What you'll learn
This topic examines how size affects biological efficiency and why multicellular organisms require specialised transport and exchange systems. You'll understand the mathematical relationship between surface area and volume, and how organisms have evolved adaptations to overcome the limitations imposed by increasing size.
Key terms and definitions
Surface area to volume ratio (SA:V) — the ratio comparing the outer surface of an organism to its total volume, which decreases as organisms get larger
Diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient
Exchange surface — a specialised surface adapted to allow efficient transfer of materials between an organism and its environment
Alveoli — tiny air sacs in the lungs providing a large surface area for gas exchange
Villi — finger-like projections in the small intestine that increase surface area for nutrient absorption
Concentration gradient — the difference in concentration between two areas, which drives diffusion
Mass transport system — a system (such as the circulatory system) that moves materials over large distances within organisms
Transpiration — the loss of water vapour from plant leaves through stomata
Core concepts
Surface area to volume ratio and cell size
Single-celled organisms like bacteria and amoeba have a large surface area compared to their volume. This high SA:V ratio means:
- Oxygen can diffuse across the cell membrane quickly enough to supply the entire cell
- Waste products like carbon dioxide can diffuse out efficiently
- Materials don't need to travel far to reach all parts of the cell
As organisms increase in size, their volume increases faster than their surface area. For a cube:
- If side length doubles from 1 cm to 2 cm
- Surface area increases by a factor of 4 (from 6 cm² to 24 cm²)
- Volume increases by a factor of 8 (from 1 cm³ to 8 cm³)
- SA:V ratio decreases from 6:1 to 3:1
Calculating SA:V ratio:
For a cube with side length a:
- Surface area = 6 × a²
- Volume = a³
- SA:V ratio = 6a² ÷ a³ = 6/a
This mathematical relationship explains why large multicellular organisms cannot rely on diffusion alone. Their relatively small surface area cannot supply enough oxygen or remove waste quickly enough for their large volume of cells.
Adaptations of exchange surfaces
Multicellular organisms have evolved specialised exchange surfaces that share common features:
Large surface area
- Increased by folding or having many small structures
- Examples: alveoli in lungs, villi in small intestine, root hairs in plants
Thin walls/short diffusion distance
- Often just one cell thick
- Reduces the distance molecules must travel
- Examples: alveolar walls are one cell thick, capillary walls are one cell thick
Good blood supply (in animals)
- Maintains steep concentration gradients
- Rapidly removes absorbed substances or delivers waste for removal
- Examples: capillary networks surrounding alveoli and villi
Moist surface (where appropriate)
- Allows gases to dissolve before diffusing
- Examples: alveolar lining, fish gills
Gas exchange in humans
The lungs are the site of gas exchange in humans. Air travels through:
- Trachea (windpipe)
- Bronchi (two branches, one to each lung)
- Bronchioles (smaller branches)
- Alveoli (air sacs where gas exchange occurs)
Alveolar adaptations:
- Approximately 300 million alveoli provide a surface area of about 70 m²
- Walls are one cell thick for short diffusion distance
- Surrounded by dense capillary networks
- Moist lining allows oxygen to dissolve
- Elastic tissue allows ventilation to maintain concentration gradients
Gas exchange process:
- Oxygen diffuses from alveolar air (high concentration) into blood capillaries (lower concentration)
- Carbon dioxide diffuses from blood (high concentration) into alveoli (lower concentration)
- Breathing movements (ventilation) maintain steep concentration gradients by bringing in fresh oxygen and removing carbon dioxide
Absorption in the small intestine
The small intestine is adapted for efficient absorption of digested food products:
Villi structure:
- Millions of finger-like projections increase surface area enormously
- Each villus contains a network of capillaries (for glucose and amino acids)
- Each villus contains a lacteal (lymph vessel for fatty acids and glycerol)
- Epithelial cells have microvilli forming a brush border, further increasing surface area
Absorption mechanisms:
- Simple sugars (glucose) and amino acids are absorbed into blood capillaries
- Fatty acids and glycerol are absorbed into lacteals
- Active transport is used alongside diffusion to maximize absorption
- Single cell layer ensures short diffusion distance
Exchange in plants
Root hair cells absorb water and mineral ions from soil:
- Long projections increase surface area in contact with soil
- Large permanent vacuole maintains water potential gradient
- Many mitochondria provide energy for active transport of mineral ions
Leaf structure for gas exchange:
- Broad, flat shape provides large surface area
- Thin structure means short diffusion distances
- Stomata (pores) allow gases to enter and exit
- Air spaces in spongy mesophyll allow gases to circulate to cells
- Guard cells control stomatal opening
Transpiration is the inevitable consequence of gas exchange in leaves:
- Water evaporates from moist cell surfaces inside the leaf
- Water vapour diffuses out through stomata
- This creates a "pull" that draws water up from roots through xylem
- Stomata close in hot, dry conditions to reduce water loss
Factors affecting transpiration rate:
- Temperature — higher temperature increases evaporation and diffusion rate
- Humidity — lower humidity increases concentration gradient, increasing rate
- Air movement — wind removes water vapour, maintaining steep gradient
- Light intensity — light causes stomata to open, increasing rate
Transport systems in multicellular organisms
Large organisms require mass transport systems because diffusion is too slow over long distances.
In animals (circulatory system):
The heart pumps blood through vessels to transport:
- Oxygen from lungs to respiring cells
- Carbon dioxide from cells to lungs
- Nutrients from small intestine to cells
- Urea from liver to kidneys
- Hormones from endocrine glands to target organs
This ensures rapid delivery over distances too large for diffusion alone.
In plants (vascular tissue):
Xylem transports water and minerals from roots to leaves:
- Dead cells form continuous tubes
- Thick, lignified walls provide support
- Transpiration pull and root pressure move water upward
Phloem transports dissolved sugars (translocation):
- From sources (usually leaves) to sinks (growing regions, storage organs)
- Contains living cells (sieve tube elements)
- Companion cells provide metabolic support
- Movement in both directions depending on season
Worked examples
Example 1: Calculating and comparing SA:V ratios
Question: A student investigates cells of different sizes using agar cubes as models.
Cube A has sides of 1 cm Cube B has sides of 3 cm
(a) Calculate the surface area to volume ratio for each cube. [4 marks] (b) Explain which cube would be more efficient at exchanging materials by diffusion. [2 marks]
Answer:
(a) Cube A:
- Surface area = 6 × 1² = 6 cm² [1]
- Volume = 1³ = 1 cm³ [1]
- SA:V ratio = 6:1 [1]
Cube B:
- Surface area = 6 × 3² = 54 cm² [1]
- Volume = 3³ = 27 cm³ [1]
- SA:V ratio = 54:27 = 2:1 [1]
(b) Cube A would be more efficient [1] because it has a larger surface area to volume ratio, meaning more surface area per unit volume for materials to diffuse across [1].
Example 2: Explaining adaptations of exchange surfaces
Question: The alveoli are the site of gas exchange in the lungs.
Describe three ways in which alveoli are adapted for efficient gas exchange. [3 marks]
Answer:
- Large surface area / many alveoli [1] — increases the area available for gas exchange
- Walls one cell thick [1] — provides short diffusion distance for oxygen and carbon dioxide
- Good blood supply / surrounded by capillaries [1] — maintains concentration gradient by bringing deoxygenated blood and removing oxygenated blood
Mark scheme note: Any three from: large surface area, thin walls, good blood supply, moist surface. Must link feature to function for full marks.
Example 3: Transpiration investigation
Question: A student investigated the rate of water uptake by a plant shoot using a potometer in different conditions.
| Condition | Distance bubble moved in 5 minutes (mm) |
|---|---|
| Normal room conditions | 45 |
| In front of a fan | 78 |
(a) Explain why water uptake increased when the plant was placed in front of a fan. [3 marks] (b) State one precaution the student should take when setting up the potometer. [1 mark]
Answer:
(a) Air movement / wind removes water vapour from around the leaf [1]. This maintains a steep concentration gradient / increases the concentration gradient between inside the leaf and the air [1]. Therefore the rate of diffusion / transpiration increases, so more water is drawn up [1].
(b) Ensure there are no air bubbles in the apparatus / seal all joints with petroleum jelly / cut the shoot underwater [1].
Common mistakes and how to avoid them
Confusing surface area with SA:V ratio — remember that as organisms get larger, surface area increases but SA:V ratio decreases. Always calculate the ratio, not just the surface area.
Stating diffusion happens "through" the blood — diffusion occurs into and out of the blood across exchange surfaces. Blood circulation is mass transport, not diffusion.
Writing that plants "breathe in" carbon dioxide — plants require carbon dioxide for photosynthesis, not respiration. All living cells respire and produce carbon dioxide, which diffuses out.
Confusing transpiration with translocation — transpiration is water loss through stomata; translocation is sugar transport through phloem. Learn which substance moves where.
Not linking structure to function in adaptation questions — always explain how the feature helps. "Large surface area" alone gets no marks; "large surface area increases rate of diffusion" earns credit.
Forgetting units in calculations — always include cm², cm³, or appropriate units when calculating surface area, volume, or ratios.
Exam technique for "B2: Scaling Up"
"Calculate" questions — show your working clearly. For SA:V calculations, write the formula, substitute numbers, and give the final ratio in simplest form. Even if your final answer is wrong, you can gain method marks.
"Explain" and "Describe" command words — "Describe" requires you to state features; "Explain" requires you to say why or how something happens. For 3-mark explanation questions, aim for a feature, the mechanism, and the effect.
Linking adaptations to efficiency — when asked about adaptations of exchange surfaces, use the key features (large surface area, thin walls, good blood supply, concentration gradient) and explicitly state how each increases the rate of exchange.
Drawing and labelling — if asked to draw gas exchange or absorption structures, include clear labels with leader lines. Annotations explaining function may earn additional marks, so read the question carefully.
Quick revision summary
Large organisms have small surface area to volume ratios, making diffusion insufficient for their needs. Specialised exchange surfaces (alveoli, villi, root hairs) compensate with large surface areas, thin walls, and mechanisms to maintain concentration gradients. Mass transport systems (circulatory system in animals, xylem and phloem in plants) move materials over long distances. Transpiration is water loss from leaves, driven by evaporation and affected by environmental conditions. Understanding SA:V calculations and being able to explain adaptations in terms of function are essential exam skills.