WJEC Biology for AS: Study and Rev Guide

Pointer Larger organisms have a greater number of cells and therefore have higher oxygen requirements. Grade boost There are some additional features, but these are NOT present in all organisms, e.g. • Good blood supply to maintain the concentration gradient ( not single-celled organisms, insects or plants). • Ventilation mechanism to maintain concentration gradient ( not single-celled organisms, worms or plants). 2.2 Adaptations for gas exchange Surface area to volume ratio Organisms exchange gases such as oxygen and carbon dioxide with the atmosphere via a gas exchange surface. The surface area of this surface determines how much can be exchanged. When an organism doubles in size, its volume (and therefore also its oxygen requirements) is cubed, but the surface area is only squared. Therefore, as organisms increase in size, a specialised gas exchange surface is required to increase the area available. Since this also increases the area available for water loss, there is always a balance to be struck between exchanging gases and water loss in terrestrial organisms. Surface area to volume ratio General characteristics of a gas exchange surface ▪ ▪ Large surface area to volume ratio ▪ ▪ Moist to allow gases to dissolve ▪ ▪ Thin to provide a short diffusion distance ▪ ▪ Permeable to gases. Unicellular organisms In single-celled organisms, e.g. Amoeba , the surface area is large enough to meet the needs of the organism and therefore materials can be exchanged directly across its thin and permeable cell surface membrane. As the cytoplasm is constantly moving, the concentration gradient is always maintained. 2 1 1 1 2 Surface area 1 × 1 × 6 sides 2 × 2 × 6 sides 4 × 4 × 6 sides = 6mm 2 = 24mm 2 = 96mm 2 Volume 1 × 1 × 1 = 1mm 3 2 × 2 × 2 = 8mm 3 4 × 4 × 4 = 64mm 3 Surface area: 6 : 1 3 : 1 1.5 : 1 Volume ratio 2 4 4 4 78 AS Biology: Study and Revision Guide