B4: Tissues and Organs
Cell specialisation, tissues, organs and organ systems
📋 Key Concepts
Cells are the basic building blocks of all living organisms. In multicellular organisms, cells become specialised for particular functions. Similar cells group together to form tissues, tissues form organs, and organs work together in organ systems.
Key Terms
- Cell specialisation - cells adapted for a specific function
- Tissue - a group of similar cells with a similar function
- Organ - a group of tissues that work together to perform a function
- Organ system - a group of organs that work together
Hierarchy of organisation:
Cells → Tissues → Organs → Organ Systems → Organism
Each level builds on the one below. Specialised cells form tissues; tissues form organs; organs form organ systems.
Cells → Tissues → Organs → Organ Systems → Organism
Each level builds on the one below. Specialised cells form tissues; tissues form organs; organs form organ systems.
📝 The Hierarchy of Organisation
Multicellular organisms are organised into a hierarchy: cells specialise, group into tissues, tissues combine into organs, and organs form organ systems. This allows large organisms to function efficiently.
| Level | Definition | Example |
|---|---|---|
| Cell | The basic unit of life, specialised for a function | Muscle cell, nerve cell, sperm cell |
| Tissue | A group of similar cells with the same function | Muscular tissue, epithelial tissue |
| Organ | A group of tissues working together | Stomach, heart, leaf |
| Organ system | A group of organs working together | Digestive system, circulatory system |
| Organism | A living individual made of organ systems | Human, oak tree |
Example 1
Question: Describe the hierarchy of organisation in a human, using the digestive system as an example.
Answer: A muscle cell is a specialised cell. Many muscle cells form muscular tissue. Muscular tissue, glandular tissue and epithelial tissue combine to form the stomach (an organ). The stomach, intestines, liver and other organs form the digestive system (an organ system). The digestive system works with other organ systems to make up the whole organism.
Example 2
Question: A student says "an organ is a group of cells that work together." Is this correct? Explain your answer.
Answer: No, this is not correct. An organ is a group of tissues that work together. A group of similar cells with the same function is a tissue. An organ contains several different types of tissue working together. For example, the stomach contains muscular tissue, glandular tissue and epithelial tissue.
📝 Animal Tissues
Animal tissues include muscular tissue (for movement), glandular tissue (for producing secretions), and epithelial tissue (for covering surfaces). Each tissue is made of cells specialised for its function.
| Tissue | Function | Cell Specialisation |
|---|---|---|
| Muscular tissue | Contracts to cause movement | Long cells that can shorten (contract) when stimulated |
| Glandular tissue | Produces and secretes substances (e.g. enzymes, hormones) | Cells with many ribosomes and a large Golgi apparatus for making and releasing secretions |
| Epithelial tissue | Covers and lines body surfaces | Thin, flat cells that form a continuous layer for protection and exchange |
Remember: These three tissues are found in the stomach. Muscular tissue churns food, glandular tissue produces digestive juices, and epithelial tissue lines the inside surface.
📝 Plant Tissues
Plant tissues include epidermal tissue (covering), palisade mesophyll (photosynthesis), spongy mesophyll (gas exchange), xylem (water transport) and phloem (sugar transport).
| Tissue | Function | Key Features |
|---|---|---|
| Epidermal tissue | Covers and protects the plant surface | Thin, flat cells; often covered by a waxy cuticle to reduce water loss |
| Palisade mesophyll | Photosynthesis | Tall, column-shaped cells packed with chloroplasts near the upper surface of the leaf |
| Spongy mesophyll | Gas exchange | Rounded cells with large air spaces between them for gases to diffuse through |
| Xylem | Transports water and dissolved minerals from roots to leaves | Dead cells forming hollow tubes; strengthened with lignin for support |
| Phloem | Transports dissolved sugars (translocation) from leaves to rest of plant | Living cells with sieve plates; companion cells provide energy |
Example 3
Question: Explain why palisade mesophyll cells are well adapted for photosynthesis.
Answer: Palisade mesophyll cells are tall and column-shaped, packed with many chloroplasts to absorb maximum light for photosynthesis. They are positioned near the upper surface of the leaf where light intensity is highest. The columns are closely packed to fit as many chloroplast-containing cells as possible in the light path.
Example 4
Question: Compare xylem and phloem tissue.
Answer: Both are plant transport tissues. Xylem transports water and minerals upwards from roots to leaves; phloem transports sugars from leaves to all parts of the plant (translocation). Xylem vessels are made of dead cells forming hollow tubes strengthened with lignin; phloem is made of living cells with sieve plates and companion cells. Xylem carries water in one direction only; phloem carries sugars in both directions.
📝 Organs: The Stomach
The stomach is an example of an organ made of multiple tissues working together. It contains muscular tissue, glandular tissue and epithelial tissue, each performing a different role in digestion.
Tissues in the Stomach
- Muscular tissue - contracts to churn and mix food with digestive juices, breaking it down mechanically
- Glandular tissue - produces and secretes digestive juices containing enzymes (e.g. protease) and hydrochloric acid
- Epithelial tissue - lines the inside of the stomach, providing a smooth surface and protecting the stomach wall from acid
Key principle: An organ contains different tissues because it needs to perform several functions. Each tissue is specialised for one of those functions.
📝 Organ Systems: The Digestive System
The digestive system is an organ system made of several organs working together to break down large food molecules into small, soluble molecules that can be absorbed into the blood.
Organs of the Digestive System
- Mouth - mechanical digestion (teeth chewing) and chemical digestion (salivary amylase breaks down starch)
- Oesophagus - moves food from mouth to stomach by peristalsis
- Stomach - churns food, produces protease enzyme and hydrochloric acid
- Small intestine - digests food and absorbs small soluble molecules into the blood
- Large intestine - absorbs water from remaining undigested food
- Rectum - stores faeces before they leave the body
- Liver - produces bile to help digest fats
- Pancreas - produces digestive enzymes (amylase, protease, lipase)
Example 5
Question: Explain how the stomach is an example of an organ.
Answer: The stomach is an organ because it is made of different tissues working together to perform the function of digestion. It contains muscular tissue (to churn food), glandular tissue (to produce digestive juices containing enzymes and acid), and epithelial tissue (to line the stomach surface). These different tissues each have their own function but work together so the stomach can digest food effectively.
❓ Practice Questions
Q1: Define the term "tissue." (1 mark)
Q2: Describe the hierarchy of organisation in a multicellular organism. (4 marks)
Q3: Name the three types of tissue found in the stomach and state the function of each. (3 marks)
Q4: Explain the difference between a tissue and an organ. Use an example in your answer. (3 marks)
Q5: Compare palisade mesophyll tissue and spongy mesophyll tissue in a leaf. (4 marks)
Q6: A student says "the stomach is made of muscle cells, gland cells and lining cells." Suggest a more scientifically accurate way to describe the stomach. (2 marks)
✅ Answers
- A tissue is a group of similar cells with the same function working together.
- Cells are the basic units and become specialised for particular functions. Similar specialised cells group together to form tissues. Different tissues combine to form organs. Organs work together in organ systems. Organ systems make up the whole organism.
- Muscular tissue - contracts to churn food; glandular tissue - produces digestive juices (enzymes and acid); epithelial tissue - lines the surface of the stomach.
- A tissue is a group of similar cells with the same function (e.g. muscular tissue is made of muscle cells). An organ is a group of different tissues working together (e.g. the stomach contains muscular tissue, glandular tissue and epithelial tissue). An organ contains multiple tissue types; a tissue contains one cell type.
- Both are tissues in the leaf. Palisade mesophyll has tall, column-shaped cells packed with chloroplasts for photosynthesis. Spongy mesophyll has rounded cells with large air spaces between them for gas exchange. Palisade is near the upper surface for maximum light absorption; spongy is below the palisade layer. Palisade has more chloroplasts; spongy has more air spaces.
- The stomach is an organ made of three types of tissue: muscular tissue, glandular tissue and epithelial tissue. The student should describe the groupings of cells as tissues rather than listing individual cell types.
🎯 Exam Tips
- Learn the hierarchy in order: cells → tissues → organs → organ systems → organism
- Do not confuse "tissue" and "organ" - a tissue has one cell type, an organ has multiple tissue types
- When asked about the stomach, always name the three tissues AND their functions
- For plant tissues, make sure you can explain WHY palisade cells have many chloroplasts (for photosynthesis) and WHY spongy mesophyll has air spaces (for gas exchange)
- Remember xylem transports water UP, phloem transports sugars in BOTH directions
- Questions often ask "Explain why X is an organ" - your answer must mention that it contains different tissues working together
🔢 Maths Skills
Mathematical Skills
Mathematical skills in this topic are minimal. Focus on interpreting diagrams and understanding hierarchical organisation. You may need to calculate the surface area to volume ratio of exchange surfaces (e.g. villi) to explain why larger surface area increases absorption.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Tissues are groups of identical cells Correct: Tissues are groups of similar cells with the same function, but some tissues contain mixed cell types (e.g. blood is a tissue containing red cells, white cells and platelets)
2. Wrong: Organs work independently Correct: Organs work together in organ systems — e.g. the stomach, liver and intestines all cooperate in the digestive system
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the hierarchy of organisation using the digestive system as an example.
The hierarchy of organisation begins with specialised cells. In the digestive system, gland cells produce enzymes and muscle cells contract to churn food. Similar cells group together to form tissues — glandular tissue produces digestive juices, muscular tissue contracts for churning, and epithelial tissue lines surfaces. Different tissues combine to form organs — for example, the stomach contains all three tissue types working together to digest food. Organs work together in organ systems — the stomach, small intestine, liver and pancreas all function together in the digestive system to break down and absorb food. The digestive system works with other organ systems to support the whole organism.
Mark scheme: 1 mark for cells with example; 1 mark for tissues with example; 1 mark for organs with example; 1 mark for organ systems with example; 1 mark for organism; 1 mark for coherent explanation of hierarchy
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A diagram shows a cross-section of a leaf. Identify the levels of organisation visible, from cell to organ system.
Answer: Individual palisade cells (cell level) group into palisade mesophyll tissue (tissue level). Multiple tissues including palisade mesophyll, spongy mesophyll, epidermal tissue, xylem and phloem combine to form the leaf (organ level). The leaf works with roots and stems in the plant transport and photosynthetic systems (organ system level).
📝 Exam Questions by Topic
🎬 Video Resources
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