B6 exchange

• larger organisms have a lower surface area to volume ratio
• smaller organisms have a higher surface area to volume ratio
• the need for specialised exchange surfaces grows as the organism increases in size, because it is more difficult for simple diffusion to work at a fast enough pace to keep the organism stable
• in single-celled organisms, substances can easily enter the cell as the diffusion distance is short, but in multicellular organisms, specialised exchange surfaces are required as the diffusion distance is so much longer
• specialised exchange surfaces have the following features:

  • a high surface area to volume ratio, increasing the rate of diffusion
  • very thin, decreasing diffusion distance so increasing the rate of diffusion
  • selectively permeable, allows certain materials to cross
  • movement of the external medium, to maintain a concentration gradient
  • transport system, to ensure movement of the internal medium, to maintain a concentration gradient
• single-celled organisms have a large surface area to volume ratio
• in single-celled organisms, oxygen is absorbed by diffusion across the cell-surface membrane, and carbon dioxide moves out by diffusion across the cell-surface membrane

  the cell wall has no impact on the diffusion of gases
• insects do not have a transport system so oxygen has to be directly transported to respiring tissues
• insects have a network of tubes which run through the body called trachea, they branch into tracheoles and their openings are called spiracles
• in insects, spiracles can be opened or closed by a valve, when open they increase the amount of water lost, so typically insects keep them closed
• in insects, gases can move through the trachea and tracheoles by diffusion, and by mass transport as the body contracts rapidly to pump gases through
• in insects, the ends of the tracheoles are filled with water, during exercise lactate is produced which decreases the water potential of cells near these ends, so water moves out of the tracheoles by osmosis, decreasing the volume, so air is drawn into the tracheoles
• the tracheal system is an efficient method of gas exchange, but it does have some limitations as it relies mostly only diffusion, which requires a short diffusion pathway, this means insects must be smaller than organisms which do not use the tracheal system
• fish have a small surface area to volume ratio, and are covered in an impermeable membrane which means gases cannot diffuse through their skin
• fish require a specialised exchange surface which is the gills
• the gills are located behind the head of the fish, they are made up of gill filaments stacked in a pile
• gill filaments are at right angles to gill lamellae which increase the surface area of the gills
• in fish, water is taken in through the mouth, forced over the gills, and released out of an opening on either side of the body
• the flow of water over and the flow of blood within the gills are in opposite directions, this is called countercurrent flow
• the countercurrent flow in gills is beneficial because:

  • blood that is already heavily oxygenated meets water with maximum oxygen, so oxygen diffuses from the water to the heavily oxygenated blood
  • blood that is deoxygenated meets water with little minimum oxygen, so oxygen diffuses from the water to the deoxygenated blood
• therefore, the countercurrent flow maximises the diffusion gradient for oxygen uptake across the whole length of the lamellae, so the maximum amount of oxygen is absorbed into the blood
• stomata are minute pores on the leaves of plants, they are surrounded by a pair of guard cells which can open and close the stoma
• opening and closing the stomata controls the rate of gas exchange but also limits water loss by evaporation
• plants which have a large number of stomata are more efficient at gas exchange as each cell is closer to a stoma so the diffusion distance is decreased
• leaves have air spaces which allow gases to move around the leaf and reach photosynthesising mesophyll cells
• plants which have mesophyll cells with a large surface area are more efficient at gas exchange as diffusion distance is decreased
• insects reduce water loss by the following adaptations:

  • small surface area to volume ratio which minimises the area over which water can be lost
  • rigid outer skeleton of chitin is covered by a waterproof cuticle
  • spiracles which can be closed
• plants reduce water loss by the adaptations:

  • waxy cuticle = forms a waterproof barrier
  • leaves which can roll up = protect the stomata, air is trapped within them, water potential increases until it is equal, then no more water can be lost
  • a layer of hair on leaves = traps moist air next to them, reducing the water potential gradient so less water is lost
  • stomata in pits = trap moist air reducing the water potential gradient so less water is lost
  • smaller surface area to volume ratio such as needles = decreases the rate of diffusion, so reduces water loss but also reduces photosynthesis
• the lungs are a pair of lobed structures with a large surface area, located in the chest cavity, that are able to inflate
• the lungs are surrounded by the rib cage which acts as protection
• external intercostal muscles and internal intercostal muscles between the ribs contract and relax to raise and lower the ribcage
• the diaphragm separates the lungs from the abdomen
• in human gas exchange, air enters the system through the nose and mouth, it travels down the trachea, then the bronchi, then the bronchioles, then the alveoli
• the trachea is a flexible airway supported by rings of cartilage
• the bronchi are two divisions of the trachea, each leading to one lung
• the bronchioles are a series of subdivisions of the bronchi, their walls are made of muscle, the muscle can constrict to control flow in and out of alveoli
• the alveoli are minute air sacs at the end of bronchioles, they have elastic fibres between them so can stretch as they fill with air and spring back to release air
• in humans, the gas exchange surface is the alveolar membrane
• ventilation is required to maintain the diffusion of gases across the alveolar epithelium, it involves moving air constantly in and out of the lungs
• when air pressure inside the lungs is lower than the atmosphere, air moves into the lungs, this process is called inspiration