The principles of organisation

Cell structureMicroscopyCell differentiation and specialisationThe principles of organisationCell divisionStem cells

flashcards | quizzes

Objectives

To define and give examples of the different levels of organisation in multicellular organisms: cells, tissues, organs, and organ systems.
To describe the relationship between these levels, explaining how cells group to form tissues, tissues form organs, and organs form organ systems.
To identify and select the most appropriate units of measurement (m, mm or µm) for structures at each level of organisation.

Organisation in multicellular organisms

Specialised Cells:

Cells are the basic building blocks of all living organisms. See the cell structure topic for more information.
In multicellular organisms, cells often specialise for specific functions. See the cell differentiation and specialisation topic for more information.

Tissues:

Cells with a similar structure and function group together to form tissues. For example:
- Muscle cells group together to form muscle tissue in animals.
- Palisade mesophyll cells group together to form palisade mesophyll tissue in the leaves of plants.

Organs:

Organs are aggregations (or groups) of tissues that work together to perform specific functions. For example:
- The lungs of animals are made of different tissues (such as epithelial, elastic and connective tissues) that work together for gas exchange in animals.
- The leaves of plants are made of different tissues (such as palisade mesophyll, spongy mesophyll and epidermal tissues) that work together for photosynthesis in plants.

Organ Systems:

Organ systems are groups of organs working together to perform complex functions. For example:
- The digestive system includes the stomach, intestines, and liver, working together to digest and absorb food.
- The plant transport system includes roots, stem, and leaves for moving water, minerals, and sugars.

Example organs and systems in humans

Maths skills - Units of measurement

Organ systems are the largest structures, followed by organs, tissues, and then cells, which are the smallest. Since the sizes vary widely, different units are needed when taking measurements.

Use meters (m) for large structures like organ systems.
Use millimeters (mm) for structures like leaves.
Use micrometers (µm) for tissues and cells.

Always choose the most appropriate unit for the size of the structure being measured.

Key words

Cell: The basic building block of all living organisms.
Specialised Cell: A cell that has a specific structure to perform a particular function (e.g., muscle cell).
Tissue: A group of cells with a similar structure and function working together (e.g., muscle tissue, palisade mesophyll).
Organ: An aggregation (group) of different tissues that work together to perform a specific function (e.g., lungs, leaf).
Organ System: A group of organs working together to perform a complex function (e.g., the digestive system, plant transport system).
Micrometer (µm): A unit of measurement appropriate for measuring cells and tissues.

Extension ideas

System Dissection and Comparison: Choose a specific organ system from the human body (e.g., the circulatory system or the respiratory system), identify at least three major organs within that system and list the function of each organ, and explain how these organs work together to perform the system's overall function.
Cellular Measurement Conversion: Research the typical size of a human red blood cell, convert this measurement into millimeters and meters, explain why the micrometer is the most practical unit for this measurement.

GCSE Biology

Topic 2: Organisation