What you need to know
- 3.1Living organisms consist of cells with shared features.
- 3.2Organisation from cells to tissues, organs and organ systems.
By the end, you should be able to…
- Distinguish the four levels of organisation.
- Explain why an organ contains more than one tissue.
- Apply the hierarchyAn arrangement of groups within larger groups, with no overlap: domain, kingdom, phylum, class, order, family, genus, species. to unfamiliar examples.
Build your understanding
Cells are the starting point
A cell is the basic structural and functional unit of living organisms. A unicellular organism carries out its life processes within one cell. Multicellular organisms use specialised cells whose structures and activities suit particular roles.
Sharing common features does not mean all cells look or behave alike. Cells can differ in shape, organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome. abundance and the proteins they produce. The gene-expression mechanisms underlying specialisation are covered later in 3C.
Tissues combine related functions
A tissue is an organised group of cells working together for a particular function, often with extracellular material. Cells in a tissue may include more than one specialised type; “all cells are identical” is too restrictive.
For example, muscle tissue contracts, while epithelial tissue forms coverings or linings involved in functions such as protection, absorption or secretion. Their organisation and connections matter as well as their individual cells.
Organs combine different tissues
An organ is a structure containing different tissues that cooperate in a particular function. The intestine contains epithelial tissue for absorption and secretion, muscle tissue for moving contents, connective tissue for support and nervous tissue involved in coordination.
The heart is not simply one large muscle cell or a single tissue. Cardiac muscle, connective tissue, lining tissues, nerves and blood vessels contribute different roles to a working pump.
Organ systems coordinate whole-body tasks
An organ system consists of organs cooperating in a major function. The digestive system includes organs involved in processing food and absorbing nutrients; the circulatory system transports substances between regions.
Follow a connected chain: intestinal epithelial cell → epithelial tissue → small intestine → digestive system. The same organ also depends on other systems: delivery of absorbed nutrients to distant cells requires circulation. The hierarchyAn arrangement of groups within larger groups, with no overlap: domain, kingdom, phylum, class, order, family, genus, species. describes cooperation, not isolated boxes.
NOT: an organ is not a tissue, and a unicellular organism is not missing “the system level” inside one cell.
- Cells can specialise and still share the features of cells
- A tissue may include a matrix the cells have made
- Name the example at each level
- Cooperation is the reason for the hierarchy, not a longer list of names
Two traps this diagram blocks
- Do not call the stomach a tissue.
- Do not say every cell in an organ is the same type.
From the diagram alone
Question
Arrange cell, tissue, organ and organ system in order, and give one example of an organ. [3]
Show a model answer
The order is cell, then tissue, then organ, then organ system. The stomach is an organ because it contains several tissues, including muscle and epithelium, working together.
Make your reasoning visible
Use a linked example
Choose one cell type, its tissue, an organ containing it and the system that organ contributes to.
Explain why it is an organ
Name at least two different tissues and how their contributions combine.
Keep organ and organelle separate
A mitochondrion is an organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome. within a cell, not an organ in an organ system.
Avoid this misconception
The claim: An organ is made of only one type of tissue.
The correction: An organ contains several tissues whose different functions work together.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Classify mitochondrion, epithelial tissue and small intestine by organisational level.
Show suggested answer
Mitochondrion: organelle. Epithelial tissue: tissue. Small intestine: organ.
Practice question
Retrieval and explanation · suggested answer
Explain why the small intestine is an organ rather than a tissue.
Show suggested answer
It contains different tissues, including epithelium and muscle, cooperating in digestion, absorption and movement of contents.
Practice question
Retrieval and explanation · suggested answer
Why does an absorptive epithelial cell depend on the circulatory system?
Show suggested answer
Absorbed nutrients must be transported away and the cell needs supplies such as oxygen; circulation connects local exchange with distant tissues.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A laboratory model contains two specialised cell types but no organised tissue structure. Does cell diversity alone make it a functional organ?
Show a hint
Consider organisation, interaction and coordinated function.
Explore the reasoning
No. Different cell types need suitable spatial organisation, connections and conditions to cooperate. Diversity is not sufficient evidence of organ-level function.
Recall this lesson
Lesson recall questions
Question 1 of 5: Put in order the argument that cells are the shared organisation of life.
Cells as a shared organisation
Practice question
Retrieval and explanation · suggested answer
Put in order the argument that cells are the shared organisation of life.
Lesson: 3A 2: Eukaryotic Cells 1: Common Cellular Structures
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Why an organoid is a model
3A · The Organisation of CellsNIDDK describes engineered kidney tissues and organoids as tools for disease research. Such models reproduce selected aspects of an organ rather than automatically replacing all its functions. Organoids…Specification 3.2Read & use the scienceClose article
Why this matters
NIDDK describes engineered kidney tissues and organoids as tools for disease research. Such models reproduce selected aspects of an organ rather than automatically replacing all its functions. Organoids bring together several cell types in a three-dimensional arrangement. That makes organisation more realistic than a single layer of cells, while still leaving out some whole-organ interactions. A researcher might use a kidney organoid to study how tissue develops or responds to a candidate treatment. They must state which function the model reproduces and which it does not. In the specification, cells form tissues, tissues form organs, and organs interact in systems; an organoid illustrates the middle steps without being the entire system.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.2 understand how the cells of multicellular organisms are organised into tissues, tissues into organs, and organs into organ systems
Use the science
Why might a model of kidney cells still need improvements before representing a whole functioning kidney?
Explore a possible response
It may lack mature organisation, blood supply, other cell types or integration with the body. The model must be judged against the particular function being studied.
Explore the source
- NIDDK: engineered kidney tissues and organoidsNational Institute of Diabetes and Digestive and Kidney Diseases
How a tissue sample becomes a clinical answer
3A · The Organisation of CellsA pathologist can inspect cells, their arrangement into tissues and the tissue’s position in an organ. The National Cancer Institute explains that a diagnosis from a biopsy draws…Specification 3.2 · 3.1Read & use the scienceClose article
Why this matters
A pathologist can inspect cells, their arrangement into tissues and the tissue’s position in an organ. The National Cancer Institute explains that a diagnosis from a biopsy draws on microscopic appearance as well as other tests. A single abnormal-looking cell is not automatically a full diagnosis; context and organisation matter. This is the same hierarchy the Unit 2 specification describes, now used in a clinical setting. Students can ask whether evidence was collected at cellular, tissue or organ level and what each level adds. The source shows the job of making an evidence-based report, not a shortcut for diagnosing disease from a classroom image.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.2 understand how the cells of multicellular organisms are organised into tissues, tissues into organs, and organs into organ systems
- 3.1 know that all living organisms are made of cells, sharing some common features
Use the science
Why might a cell image alone be less informative than a tissue section for a pathologist?
Explore a possible response
A tissue section preserves the arrangement and relationships among cells. Those patterns can help distinguish normal from abnormal tissue and link findings to an organ.
Explore the source
- Pathology reports and stained tissue sectionsNational Cancer Institute