4.2.1Levels of organisation
Cells are the basic building blocks of all living organisms, and larger organisms are built by grouping them.
A tissue is a group of cells with a similar structure and function. Organs are aggregations of tissues performing specific functions. Organs are organised into organ systems, which work together to form organisms.
Those three sentences are the whole of this sub-topic, and they are worth learning word for word — the definitions themselves are what gets marked.
- Tissue
- A group of cells with a similar structure and function — muscle tissue, for example.
- Organ
- An aggregation of tissues performing a specific function, such as the stomach or the heart.
- Organ system
- A group of organs working together, such as the digestive system.
It is worth having a feel for the sizes as well as the order. A typical animal cell is a few tens of micrometres across; a tissue is large enough to see, measured in millimetres; an organ such as the stomach or a leaf is measured in centimetres.
How big the jump is from one level to the next varies enormously — a leaf and a heart are both organs, and neither is a fixed multiple of the tissue inside it — but the direction is always the same. That is what matters: as an organism gets larger its surface area to volume ratio falls, which is why a large organism needs a transport system and a single cell does not.
The same four levels describe a plant. Palisade mesophyll is a tissue; the leaf is an organ built from several tissues; the roots, stem and leaves together form the transport organ system; and those systems make the plant. Being able to move between an animal example and a plant one is worth practising, because a question will often give you the unfamiliar one.
Why organise at all? A single cell exchanges everything it needs straight through its own surface. Once an organism is more than a few cells thick that stops working — the surface area to volume ratio is too small — so cells specialise, group into tissues that do one thing well, and organs and systems appear to move materials between them. Organisation is the consequence of getting big.
Common mistake
The stomach is an organ, not a tissue — it contains several tissues, including muscle and glandular tissue. If something is made of more than one kind of tissue and does one specific job, it is an organ.
Before you practise
- A tissue is a group of cells with a similar structure and function.
- An organ is an aggregation of tissues performing a specific function.
- Organ systems are groups of organs working together to form organisms.
- Each level is larger than the last, which is why big organisms need transport systems.