Structural Organisation and Function in Animals
This domain examines how cells are organised into tissues, tissues into organs and organs into systems, and how structure predicts function. Questions often compare tissue types or trace a substance through a system.
Tissues
- Epithelium: cells closely packed with little matrix; squamous, cuboidal, columnar and ciliated types cover, line and secrete.
- Connective tissue: abundant extracellular matrix; loose and dense connective tissue, cartilage, bone and blood. Fibres include collagen and elastin.
- Muscle: skeletal (striated, voluntary, multinucleate); cardiac (striated, involuntary, intercalated discs); smooth (non-striated, involuntary, spindle-shaped).
- Nervous tissue: neurons conduct impulses; neuroglia support, insulate and nourish.
Systems and structure-function links
- Digestive tract: villi and microvilli increase absorptive surface; liver secretes bile; pancreas supplies enzymes.
- Circulation: open system in arthropods and most molluscs; closed in annelids and vertebrates. Fish have a two-chambered heart, amphibians three, mammals and birds four.
- Respiration: gills, lungs, skin and insect tracheae each increase gas-exchange surface.
- Excretion: protonephridia, nephridia, Malpighian tubules and kidneys; the nephron uses filtration, reabsorption and secretion.
Worked example
A frog has a three-chambered heart: two atria and one ventricle. Reasoning: oxygenated blood returns to the left atrium, deoxygenated blood to the right atrium, and both enter a single ventricle, so some mixing occurs. This is double circulation with incomplete separation, unlike the completely separated four-chambered mammalian heart.
Traps
Cardiac muscle is striated but involuntary. Open circulation is not absence of circulation. Not every vertebrate has four chambers. Villi increase surface area, not gut length. Insect tracheae are not lungs.