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Transport Mechanisms in Different Organisms (20)
All organisms need to exchange oxygen and carbon dioxide with their surroundings for respiration or
in plants for photosynthesis. According to Fick's law we know that the rate of diffusion depends on
the surface area, concentration difference and distance of pathway.
The requirements for respiration depend on the mass or volume of the organism, which is known as
the surface area: volume ratio. As organisms get bigger their volume and surface area both get
bigger, but not by the same amount. As organisms become bigger it becomes more difficult for them
to exchange materials with their surroundings.
Organisms which are very large have to be multicellular, which means that their bodies are composed
of many small cells, rather than one big cell: this can exchange materials quickly and independently.
Large organisms therefore need specialised exchange systems with a large surface area. These
systems include lungs, gills, roots and leaves.
Small organisms don't have specialised gas exchange systems like lungs or gills, but instead simply
exchange gases through the surface of their bodies. To maximise their rate of gas exchange they
have developed particular body shapes to increase their surface area: volume ratio.
Single-celled Organisms like bacteria or Amoeba, have a large surface area: volume ratio, so they can
exchange gases quickly and directly though their cell surface which is covered only by a cell-surface
membrane. Oxygen diffuses across their body in the same way carbon dioxide from respiration
Insects are fairly small, but they are also very active, so they need to respire quickly. They have a
rigid exoskeleton, which is waterproof to prevent the insects drying out, but it also prevents gas
exchange. Insects increase their rate of gas exchange by having openings called spiracles, which lead
to a network of tubes called tracheae, which branch into many smaller tracheoles that carry air
directly to the cells. These tracheae and tracheoles are held open by rings of hard chitin. The
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ends of the tracheoles oxygen diffuses directly into the cells, and carbon dioxide diffuses out, down
their concentration gradients. When the insect is at rest, water diffuses out of its cells into the ends
of the tracheoles, just as it does in human alveoli. This reduces the surface area in contact with the
cells and reduces the rate of diffusion. To overcome problems of water loss some insects have hairs
around the spiracles, and some can close their spiracles when they are inactive.…read more