Cellular Organisation — Cells, Tissues, Organs and Systems

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.6 — Cell division, diversity and cellular organisation (refer to the official OCR H420 specification document for exact wording). It develops the organisation of cells into tissues, organs and organ systems in both animals and plants, completing the Membranes, Cell Division and Cellular Organisation module.

Multicellular organisms are not just piles of specialised cells — they are carefully organised hierarchies. Cells group together into tissues, tissues combine to form organs, organs work together in organ systems, and organ systems make up a whole organism. This lesson develops the OCR H420 Module 2.1.6 content on the organisation of cells into tissues, organs and organ systems, and completes the Membranes, Cell Division and Cellular Organisation module.

1. The Hierarchy of Organisation

The basic hierarchy in multicellular organisms is:

Cell → Tissue → Organ → Organ System → Organism

graph LR
  A["Specialised cells<br/>e.g. epithelial cell"] --> B["Tissue<br/>e.g. squamous epithelium"]
  B --> C["Organ<br/>e.g. lung"]
  C --> D["Organ system<br/>e.g. respiratory system"]
  D --> E[Organism]

Each level has its own definition.

Key Definitions:

Tissue: A group of cells of similar type working together to perform a particular function.

Organ: A collection of different tissues that work together to perform a particular function.

Organ System: A group of organs that work together to perform a coordinated set of functions.

This organisation allows complex multicellular life: specialised cells do the work, tissues coordinate their efforts, organs integrate multiple activities, and organ systems manage entire physiological processes.

2. Tissues — Examples in Animals

2.1 Epithelial Tissue

Epithelial tissues form continuous sheets of cells that cover body surfaces, line cavities and make up glands. Their key features are tightly packed cells with little intercellular material, resting on a basement membrane (a thin layer of protein fibres and polysaccharides).

Two forms you must know:

Squamous epithelium consists of a single layer of flat, thin cells resting on a basement membrane. Because it is so thin, it is the chosen barrier wherever rapid diffusion is required:

Alveolar walls (where O₂ and CO₂ diffuse between air and blood)
Capillary walls (endothelium — a single layer of flat cells)
Lining of the blood vessels and heart chambers
Parts of the nephron (Bowman's capsule)

Ciliated epithelium consists of column-shaped cells with cilia on their apical (outward-facing) surface. It is specialised for moving substances along a surface:

Lining of the trachea and bronchi — the cilia sweep mucus (produced by interspersed goblet cells) carrying trapped dust and pathogens up towards the throat.
Oviducts — the cilia move the ovum from ovary to uterus.

Both form continuous barriers and both rest on a basement membrane. The key difference is their shape and function.

2.2 Connective Tissue

Connective tissues support, connect and separate other tissues. They typically consist of widely spaced cells embedded in an extracellular matrix of fibres (collagen, elastin) and ground substance.

Examples include:

Cartilage — a firm but flexible connective tissue found at the ends of bones, in the nose, ear, trachea (where it keeps the airways open) and between vertebrae. It contains cells called chondrocytes embedded in a matrix rich in collagen and elastic fibres. It is avascular (no blood supply), which is why cartilage injuries heal slowly.

Bone — the hardest connective tissue, consisting of cells (osteocytes) in a matrix mineralised with calcium phosphate.

Blood — a fluid connective tissue with a matrix (plasma) containing cells (erythrocytes, leukocytes, platelets).

2.3 Muscle Tissue

Muscle tissue is specialised for contraction. Three types:

Skeletal (striated) muscle — attached to bones; under voluntary control; contains sarcomeres of actin and myosin in a striped pattern.
Smooth muscle — in the walls of hollow organs (gut, blood vessels); involuntary; no striations.
Cardiac muscle — in the heart; involuntary; striated; cells connected by intercalated discs that coordinate contraction.

All muscle tissue contains actin and myosin filaments that slide past each other to produce force.

2.4 Nervous Tissue

Nervous tissue consists of neurones (for conducting electrical impulses) and glial cells (supporting cells). Found in the brain, spinal cord and peripheral nerves. It transmits information at high speed.

3. Tissues — Examples in Plants

Plants have a distinct set of tissues suited to their lifestyle.

3.1 Xylem Tissue

Xylem transports water and mineral ions from roots to shoots. Mature xylem vessels consist of dead, hollow cells with their end walls broken down, forming continuous tubes. The walls are reinforced with lignin — a strong, waterproof polymer — deposited in rings, spirals or nets that resist collapse under the tension of water being drawn upward. Xylem also provides structural support to the plant.

Cell types in xylem:

Xylem vessels (the main conducting cells)
Tracheids (present in all vascular plants)
Fibres (providing extra support)
Parenchyma (storage)

3.2 Phloem Tissue

Phloem transports sugars (mainly sucrose) from sources (e.g. leaves) to sinks (e.g. fruits, roots, growing shoots) — a process called translocation. Unlike xylem, phloem cells are living.

Cell types:

Sieve tube elements — the main conducting cells; elongated, with perforated end walls called sieve plates. They are unusual in that they lack a nucleus and most organelles at maturity.
Companion cells — closely associated with sieve tube elements, they retain their nuclei and control the metabolism of the sieve tubes via connecting plasmodesmata. Companion cells have many mitochondria to supply ATP for loading sucrose.

3.3 Epidermis

The epidermis is a single layer of cells covering the surface of young plants. Functions include protection, gas exchange (via stomata), water regulation (via the waxy cuticle) and sensing. Specialised epidermal cells include guard cells and root hair cells.

3.4 Meristematic Tissue

Meristems contain undifferentiated cells that continue to divide throughout the plant's life, generating new tissue (see Lesson 11 — stem cells).

4. Organs — Examples

An organ is a structure made of several different tissues working together to perform a particular function. Examples:

4.1 The Lung (Animal Organ)

Tissues present:

Squamous epithelium (alveolar walls and capillary endothelium) — for gas exchange
Ciliated epithelium with goblet cells (lining trachea and bronchi) — for removing trapped particles
Cartilage (rings in the trachea and bronchi) — to keep airways open
Smooth muscle (in bronchioles) — to adjust airway diameter
Connective tissue and elastic fibres — to allow recoil during exhalation
Blood vessels (capillaries surrounding alveoli) — for gas transport

4.2 The Leaf (Plant Organ)

Tissues present:

Upper epidermis — protection; waxy cuticle reduces water loss
Palisade mesophyll — main site of photosynthesis
Spongy mesophyll — loosely packed cells with air spaces for gas exchange
Xylem (in veins) — water supply
Phloem (in veins) — sugar export
Lower epidermis with stomata — gas exchange

4.3 The Stomach

Epithelium — with gastric glands secreting acid and enzymes
Smooth muscle — churns food
Connective tissue and blood vessels — support and transport

4.4 The Heart

Cardiac muscle — contraction
Connective tissue, valves, chordae tendineae
Endothelium (lining chambers) — squamous epithelium
Nervous tissue — pacemaker cells and autonomic innervation

5. Organ Systems

An organ system comprises several organs that work together. A human body has roughly eleven:

Organ system	Main organs	Key function
Digestive	Mouth, stomach, intestines, liver, pancreas	Breakdown and absorption of food
Respiratory	Trachea, bronchi, lungs, diaphragm	Gas exchange with air
Circulatory	Heart, arteries, veins, capillaries	Transport of materials
Nervous	Brain, spinal cord, peripheral nerves	Rapid communication
Endocrine	Glands (pituitary, thyroid, pancreas, adrenals)	Hormonal communication
Urinary	Kidneys, ureters, bladder, urethra	Excretion and osmoregulation
Muscular	Skeletal, smooth and cardiac muscles	Movement and support
Skeletal	Bones, cartilage, ligaments, tendons	Structural support
Immune	Lymph nodes, spleen, bone marrow, thymus	Defence against pathogens
Reproductive	Ovaries, testes, associated ducts and organs	Production of gametes and offspring
Integumentary	Skin, hair, nails	Barrier, thermoregulation

Organ systems are interdependent. The circulatory system cannot work without the respiratory system to oxygenate blood, or the digestive system to supply nutrients, and so on.

6. The Organism

The highest level of the hierarchy is the organism — a complete individual with coordinated organ systems. All the processes studied in A-Level biology — homeostasis, respiration, response to stimuli — depend on the integration of many organ systems working together in one body.

Beyond the organism, biology also studies populations, communities, ecosystems and the biosphere, but these are outside the scope of this module.

7. Putting the Module Together

This final lesson completes a conceptual journey that began with the fluid mosaic membrane. Tracing the chain:

Membranes (Lessons 1–6) give cells control over what enters and leaves, and enable signalling.
The cell cycle (Lesson 7) and mitosis (Lesson 8) produce new cells for growth, repair and asexual reproduction.
Meiosis (Lesson 9) produces varied gametes for sexual reproduction.
Cell specialisation (Lesson 10) allows cells to take on specific roles.
Stem cells (Lesson 11) are the source from which specialised cells arise.
Cellular organisation (this lesson) builds tissues, organs and organ systems from those specialised cells.

Every step up the hierarchy — from a single cell to a full organism — depends on the correct functioning of membranes, the fidelity of cell division, and the regulated differentiation of stem cells. Understanding this flow of ideas will serve you well not just in Module 2 but throughout A-Level Biology.

8. Common Exam Mistakes

Confusing tissue and organ. A tissue is cells of one main type; an organ contains several different tissues.
Calling the lining of blood vessels a tissue. It is a squamous epithelium — but the blood vessel as a whole is an organ, containing multiple tissues.
Treating xylem as a single cell type. It contains vessels, tracheids, fibres and parenchyma — several cell types working as a tissue.
Forgetting that phloem cells are alive at maturity (unlike xylem).
Saying the lungs are a "tissue". They are an organ in the respiratory organ system.
Confusing the hierarchy order — always go cell → tissue → organ → organ system → organism.
Writing that a "leaf is a tissue". It is a plant organ containing multiple tissues.
Not mentioning the basement membrane when describing epithelial tissue.

9. Exam-Style Questions

Define the terms tissue, organ and organ system, giving an example of each. (6)
Describe three tissues found in the leaf, explaining the function of each. (5)
Compare the structure and function of xylem and phloem tissue. (4)
Explain why the walls of alveoli and capillaries are squamous epithelium. (3)

Model answer for (4): "Both alveolar and capillary walls are squamous epithelium because squamous epithelium consists of a single layer of flattened cells, giving a very short diffusion distance between air in the alveolus and blood in the capillary. This maximises the rate of diffusion of oxygen into the blood and carbon dioxide out of the blood, as described by Fick's law. The thinness and flatness of the cells make the combined alveolar-capillary membrane only about 0.5 μm thick, ideal for efficient gas exchange."

Summary

Multicellular organisms are organised as cell → tissue → organ → organ system → organism.
A tissue is a group of similar cells performing a particular function (e.g. squamous epithelium, xylem).
An organ contains several tissues working together (e.g. lung, leaf).
An organ system is a group of organs performing a coordinated function (e.g. respiratory, circulatory).
Key animal tissues include squamous and ciliated epithelia, cartilage, muscle, nervous tissue and blood.
Key plant tissues include xylem (dead, lignified, water transport), phloem (living, sucrose transport), epidermis and meristems.
Every level of the hierarchy depends on the correct functioning of membranes, cell division and specialisation — the topics of this entire module.

10. Comparison Table — Animal Tissue Types

Tissue type	Key feature	Examples	Function
Squamous epithelium	Flat, thin single layer; basement membrane	Alveoli, capillaries, Bowman's capsule	Short diffusion distance for gas + filtrate exchange
Ciliated epithelium	Columnar with apical cilia; goblet cells interspersed	Trachea, bronchi, oviducts	Mucociliary clearance; ovum transport
Connective — cartilage	Chondrocytes in collagen/elastin matrix; avascular	Joints, trachea, ear, nose, intervertebral discs	Flexible support; airway patency
Connective — bone	Osteocytes in calcium-phosphate matrix	Skeleton	Rigid support; calcium store; haematopoiesis
Connective — blood	Cells (RBCs, WBCs, platelets) in plasma matrix	Throughout body	Transport; defence; clotting
Muscle — skeletal	Striated, sarcomeres, voluntary	Bones	Movement
Muscle — smooth	No striations, involuntary	Gut, blood vessels	Peristalsis; vasoconstriction
Muscle — cardiac	Striated, involuntary, intercalated discs	Heart	Pumping blood
Nervous	Neurones + glial cells	Brain, spinal cord, peripheral nerves	Rapid signal transmission

Lesson 12: Cellular Organisation — Cells, Tissues, Organs and Systems