VCE Biology (Year 11) · Unit 1 — Cells and Systems · 11m read

Plants and Animals — Gas Exchange and Transport

Ever wondered why a leaf's underside looks totally different from the top? Plants and animals need to swap gases with their environment, but they've evolved wildly different structures to do it—stomata in leaves versus lungs in your chest. You'll explore how these surfaces are actually built the same way for maximum efficiency, and understand why getting oxygen in and carbon dioxide out is literally what keeps every living thing alive.

Learning objective: Compare structures for gas exchange and transport in plants and animals.

By the end of this lesson you can:

• I describe stomata and the human respiratory system.
• I compare plant and animal transport.
• I link structure to efficient exchange.

Transport systems are delivery networks. Blood vessels and xylem/phloem are roads delivering oxygen, water and nutrients to every cell.

Efficient exchange in two kingdoms

REQUIREMENTS FOR EFFICIENT EXCHANGE:
• Large SURFACE AREA.
• Thin membrane (short diffusion distance).
• Moist environment.
• Steep concentration gradient maintained.
• Close proximity to transport system.

HUMAN RESPIRATORY SYSTEM:
• NOSE → PHARYNX → LARYNX → TRACHEA → BRONCHI → BRONCHIOLES → ALVEOLI.
• ALVEOLI: 500 million, total SA ~70m².
• Thin walls + rich capillary network = fast O₂/CO₂ exchange.

MECHANICS OF BREATHING:
• Diaphragm + intercostal muscles.
• Inhale: diaphragm flattens, chest expands, pressure drops → air in.
• Exhale: opposite.

PLANT GAS EXCHANGE:
• Through STOMATA on leaves.
• Guard cells open/close stomata.
• Opens during day: CO₂ in, O₂ out (photosynthesis).
• Opens at night: O₂ in, CO₂ out (respiration).
• Lenticels in stems also allow exchange.

HUMAN CIRCULATORY SYSTEM:
• Heart → arteries → arterioles → capillaries → venules → veins.
• Two circuits: pulmonary (to/from lungs) and systemic (to/from body).
• Blood carries O₂, CO₂, nutrients, hormones, heat.

PLANT TRANSPORT:
• XYLEM — water + minerals from roots to leaves (upward only).
• PHLOEM — sugars (products of photosynthesis) to rest of plant (both directions).
• Transport powered by TRANSPIRATION (water pull from leaves) and active processes.

COMPARISON:
• Both use branching networks + thin surfaces.
• Animals have a pump (heart); plants rely on pressure / transpiration.
• Plants lack an internal circulatory fluid like blood.

EXCHANGE IN SMALL ORGANISMS:
• Bacteria, protists → direct diffusion (high SA:V).
• No need for specialised systems.

SPECIALISED STRUCTURES:
• GILLS (fish) — thin plates, counter-current water flow.
• TRACHEAE (insects) — air tubes direct to tissues.

EVOLUTIONARY PRINCIPLE:
• Larger organisms need specialised exchange + transport systems.

Exchange principles

• Big SA: More exchange.
• Thin: Shorter distance.
• Gradient: Maintains flow.

Quick reference

| Organism | Structure | Function |
|---|---|---|
| Animals | Lungs/alveoli | Gas exchange |
| Animals | Heart and vessels | Circulation |
| Plants | Stomata | Gas exchange |
| Plants | Xylem | Water transport |
| Plants | Phloem | Sugar transport |

Worked example (Medium)

Compare how O₂ reaches working muscles in a human and chloroplasts in a plant leaf.

1. Human. Breathed in → alveoli → haemoglobin → blood → muscles → diffuses into cells.
2. Plant. Atmospheric O₂ enters through stomata → intercellular air spaces → diffuses into leaf cells.
3. Contrast. Humans use circulation + carrier proteins; plants rely on diffusion + intercellular spaces.

Answer: Both maximise exchange via surface area + short distance, but transport mechanisms differ.

Worked example (Hard)

A fish uses counter-current flow in its gills for gas exchange. Explain why this is more efficient than concurrent (parallel) flow and relate to the principles of efficient exchange.

1. Counter-current defined. In counter-current flow, water passes over the gill lamellae in the OPPOSITE direction to blood flow through the gill capillaries.
2. Gradient maintenance. Because the fluids flow in opposite directions, there is ALWAYS a concentration gradient for O₂ along the entire length of the exchange surface — O₂ continuously diffuses from water into blood.
3. Concurrent comparison. In parallel flow, the gradient would equalise partway along — diffusion would stop once concentrations match. Counter-current allows up to 80-90% O₂ extraction vs ~50% in concurrent.
4. Link to principles. Counter-current maximises the concentration gradient (one of the key principles of efficient exchange), alongside thin lamellae (short distance) and large gill surface area.

Answer: Counter-current flow maintains a continuous concentration gradient along the gill, enabling far greater O₂ extraction than parallel flow — a key adaptation for aquatic respiration.

Common mistakes

• Saying plants have no transport system Plants DO have a transport system: xylem (water, minerals upward) and phloem (sugars in both directions). They just lack a pump like the heart. Fix: Say "plants lack a circulatory pump" rather than "plants have no transport system".
• Confusing xylem and phloem Students swap which vessel carries which substance. Xylem = water/minerals (one-way up); phloem = sugars (both directions). Fix: Mnemonic: Xylem = eXits upward (water up); Phloem = Food (sugars).

Why insects breathe differently — the tracheal system

While mammals rely on lungs and blood to deliver oxygen, insects use a completely different strategy: the tracheal system. A network of air-filled tubes called tracheae branches throughout the insect's body, delivering oxygen directly to cells without using blood as an intermediary.

Air enters through openings called spiracles on the insect's exoskeleton. These can open and close to regulate gas exchange and reduce water loss. The tracheae branch into progressively smaller tubes called tracheoles, which are so fine they penetrate individual tissues — some even indent into muscle cells.

This system is incredibly efficient for small organisms because the diffusion distance is extremely short. However, it imposes a fundamental size limit on insects. Because the tracheal system relies on passive diffusion (with some active ventilation in larger species), it cannot deliver enough oxygen to very large bodies. This is why insects today are relatively small compared to vertebrates.

Interestingly, during the Carboniferous period (~300 million years ago), atmospheric oxygen levels reached 35% (vs 21% today). This higher O₂ concentration allowed diffusion to work over longer distances, enabling giant insects like *Meganeura* (dragonflies with 70cm wingspans). The tracheal system beautifully illustrates how exchange surface design limits and enables body size across evolution.

Practice questions

1. Plant xylem transports mainly:

   1. Sugars
   2. Oxygen
   3. Water and minerals
   4. Nitrogen gas

   (1 marks)

2. Why are alveoli adapted for efficient gas exchange?

   (3 marks)

Summary

• Describe stomata and the human respiratory system.
• Compare plant and animal transport.
• Link structure to efficient exchange.