B3.1 Gas exchange

Gas exchange is the diffusion of oxygen, carbon dioxide or other gases between an organism and its environment.

Cells need oxygen for aerobic respiration and must remove carbon dioxide. Diffusion is effective only across a surface where a gradient exists and the path is short enough.

Check: gas; source and destination; concentration or partial-pressure gradient; distance crossed.

Oxygen moves from alveolar air into blood while carbon dioxide moves in the opposite direction.

Gas exchange is not the same as ventilation or circulation: those processes help maintain the gradient but are not the crossing event.

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.

Net gas diffusion continues when ventilation, blood flow or photosynthetic uptake continually removes gas from one side of the surface.

If both sides approach the same concentration, random movement continues but net transfer falls. Renewal of air or fluid restores the difference that drives net movement.

Trace the chain: gas enters; a gradient drives diffusion; ventilation, circulation or metabolism renews the gradient.

Fresh air reaching an alveolus keeps oxygen higher in the air than in incoming blood, sustaining oxygen uptake.

A concentration gradient is not maintained by diffusion itself; another flow or reaction must renew it.

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.

Ventilation moves fresh air into and stale air out of the lungs, renewing alveolar concentration gradients.

Inspiration increases thoracic volume and lowers pressure so air enters; expiration reverses the pressure difference. Elastic recoil and muscle action return the lungs toward resting volume.

Trace: thoracic volume change; pressure change; air movement; refreshed alveolar gradient.

When the diaphragm contracts and flattens, thoracic volume rises, pressure falls and air flows into the lungs.

Air moves because of pressure differences, not because the diaphragm pushes air directly.

Spirometry records changes in air moved during breathing, allowing volumes such as tidal and vital capacity to be estimated.

A spirometer measures the volume entering or leaving the apparatus over time. It cannot directly measure residual volume that remains after maximal expiration, so some capacities need indirect methods.

Interpret a trace by identifying baseline, peaks and troughs, then match the measured change to the volume definition.

The difference between a maximal inhalation and maximal exhalation trace estimates vital capacity, not total lung capacity.

A larger trace does not automatically mean healthier lungs; effort, technique and the exact volume being measured matter.

Leaves use stomata and internal air spaces to exchange carbon dioxide and oxygen, while a waxy cuticle and guard cells limit water loss.

Spongy mesophyll air spaces expose moist cell surfaces to gases. Stomata open when carbon dioxide uptake is valuable but closing reduces transpiration, creating a real trade-off.

Explain a leaf adaptation by linking: route for gas; photosynthetic demand; water-loss cost.

Opening stomata in daylight can increase carbon-dioxide entry for photosynthesis but also increases water-vapor loss.

Stomata do not maximize gas exchange all the time; guard-cell control balances carbon gain against dehydration.

In a dicot leaf, air spaces and mesophyll arrangement create short routes from stomata to photosynthetic cells, while vascular tissue supplies and removes materials.

Palisade cells near the upper surface capture light; spongy mesophyll spaces allow gases to spread internally; veins connect the leaf to water and sugar transport.

Read a leaf cross-section by mapping: surface protection; photosynthetic tissue; air spaces; vascular bundle; stomatal route. Then link each position to movement.

Carbon dioxide entering a lower-surface stoma diffuses through spongy-air spaces before reaching mesophyll cells, where photosynthesis consumes it.

A tissue’s position is not its function by itself; explain how its structure changes a diffusion or transport path.

Transpiration is the loss of water vapor from leaves, mainly through stomata, and it helps create a pull that draws water upward through xylem.

Evaporation lowers water potential in leaf air spaces. Cohesion between water molecules transmits tension through the continuous xylem column, linking leaf loss to root uptake.

Trace: evaporation; lower leaf water potential; cohesion/tension; upward xylem flow.

On a dry, windy day, faster evaporation can increase transpiration and the pull on the xylem stream.

Transpiration is water loss, not water absorption; roots supply water but stomata regulate the major exit.

Stomatal density is the number of stomata per unit leaf area; comparing densities can indicate potential routes for gas exchange and water loss.

More stomata can increase the maximum conductance of a leaf, but actual exchange also depends on aperture, humidity, wind, light and gradients.

Compare samples using: count; same area; same surface; then separate potential density from actual opening.

A leaf with 20 stomata per mm² has twice the density of one with 10 per mm², but it may lose less water if most pores are closed.

Stomatal density is not a direct measurement of photosynthesis or transpiration rate.

Haemoglobin binds oxygen reversibly in lungs and releases it in tissues, with its subunits enabling efficient loading and unloading.

In the lungs, high oxygen partial pressure favors binding. In respiring tissues, lower oxygen and altered conditions favor release, so haemoglobin transports rather than permanently stores oxygen.

Interpret a binding claim by checking: oxygen partial pressure; affinity; loading site; unloading tissue. These conditions change along the circulation.

Blood leaving the lungs carries more oxyhaemoglobin than blood arriving from oxygen-consuming muscle because the lungs provide a higher oxygen partial pressure.

High haemoglobin affinity is not always best: oxygen must also be released where cells need it.

The Bohr shift is the reduced oxygen affinity of haemoglobin when carbon dioxide rises and pH falls, promoting oxygen release in respiring tissues.

Respiration produces carbon dioxide, which forms carbonic acid and increases H⁺. These changes stabilize the lower-affinity form of haemoglobin, shifting the dissociation curve right.

Trace: more respiration; CO₂ and H⁺ rise; affinity falls; unloading increases.

During exercise, muscle CO₂ production rises, so haemoglobin releases more oxygen at the same tissue partial pressure.

The Bohr shift changes affinity, not the amount of haemoglobin or the oxygen concentration in the air.

An oxygen dissociation curve plots haemoglobin saturation against oxygen partial pressure and shows how readily oxygen binds or is released.

The sigmoidal shape reflects cooperative binding: binding one oxygen changes haemoglobin shape and makes later binding easier. Curve position reveals affinity; a right shift means lower affinity.

Read the graph by checking: partial pressure; saturation; steep unloading region; curve shift.

A right-shifted curve reaches lower saturation at a given oxygen pressure, so it can unload more oxygen in active tissue.

A curve does not show oxygen flow or blood volume by itself; it shows binding relationship under specified conditions.