B3.1 Gas exchange

Gas exchange is the passive diffusion of respiratory gases between an organism and its environment: oxygen is acquired for aerobic respiration and carbon dioxide is removed.

A small unicellular organism has a high surface area-to-volume ratio and every part is close to its surface, so diffusion across its membrane can meet demand. As organism size increases, SA:V decreases and the distance from interior cells to the exterior increases.

Large multicellular organisms therefore need specialized exchange surfaces plus transport systems. The exchange surface shortens the diffusion path; ventilation and circulation maintain gradients and connect the surface to internal cells.

Oxygen can diffuse directly into a small unicellular eukaryote, but in a mammal it first crosses an alveolar surface and is then carried by blood to cells far from the exterior.

Gas exchange is the crossing of a surface. Ventilation moves air, circulation transports gases inside the organism, and cell respiration consumes oxygen and produces carbon dioxide.

A good gas-exchange surface has a large area, a short diffusion distance and a moist barrier that gases can dissolve in.

A large area provides more parallel routes, thinness shortens travel time and moisture allows gas molecules to enter solution before crossing cells. Maintaining a gradient completes the design.

Evaluate a surface using: area; thickness; moisture; permeability; and gradient maintenance.

Alveoli combine a huge surface, one-cell-thick epithelium and moist lining, so oxygen can diffuse rapidly into nearby capillaries.

Large area alone is insufficient if the barrier is thick or the gradient is lost.

Animal gas-exchange surfaces maintain steep concentration gradients by continuously renewing the external medium and transporting gases away or toward the internal side.

Dense networks of blood vessels give a large contact area, and continuous blood flow brings oxygen-poor blood to the surface while carrying oxygenated blood away. This prevents the two sides from approaching equilibrium.

Ventilation renews air over lung surfaces and water over gill surfaces. Together, ventilation and circulation keep oxygen higher in the environment than in incoming blood and carbon dioxide higher in incoming blood than in the environment.

Fresh alveolar air has a higher oxygen partial pressure than venous blood arriving in adjacent capillaries; continuous airflow and blood flow preserve this difference while oxygen diffuses.

Diffusion follows a gradient but does not maintain it. Without ventilation and continuous flow, net exchange slows as concentrations approach equilibrium.

Mammalian lungs use branching airways, many alveoli, thin epithelium and dense capillaries to maximize gas exchange.

Branching distributes air, alveoli create area, the thin alveolar-capillary barrier shortens diffusion distance and blood flow carries gases away. Elastic tissue and surfactant support repeated ventilation.

Link adaptation to function: bronchioles distribute; alveoli add area; thin walls shorten distance; capillaries maintain gradients.

A red blood cell passing through an alveolar capillary encounters oxygen-rich air across a very thin moist barrier.

Lung adaptations work together; naming one feature without its mechanism does not explain efficient exchange.

Lung ventilation results from muscles changing thoracic volume, which changes pressure relative to the atmosphere and causes air to flow.

During inspiration, the diaphragm contracts and flattens while external intercostal muscles contract, moving the ribs up and out. Thoracic volume increases, pressure falls below atmospheric pressure and air enters.

During quiet expiration these muscles relax and elastic recoil lowers thoracic volume, raising pressure so air leaves. During forced expiration, internal intercostals pull the ribs down and in while abdominal muscles push the diaphragm upward.

Sequence for inhalation: diaphragm contracts + ribs move up/out → thoracic volume increases → intrapulmonary pressure decreases → air flows into the lungs down the pressure gradient.

The diaphragm does not pull air directly. It changes thoracic volume; the resulting pressure difference moves air. Abdominal muscles are especially important in forced, not quiet, expiration.

A spirometer records air volume moved over time. Tidal volume is the air moved in a normal breath; inspiratory and expiratory reserve volumes are the additional amounts moved by maximal inspiration or expiration.

On a trace, vertical differences represent volume and the horizontal axis represents time. Identify the normal peak-to-trough change for tidal volume, then measure from a normal limit to the corresponding maximal limit for each reserve.

Measure the vertical distance from maximal inspiration to maximal expiration to obtain vital capacity; it should equal the sum of the three component volumes measured from the same calibrated trace.

Simple spirometry cannot directly measure residual volume because that air never leaves the lungs. Do not confuse a spirometer with a respirometer, which measures aspects of respiration rather than lung ventilation.

A leaf provides short internal routes for carbon dioxide and oxygen while limiting uncontrolled water loss through its exposed surface.

The transparent epidermis protects the leaf, and its waxy cuticle reduces evaporation. Stomata form adjustable pores; guard-cell turgor changes their aperture to balance carbon-dioxide entry with water-vapour loss.

Spongy mesophyll contains connected air spaces that expose large moist cell surfaces to gases. Veins deliver water needed by mesophyll cells and transport products away, while stomata connect the internal air spaces to the atmosphere.

When guard cells open a stoma in light, carbon dioxide diffuses through the pore and air spaces to photosynthesizing mesophyll, but water vapour can diffuse out along the same route.

The cuticle is a barrier that reduces water loss rather than the main gas-entry route. Stomatal opening is regulated, so leaves do not maximize gas exchange continuously.

A plan diagram of a dicot leaf transverse section shows the relative distribution of tissues from upper to lower surface, without drawing individual cells.

The usual sequence is cuticle and upper epidermis → palisade mesophyll → spongy mesophyll with air spaces → lower epidermis with stomata. Vascular bundles lie within the mesophyll, with xylem generally nearer the upper surface and phloem nearer the lower.

Use clear single lines, preserve relative layer thickness and label tissue regions. Include cuticle, upper and lower epidermis, palisade and spongy mesophyll, air spaces, a vascular bundle with xylem and phloem, and stomata/guard cells in the epidermis.

Carbon dioxide entering a lower-surface stoma follows connected spongy-mesophyll air spaces to photosynthetic cells, while xylem in a nearby vein supplies water.

A plan diagram records tissue distribution, not fine cellular detail: do not shade, sketch every chloroplast or replace relative positions with a list of structures.

Transpiration is water loss from a plant: water evaporates from moist mesophyll cell walls and the vapour diffuses through leaf air spaces and out through stomata.

Gas exchange exposes moist internal surfaces to the atmosphere, so stomatal opening for carbon dioxide entry also permits water-vapour loss. A steeper water-vapour gradient or more open stomata increases the rate.

Higher temperature increases evaporation; lower humidity steepens the vapour gradient; wind removes the humid boundary layer; light commonly promotes stomatal opening. Water stress can close stomata and reduce transpiration.

A warm, dry, windy leaf usually transpires faster than a cool leaf in still, humid air because evaporation is faster and the external boundary layer is continually replaced.

A potometer measures water uptake as an estimate, not transpiration directly; water may also be used in growth or photosynthesis. State which factor changes evaporation, gradient or stomatal aperture.

Stomatal density is the number of stomata per unit leaf area, determined from a micrograph or a leaf-surface cast with a calibrated field of view.

Count only stomata within a known sampled area and divide by that area. Use a consistent boundary rule so stomata touching an edge are not counted twice across adjoining fields.

Repeat counts in several randomly or systematically selected fields from the same leaf surface, calculate each density and report a mean; repeat across leaves when comparing plants.

One field may not represent a biologically variable leaf. Replication increases reliability, while magnification alone is insufficient unless the real field area has been calibrated.