Because the amount of oxygen in water is about 1/20th compared to air, fish need a highly efficient means of oxygen extraction. To this end they combine high SA to vol with something called counter-current gas exchange.
Counter current flow means that the blood and the oxygen flow in opposite directions to each other (shown on the left hand side of the figure below). If the blood and water flowed in the same direction, it would be called concurrent flow. 
The real benefit of counter current flow is the ability of the blood to always have a slightly lower concentration than the water flowing over it. This means a concentration gradient is established for longer and more oxygen is removed from the water to the blood for the fish. If blood and water flowed in the same direction, then the concentrations would be equal at some stage and no further gas exchange could take place.
Plant Gas exchange
In the leaf, gas exchange structures are the stomates. These not only allow O2 out and CO2 in, but also allow minerals to be transported up the plant as transpiration acts as the driving force for the movement of water up the plant.
Stomates in plants that live in warmer drier climates like Australia tend to open in the morning, close down in the middle of the day, and then reopen in the afternoon. This way, water loss is reduced in the warmest part of the day, but light intensity is not compromised. Stomates are generally on the underside of the leaves away from direct sunlight which also helps with water conservation.
Lenticels are corky areas of loosely packed cells that are located on branches, stems, and the trunks of plants. They are able to let oxygen into these parts of the plants that cannot perform photosynthesis to be used in respiration and allow CO2 to escape.
When it comes to the roots of plants, most absorb oxygen through from air spaces within the soil. This is why over-watered plants die, because the roots literally drown. However, some plants live in areas where air can never get into the soil to allow them to efficiently exchange oxygen and CO2. The best example of this are mangroves. To compensate for this they have snorkel like extensions on their roots called pnematophores.
No comments:
Post a Comment