Organs: structure and function explained

Organisation • Principles of organisation

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What is the effect of increased organ surface area?

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Increased surface area raises the rate of exchange of materials, so organs with larger surface areas perform absorption or secretion more efficiently.

Key concepts

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Clear definition of an organ

An organ is a group of tissues that perform a specific function for an organism. Tissues within an organ have different structures and roles but act together so the organ fulfils a single, recognisable job. The hierarchical organisation places organs above tissues and below organ systems, so organs connect cell-level processes to whole-body activities.

Types of tissues in animal organs

Animal organs commonly contain muscle tissue, epithelial tissue and connective tissue. Muscle tissue produces force and movement, epithelial tissue forms protective or secretory linings, and connective tissue supports and links structures. The presence of nerve tissue in some organs provides electrical coordination so contractions or secretions occur at the correct time. The combination of these tissues produces the organ’s overall function.

Plant organs and their component tissues

Plant organs include roots, stems (shoots) and leaves. Roots contain epidermis with root hair cells for water and mineral uptake, and internal vascular tissues (xylem and phloem) for transport. Leaves contain mesophyll tissues for photosynthesis and vascular bundles to move sugars and water. Organ-level adaptations result from specific arrangements of these tissues to meet transport, support and photosynthetic demands.

Structure–function links and limiting factors

Organ function depends on tissue composition and supply lines such as blood or xylem/phloem. Adequate blood supply delivers oxygen and nutrients and removes wastes; without it, organ performance declines. Small diffusion distances, large surface area and rich capillary networks increase transport efficiency into organ tissues. Limitations arise when supply does not match demand, when tissue damage reduces function, or when organ size and shape restrict exchange rates.

Organs within organ systems

Multiple organs combine into organ systems that perform broader tasks. For example, the digestive system contains the pancreas, liver and intestines; each organ performs a specific step and the system depends on coordinated organ actions. Cause leads to effect: tissue specialisation causes organ-level capability, organ-level capability causes system-level processes, and system-level processes maintain organismal homeostasis.

Key notes

Important points to keep in mind

An organ consists of two or more tissue types arranged to perform a specific function.

Organs sit between tissues and organ systems in the organisational hierarchy.

Muscle tissue provides movement, epithelial tissue lines and protects, connective tissue supports, nerve tissue coordinates.

Adequate blood (or xylem/phloem) supply is essential for organ performance; poor supply limits function.

Surface area and diffusion distance determine exchange efficiency for organs involved in absorption or gas exchange.

Organs combine into systems; loss of one organ’s function can impair the whole system.

Plant roots, stems and leaves are organs adapted for absorption, transport and photosynthesis respectively.