Adaptations for gas exchange : complete notes

detailed revision notes wjec syllabus orientated with diagrams and pictures

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2.2 Adaptations for gas exchange
1. Living things need to obtain materials such as carbon dioxide and oxygen from the environment and
remove waste from their cells to the environment.
Organisms are differently adapted so gas exchange can take place whether it be in
water or on land. In particular oxygen is needed to convert organic molecules into
energy through the process of respiration.
NB: Gas exchange is the process by which oxygen reaches cells and waste products
are removed, don't confuse with respiration which is energy production in cells.
2. Requirements may be proportional to volume however diffusion is proportional to surface area
3. In large organisms the surface area to volume ratio is much less than in very small organisms.
As organisms get larger their volume to surface area increases meaning they cannot
rely on diffusion alone as the diffusion path would be too long.
4. In small, unicellular organisms the surface area to volume ratio is so large that diffusion
through the body surface is sufficient to supply their needs.
An example of this is an amoeba where the cell
membrane acts as the exchange surface, It is thin and
moist so is efficient at its job.
5. Also, distances within the body are small and transport by
diffusion is again sufficient, e.g. Amoeba,
its size and lifestyle in water enables diffusion to supply its
needs. The cell membrane has a large diffusion path
and large surface area to volume ratio.
6. Larger, multicellular organisms may have a surface area to volume ratio which is too small
to supply all their needs.
Multicellular organisms are an aggregation of cells. Cells aggregate together to
increase their size but decreases their surface area to volume ratio and the diffusion
path increases also materials are needed to be exchanged between different organs as
well as the organism and the environment. So diffusion is no longer a viable process of
exchange.
7. These organisms therefore possess special surfaces for gaseous exchange, gills for
aquatic environments, lungs for terrestrial environments.
Gas exchange surfaces such as the gills of a fish, the alveoli in the lungs of a mammal, the
trachae of an insect and the spongy mesophyll cells in the leaves of a plant are effective
exchange surfaces.
8. These exchange surfaces have particular properties to aid diffusion: large surface area
thin, moist, permeable surface.
In order to achieve the maximum rate of diffusion a respiratory surface must:
have a large surface area compared to volume

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The large moist area for gaseous exchange is a region of potential water loss.
10. Earthworms are multicellular, terrestrial animals restricted to damp areas. A moist body
surface for diffusion, with a circulatory system and blood pigments, increase efficiency of
gaseous exchange sufficient for a slow moving animal.
Simple multicellular animals such as worms have a low oxygen requirement as they
have an extremely low metabolic rate as they move very slowly.…read more

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Fish can be further classified into two groups according to the material that makes up their
skeleton.
Cartilaginous fish:
Skeletons are made entirely of cartilage.
Normally live in the sea e.g. shark
Characterised by five gill clefts that
open at five gill slits.
Gas exchange involves parallel flow.
Therefore blood travels through the capillaries in the same direction as the sea water.
This is relatively inefficient as a diffusion gradient is not maintained.…read more

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The gills have an extensive network of blood capillaries to allow efficient diffusion and
haemoglobin for oxygen carriage.
14. Compared with parallel flow, counter current flow increases efficiency because the
diffusion gradient between the adjacent flows is maintained over the whole surface.
Blood flows between the gill plates under pressure in the opposite direction the blood in the
capillaries. The blood always meets water with higher oxygen content than itself. It removes
80% of the oxygen from the water.…read more

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The human respiratory system includes epiglottis, trachea, bronchi, bronchioles, alveoli,
pleural membranes, ribs, intercostal muscles, diaphragm. These are involved in two
functions: ventilation andexchange of gases.
Lungs are enclosed within the Thorax , an air tight cavity
At the base of the thorax is a sheet of muscles, the Diaphragm.
Air is drawn into the lungs via the trachea which is strengthened by rings of
cartilage.…read more

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They are thin and moist providing a short diffusion path and a medium for
gasses to dissolve into.
Intercostal muscle contract and relax to move the ribcage up and down.
two pleural membranes surround the pleural cavity membranes will secrete
pleural fluid which acts as a lubricant , reducing friction between the lungs and
the ribcage.
goblet cells of the cilliated epithelium that produces this mucus.…read more

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Plants rely entirely on diffusion for the exchange of gases. Leaves are therefore thin to
shorten distances for diffusion and have a large surface area and are permeated by air
spaces.
Plants photosynthesise during the day and respire at night.
the leaf is thin and flat with a large surface area
Spongy mesophyll tissue allows gases to circulate.
plant tissue contains air spaces
stomata pores on lower epidermis.
25.…read more

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Palisade cells are arranged so that the light energy has few cell walls to pass through.
In dim light the chloroplasts will move to the top of the cells to maximise the light
absorbed.
28. Light can pass through to the spongy mesophyll. The spaces between mesophyll cells
allow carbon dioxide to diffuse to the cells and oxygen can diffuse away. The cells are moist
so gases can dissolve.
29. Leaves have a cuticle to prevent water loss which also reduces gaseous exchange.
30.…read more

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Xerophytes may open stomata at night instead of during the day in order to conserve water, whilst
other plants may close stomata during the day or night under drought conditions.
Xerophytes are plants which have adapted to live under conditions of low water
availability. An example is marram grass which has a thick cuticle with no stomata on
the outer surface, sunken stomata and hairs to reduce the water potential gradient of
inner surface.…read more

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