AQA Biology A-levelTopic 3: Organisms exchange substances with their environment Notes

AQA Biology A-levelTopic 3: Organisms exchange substances with their environment Notes 1 AQA Biology A-levelTopic 3: Organisms exchange substances with their environment Ntion ExchangeThe need for specialised exchange surfaces arises as the size of the organism, and its surface area to volume ratioincreases. In the case of single celled organisms, the substances can easily enter the cell as the distance that needs to be crossed over is short. However, in multicellular organismsthat distance is much larger due to a higher surface area to volume ratio. As a result of that, multicellular organisms required specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen. Features of an efficient exchange surface include large surface area, for instance the root hair cellsor folded membranes, such as those of the mitochondria. An efficient exchange surface should also be thin to ensure that the distance that needs to be crossed by the substance is short. The exchange surface also requires a good blood supply/ventilationto maintain a steep gradient, for example that of the alveoli. Ventilation and gas exchange in fish, insects and plants.Fish Fish have a small surface area to volume ratiofor gas exchange, apart from this they also have an impermeable membrane so gases can’t diffuse through their skin. This means that fish therefore need a specialised gas exchange surface.Bony fishhave four pairs of gills,each gill supported by an arch. Along each arch there are multiple projections called gill filaments,with lamellae on them which participate in gas exchange. Blood and water flow across the lamellae in a counter current directionmeaning they flow in the opposite direction to one another. This ensures that a steep diffusion gradient is maintained so that the maximum amount of oxygen is diffusing into the deoxygenated blood from the water. The projections are held apartby waterflow.Therefore, in the absence of water they stick together, thus meaning fish cannot survive very long out of water. Ventilation is required to maintain a continuous unidirectional flow.Ventilation begins with the fish opening its mouth followed by lowering the floor of buccal cavity. This enables water to flow in. Afterwards, fish closes its mouth, causing the buccal cavity floor to raise, thus increasing the pressure. The water is forced over the gill filaments by the difference in pressurebetween the mouth cavity and opercular cavity. The operculum acts as a valve and pumpand lets water out and pumps it in. Te r r e s t r i a l I n s e c t s Insects do not possess a transport system therefore oxygen needs to be transported directly totissues undergoing respiration. This is achieved with the help of spiracles, small openings of tubes, either bigger tracheaor smaller tracheoles,which run into the body of an insect and supply it with the required gases. Gases move in and out through diffusion, mass transport as a result of muscle contraction and as a result of volume changes in the tracheoles. tion Plants Finally plants are adapted to efficient gas exchange through many adaptations in their leaves. Leaves have many small holes called stomata which allow gases to enter and exit the leaves. The large number of these means no cell is far from the stomata, reducing the diffusion distance. Leaves also possess air spaces to allow gases to move around the leaf and easily come into contact with photosynthesising mesophyll cells. Mammalian gaseous exchange systemThe lungs are a pair of lobed structures with a large surface arealocated in the chest cavitythat are able to inflate. The lungs are surrounded by the rib cagewhich serves to protect them. A lubricating substance is secreted to prevent friction between the rib cage and lungs during inflation and deflation. External and internal intercostal musclesbetween the ribs contract to raise and lower the ribcage respectively. A structure called the diaphragmseparates the lungs from abdomen area.The air enters through the nose, along the trachea, bronchi and bronchioles which are structures well adapted to their role in enabling passage of air into the lungs. The gaseous exchange takes place in the walls of alveoli, which are tiny sacs filled with air. The trachea, bronchi and bronchioles enable the flow of air into and out of the lungs. The airways are held open with the help of a rings of cartilage, incomplete in the trachea to allow passage of food down the oesophagusbehind the trachea.Trachea and bronchiare similar in structure, with the exception of size – bronchi are narrower. They are composed of several layerswhich together make up a thick wall. The wall is mostly composed of cartilage, in the form of incomplete C rings. The inside surface of the cartilage is a layer of glandular and connective tissue, elastic fibres, smooth muscle and blood vessels.This is referred to as the ‘loose tissue’. The inner lining is an epithelial layer composed of ciliated epithelium and goblet cells.The bronchioles are narrower than the bronchi. Only the larger bronchioles contain cartilage. Their wall is made out of smooth muscle and elastic fibres. The smallest of bronchioles have alveoli clusters at the ends. The alveoli are adapted for transport for the following reasons: -The alveoli are very thin being only around one cell thick. These are surrounded by capillarieswhich are also only one cell thick. This reduces the diffusion pathway for gases. -The constant blood supply by capillaries means that a steep concentration gradient is constantly maintained. -The are a large number of alveoli (~300 million), collectively giving a surface area of ~tion