B6: Principles of Organisation
Cell hierarchy, tissues, organs, organ systems, and plant tissues
Key Definitions
Cell — The basic building block of all living organisms; the smallest unit of life.
Tissue — A group of similar cells with a similar structure and function working together.
Organ — A group of different tissues that work together to perform a specific function.
Organ system — A group of organs that work together to perform a major life function.
Organism — A living individual made up of organ systems working together.
Tissue — A group of similar cells with a similar structure and function working together.
Organ — A group of different tissues that work together to perform a specific function.
Organ system — A group of organs that work together to perform a major life function.
Organism — A living individual made up of organ systems working together.
The Hierarchy of Organisation
Cell → Tissue → Organ → Organ System → Organism
Cell — Individual cells are specialised for specific functions (e.g. muscle cell, nerve cell, root hair cell).
Tissue — A group of similar cells working together (e.g. muscle tissue = many muscle cells contracting together; glandular tissue = cells that secrete substances).
Organ — Multiple tissues combine (e.g. the stomach contains muscular tissue for churning, glandular tissue for digestive juice secretion, and epithelial tissue for lining).
Organ system — Organs work together (e.g. the digestive system includes the mouth, oesophagus, stomach, liver, pancreas, small intestine, large intestine).
Organism — All organ systems function together in one living individual (e.g. a human).
Tissue — A group of similar cells working together (e.g. muscle tissue = many muscle cells contracting together; glandular tissue = cells that secrete substances).
Organ — Multiple tissues combine (e.g. the stomach contains muscular tissue for churning, glandular tissue for digestive juice secretion, and epithelial tissue for lining).
Organ system — Organs work together (e.g. the digestive system includes the mouth, oesophagus, stomach, liver, pancreas, small intestine, large intestine).
Organism — All organ systems function together in one living individual (e.g. a human).
Example 1: From Cell to Organ System — The Digestive System
Cell: Enzyme-producing cell in the pancreas → Tissue: Glandular tissue in the pancreas → Organ: The pancreas (produces digestive enzymes and hormones) → Organ system: The digestive system (breaks down food and absorbs nutrients) → Organism: The human body.
Example 2: From Cell to Organ System — The Circulatory System
Cell: Cardiac muscle cell → Tissue: Cardiac muscle tissue in the heart wall → Organ: The heart (pumps blood around the body) → Organ system: The circulatory system (transports oxygen, nutrients, and waste) → Organism: The human body.
Animal Tissues
| Tissue | Structure | Function | Example Location |
|---|---|---|---|
| Epithelial tissue | Closely packed cells forming sheets or layers; some have cilia or microvilli | Covers body surfaces and lines cavities; protects; absorbs; secretes | Skin, gut lining, airway lining |
| Glandular tissue | Cells that contain many ribosomes and vesicles for producing and releasing substances | Produces and secretes hormones, enzymes, mucus, or other substances | Salivary glands, pancreas, stomach lining |
| Muscular tissue | Elongated cells containing contractile proteins (actin and myosin) | Contracts to cause movement; can be skeletal, smooth, or cardiac | Heart wall, stomach wall, skeletal muscles |
| Connective tissue | Cells embedded in a matrix (e.g. collagen fibres, calcium phosphate) | Supports, connects, and binds other tissues; provides strength | Bones, tendons, ligaments, blood |
Plant Tissues
| Tissue | Structure | Function |
|---|---|---|
| Epidermal tissue | Single layer of closely packed cells; often covered by a waxy cuticle | Covers and protects the leaf; cuticle reduces water loss; may include guard cells |
| Palisade mesophyll | Tall, column-shaped cells packed with chloroplasts; near the upper surface of the leaf | Main site of photosynthesis; maximises light absorption |
| Spongy mesophyll | Irregularly shaped cells with many air spaces between them | Allows gas exchange (CO₂, O₂, water vapour) within the leaf; some photosynthesis |
| Xylem | Dead, hollow tubes strengthened with lignin; no cytoplasm | Transports water and mineral ions from roots to leaves; provides structural support |
| Phloem | Living cells with sieve plates and companion cells | Transports dissolved sugars (translocation) from leaves to all plant parts |
| Meristem tissue | Small, unspecialised cells with thin walls and no vacuole; at root/shoot tips | Produces new cells by mitosis; can differentiate into any plant cell type |
Example 3: The Stomach as an Organ
The stomach is an organ made of multiple tissues:— Muscular tissue — The thick muscular wall contracts to churn and mix food with digestive juices (mechanical digestion).
— Glandular tissue — Produces gastric juice containing hydrochloric acid (kills bacteria, provides optimum pH for protease) and protease enzyme (digests proteins into amino acids).
— Epithelial tissue — Lines the inside of the stomach, protecting underlying tissue from acid damage and allowing absorption of some substances.
These tissues work together so the stomach can perform its function: digesting food.
Example 4: The Leaf as a Plant Organ
A leaf is a plant organ made of multiple tissues:— Epidermal tissue — Covers the leaf; waxy cuticle reduces water loss; guard cells control stomata for gas exchange.
— Palisade mesophyll — Many chloroplasts for photosynthesis; near the upper surface for maximum light absorption.
— Spongy mesophyll — Air spaces for gas exchange between the leaf and the environment.
— Xylem — In the veins (vascular bundles), transports water to the leaf for photosynthesis.
— Phloem — In the veins, transports sugars produced by photosynthesis away from the leaf to the rest of the plant.
The Digestive System as an Organ System
The digestive system is an organ system made of multiple organs working together:
— Mouth — Mechanical digestion (teeth chewing); chemical digestion (amylase in saliva breaks down starch).
— Oesophagus — Transports food to the stomach by peristalsis (muscular contractions).
— Stomach — Churns food; produces protease and hydrochloric acid.
— Liver — Produces bile (emulsifies fats, neutralises acid).
— Pancreas — Produces amylase, protease, and lipase; also produces insulin.
— Small intestine — Digestion and absorption; villi increase surface area.
— Large intestine — Absorbs water from remaining food.
— Rectum — Stores faeces before egestion.
— Mouth — Mechanical digestion (teeth chewing); chemical digestion (amylase in saliva breaks down starch).
— Oesophagus — Transports food to the stomach by peristalsis (muscular contractions).
— Stomach — Churns food; produces protease and hydrochloric acid.
— Liver — Produces bile (emulsifies fats, neutralises acid).
— Pancreas — Produces amylase, protease, and lipase; also produces insulin.
— Small intestine — Digestion and absorption; villi increase surface area.
— Large intestine — Absorbs water from remaining food.
— Rectum — Stores faeces before egestion.
Exam tip: When asked to describe the hierarchy of organisation, always use a specific example. For instance: "Cells → Tissues → Organs → Organ Systems → Organism. For example, a muscle cell is part of muscular tissue in the stomach (organ), which is part of the digestive system (organ system)."
Practice Questions
1. Foundation Define the terms tissue, organ, and organ system.
Tissue: a group of similar cells with a similar structure and function working together. Organ: a group of different tissues that work together to perform a specific function. Organ system: a group of organs that work together to perform a major life function.
2. Foundation Name three tissues found in the stomach and state the function of each.
1) Muscular tissue — contracts to churn and mix food with digestive juices. 2) Glandular tissue — produces digestive juices containing enzymes and acid. 3) Epithelial tissue — lines the stomach, protecting underlying tissue from acid.
3. Higher Explain the difference between palisade mesophyll and spongy mesophyll tissue in a leaf.
Palisade mesophyll consists of tall, tightly packed cells containing many chloroplasts, positioned near the upper surface of the leaf to maximise light absorption for photosynthesis. Spongy mesophyll consists of loosely packed irregular cells with large air spaces between them, allowing gases (CO₂, O₂, water vapour) to diffuse freely within the leaf for gas exchange. Palisade mesophyll is primarily for photosynthesis; spongy mesophyll is primarily for gas exchange.
4. Higher Describe the hierarchy of organisation using the heart as an example.
Cardiac muscle cell (cell) → Cardiac muscle tissue (tissue) → The heart (organ) → The circulatory system (organ system) → The human (organism). The cardiac muscle cells work together as cardiac muscle tissue, which is one of several tissues in the heart. The heart works with blood vessels and blood as the circulatory system, which works with other organ systems in the human body.
5. Foundation What is the function of meristem tissue in plants?
Meristem tissue is found at the tips of roots and shoots. It consists of unspecialised cells that divide rapidly by mitosis and can differentiate into any type of plant cell. This allows the plant to continue growing throughout its life.
🔢 Maths Skills
Mathematical Skills
Interpreting hierarchy diagrams: practise reading diagrams that show the organisation hierarchy (cell → tissue → organ → organ system → organism). Identify each level and count how many levels are shown. For example, a diagram showing "cardiac muscle cell → cardiac muscle tissue → heart → circulatory system → human" has 5 levels.
⚠️ Common Misconceptions
Watch Out!
Students often think tissues are groups of identical cells. Wrong: Tissues can only contain one cell type Correct: Tissues can contain multiple cell types working together for a shared function
Students often think organs work independently. Wrong: Each organ functions on its own Correct: Organs work together in organ systems to perform major life functions
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the hierarchy of organisation from cells to organ systems using the digestive system.
Cells are the basic building blocks — for example, an enzyme-producing cell in the pancreas. A group of similar cells working together forms a tissue — glandular tissue in the pancreas secretes digestive enzymes. Different tissues combine to form an organ — the pancreas contains glandular tissue, blood vessel tissue and connective tissue, all working together to produce and release enzymes and hormones. Organs work together in an organ system — the pancreas is one of several organs (mouth, oesophagus, stomach, liver, small intestine, large intestine) that make up the digestive system, which breaks down food and absorbs nutrients. Each level builds on the one below to carry out increasingly complex functions.
Mark scheme: 1 mark for defining each level (cell, tissue, organ, organ system); 1 mark for a correct digestive system example at each level; 1 mark for explaining how levels build on each other; 1 mark for stating the function of each level; 1 mark for using scientific terminology throughout; 1 mark for a logical, structured answer
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Given a diagram showing the hierarchy of organisation in an unfamiliar organism, identify each level (cell, tissue, organ, organ system) and explain how the structure at each level contributes to the organism's overall function. For example, if given a diagram of a plant leaf showing palisade cells → palisade mesophyll → leaf → shoot system, explain why the palisade cells are packed with chloroplasts, how the tissue maximises light absorption, and how the leaf organ coordinates photosynthesis and gas exchange.
📝 Exam Questions by Topic
🎬 Video Resources
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