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Summary AQA A level Biology Paper 1 Topic 3 notes
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A detailed summary of the AQA A level Biology paper 1 topic 3 which has been edited to learn help the content and the mark scheme too.
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🌿T3: organisms exchange
substances with their
environment
3.3.1 surface area to volume ratio
who has and needs a larger SA:V and why?
-smaller organisms have a larger SA:V which allows for efficient
exchange
-organisms with a high metabolic rate need larger SA:V
what do organisms have to adapt?
-flattened surface so no cell is far from the surface
-specialised exchange surfaces with large areas to increase SA:V
(lungs and gills)
what are the features of specialised exchange systems?
-large SA:V ration to increase rate of exchange
-very thin to reduce diffusion distance
-selectively permeable
-larger organisms have transport systems and counter current flow
3.3.2 gas exchange
how does gas exchange occur in single celled organisms?
-they are small and have large SA:V so they can exchange gases by
diffusion across the cell membrane
what is the structure of an insect gas exchange system?
-network of tubes called tracheae which are supported by strengthened
rings to prevent them from collapsing
T3: organisms exchange substances with their environment 1
, -these divide into smaller dead-end tubes called tracheoles
-they have small holes on their surface called spiracles which is where
gases exchange through but these are kept close most of the time to
reduce water loss
-
give three ways in which respiratory gases move in and out
along a diffusion gradient
when cells are respiring, oxygen is used up and so its
concentration towards the ends of the tracheoles falls which
creates diffusion gradient which causes oxygen to diffuse from
atmosphere to cells.
co2 is also produced so it creates a diffusion gradient in
opposite direction allowing it to diffuse out to the atmosphere
mass transport
-contraction of muscles in insects squeezes trachea enabling mass
movement of gases
the ends of tracheoles are filled with water
-during major activity, cells anaerobically respire which produces
soluble lactate which lowers water potential of cells so water moves
into cells by osmosis
-volume of water in tracheoles decreases drawing more air into
them so diffusion is faster because it take place in a gaseous
T3: organisms exchange substances with their environment 2
, environment but this increases water evaporation
how is water loss limited in insects?
-small SA:V to minimise the area over which water is lost
-waterproof covering made of chitin that is covered with a waterproof
cuticle
-spiracles close to reduce water loss
what is the structure of a gill?
-gill filaments- stacked disks
-at right angles to the filaments are gill lamellae to increase SA
-water is taken in through mouth and forced over gills and out through
an opening on each side of body
explain what counter current flow is and why it is important?
-the blood and water flow in opposite directions
-blood with high conc of o2 meets water with high conc of o2 so
oxygen diffuses from water to blood
-blood with little oxygen meets water with little oxygen so it diffuses
from water to blood
-diffusion gradient is maintained across the length of whole gill
how does the structure of a leaf help gas exchange?
-many stomata so no cell is far from the stoma which reduces diffusion
distance
-many interconnecting air spaces which occur in the mesophyll so
gases can easily come in contact with the mesophyll cells
-large SA of mesophyll cells for rapid diffusion
T3: organisms exchange substances with their environment 3
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