calling xylem an organ or calling the leaf a tissue. Learn the order and check which level the question is actually asking about.

Examiner tip. Examiners report that a few candidates confused organs with tissues. The single safest thing to fix: a tissue is similar cells; an organ contains several different tissues. If your answer to "what is an organ?" contains the words "similar cells", it is wrong.

Organelles and cells

Organelles

An organelle is a structure inside a cell that carries out a particular job. They are the smallest level of organisation you need to name.

Bacterial cells are cells, but they have no nucleus and no mitochondria or chloroplasts — their genetic material is a circular chromosome plus plasmids.

Cells

A cell is the basic unit from which all living organisms are built. Cells are specialised — their structure suits their function:

• Root hair cell — long extension gives a large surface area for absorbing water and mineral ions; no chloroplasts, because there is no light underground.
• Palisade cell — packed with chloroplasts and at the top of the leaf, where the light is.
• Red blood cell — no nucleus, so there is more room for haemoglobin to carry oxygen.
• Sperm cell — a tail for swimming and many mitochondria to release the energy for it.

Each of these is still just one cell. A group of them of the same type is the next level up: a tissue.

Common mistake: naming an organelle when the question asks for a cell, or vice versa. "Chloroplast" is an organelle; "palisade cell" is a cell.

Tissues, organs and organ systems

Tissue

A tissue is a group of cells with similar structure and function working together.

• Muscle tissue — cells that all contract
• Glandular tissue — cells that all secrete enzymes or hormones
• Epithelial tissue — cells that cover and line surfaces
• Xylem — cells that carry water and mineral ions up the plant
• Phloem — cells that transport sucrose and amino acids
• Mesophyll — leaf cells that photosynthesise

Organ

An organ is made of several different tissues working together to carry out a particular function.

The key idea: an organ is a mixture of tissue types. That mixture is what makes it an organ rather than a tissue.

Organ system

An organ system is a group of organs working together to carry out a major function of the body.

All the systems together make the organism.

Common mistake: writing "the digestive system is a group of tissues" or "the stomach is a group of similar cells". Match the word to the right level: cells → tissue, tissues → organ, organs → system.

Examiner tip. Examiners report that a significant number of candidates confused the definition of a tissue with organs and suggested that organs are collections of similar cells. Write the two definitions out so they contrast: tissue = similar cells working together; organ = several different tissues working together.

Answering questions on levels of organisation

This objective says describe — so you are being asked to state what the levels are, in order, with examples. You are not being asked to explain why organisms are organised this way.

Command words

So:

• Describe the levels of organisation → "organelles make up cells, cells of a similar type make up tissues, different tissues make up organs, organs make up organ systems."
• Explain why the stomach is an organ → "because it is made of several different tissues (muscle, glandular and epithelial) working together to digest food."

Common mistake: examiners repeatedly report candidates confusing 'describe' and 'explain' — either giving a description when a reason is wanted, or padding an explanation with description that earns nothing. Read the command word before you start writing.

Use the key terms

Marks here are given for the precise words: organelle, cell, tissue, organ, organ system. Vague answers like "small bits inside" or "a group of things that do a job" do not score. Name the level, then give a named example.

Quick self-test

For each, say which level it is:

1. Mitochondrion — organelle
2. Xylem — tissue
3. Leaf — organ
4. Root hair cell — cell
5. Circulatory system — organ system
6. Muscle — tissue
7. Stomach — organ
8. Nucleus — organelle
9. Heart — organ
10. Digestive system — organ system

If you can do all ten and give both definitions (tissue vs organ) without hesitating, this sub-topic is done.

Examiner tip. Examiners report that where candidates lost credit it was typically from confusing command words, not giving enough depth, and not using key terms. On this topic, always use the spec's own words — organelle, cell, tissue, organ, organ system — rather than describing them loosely.