Cell organisation: tissues, organs and systemsEdexcel A-Level Biology A: Revision notes
Section 1
Levels of organisation
In a multicellular organism, cells are organised into a hierarchy:
cell, tissue, organ, organ system, organism
Each level is made of the one below it, and each has its own function. The organisation lets different parts of the organism carry out different jobs at the same time.
Section 2
Specialised cells
During development, cells differentiate: they become specialised for a particular function by expressing some genes and not others. Examples are ciliated epithelial cells (move mucus), goblet cells (secrete mucus), palisade mesophyll cells (photosynthesis) and xylem vessels (transport water).
Specialisation gives division of labour: each cell type does one job well, so the organism works more efficiently than if every cell did everything.
Section 3
Tissues
A tissue is a group of similar cells, with any extracellular material, that work together to carry out a particular function.
- Animal examples: squamous epithelium, ciliated epithelium, smooth muscle, cardiac muscle, connective tissue.
- Plant examples: xylem (transports water and mineral ions), phloem (transports sucrose), palisade mesophyll (photosynthesis).
A tissue is a group of similar cells. If a structure contains several different tissues, it is an organ, not a tissue.
Section 4
Organs
An organ is a group of different tissues, arranged together, that work together to carry out a particular function.
Small intestine: epithelium (absorbs nutrients), smooth muscle (peristalsis), connective tissue with blood vessels, and nervous tissue.
Stomach: glandular epithelium (secretes acid, pepsin and mucus), smooth muscle (churns food), connective tissue.
Leaf: epidermis, palisade and spongy mesophyll, and xylem and phloem in the veins.
Section 5
Organ systems
An organ system is a group of organs that work together to carry out a major function. Examples:
- Digestive system: mouth, oesophagus, stomach, small intestine, liver, pancreas.
- Gas exchange system: trachea, bronchi, lungs.
- Circulatory system: heart and blood vessels.
- Plant shoot system: stem, leaves and flowers.
An organism is made of several organ systems working together.
Section 6
Why is this organisation useful?
Large organisms have a small surface area to volume ratio, so they cannot rely on diffusion alone and need specialised exchange and transport systems. Organising cells into tissues, organs and systems means:
- each tissue is adapted for its job (division of labour)
- different functions occur at the same time
- each system can be controlled and repaired independently.
In an organisation question, always name a specific example at each level, for example ciliated cell, ciliated epithelium, trachea, gas exchange system.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Cell organisation: tissues, organs and systems
- The wall of the small intestine is made of several types of cell. The inner lining is a layer of epithelial cells with microvilli that absorb nutrients. Beneath it is connective tissue containing blood vessels, and around the outside are layers of smooth muscle that contract to move food along.Explain why the small intestine is an organ rather than a tissue.2 marks
- A leaf from a privet shrub has an upper epidermis, a layer of palisade mesophyll, a layer of spongy mesophyll, vascular bundles containing xylem and phloem, and a lower epidermis with stomata surrounded by guard cells.Explain the advantage to a large multicellular plant of having its cells organised into specialised tissues.2 marks
- The wall of the stomach contains glandular epithelium, whose cells secrete hydrochloric acid, the enzyme pepsin and mucus. It also contains layers of smooth muscle, connective tissue with blood vessels, and nervous tissue.Describe how cells are organised into an organ system, using the stomach as an example.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).