Cells and organisation · GCSE Biology
Levels of organisation
Explain how cells form tissues, organs and systems for GCSE Biology, with examples and links to stem cells and specialisation.
Cell → tissue (same type working together) → organ (tissues for a job) → organ system → organism. Stem cells differentiate to build specialised tissues.
The important bits
What you need to know
- 1
Multicellular organisms are organised in levels: cells are the basic unit; tissues are groups of similar cells working together; organs are structures made of tissues; organ systems are groups of organs working together.
- 2
A tissue has a specific function because its cells are specialised. Example: muscular tissue contracts; epithelial tissue covers surfaces; glandular tissue secretes chemicals.
- 3
An organ is made of several tissues working together. The stomach has muscular tissue (churning), glandular tissue (enzymes and acid) and epithelial tissue (lining).
- 4
Organ systems carry out major life processes. The digestive system includes the stomach, intestines, liver and pancreas working together to digest and absorb food.
- 5
Specialisation means cells develop different structures to perform specific jobs — the same DNA is expressed differently in different cell types.
- 6
Stem cells are unspecialised cells that can divide and differentiate into specialised cells, building tissues and organs during growth and repair.
- 7
Embryonic stem cells can become any cell type; adult stem cells (e.g. in bone marrow) are more limited. Plant meristems remain unspecialised throughout life.
- 8
Understanding levels of organisation helps explain how structure at one level supports function at the next — e.g. alveoli (tissue level) in lungs (organ) in the gas exchange system.
Quotations worth analysing
Short evidence. Real method.
“A tissue is a group of similar cells that work together to perform a shared function.”
“Similar cells” and “shared function” are both needed. A whole organ is not a tissue.
“The stomach is an organ made of muscular, glandular and epithelial tissues.”
Name at least two tissues and link each to a function when explaining an organ.
“Stem cells can differentiate into specialised cells during growth and repair.”
Differentiation is how organisation levels are built from unspecialised starting cells.
Go deeper
Build the ladder with real examples
Cell: palisade mesophyll cell with chloroplasts. Tissue: palisade mesophyll tissue (many such cells) for photosynthesis. Organ: leaf (also has epidermis, veins, stomata). System: gas exchange / photosynthesis in the plant as a whole. Animal path: ciliated epithelial cell → ciliated epithelium in trachea → lung (organ) → gas exchange system with heart and blood vessels. Examiners want you to move up the ladder in order without skipping a level. “The heart is a tissue” is an instant loss.
Go deeper
Specialisation and stem cells recap
Every body cell has the same genes, but only some are switched on. Muscle cells express contractile proteins; red blood cells express haemoglobin and lose their nucleus. Stem cells divide by mitosis; one daughter may remain a stem cell, the other differentiates when certain genes activate. This links division topic to organisation: mitosis supplies cells that then specialise into tissues. Cancer is uncontrolled division without normal differentiation — organisation breaks down.
Go deeper
Why organisation matters for function
Isolated cells cannot form a stomach’s churning action — you need organised muscle layers and glands in the right place. Systems coordinate organs: digestion needs peristalsis, enzymes, bile and absorption together. A six-mark question might show a hierarchy diagram with a missing label — practise filling cell, tissue, organ, system. Plants follow the same logic: xylem tissue in roots, stems and leaves as part of the transport system.
See the idea in action
Explain how the stomach is organised (4 marks). The stomach is an organ. It contains muscular tissue that contracts to churn food. Glandular tissue secretes protease enzymes and hydrochloric acid. Epithelial tissue lines the stomach, protecting underlying tissues and allowing secretion. Together these tissues enable mechanical and chemical digestion as part of the digestive system.
Exam technique
Turn knowledge into marks
When defining tissue vs organ, always give an example with named tissues. Link stem cells to differentiation when organisation levels are mentioned in the same question.
Common mistakes
Do not give these marks away
- 01
Calling an organ a tissue (e.g. “muscle tissue is an organ” without naming multiple tissues in a structure).
- 02
Saying all cells in an organism are identical in structure and function.
- 03
Placing organ systems below organs in the wrong order, or skipping the tissue level.
Which is the correct order of organisation in animals, from smallest to largest?
AOrgan → tissue → cell → organ system
BCell → tissue → organ → organ system
CTissue → cell → organ system → organ
DCell → organ → tissue → organ system
Show the answer
Cell → tissue → organ → organ system. Cells form tissues, tissues form organs, organs work together in organ systems. The organism is the whole living thing.
Quick questions
If this is the bit you searched
What is a tissue in biology?
A group of similar cells working together to perform a specific function, such as muscular or epithelial tissue.
What is the difference between an organ and a tissue?
A tissue is one type of cell group with one main job. An organ is made of several tissues working together, e.g. the stomach.
How do stem cells relate to levels of organisation?
They divide and differentiate into specialised cells that form tissues, organs and systems during growth and repair.
Give an example of an organ system.
The digestive system, made of organs including the stomach, small intestine and liver, working together to digest food.