From cells to systems

Levels of organisation in multicellular organisms: cells, tissues, organs, organ systems and organisms. The four main animal tissue types, the major human body systems and how they work together, and the same levels in plants.

Eat a sandwich and at least six body systems get involved. Your nervous system notices you're hungry, your skeleton and muscles lift it to your mouth, your digestive system breaks it down, your circulatory system carries the glucose away, your respiratory system brings in the oxygen to release its energy, and your urinary system clears out the wastes afterwards. None of them could keep you alive on its own.

Levels of organisation

A multicellular organism is organised in levels, each built from many units of the level below:

From a single cell to a whole organism.

Cell: the basic unit of life, for example a muscle cell.
Tissue: a group of similar cells working together to do one job, for example muscle tissue, which contracts.
Organ: a structure made of several different tissues that work together, for example the stomach, which has muscle tissue to churn food, lining tissue to make digestive juices, nerve tissue to control it and blood vessels to supply it.
Organ system: a group of organs that work together to carry out a major function, for example the digestive system (mouth, oesophagus, stomach, intestines, liver and pancreas).
Organism: a complete living thing, made of organ systems working together.

Four kinds of animal tissue

Tissue	Job	Examples
Epithelial	Covers and lines surfaces	Outer layer of skin; lining of the mouth, gut and air passages
Muscle	Contracts to cause movement	Arm muscles, the heart, the walls of the stomach
Nerve	Carries electrical messages	Brain, spinal cord, nerves
Connective	Supports, joins and protects other tissues	Bone, cartilage, tendons, fat, blood

InteractiveBuild the chain

Try this

Choose a system and build it from the bottom: pick the cell, then the tissue, the organ and the system.
Some tiles are the right level but from the wrong system. Read the feedback when you slip.
Can you build all six systems without a mistake?

Human body systems

System	Main organs	Main job
Digestive	Mouth, oesophagus, stomach, small and large intestines, liver, pancreas	Breaks food down into small nutrients and absorbs them into the blood
Circulatory	Heart, blood vessels, blood	Carries oxygen, nutrients and wastes around the body
Respiratory	Nose, trachea, lungs, diaphragm	Takes in oxygen and removes carbon dioxide
Nervous	Brain, spinal cord, nerves, sense organs	Detects changes and coordinates the body's responses
Skeletal	Bones, cartilage, ligaments	Supports and protects the body, allows movement, makes blood cells
Muscular	Skeletal muscles, tendons	Moves the body by pulling on bones
Urinary (excretory)	Kidneys, ureters, bladder	Filters wastes out of the blood and removes them as urine
Integumentary	Skin, hair, nails	Protects the body, helps control temperature, senses touch
Reproductive	Ovaries, uterus; testes	Produces sex cells and, in females, supports a developing baby

Systems depend on each other. The circulatory system is the delivery network for all the others: it carries oxygen from the respiratory system and nutrients from the digestive system to every cell, then carries carbon dioxide back to the lungs and other wastes to the kidneys. The nervous system speeds up your heart and breathing when you exercise. The skeletal and muscular systems are so closely linked that they are often treated as one, the musculoskeletal system.

Plants have levels too

Plants are organised the same way. Palisade cells make up the photosynthesising tissue in a leaf; a leaf is an organ made of that tissue plus a protective outer layer, transport tissue (veins) and more. Plants have two main organ systems:

The shoot system, above ground: stems, leaves, flowers and fruit. It makes food by photosynthesis and carries out reproduction.
The root system, below ground: anchors the plant and absorbs water and minerals.

Two transport tissues run through both: xylem carries water and minerals up from the roots, and phloem carries sugars from the leaves to wherever they are needed.

Test yourself

A group of similar cells working together to do one job is
Which list is in order from smallest to largest?
The kidneys belong to the
Which system carries glucose from the digestive system to muscle cells?
Which tissue carries water up from the roots of a plant?

Name each level of organisation.

Which level is each one?

4 marks

Using an example, describe how cells, tissues, organs and systems are related. Then explain why the circulatory system is needed by every other body system.

Your answer