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Organisms exchange substances with their environment (AQA AS
Biology) PART 2 of 5 TOPICS
Gas exchange is when oxygen is reached to the respiring cells and waste products are removed.
Single celled organisms such as amoeba have a thin membrane that is moist.
Exchange surfaces have a large surface area, a thin membrane, most are moist for gases to dissolve
and a mechanism to maximise the diffusion gradient. Organisms that have their exchange systems
inside reduces water so moisture remains. They have good blood supply if the organism is an animal.
There are four gas exchange systems that you need to know:
Lungs: Air enters from the nose and is cleaned by the cilia capturing dust particles and other
irritants. It is warmed and moistened and is sent through the trachea then through the
bronchus to the bronchioles and then to the alveoli. This is where the gas exchange takes
place. Deoxygenated blood passes next to the alveoli which has a low concentration of oxygen
compared to the alveoli producing a concentration gradient. Oxygen diffuses into the blood
down its concentration gradient from the alveoli. CO2 leaves the deoxygenated blood down
its concentration from the blood to the alveoli as the blood has a higher concentration of CO2
than the alveoli. This waste gas is then expired. NB: There are different thorax movements
that you need to know which is described as the following - Inhalation causes the diaphragm
to contract and flatten and the external intercostals muscles to contract pulling the rib cage
up and out. This increases the volume of the thorax which decreases the pressure so air can
move in down its pressure gradient. Exhalation causes the diaphragm to relax and lift and the
internal intercostals muscles to contract pulling the rib cage down and in. This decreases the
volume of the thorax which increases the pressure so air can move out down its concentration
gradient. Alveoli are one cell thick for short diffusion pathway and there are many alveoli
present in the lungs creating a large surface area. Constant ventilation replaces air which
maintains the concentration gradient.
Fish: Water moves in through the mouth with a high concentration of oxygen than the blood.
When the water passes through next to the deoxygenated blood a concentration gradient is
created where oxygen diffuses into the blood down its concentration gradient. CO2 moves
from the blood into the water down its concentration gradient because the blood has a higher
concentration of CO2 compared to the water. The water with the waste gas moves out
through the operculum. NB: The method on the direction of flow of blood and water must
be known which is as follows - There is a counter current flow of blood and water meaning
the movement of blood one way means the water will move the other way. This is because a
steep concentration gradient is maintained for efficient gas exchange. The gill filaments that
the gases diffuse through are two cells thick so the diffusion pathway is short and have a large
surface area as they have gill lamellae .
Plants: Gases diffuse in and out of the leaf down their concentration gradients where oxygen
diffuses in and CO2 diffuses out. The wind replaces the air accumulating the stomata which
maintains a concentration gradient.
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Insects: These organisms have a tracheal system which is similar to plants. Oxygen diffuses in
through the spiracles into the tracheae and then into the trachieoles to the respiring cells.
Waste gases then exit through the spiracles.
Xeromorphic plants such as xerophytes reduce water loss as they have a thicker cuticle than the
normal plants and have sunken stomata which means water droplets can accumulate round the
stomata reducing water loss as it prevents a concentration gradient being made.…read more