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How cells and organisms carry out exchanges with their external environment to maintain their
internal environment?
The exchange of substances in cells and organisms is vital for their survival
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The majority
of cells use diffusion to achieve exchange however active transport is required in some processes of
exchange
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However, some organisms require specialized exchange surfaces as diffusion alone cannot
carry out effective exchange with their external environment to maintain their internal environment
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Firstly, the gas-exchange system in humans includes specialised organs for exchange known as lungs
to ensure efficient gas exchange between the air and the blood
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Ventilation is the system by which humans
are able to take in and release gas
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The process
of inspiration happens when external intercostal muscles contract causing the ribs and sternum
move up and out
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The process of expiration takes place when external intercostal
muscles relax leading to the ribs and sternum to move down and inwards
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The air pressure in the alveoli is more than the atmospheric pressure causing air to be
forced out
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The alveoli are adapted, to ensure the
internal demand for oxygen is met, by having a thin exchange surface as the alveolar epithelium is
only one cell thick meaning it has a short diffusion pathway
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There is also
a high concentration gradient due to the alveoli being surrounded by a network of capillaries
meaning a constant blood supply, which is the movement of the internal medium, and the
ventilation of air known as movement of the environmental medium
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Oxygen diffuses across the alveolar epithelium and capillary
endothelium into the haemoglobin within red blood cells
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However, gas exchange in single celled organisms and insects is varied compared to humans
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They are
small therefore meaning a larger surface area to volume ratio but also have a thin surface and short
diffusion pathway meaning there is no need for a gas exchange system
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Air moves into the trachea through pores on the surface known as spiracles, which
use rhythmic abdominal movements to move air in and out
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Oxygen moves down the concentration gradient towards the cells for which the trachea branch off
into smaller traceless which have thin and permeable walls which extend throughout the body
tissues of the insect
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Carbon dioxide
produced by respiring cells creates a diffusion gradient in the opposite direction causing carbon
dioxide to move down its own concentration gradient towards to spiracles where it is released into
the atmosphere
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Respiration in muscle cells during periods of major activity may
be anaerobic producing a lactate
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The water volume in the ends is reduced
and in doing so draws air in causing the final diffusion pathway is in gas rather than a liquid phase
and therefore diffusion is more rapid
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Fish use a counter-current system for gas exchange as the concentration of oxygen in water is less
than in air
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Water, containaing oxygen,
enters the fish through its mouth and passes through the gills
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Surface area is further increased by lamellae, at right angles to the filaments, which also have many
blood capillaries and a thin surface layer of cells to speed up the rate of diffusion
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This enables a larger concentration gradient between water and the blood
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Whereas blood with little oxygen in it meets water which has had the
majority of its oxygen removed causing diffusion of oxygen from the water to the blood
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Dicotyledonous plants exchange gases at the surface of the
mesophyll cells
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Most of the
gaseous exchanges occur in the leaves with the main exchange surface being of the mesophyll cells
in the leaf
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There are many very small pores known as stomata, surrounded by guard cells, which provide
a short diffusion pathway
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In addition to plants and insect having specialised exchange surfaces they also have adaptations to
control water loss as exchanging gases tends to lead to the organism losing water
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They also have a waterproof, waxy cuticle over their
body and tiny hairs around the spiracles causing a reduction in evaporation
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However, some plants are specifically
adapted to live and survive in warm, dry and windy habitats
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On the other hand, many organisms have specialised exchange surfaces for the exchange of gas and
water but also for the absorption of products of digestion
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Another mechanism to increase the rate of
exchange is by increasing the number of protein channels and carrier proteins in any given area of
membrane
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Therefore the substances move
down the gradient by facilitated diffusion from inside the ileum into the blood
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As
diffusion causes equilibrium in and out of the ileum active transport is required to transfer the rest
of the glucose and AA in the ileum into the blood
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To conclude, every organism and cell has different ways of exchanges substances with their external
environment to maintain their internal environment
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They have developed adaptations to be efficient in gaining the substances
required for the internal environment to be efficient

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