Principles of organisation — AQA GCSE Biology
Test yourself on Principles of organisation with AQA GCSE practice questions.
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Principles of organisation explained
All living organisms are made of cells, and the cell is the smallest unit that can carry out all the processes we associate with life.
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Some organisms, such as bacteria and amoeba, are unicellular and consist of one cell that performs every life process. Most plants and animals are multicellular, and their cells become specialised so that different jobs are shared between different cell types. For example, a red blood cell has no nucleus and is packed with haemoglobin to carry oxygen, while a root hair cell has a long extension to increase surface area for water and mineral uptake. Understanding that cells are the basic building blocks allows you to explain how larger structures such as tissues, organs and organ systems are organised, and why cell specialisation is important in multicellular organisms.
A tissue is a group of cells with a similar structure and function.
In multicellular organisms, cells that share a similar structure and carry out the same job are grouped together to form a tissue. Working as a tissue allows a job to be done more effectively than a single cell could manage alone. For example, muscular tissue contains muscle cells that contract to move parts of the body, glandular tissue contains secretory cells that release substances such as enzymes, and epithelial tissue covers and protects surfaces. In plants, palisade mesophyll tissue contains many chloroplasts for photosynthesis, while xylem tissue transports water and mineral ions. Recognising tissues as the level between cells and organs helps you explain how the organ systems of a plant or animal are built up and how each level contributes to the functioning of the whole organism.
Organs are aggregations of tissues performing specific functions.
An organ is a structure made from several different tissues that work together to carry out a particular function. Because each tissue contributes a different job, the organ can perform a task that no single tissue could achieve alone. For example, the stomach contains muscular tissue to churn food, glandular tissue to secrete digestive juices, and epithelial tissue to line its inner and outer surfaces. In plants, a leaf is an organ containing palisade mesophyll tissue for photosynthesis, xylem and phloem for transport, and epidermal tissue for protection. Organs are themselves grouped into organ systems, such as the digestive system or the transport system in plants, so understanding organs helps explain how whole organisms function.
Organs are organised into organ systems, which work together to form organisms.
This statement describes the final levels of the organisational hierarchy that runs from cells through tissues and organs to whole organisms. A tissue is a group of similar cells carrying out a shared function, such as muscular tissue. Several tissues combine into an organ, for example the stomach, which contains muscular tissue, glandular tissue and epithelial tissue. Organs then cooperate as an organ system: the stomach, small intestine, large intestine, liver and pancreas form the digestive system. Each system carries out a major life process, and the systems interact so that the organism functions as a coordinated whole. For instance, the digestive system supplies glucose that the circulatory system transports and that muscles use in respiration. Understanding this hierarchy lets you explain how damage at one level, such as faulty insulin-producing cells, affects the whole organism.
Your focus
- State that cells are the basic building blocks of all living organisms.
- Describe examples of specialised cells and relate their structure to their function.
- Explain how cells are organised into tissues, organs and organ systems in multicellular organisms.
Show all 12 objectives
- Define a tissue as a group of cells with a similar structure and function.
- Identify named examples of tissues in plants and animals and state their functions.
- Explain how tissues contribute to the structure and function of organs and organ systems.
- Define an organ as an aggregation of tissues performing a specific function.
- Describe named examples of organs and identify the tissues they contain.
- Explain how different tissues within an organ work together to carry out the organ's function.
- State the order of the levels of organisation from cell to organism.
- Give a named example of a tissue, an organ and an organ system.
- Explain how organs within a system cooperate to carry out a function.
Principles of organisation exam tips
Marking Points
- State that all living organisms are composed of cells and that the cell is the basic unit of life.
- Recognise that some organisms are unicellular while others are multicellular.
- Explain that multicellular organisms contain many cells that may be specialised for particular functions.
- Link cell specialisation to the efficient functioning of the whole organism, for example exchange surfaces or transport.
- Use examples of specialised cells, such as red blood cells, nerve cells, root hair cells or sperm cells, and relate structure to function.
- Recognise the hierarchy from cells to tissues, organs and organ systems in multicellular organisms.
- Define a tissue as a group of cells with a similar structure and function.
- Give named examples of tissues, such as muscular, glandular, epithelial, xylem or palisade mesophyll tissue.
- Explain that cells in a tissue work together to carry out a shared function more efficiently.
- Relate the structure of the tissue to its function, for example muscle tissue contains contractile cells that shorten to produce movement.
- Place tissue correctly in the hierarchy: cells form tissues, tissues form organs, organs form organ systems.
- Distinguish tissue from organ by noting that an organ contains several different tissues working together.
- Define an organ as an aggregation of different tissues performing a specific function.
- Give named examples of organs, such as the stomach, heart, leaf or root, and state their functions.
- Identify at least two different tissues within a named organ and describe the contribution of each.
- Explain that the combination of tissues allows the organ to carry out a function that individual tissues could not perform alone.
- Place organs correctly in the hierarchy: cells form tissues, tissues form organs, organs form organ systems.
- Link organ function to the needs of the whole organism, for example the heart pumping blood to deliver oxygen and glucose.
- Cells are the basic building blocks of living organisms and become specialised for particular functions.
- A tissue is a group of similar cells that work together to carry out a shared function.
- An organ is a structure made of several different tissues working together, such as the stomach or heart.
- An organ system is a group of organs that cooperate to carry out a major function, such as digestion or gas exchange.
- Organ systems interact so that the organism can carry out all the processes needed for life.
- Examples should be used to show the sequence: cell, tissue, organ, organ system, organism.
Examiner Tips
- 💡Use the phrase basic building blocks of all living organisms when defining a cell, then add a specific example to show understanding.
- 💡When describing a specialised cell, always link one structural feature to one function, such as many mitochondria in a muscle cell for energy release.
- 💡If asked about organisation, work upwards in order: cell, tissue, organ, organ system, organism.
- 💡Learn at least two animal and two plant tissue examples with their functions so you can answer both context and definition questions.
- 💡When defining a tissue, include both parts of the definition: similar structure and shared function.
- 💡Use the hierarchy cell, tissue, organ, organ system, organism to structure answers about levels of organisation.
- 💡When asked about an organ, name at least two tissues and explain the role of each in the organ's function.
- 💡Use the stomach or leaf as a prepared example, since both contain several clearly different tissues.
- 💡Keep the hierarchy in order and use it to check that your answer places the organ at the correct level.
- 💡Use named examples at each level, such as muscle cell, muscular tissue, stomach, digestive system, human.
- 💡When asked to explain, link each level to the function it performs rather than only listing the levels.
- 💡Check that any example you give genuinely belongs to the level named in the question.
Common Mistakes
- Thinking that cells are only found in animals; the correction is that all living organisms, including plants, fungi and bacteria, are made of cells.
- Assuming every cell in a multicellular organism is identical; the correction is that many cells are specialised in structure and function.
- Confusing the cell with a tissue or organ; the correction is that a cell is the smallest living unit, while tissues and organs are groups of cells working together.
- Calling a single cell a tissue; the correction is that a tissue is a group of similar cells working together.
- Describing an organ as a tissue; the correction is that an organ is made of several tissues and performs a specific function.
- Assuming all cells in a tissue are identical in every way; the correction is that they share a similar structure and function, which may still vary in detail.
- Describing an organ as a group of identical cells; the correction is that an organ contains several different tissues.
- Confusing an organ with an organ system; the correction is that an organ system is a group of organs working together.
- Listing tissues without linking them to the organ's function; the correction is to explain how each tissue contributes to the overall job of the organ.
- Treating an organ as a single tissue; correct this by stating that an organ contains several tissues, for example the stomach has muscular and glandular tissue.
- Confusing an organ with an organ system; correct this by noting that an organ system is a group of organs, such as the digestive system, not one structure.
- Describing the hierarchy in the wrong order; correct this by rehearsing the sequence cell, tissue, organ, organ system, organism.