How are cells organised into tissues, organs and organ systems to keep an organism alive?
The levels of organisation from cells to tissues, organs and organ systems, the meaning of each level, and how the digestive system is an example of an organ system, including the action of enzymes.
A focused answer to AQA GCSE Biology 4.2.1, covering the levels of organisation from cells to tissues, organs and organ systems, with the digestive system as the worked example.
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What this dot point is asking
AQA wants you to describe how cells are organised into tissues, tissues into organs, and organs into organ systems, define each level, and use the digestive system as a worked example of an organ system carrying out an essential function.
The levels of organisation
The order runs: cells, then tissues, then organs, then organ systems, and finally the whole organism. For example, muscular tissue and glandular tissue combine in the stomach (an organ), which works with the mouth, oesophagus, liver, pancreas and intestines as the digestive system (an organ system). This organisation lets specialised cells carry out specific jobs while working together for the whole organism.
The digestive system as an organ system
The digestive system breaks down food so the nutrients can be absorbed into the blood. Enzymes are central to this, because most food molecules are too large to be absorbed and must be broken into small, soluble molecules:
- Carbohydrases (such as amylase) break down carbohydrates into simple sugars.
- Proteases break down proteins into amino acids.
- Lipases break down lipids (fats) into fatty acids and glycerol.
The small, soluble products of digestion are absorbed into the blood through the wall of the small intestine, which is adapted with a large surface area (villi), a thin wall and a good blood supply, exactly the features that make any exchange surface efficient.
The digestive system is a good example of why organisms are organised into systems at all. No single cell could digest a meal, but specialised cells grouped into tissues (muscular and glandular), built into organs (the stomach, pancreas, liver and intestines) and linked into a system, can carry out the whole process from chewing to absorption. Each level adds a capability the level below could not provide on its own. AQA expects you to be able to take an organ such as the stomach, name the tissues it contains, and explain how those tissues let the organ do its job within the wider system, which is the same structure-to-function reasoning used throughout the organisation topic.
Try this
Q1. Define the term tissue. [1 mark]
- Cue. A group of cells with similar structure and function working together.
Q2. Put these in order of increasing size: organ, cell, organ system, tissue. [1 mark]
- Cue. Cell, tissue, organ, organ system.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20183 marksDescribe the levels of organisation in the human body, from cells up to organ systems, using an example for each level.Show worked answer →
A 3-mark describe question rewards each level defined with an example.
A cell is the basic building block, for example a muscle cell. A tissue is a group of cells with a similar structure and function working together, for example muscular tissue. An organ is a group of different tissues working together to perform a function, for example the stomach, which contains muscular and glandular tissue. An organ system is a group of organs working together to perform a major function, for example the digestive system, which includes the stomach, intestines, liver and pancreas.
Markers reward the correct order (cell, tissue, organ, organ system) with a sensible example at each level.
AQA 20214 marksThe stomach is an organ in the digestive system. Explain how the different tissues in the stomach allow it to carry out its function in digestion.Show worked answer →
A 4-mark explain question rewards tissues linked to the stomach's job.
The stomach contains muscular tissue, which contracts to churn and mix the food with digestive juices, helping to break it up physically. It contains glandular tissue, which produces and releases digestive enzymes (such as protease) and hydrochloric acid; the enzymes chemically digest the food and the acid kills pathogens and provides the right pH for the enzyme to work. Epithelial tissue lines the stomach and protects it.
Markers reward muscular tissue churning the food, glandular tissue making enzymes and acid, and linking these to digestion.
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Sources & how we know this
- AQA GCSE Biology (8461) specification — AQA (2016)