Describe gas exchange and internal transport in plants (stomata, xylem, phloem, transpiration and the cohesion-tension theory) and animals (alveoli, gills, open and closed circulatory systems, the human circulatory system)

What this dot point is asking

QCAA expects you to describe how plants and animals exchange respiratory gases with the environment and move materials internally. You need to handle the named structures (stomata, xylem, phloem, alveoli, gills, heart chambers), the cohesion-tension theory and the open vs closed distinction.

The answer

A multicellular body cannot rely on diffusion across the body surface alone. Specialised gas exchange surfaces and internal transport systems supply every cell with oxygen and nutrients and remove waste.

Gas exchange in plants

Stomata. Small pores in the leaf epidermis, mainly on the lower surface, bounded by two guard cells. Open during the day for CO2 to diffuse in (for photosynthesis) and O2 to diffuse out; close at night and during water stress to conserve water.

Guard cell mechanism. When guard cells are turgid (high water potential, K+ accumulated), they bow outward and the stomatal pore opens. When they lose water, they collapse together and the pore closes.

Mesophyll. Inside the leaf, palisade and spongy mesophyll cells expose a large moist surface area to the air spaces. Gases dissolve in the moist cell wall and diffuse into the cell.

Plants do not have a dedicated respiratory system. Every photosynthetic and non-photosynthetic cell exchanges gases locally through air spaces, stomata or, in roots, lenticels and root hairs.

Internal transport in plants

Plants have two vascular tissues, both organised into vascular bundles.

Xylem. Carries water and dissolved minerals from roots to leaves in one direction (upward).

• Composed of dead, hollow, lignified vessel elements and tracheids.
• No end walls in vessels (angiosperms); cell contents removed at maturity.
• Lignin reinforces the walls so vessels resist collapse under tension.

Phloem. Carries dissolved sugars (mainly sucrose) and other organic molecules from sources (leaves) to sinks (roots, fruits, growing tissues) bidirectionally as needed.

• Composed of sieve tube elements (living but lacking nuclei) connected end to end through sieve plates.
• Each sieve tube element is supported by an adjacent companion cell that provides metabolic services.
• Translocation is driven by an active loading of sucrose at the source, creating an osmotic gradient and bulk flow towards the sink (the pressure-flow model).

The cohesion-tension theory of transpiration

Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through stomata. It is the engine that pulls water up the xylem.

1. Water evaporates from the moist cell walls in the mesophyll and exits through open stomata.
2. Evaporation lowers water potential in the mesophyll, pulling water out of the xylem in the leaf veins.
3. Cohesion between water molecules (hydrogen bonding) holds the water column together; the pull is transmitted down the continuous xylem from leaves to roots.
4. Adhesion between water and the lignified xylem walls supports the column against gravity.
5. Water enters root hair cells by osmosis from the soil to replace the water drawn upward.

Factors increasing transpiration: light, high temperature, low humidity, wind. Factors decreasing it: stomatal closure, waxy cuticles, sunken stomata in xerophytes.

Gas exchange in animals

Animals concentrate exchange at specialised surfaces with high SA:V, thin walls and good blood supply.

Lungs (mammals, including humans). Bronchi branch into bronchioles ending in millions of alveoli. Each alveolus is a single-cell-thick sac wrapped in capillaries. Total exchange surface around 70 square metres. O2 dissolves in the moist alveolar lining and diffuses across the alveolar and capillary walls into red blood cells, where it binds haemoglobin. CO2 diffuses the opposite way.

Gills (fish). Stacks of filaments, each carrying many lamellae. Water flows over the lamellae in the opposite direction to blood flow inside (counter-current exchange), maintaining a steep oxygen gradient along the whole length of the lamella.

Tracheal system (insects). Air enters through spiracles and travels through branching tracheae and tracheoles directly to tissues. No blood is involved in gas transport; the haemolymph carries nutrients only.

Internal transport in animals

Open circulatory systems. A heart pumps blood (haemolymph) into the haemocoel, bathing tissues directly. Low pressure, slow flow. Adequate for small, slow-moving animals (most arthropods, most molluscs).

Closed circulatory systems. Blood is confined to vessels (arteries, capillaries, veins) and pumped at high pressure. Supports higher metabolic rates. Found in vertebrates, cephalopods and annelids.

The human circulatory system. A double closed circulation.

• Pulmonary circuit. Right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs, returns oxygenated blood via the pulmonary vein to the left atrium.
• Systemic circuit. Left ventricle pumps oxygenated blood through the aorta to the body, returns deoxygenated blood via the venae cavae to the right atrium.
• Four chambers. Right atrium, right ventricle, left atrium, left ventricle. Tricuspid and bicuspid (mitral) valves prevent backflow within the heart; semilunar valves at the artery exits prevent backflow into the ventricles.
• Vessel types. Arteries (thick muscular walls, high pressure, away from heart), capillaries (one cell thick, site of exchange), veins (thinner walls, low pressure, valves, return blood to heart).

Common traps

Calling phloem one-way. Phloem is bidirectional; xylem is one-way upward.

Forgetting cohesion or adhesion. QCAA mark schemes for cohesion-tension answers require both.

Reversing alveolar wall thickness. Alveolar and capillary walls are each one cell thick to minimise diffusion distance.

Confusing pulmonary artery and vein with their oxygen content. The pulmonary artery carries deoxygenated blood (away from the heart); the pulmonary vein carries oxygenated blood (towards the heart). The exception to the "arteries carry oxygenated, veins deoxygenated" rule.

Cross-link to Year 12 assessment

Exchange and transport return in Unit 2 (the kidney's transport surfaces in osmoregulation; the role of the circulatory system in delivering immune cells and hormones), in Unit 3 IA1 data tests on photosynthesis and ecosystem productivity, and in Unit 3 IA2 student experiments measuring transpiration rates as a function of light, temperature or humidity.

In one sentence

Plants exchange gases through stomata and move water upward through xylem under cohesion-tension generated by transpiration while sugars are translocated through phloem by pressure flow; animals exchange gases at folded thin-walled surfaces (alveoli, gills, tracheoles) and circulate blood through open or closed systems, with humans using a double closed circulation through a four-chambered heart.