Cellular Organisation — Cells, Tissues, Organs and Systems

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 covers OCR A-Level Biology A specification point 2.1.6 (f) — 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:

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