EXCHANGE AND TRANSPORT

You will find out about:  The relationship between the size of an organism or structure and surface area to volume ratio.  Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as the ratio reduces.  Gas exchange in a single-celled organism, insect, fish and leaf  Candidates should be able to use their knowledge and understanding of the principles of diffusion to explain the adaptations of gas exchange surfaces.  Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants.

Length of sides mm Surface area mm 2 Volume mm 3 Surface area to volume ratio

Amoeba Large SA:V enabling it to exchange gases by simple diffusion and the gases can get to the centre of the cell.

Flatworm Exchange gases across their surfaces. They can manage with this as they are inactive Their flattened shape provides a relatively large SA:V. No cell is ever that far from the surface.

Insects and gas exchange The air containing oxygen moves through the spiracles down the tracheal tubes that divide into finer tracheoles. The gases in the air move by diffusion across the moist lining of the tracheoles directly to and from tissues.

Spiracles Have valves so they can open and close to reduce water loss Open when carbon dioxide levels within the tracheae increase

Fish  Water flows in through mouth, over the gills then out through a gill slit or operculum on each side of the head.  Oxygen in the water diffuses across the gill lamellae into blood caplillaries  This occurs effectively because of the counter current blood flow through the gill lamellae

Counter Current flow is more efficient as the concentration gradient is maintained along the length of the gill lamellae. Water flow across lamellae. Max Oxygen Blood Flow. Low oxygen High Oxygen Oxygen diffusion

Features of specialised exchange surfaces:  Large surface area  Very thin to reduce diffusion distance  Movement of external medium eg air to maintain diffusion gradient  Movement of blood to maintain diffusion gradient

Xerophytes are plants that live in very dry conditions and experience water shortage and so need to have adaptations to limit their water loss. Adaptations are to limit water loss, increase water uptake, and store water.

leaves reduced to spines. Photosynthesis occurs in stem cells Stem holds water Extensive roots

Adaptations to reduce water loss  A thick waxy cuticle to reduce evaporation over the leaf surface.  Rolling up of leaves to protect the lower surface from outside. Water gets trapped in this area so the air here is relatively humid, reducing diffusion from the leaf.  Hairy leaves –especially on the lower surface to trap moist air nest to the leaf surface.  Stomata in pits or grooves-these again trap moisture next to the leaf and reduce the diffusion of water from the leaf.  A reduced surface area to volume ratio of the leaves. Having leaves that are short and round rather than broad and flat.

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