Principles of Organisation

In AQA GCSE Biology, Organisation refers to the way living things are structured so that they can carry out the processes of life efficiently. This lesson introduces the fundamental hierarchy of organisation in multicellular organisms — from the smallest unit of life, the cell, all the way up to complete organ systems. Understanding this hierarchy is essential because it underpins every other topic in this unit.

The Hierarchy of Organisation

All multicellular organisms are built from the same basic structural plan. The hierarchy runs from the simplest level to the most complex:

graph TD
    A[Cells] --> B[Tissues]
    B --> C[Organs]
    C --> D[Organ Systems]
    D --> E[Organism]

Level	Definition	Example in Animals	Example in Plants
Cell	The basic building block of all living organisms	Red blood cell	Palisade mesophyll cell
Tissue	A group of similar cells working together to perform a specific function	Muscle tissue	Xylem tissue
Organ	A structure made up of different tissues working together to perform a particular function	Heart	Leaf
Organ system	A group of organs working together to perform a major life process	Circulatory system	Transport system
Organism	An entire living individual made up of multiple organ systems working together	Human	Oak tree

Exam Tip: Learn the hierarchy in order — Cells, Tissues, Organs, Organ Systems, Organism. A common exam question asks you to put these in order or to define each level.

Cells and Specialisation

A cell is the smallest unit that can carry out all of the processes of life. In multicellular organisms, cells become specialised — they develop particular features that make them efficient at performing a specific job.

Examples of Specialised Animal Cells

Cell	Specialisation	Function
Red blood cell	Biconcave disc shape (large surface area), no nucleus (more room for haemoglobin), contains haemoglobin	Transports oxygen around the body
White blood cell	Can change shape, some produce antibodies, some engulf pathogens	Defends the body against disease
Nerve cell (neurone)	Long axon, branched dendrites, myelin sheath for insulation	Transmits electrical impulses rapidly
Sperm cell	Streamlined head, acrosome with enzymes, many mitochondria in the mid-piece, tail (flagellum)	Swims to and fertilises the egg cell
Muscle cell	Contains many mitochondria, protein fibres that can contract	Contracts to produce movement

Examples of Specialised Plant Cells

Cell	Specialisation	Function
Root hair cell	Long hair-like projection (large surface area), thin cell wall, no chloroplasts	Absorbs water and mineral ions from soil
Palisade mesophyll cell	Packed with chloroplasts, tall column shape, near upper surface of leaf	Main site of photosynthesis
Guard cell	Kidney-shaped, unevenly thickened cell wall, chloroplasts present	Opens and closes stomata to control gas exchange and water loss
Xylem cell	Dead, hollow, no end walls, lignified walls	Transports water and minerals from roots to leaves
Phloem cell	Sieve plates with pores, companion cells provide energy	Transports dissolved sugars (translocation)

Exam Tip: When describing a specialised cell, always link the structural feature to the function. For example, do not just say "a red blood cell has no nucleus." Say "a red blood cell has no nucleus, which leaves more space for haemoglobin, allowing it to carry more oxygen."

Tissues

A tissue is a group of cells with a similar structure and function working together to carry out a particular job. Multicellular organisms have many different types of tissue.

Animal Tissues

Tissue	Description	Location
Muscular tissue	Contains cells that can contract and relax to produce movement	Walls of the heart, stomach, intestines, limbs
Glandular tissue	Contains cells that produce and secrete useful substances such as enzymes and hormones	Stomach lining, salivary glands, pancreas
Epithelial tissue	Covers the outside of the body and lines the inside of organs and tubes	Skin, lining of the small intestine, air sacs in lungs
Nervous tissue	Contains neurones and supporting cells that transmit electrical impulses	Brain, spinal cord, nerves throughout the body

Plant Tissues

Tissue	Description	Location
Epidermal tissue	Outer layer of cells that covers and protects the plant	Surface of leaves, stems, roots
Palisade mesophyll	Packed with chloroplasts; the main tissue for photosynthesis	Upper part of the leaf
Spongy mesophyll	Loosely packed cells with air spaces to allow gas exchange	Lower part of the leaf
Xylem	Hollow, dead tubes reinforced with lignin	Throughout the stem, root and leaf
Phloem	Living tubes with sieve plates and companion cells	Throughout the stem, root and leaf
Meristem	Rapidly dividing unspecialised cells found at growing points	Root tips, shoot tips

Organs

An organ is a structure made up of a group of different tissues that work together to perform a specific function.

The Stomach as an Example Organ

The stomach is an excellent example to illustrate how tissues combine to form an organ:

graph TD
    A[Stomach — An Organ] --> B[Muscular tissue]
    A --> C[Glandular tissue]
    A --> D[Epithelial tissue]
    B --> B1[Churns and mixes food with digestive juices]
    C --> C1[Produces enzymes like pepsin and hydrochloric acid]
    D --> D1[Lines the inside and outside of the stomach for protection]

This shows that the stomach needs all three types of tissue working together in order to carry out its function of digesting food.

Exam Tip: Be prepared to explain how the stomach contains muscular, glandular and epithelial tissue. This is a classic 4-mark question that tests your understanding of the tissue-to-organ relationship.

Organ Systems

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

Organ System	Key Organs	Function
Digestive system	Mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas	Breaks down food and absorbs nutrients
Circulatory system	Heart, blood vessels (arteries, veins, capillaries), blood	Transports substances around the body
Respiratory system	Lungs, trachea, bronchi, bronchioles, alveoli, diaphragm	Exchanges oxygen and carbon dioxide
Nervous system	Brain, spinal cord, nerves, receptors	Detects stimuli and coordinates responses
Excretory system	Kidneys, ureters, bladder, urethra	Removes metabolic waste products
Reproductive system	Ovaries, testes, uterus (in females)	Produces offspring
Musculoskeletal system	Bones, muscles, tendons, ligaments	Support, protection and movement

Organisms

The highest level of organisation is the organism itself. An organism is a single complete living individual in which all organ systems work together to maintain life processes. In the human body, all of the organ systems listed above must cooperate, for example:

The digestive system breaks down food into soluble nutrients
The circulatory system transports those nutrients to every cell
The respiratory system provides oxygen for cellular respiration in every cell
The excretory system removes the waste products (urea, carbon dioxide) that these cells produce

This interdependence is why damage to one organ system often has consequences throughout the entire body.

Summary

Living organisms are organised into a hierarchy: cells, tissues, organs, organ systems, organisms
Cells are the basic building blocks of life and become specialised to carry out particular functions
A tissue is a group of similar cells working together
An organ is made of different tissues working together (e.g. the stomach contains muscular, glandular and epithelial tissue)
An organ system is a group of organs working together to carry out a major life function
All organ systems work together to make a functioning organism
Always link structure to function when describing specialised cells

Exam Tip: Practise writing definitions for each level of organisation. In the exam, you may be given a diagram and asked to label or identify each level. Make sure you can give a named example for each level in both animals and plants.