aliases:

Gas Exchange

AQA spec ref: 3.3.2 - Gas exchange

Gas exchange is the process by which organisms obtain oxygen for aerobic respiration and remove carbon dioxide. The mechanisms used vary enormously across organisms - from simple diffusion across a plasma membrane in single-celled organisms, to elaborate specialised organs in large multicellular organisms. In every case, the underlying principle is the same: maximising the rate of diffusion by maximising surface area, minimising diffusion distance, and maintaining steep concentration gradients. See Surface Area to Volume Ratio.

Single-Celled Organisms

In organisms such as Amoeba and bacteria, the cell is small enough that the surface area to volume ratio is large. Every cell is close to the external environment, so diffusion distances are short. Gases diffuse directly across the plasma membrane - no specialised surface is needed. The concentration gradient is maintained by continuous consumption of O₂ in respiration and production of CO₂.

Gas Exchange in Insects

Insects are terrestrial and have a waterproofed exoskeleton (cuticle) that prevents water loss - but this also blocks gas exchange across the body surface. Instead, insects have evolved an internal network of air-filled tubes: the tracheal system.

Structure

• Spiracles - small openings in the exoskeleton (along the thorax and abdomen) that can be opened or closed by valves (sphincter muscles). Opening allows gas exchange; closing reduces water loss.
• Tracheae - large, reinforced air tubes supported by chitinous rings (spirals of chitin) that prevent collapse. They branch repeatedly.
• Tracheoles - fine, fluid-filled terminal branches (diameter ~1 μm) that penetrate directly between cells and even into cells in some cases. This is where actual gas exchange occurs.

Mechanism

Oxygen diffuses from the air down the tracheal system along a concentration gradient. CO₂ diffuses in the opposite direction. In resting insects, ventilation is minimal - diffusion alone is sufficient. During activity, abdominal pumping movements ventilate the tracheae, increasing the flow of air and maintaining steeper gradients.

Adaptations

• Short diffusion distance - tracheoles deliver O₂ directly to metabolically active cells, bypassing the blood entirely (insect blood - haemolymph - does not transport O₂)
• Large surface area - the branching tracheole network creates a huge combined surface area
• Fluid at tracheole tips - gases dissolve in the tracheole fluid before diffusing across the cell membrane. During intense activity, lactic acid reduces the water potential of the fluid → water is drawn out by osmosis → more air penetrates further into the tracheole → O₂ closer to cells

Gas Exchange in Fish

Fish live in water and extract dissolved O₂ using gills. The challenge is that water contains far less dissolved O₂ than air (~7 mg/L vs 210 mg/L in air), so the exchange system must be highly efficient.

Gill Structure

• Gill arches - bony/cartilaginous supports inside the gill chamber
• Gill filaments - thin, flat projections from each gill arch; run perpendicular to water flow
• Gill lamellae (secondary lamellae) - tiny plate-like projections off each filament; enormously increase surface area; walls are one cell thick; closely associated with capillaries

Counter-Current Flow

The key adaptation of fish gills is counter-current exchange: water flows over the lamellae in one direction, while blood flows through the capillaries in the opposite direction. This means:

• Blood that has already loaded much O₂ meets water that still has a relatively high O₂ concentration→can continue to load more O₂
• Blood just entering the lamellae meets water that has already given up some O₂ but still has more than the blood → loading begins immediately

The result: blood can extract up to 80 - 90% of the dissolved O₂ from the water. A parallel flow system (blood and water flowing in the same direction) would equilibrate quickly - the maximum extraction would be ~50%.

Ventilation

Water is drawn in through the mouth and expelled through the operculum (gill cover). The pressure differential between the mouth cavity and the opercular cavity maintains a near-continuous flow of water over the gills, sustaining the concentration gradient.

Gas Exchange in Dicotyledonous Plants

Plants exchange O₂ and CO₂ for photosynthesis and respiration. The leaf is the primary organ of gas exchange.

Leaf Structure for Gas Exchange

• Stomata - pores in the epidermis (mostly lower surface in mesophytes) formed between two guard cells. Stomata open during the day (when CO₂ is needed for photosynthesis) and close at night to reduce water loss.
• Air spaces in the spongy mesophyll - interconnected air spaces form a large internal surface area for gas exchange. Cells lining these spaces have a moist surface through which gases dissolve and diffuse.
• Thin cell walls and short diffusion distances - mesophyll cells are close to the air spaces; CO₂ has only a short distance to diffuse to reach the chloroplast stroma.

Guard Cell Mechanism

Guard cells control stomatal aperture by changing their turgor pressure:

Opening (during the day):

• Light stimulates active transport of K⁺ ions into guard cells
• Water potential of guard cells decreases→water enters by osmosis→guard cells become turgid
• Due to their uneven thickening (inner wall thicker than outer), turgid guard cells bow outward→stoma opens

Closing (at night / water stress):

• K⁺ leaves guard cells → water potential rises → water leaves by osmosis → guard cells become flaccid → stoma closes

ABA (abscisic acid) is produced under water stress → triggers K⁺ efflux from guard cells → stomata close → reduces transpiration.

Pathway of CO₂ into a Leaf

Stomata open → CO₂ diffuses in along concentration gradient → dissolves in moisture on cell surface → diffuses into mesophyll cells → used in Calvin cycle in chloroplast stroma. O₂ produced by photosynthesis diffuses in reverse.

Gas Exchange in Mammals

Mammals are large endotherms with high metabolic rates - they require large amounts of O₂ and produce large amounts of CO₂. The mammalian lung is the specialised organ of gas exchange.

Lung Structure

• Trachea→bronchi→bronchioles→alveolar ducts→alveoli
• Alveoli - tiny air sacs (~200 - 300 μm diameter); approximately 300 - 500 million in human lungs → total surface area ~70 m²
• Type I pneumocytes - very thin, flat cells forming the alveolar wall; ~0.2 μm thick. This is the gas exchange surface.
• Type II pneumocytes - cuboidal cells that produce surfactant - a phospholipid mixture that lowers surface tension of the alveolar fluid, preventing the alveoli from collapsing on exhalation
• Capillary network - dense network of capillaries surrounds each alveolus; walls are one endothelial cell thick; ~0.1 μm. Total diffusion distance (type I cell + capillary wall) = ~0.3 - 0.5 μm

Adaptations of Alveoli

• Enormous surface area - 300 - 500 million alveoli provide ~70 m² (the size of a tennis court)
• Very short diffusion distance - alveolar epithelium + capillary endothelium together ~0.3 μm
• Steep concentration gradient - maintained by ventilation (breathing) and perfusion (blood flow):
• Ventilation brings fresh air (high O₂, low CO₂) continuously into alveoli
• Blood flow removes O₂-rich blood and brings deoxygenated blood continuously
• Moist surface - gases dissolve in the fluid lining before diffusing
• Surfactant - prevents alveolar collapse; without it (e.g. in premature babies lacking surfactant - respiratory distress syndrome), alveoli collapse on exhalation

Ventilation (Breathing)

Ventilation maintains the concentration gradient at the alveolar surface.

Inspiration (active):

• External intercostal muscles contract→ribs move up and out
• Diaphragm contracts→flattens downward
• Thoracic volume increases→pressure falls below atmospheric→air flows in

Expiration (passive at rest):

• External intercostal muscles relax→ribs fall by gravity
• Diaphragm relaxes→domes upward
• Thoracic volume decreases→pressure rises above atmospheric→air flows out

Forced expiration uses internal intercostal muscles and abdominal muscles actively.

Lung Volumes

• Tidal volume - volume of air in one normal breath (~500 mL)
• Vital capacity - maximum air in one breath (in - out), typically 3 - 5 L
• Residual volume - air remaining after maximum exhalation (~1.5 L); prevents alveolar collapse

Summary

AQA Exam Tips

• Counter-current in fish: always explain why it's more efficient than parallel flow. "Counter-current maintains a diffusion gradient along the entire length of the lamella; parallel flow would equilibrate quickly, allowing only ~50% extraction."
• Fick's law: every exchange surface question is ultimately about maximising SA, minimising thickness, maintaining concentration gradient. Frame your answers around these three variables.
• Surfactant function: "reduces surface tension of fluid lining alveoli → prevents alveoli collapsing on exhalation → maintains large surface area for gas exchange." Don't just say it "helps gases dissolve."
• Guard cells: examiners often ask for the mechanism in detail - include K⁺ transport, osmosis, turgor pressure, and the uneven cell wall thickening causing the curved opening.
• Type I vs Type II pneumocytes: type I = thin, gas exchange; type II = cuboidal, surfactant production. Know both types and which is which.
• Insect tracheal system: key point is that O₂ is delivered directly to cells - it does not rely on blood transport. This bypasses the need for a respiratory pigment (haemoglobin) in insects.