Structural Organisation in Animals

The One Thing

Animal bodies are organised hierarchically: specialised cells form tissues, tissues form organs, and organs cooperate within organ systems. The structure of each tissue is adapted to its function, while the organisation of systems reflects the animal’s habitat and mode of life.

Reactions, Processes and Experiments

Key Terms

• Levels of organisation: The arrangement of the animal body from cells to tissues, organs, organ systems, and the complete organism.
• Animal tissue: A group of similar cells, usually having a common origin, that perform a particular function.
• Epithelial tissue: Closely packed tissue that covers body surfaces, lines internal organs and cavities, and forms glands.
• Simple epithelium: Epithelium made of a single cell layer, generally suited to absorption, secretion, or exchange.
• Squamous epithelium: Thin, flat epithelium that permits diffusion and filtration; found in alveoli and blood vessel walls.
• Cuboidal epithelium: Cube-shaped epithelium involved mainly in secretion and absorption; found in kidney tubules and gland ducts.
• Columnar epithelium: Tall epithelial cells specialised for absorption and secretion; found in parts of the digestive tract.
• Ciliated epithelium: Epithelium with cilia that move materials over the cell surface, such as mucus in the respiratory tract.
• Glandular epithelium: Modified epithelial tissue that produces and releases enzymes, hormones, mucus, or sweat.
• Connective tissue: Tissue that connects, supports, binds, protects, stores, or transports materials.
• Areolar tissue: Loose connective tissue that fills spaces between organs, supports internal structures, and helps in tissue repair.
• Adipose tissue: Fat-containing connective tissue that stores energy, insulates the body, and cushions organs.
• Dense connective tissue: Strong, fibre-rich connective tissue; tendons connect muscles to bones and ligaments connect bones to bones.
• Cartilage: Flexible supporting connective tissue found in the nose, ear, trachea, and joints.
• Bone: Hard connective tissue forming the skeleton, supporting the body, protecting organs, and providing muscle attachment.
• Blood: Fluid connective tissue consisting of plasma and formed elements that transports gases, nutrients, wastes, hormones, and immune cells.
• Lymph: Fluid connective tissue that returns tissue fluid to the blood and supports immune defence and fat absorption.
• Muscular tissue: Contractile tissue responsible for body movement or movement of substances within the body.
• Skeletal muscle: Striated, voluntary muscle attached to bones.
• Smooth muscle: Non-striated, involuntary muscle in the walls of internal organs such as the intestine and blood vessels.
• Cardiac muscle: Striated, involuntary muscle found only in the heart and adapted for rhythmic contraction.
• Nervous tissue: Tissue specialised for receiving stimuli, generating nerve impulses, and coordinating body activities.
• Neuron: The structural and functional unit of nervous tissue, consisting mainly of a cell body, dendrites, and an axon.
• Earthworm: A segmented annelid with a closed circulatory system, nephridia, a complete digestive tract, and a body supported by setae and a hydrostatic skeleton.
• Setae: Small earthworm bristles that assist anchorage and locomotion.
• Nephridia: Earthworm excretory structures that remove nitrogenous wastes and help maintain water and salt balance.
• Cockroach: A segmented arthropod with a chitinous exoskeleton, jointed appendages, an open circulatory system, tracheal respiration, and Malpighian tubules.
• Exoskeleton: A hard external covering that protects the cockroach, supports the body, and provides muscle attachment sites.
• Tracheal system: A network of insect air tubes that delivers oxygen directly to tissues through spiracles.
• Malpighian tubules: Cockroach excretory structures that remove nitrogenous wastes, mainly uric acid, from the haemolymph.
• Frog: A vertebrate amphibian with a bony endoskeleton, closed circulation, lungs and skin for respiration, and organs adapted to aquatic and terrestrial life.
• Metamorphosis: A major change in body form during development, such as the transformation of a frog tadpole into an adult frog.
• Cloaca: A common chamber into which the digestive, urinary, and reproductive tracts open in frogs.
• Open circulatory system: A system in which haemolymph is not always confined to blood vessels and directly bathes tissues.
• Closed circulatory system: A system in which blood remains within vessels and exchanges materials with tissues through capillaries or vessel walls.
• Poikilothermic: Having a body temperature that varies with environmental temperature.
• Hydrostatic skeleton: Support provided by fluid pressure within the body, as in the earthworm.
• Haemolymph: The circulating fluid of an open circulatory system, such as that of the cockroach.
• Gastric or hepatic caeca: Cockroach digestive structures that assist in digestion and absorption.
• Typhlosole: An internal fold in the earthworm intestine that increases absorptive surface area.

Easily Confused

• Simple epithelium vs compound epithelium: Simple epithelium has one cell layer and is suited to absorption, secretion, or exchange; compound epithelium has multiple layers and mainly provides protection.
• Tendon vs ligament: A tendon connects muscle to bone; a ligament connects bone to bone.
• Skeletal muscle vs smooth muscle: Skeletal muscle is striated and voluntary; smooth muscle is non-striated and involuntary.
• Smooth muscle vs cardiac muscle: Both are involuntary, but smooth muscle is non-striated and occurs in internal organs, whereas cardiac muscle is striated and occurs only in the heart.
• Open circulation vs closed circulation: In open circulation, haemolymph is not always confined to vessels; in closed circulation, blood remains within vessels.
• Earthworm respiration vs cockroach respiration: Earthworms exchange gases through moist skin; cockroaches use spiracles, tracheae, and tracheoles.
• Earthworm circulation vs cockroach circulation: Earthworms have a closed circulatory system; cockroaches have an open circulatory system.
• Cockroach respiration vs cockroach circulation: The tracheal system delivers oxygen directly to tissues, while the open circulatory system circulates haemolymph and does not transport oxygen to tissues.
• Nephridia vs Malpighian tubules: Nephridia excrete wastes in earthworms; Malpighian tubules excrete mainly uric acid in cockroaches.
• Tadpole excretion vs adult frog excretion: Tadpoles mainly excrete ammonia; adult frogs mainly excrete urea.
• Frog lungs vs frog skin: Adult frogs use lungs on land and moist skin in water or during rest; the skin also contributes to respiration.
• Cloaca vs urinary bladder: The cloaca is a common chamber for three body systems; the urinary bladder temporarily stores urine.

What Gets Asked

• Identify the level of organisation in a sequence such as cells → tissues → organs → organ systems → organism. The mark-losing error is omitting the distinction between an organ and an organ system.
• Classify epithelial tissues from structure and function. The key distinctions are single versus multiple layers and the shapes or specialisations of cells; compound epithelium should not be described as primarily absorptive when its stated role is protection.
• Compare connective tissues and their functions. Questions may require distinguishing tendons from ligaments, or explaining the transport and defence roles of blood and lymph.
• Compare skeletal, smooth, and cardiac muscle. The required contrasts are striations, voluntary or involuntary control, location, and rhythmic contraction in cardiac muscle.
• Describe organisation in earthworms, cockroaches, or frogs. Specific features may include earthworm setae, nephridia and gizzard; cockroach exoskeleton, tracheal system and Malpighian tubules; or frog metamorphosis, cloaca and three-chambered heart.
• Compare respiratory, circulatory, and excretory systems in representative animals. Common errors include stating that cockroach blood transports oxygen, confusing earthworm closed circulation with cockroach open circulation, or giving ammonia as the main nitrogenous waste of adult frogs.