This document summarises Unit 3 on Exchange fir the AQA specification of biology AS Level. It is tested in paper 1 of the specification and is one out of the 5 sections that are examined in paper 1. This document provides a brief summary with all of the key points and sometimes a few pneumonics to ...
Tissue Fluid is the fluid around cells in multicellular organisms.
Exchange happens in 2 ways.
● Passive (no metabolic energy required) by diffusion and osmosis
● Active(metabolic energy required) by Active Transport
Smaller organisms generally have larger surface area to volume ratio and vice versa.
They do not need a complex transport structure as they can just use passive transport.
Surface area of a sphere: 4πr squared.
Volume of a sphere: 4 over 3 Pi r cubed.
Features of passive transport:
● Flattened shape so cells are close to the surface
● Specialised exchange surfaces
Features of Specialised exchange surfaces:
● Large surface area to volume ratio
● Thin to decrease diffusion distance
● Selectively Permeable
● Movement of environmental medium(e.g Air moves in and out of animals and air
around plants)
● Transport system(internal medium)(e.g Blood)
Diffusion is proportional to Surface area(difference in concentration)
Divided by Length of diffusion pathway.
Exchange surfaces are inside organisms to prevent damage and dehydration because they
are thin.
SubSection 6.2 summary
Single celled organisms are small and therefore can use diffusion to absorb Oxygen. Due to
the large surface area to volume ratio.
Cell wall is not a barrier to the release and absorption of co2 and oxygen.
The trachea divides into tracheoles. The trachea is supported by strengthened rings to
prevent it from collapsing. The tracheoles are in contact with all the respiring tissues of the
insect so there is a short diffusion pathway for all the cells of the insect.
When cells are respiring they use up the oxygen> This means that there is a concentration
gradient between the oxygen inside the insect and the oxygen outside. Carbon dioxide is
produced when the cells are respiring which means that there is a higher concentration of
carbon dioxide inside the insect and a lower concentration of carbon dioxide outside the
insect. Diffusion is faster in air than in water.
,The trachea muscles contract to make the air move faster and in larger quantities, speeding
up the process.
When the insect needs more oxygen, The muscle cells around the tracheoles respire
anaerobically and produce lactate which is soluble. This decreases the water potential inside
the muscle cells, so that there is a higher water concentration in the tracheoles. The water
will move out of the tracheoles into the muscle fibres. This means that more oxygen will
enter the tracheoles because the volume of water decreases in the tracheoles.
Gases enter and leave through the spiracles. The spiracles have valves which open and
close. The spiracles open to allow gas exchange and close to prevent water from
evaporating from the insect.
Insects have to be small because they need a large surface area to volume ratio so that they
can have a short diffusion pathway for exchange to be effective.
The diffusion pathway in insects for oxygen to enter the insect is the Oxygen enters via
spiracles and travels along the trachea, and then to the tracheoles. The oxygen is absorbed
in the tracheoles and the concentration gradient changes
This difference in concentration gradient internally and externally makes the Oxygen
continue to diffuse into the insect.
For The release of Carbon Dioxide, Carbon Dioxide concentration increases in the
tracheoles, creating a gradient between the co2 in the insect and the co2 outside of the
insect. This makes the co2 produced diffuse outwards.
The trachea in insects can contract, causing air to move out and in of the insects much
faster. This is called mass transport.
When Insects are carrying out major activity, The muscle cells around the trachea carry out
anaerobic respiration. This makes lactate, which is soluble and which makes the water
potential of the cells around the trachea drop. This makes the fluid in the trachea move into
these cells as the gradient has changed. This makes the volume of the water in the trachea
decrease and even more air is drawn into the trachea. This increases the rate at which
diffusion happens in the cell but also increases the rate of water evaporation.
Gases enter and leave the insects via the spiracles. Water evaporation also takes place via
the spiracles. The spiracles have valves which open and close periodically. They open to
allow oxygen to diffuse in and close to prevent water vapour from leaving.
Limitations of the insect exchange system is that it limits the possible size an insect can be
as the surface area to volume ratio of the insect has to be large to allow for efficient transport
via diffusion.
SubSection 6.3 summary
Fish have a waterproof outer covering. They also have a small surface area to volume ratio.
Their adaptation to this is their gills.
The gills are behind the head and are made of layers called gill filaments.
, { I made that myself guys!!!!!!!!!!) 😀
ANYWAY KEEP STUDYING! NOPE WE ARE NOT PROCRASTINATING! WE GETTING
THOSE A’S! WE DON'T WANNA BE BROKE! WE WANNA GO ON HOLIDAYS TO WHITE
BEACH ISLANDS AND SHOP DESIGNERS!!!!!
ANYWAY……… Let’s Continue
IT IS IMPORTANT… That the direction of water flow and the direction of oxygenated blood is
different. This is called Countercurrent flow. This maintains a concentration gradient
between Oxygen in the blood and Oxygen in the water, favouring the movement of oxygen
into the blood across the whole lamellae.
In countercurrent flow, 80% of the Oxygen in the water is absorbed.
When Oxygenated blood flows in the same direction as the water flow, this is called
concurrent flow.(I put it in red so u guyz remember its the bad one)
It does not maintain the concentration gradient. Instead, Oxygen in the blood and Oxygen in
the Water reach an equilibrium, preventing Oxygen from being diffused to the maximum. In
concurrent flow, 50%(half) of the Oxygen in the water is absorbed into the blood.
SubSection 6.4 summary
Plants carry out two important reactions. Respiration and photosynthesis.
In photosynthesis, plants take in carbon dioxide and produce oxygen. In respiration, plants
take in oxygen and release carbon dioxide.
Sometimes, the gases produced in one of the reactions can be used for the other reaction,
reducing the amount of gas exchange with the external air.
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