$6 \times 4^2 = 96$, so the surface area is 96 cm². 2. Calculate the volume by cubing the side length: $4^3 = 64$, so the volume is 64 cm³. 3. Write the ratio as surface area : volume, so the ratio starts as 96:64. 4. Simplify the ratio by dividing both numbers by 32: $96 \div 32 = 3$ and $64 \div 32 = 2$, so the SA:V ratio is 3:2. 5. Interpret it: this cube has less surface area per unit volume than a smaller cube, so diffusion alone would be less effective. For SA:V questions, calculate surface area and volume separately first. Then write the ratio with **surface area first**, and simplify if possible. ## Why multicellular organisms need exchange surfaces A **multicellular organism** is made of many cells. Large multicellular organisms have two diffusion problems: - Their surface area to volume ratio is relatively small. - Many cells are deep inside the body, far away from the outside environment. Diffusion is effective over short distances, but it is too slow to supply every cell in a large organism by diffusion through the outer body surface alone. An **exchange surface** is a specialised area where substances move into or out of an organism. A **transport system** carries substances around the organism, such as the blood in mammals. Effective exchange surfaces usually have: - a **large surface area**, so more particles can cross at once - a **thin membrane**, giving a short diffusion path - in animals, an efficient **blood supply**, which carries substances away or brings them in - for gaseous exchange in animals, **ventilation**, meaning movement of fresh air or water over the exchange surface Alveoli are tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged. 1. Millions of alveoli provide a very large surface area, so many gas molecules can diffuse at the same time. 2. The alveolus wall is very thin, so the diffusion path is short. 3. A rich capillary blood supply carries oxygen away and brings carbon dioxide to the alveoli, maintaining steep concentration gradients. 4. Ventilation refreshes the air in the alveoli, keeping oxygen concentration high and carbon dioxide concentration low. ## Examples of specialised exchange surfaces The same exchange-surface ideas appear in different organisms. | Exchange surface | Main substances exchanged | Key adaptations | |---|---|---| | Mammal lungs: alveoli, tiny air sacs | Oxygen into blood; carbon dioxide out | Large surface area, thin walls, rich capillary supply, ventilation | | Small intestine: villi, finger-like projections | Digested food molecules into blood | Large surface area, thin surface, good blood supply to carry absorbed molecules away | | Fish gills: folded surfaces in the gill | Oxygen from water into blood; carbon dioxide out | Large surface area, thin surface, good blood supply, water constantly moved over gills | | Plant roots: root hair cells | Water and mineral ions from soil | Long root hairs give large surface area and close contact with soil particles | | Plant leaves: stomata, tiny pores | Carbon dioxide in; oxygen and water vapour out | Broad, thin leaves, air spaces inside the leaf, many stomata for gas diffusion | In plants, some transport processes you meet later are not diffusion: water uptake involves osmosis, and mineral ion uptake can involve active transport. For this section, focus on the shared exchange-surface features: large surface area and short diffusion distance. Do not just list a feature, such as “alveoli have capillaries”. Add the reason: capillaries maintain a steep concentration gradient by carrying substances away or bringing them in. 1. Define diffusion using the full idea: **net movement from higher concentration to lower concentration down a concentration gradient**. 2. For rate questions,