Exchange surfaces and transport in plants (B2.1)

This topic links the mathematics of surface area to volume ratio (SA:V) to real biological structures — the lungs, the gut, and leaves. It also covers the two transport tissues in plants. Expect a 6-mark on adaptations of exchange surfaces and a calculation or graph on transpiration.

Surface area to volume (SA:V) ratio

As an organism gets bigger, its volume increases faster than its surface area. Small organisms (single-celled) have a high SA:V — diffusion across the surface is fast enough. Large organisms have a low SA:V — diffusion alone cannot supply every cell quickly enough.

Cube example:

Side length	SA (cm²)	Volume (cm³)	SA:V
1 cm	6	1	6:1
2 cm	24	8	3:1
4 cm	96	64	1.5:1

Large organisms therefore need specialised exchange surfaces and transport systems.

Adaptations of exchange surfaces (general rules)

To maximise exchange rate, exchange surfaces are:

• Large surface area — more particles can move across at once.
• Thin — short diffusion distance.
• Moist — gases dissolve and diffuse faster.
• Good blood (or fluid) supply — maintains concentration gradients.

The alveoli (lung exchange surface)

The lungs contain around 700 million alveoli — tiny air sacs.

Adaptations:

• Folded structure → huge surface area (~70 m²)
• Very thin walls (one cell thick) → short diffusion path
• Moist surfaces → gases dissolve easily
• Rich capillary network → blood constantly replenished → steep O₂/CO₂ gradient maintained

Gas exchange: O₂ diffuses from alveolar air into blood; CO₂ diffuses from blood into alveolar air.

The villi (gut exchange surface)

The small intestine's inner lining is covered with villi — finger-like projections, each covered in microvilli (forming the "brush border").

Adaptations:

• Large surface area (villus folds + microvilli)
• One-cell-thick epithelium → short diffusion distance
• Good blood supply per villus → rapid removal of absorbed nutrients → maintained gradient
• Lacteals (lymph vessels) for fat absorption

Digested glucose and amino acids → absorbed by diffusion and active transport into blood capillaries. Fatty acids and glycerol → into lacteals.

Transport in plants

Xylem — water and mineral transport

• Carries water and dissolved mineral ions from roots to leaves (and the rest of the plant), in one direction only.
• Xylem vessels are dead, hollow, lignified tubes stacked end-to-end.
• Movement driven by transpiration (evaporation of water from leaves).

Phloem — sugar transport

• Carries dissolved sugars (mainly sucrose) made by photosynthesis from leaves to the rest of the plant.
• Movement can be in both directions (up and down).
• Translocation is the term for this movement.
• Phloem cells are alive (sieve cells + companion cells).

Transpiration

Transpiration is the evaporation of water from leaves. Water evaporates from the mesophyll cells into the air spaces, then exits through stomata (pores).

Factors that increase transpiration rate:

1. Higher temperature — more kinetic energy → faster evaporation.
2. Lower humidity — steeper water vapour concentration gradient → faster evaporation.
3. Higher light intensity — stomata open wider in light (for CO₂ entry for photosynthesis).
4. Increased wind speed — removes water vapour from around the leaf → steeper gradient.

Required practical: the potometer

A potometer measures the rate of water uptake (as a proxy for transpiration rate). A bubble in a capillary tube moves as the plant draws up water.

⚠ The potometer measures water UPTAKE, not directly evaporation, but the two are closely linked (uptake ≈ transpiration + small amount of growth).

Common Gateway-paper mistakes

1. Confusing xylem (water) with phloem (sugar).
2. Saying phloem is dead — xylem is dead; phloem is alive.
3. Forgetting that increased wind speed increases transpiration.
4. Thinking stomata open at night — they generally close to reduce water loss when not photosynthesising.
5. Misremembering SA:V: "bigger organism = lower SA:V ratio" (not higher).