Levels of OrganisationEdexcel IGCSE Biology: Revision notes
Section 1
What are the levels of biological organisation?
Living organisms are organised in a hierarchy, from the smallest structural units up to whole systems. The order, from smallest to largest, is:
- Organelles
- Cells
- Tissues
- Organs
- Systems
Each level is built from units of the level below it.
If asked to state the levels of organisation, list them in the correct order for full marks — examiners award one mark per correctly placed term.
Section 2
Organelles and cells
An organelle is a specialised structure inside a cell, such as the nucleus or a mitochondrion, that carries out a particular job. A cell is the smallest unit that can carry out all the processes of life on its own — it is built from many different organelles working together.
Think of a cell as a factory: each organelle is a department (the nucleus is the office giving instructions, mitochondria are the power plant).
Section 3
Tissues
A tissue is a group of similar cells that work together to perform a shared function. Examples include:
- Muscle tissue (animals) — contracts to cause movement
- Epithelial tissue (animals) — lines surfaces, e.g. the gut
- Xylem (plants) — transports water and mineral ions
- Phloem (plants) — transports sucrose and amino acids
Section 4
Organs and organ systems
An organ is made of two or more different tissues working together for a specific function. For example, the heart contains muscle tissue (to contract), connective tissue (to hold it together) and nervous tissue (to coordinate contraction).
An organ system is a group of organs working together to carry out a major function of the body. For example, the digestive system includes the mouth, oesophagus, stomach, small intestine, large intestine and pancreas, all working together to digest and absorb food.
The leaf is a plant organ: it contains epidermal tissue, palisade and spongy mesophyll tissue, and xylem and phloem tissue, all working together for photosynthesis and transport.
Must Know
- The order from smallest to largest is: organelles → cells → tissues → organs → systems
- A tissue is a group of similar cells with a shared function
- An organ is made of two or more different tissues
- A system is a group of organs working together
- Example: the heart (organ) is built from muscle, connective and nervous tissue, and is part of the circulatory system
That's the notes covered.
Carry on to the next subtopic.