Level of organisation

Every living organism is organised in a series of levels, each one bigger than the last and built from the level below it:

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

• A few organelles combine to build a cell.
• Many similar cells combine to build a tissue.
• Several different tissues combine to build an organ.
• Several organs working together make an organ system.
• All the organ systems together make the whole organism.

Think of it like building a house: bricks (cells) make a wall (tissue), walls make a room (organ), rooms make a floor (organ system), and all the floors make the whole house (organism).

Exam tip. A very common 1-mark question is "Put these in order of increasing size" or "Which level is made of different tissues?". Learn the order and the definition of each level, not just the words.

Organelle — a small specialised structure inside a cell that carries out a particular job. It is not a living organism on its own.

• Example: a ribosome (an organelle where proteins are made).
• Other examples you have met: nucleus, mitochondria, chloroplast.

Cell — the basic unit of any living organism; the smallest structure that can carry out all the processes of life. Cells contain organelles.

• Animal example: a muscle cell or a red blood cell.
• Plant example: a palisade cell (a tall, chloroplast-packed cell near the top of a leaf that does most of the photosynthesis).

So the first two levels are: organelles work inside a cell, and the cell is the basic building block of every tissue, organ and organism.

Watch out. An organelle is part of a cell — it is smaller than a cell. Don't write that a ribosome is a cell.

Tissue — a group of similar cells that work together to carry out one particular function.

• Muscle tissue — cells that contract to produce movement.
• Epithelial tissue — cells that cover and line surfaces (e.g. lining the gut or covering the skin).
• Xylem tissue (in plants) — cells that transport water up the plant and give support.

Organ — a structure made of several different tissues that work together to carry out a specific function.

• Heart (animal organ) — contains muscle tissue (to pump), epithelial tissue (lining the chambers) and other tissues; it pumps blood.
• Leaf (plant organ) — contains palisade tissue, xylem and other tissues; it carries out photosynthesis.

The key difference to learn:

Exam tip. The discriminating word is "different". An organ is different tissues working together; a tissue is similar cells. Examiners reward this exact wording.

Organ system — a group of organs that work together to carry out a major life process.

• Digestive system — mouth, stomach, small intestine, etc., working together to break down and absorb food.
• Circulatory system — heart and blood vessels, working together to transport blood around the body.

Organism — a complete individual living thing, made of all its organ systems working together. Examples: a human, an oak tree, a housefly.

So the full picture, top to bottom, is:

Watch out. Don't muddle organ and organ system. The heart is an organ; the heart plus the blood vessels working together is the circulatory system (an organ system).

The levels of organisation are the same idea in both plants and animals — only the examples change:

This matters because Paper 1 questions can give you a plant example (e.g. "Is a leaf an organ or a tissue?") and expect you to apply the same definitions you would use for an animal.

Quick check: a leaf contains several different tissues (palisade tissue, xylem, etc.), so a leaf is an organ — exactly like the heart in an animal.

Exam tip. Whenever you are unsure which level something is, ask: "Is it similar cells (tissue) or different tissues working together (organ)?" That one question answers most classification marks.