3.3.2 Gas exchange

Four organism groups solve the same gas exchange problem in four different ways. Insects pipe air directly to tissues. Fish flow water and blood in opposite directions. Plants open and close their leaves to the atmosphere. Mammals build a vast folded internal surface and pump air in and out of it. Each system follows the same three-feature template, built differently because each body's constraints differ.

Insects deliver air directly to respiring tissues through branching tracheae.

Insects carry no respiratory pigment in haemolymph and do not use blood for gas transport. Spiracles along the thorax and abdomen open into tracheae, which branch into finer tracheoles that reach individual respiring cells. The lumen is air-filled, so diffusion is much faster than it would be through tissue fluid.

Write diffusion as the named mechanism, not gas exchange. So gas exchange can occur scores zero on the explanation mark; AQA require so oxygen can diffuse to cells.

Pitfall — Insect tracheal answers — three traps

Insect tracheal answers — three traps.

Don't name blood. Haemolymph has no transport role in insects; naming it caps the mark at 2 regardless of the rest of the answer.

Don't name spiracles as an efficient-exchange adaptation. Spiracles are gateways with a separate water-conservation role; the credited adaptations are the tracheoles.

Don't drift into water-conservation features. Waxy cuticle and closeable spiracles belong to a different topic; stay on branching, thin walls, air-filled lumen, mass flow.

The tracheal architecture only works while every cell sits close to a tracheole. As body size grows, diffusion distances through the air-filled tubes start to become limiting, which is one proposed reason insects do not reach vertebrate body sizes.

Fish gills use counter-current flow to extract oxygen from water.

Bony fish carry four pairs of gills, each gill arch lined with rows of filaments, each filament covered with small projections called lamellae. The lamellae are the actual exchange surface; together they present an enormous total surface area to the water that is drawn continuously across them by ventilation.

Write slower rate of gas exchange, or slower diffusion. Less diffusion and less gas exchange are explicit rejects; less is a quantity, AQA require a rate.

1. Blood and water flow across each lamella in opposite directions.
2. The concentration gradient for oxygen is maintained along the entire length of the lamella, because water that has already given up some oxygen still has more than the blood it is now passing, so diffusion continues.
3. The fish extracts a substantially higher fraction of the available oxygen than parallel flow would allow; in bony fish, above 80% is reported.

Counter-current needs both halves: opposite directions, and the gradient maintained along the full length of the lamella. Either alone scores half the marks; the maintained-gradient half is the more commonly dropped of the two.

On justify-from-data questions, the stem often gives oxygen concentrations measured along the gill. The credited answer names that blood leaving the gill carries higher oxygen than water leaving the gill, which is the data signature of counter-current flow. The mechanism prose and the data answer share vocabulary but score under different mark points.

Plants exchange gases through stomata and the air-space network inside the leaf.

Plants need carbon dioxide for photosynthesis in the light, and exchange oxygen and carbon dioxide for respiration at all times. The architecture has to solve two problems at once: deliver enough gas to the photosynthesising tissue, and limit water loss to the atmosphere. Small adjustable pores and a large interior surface solve both halves.

Write mesophyll-cell surface area. The whole-leaf surface area answer is rejected; the credit is for the surface area each individual mesophyll cell presents to the interior air space.

On slow-growth questions where the stem says use your knowledge of gas exchange, the credited chain runs: stomata close, so less carbon dioxide enters, so less photosynthesis, so less growth. The water-as-reactant-for-photosynthesis route is biochemically defensible but scores zero here; the stem instruction selects the route.

Write less photosynthesis, not no photosynthesis. Closing stomata reduces but does not eliminate carbon dioxide entry; no photosynthesis is an explicit reject.

Mammalian airways condition the air, and alveoli build a vast thin exchange surface.

Air passes through the nose, the trachea, the bronchi, and the bronchioles before it reaches the alveoli. The airway wall is layered: cartilage on the outside, smooth muscle and elastic fibres in the middle, ciliated epithelium with goblet cells on the inside. Each tissue has a working role, and the proportions shift along the airway as cartilage thins and smooth muscle becomes more dominant toward the smaller bronchioles.

Write bronchiole for the smallest human airway, not tracheole. Tracheoles are an insect structure; using tracheoles in a human gas-exchange answer is rejected.

At the alveoli, the architecture trades absolute size for multiplied total area. There are roughly 300 million small alveoli, collectively around 70 m² of exchange surface. The alveolar wall is one cell thick (squamous epithelium), and the surrounding capillary wall is also one cell thick. The diffusion pathway from alveolar air to capillary blood is two cell layers.

Ventilation creates the pressure differences that move air in and out of the lungs.

The alveolar surface would self-limit without a way to renew the gas composition of the alveolar air. Ventilation is the bulk-flow movement of air in and out of the lungs, driven by pressure differences across the thoracic cavity. Inspiration is active; expiration at rest is largely passive, with elastic recoil contributing to the pressure rise.

1. The diaphragm contracts and flattens from its resting dome shape.
2. The external intercostal muscles contract, pulling the ribcage upward and outward.
3. The volume of the thoracic cavity increases as a result of the two movements combined.
4. Pressure inside the lungs falls below atmospheric pressure, and air flows down the pressure gradient into the lungs.

Write thoracic cavity for the volume that changes. Chest and chest cavity are explicit rejects on this section's questions.

Ribs move upward and outward; ribs do not contract or relax. The verbs apply to muscles, not bones. And rib movement is caused by the intercostal muscles, not by the diaphragm; diaphragm contracts so the ribs move up is rejected.

Expiration at rest is largely passive. The external intercostals relax, the ribcage moves down and in, and the diaphragm relaxes back to its dome shape, pushed upward by abdominal organ pressure. Thoracic cavity volume decreases; lung pressure rises above atmospheric; air flows outward. Elastic recoil of lung tissue contributes without active muscle work.

Key terms

• gas exchange
• diffusion
• short diffusion distance
• surface area
• counter-current
• gradient
• bronchiole
• alveolar epithelium
• thoracic cavity
• stomata close
• photosynthesis