Unit 3 - Organisms exchange substances with their environment
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Summary AQA A LEVEL BIOLOGY CHAPTER 6 EXCHANGE NOTES
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Unit 3 - Organisms exchange substances with their environment
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AQA
I created these notes throughout y12&13 and used them for my A level 2022 exams. I got a grade 9 in GCSE and am awaiting for my a level biology result. These notes were made using the CGP revision guide, AQA Biology textbook and class notes in order to cover everything in the course. I have also ad...
Unit 3 - Organisms exchange substances with their environment
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6.1 EXCHANGE BETWEEN ORGANISMS AND
THEIR ENVIRONMENT
09 October 2020
09:54
Smaller organisms have a larger SA:V ratio, allowing efficient exchange across their
body surface.
● Cells need to take on oxygen and nutrients.
● Cells need to excrete waste products i.e urea and carbon dioxide.
● Most organisms need to stay at a constant temperature, so heat needs to be
exchanged.
With a lower SA:V ratio, it would take too long for substances to diffuse from the
surface to the centre of the organism.
● Bigger distance between cells in centre and the surface.
● Difficult to exchange enough substances to supply a larger volume organism
with a lower SA.
Multicellular organisms use specialised exchange organs.
Some multicellular organisms have flattened shape so no cell is far from the surface.
Multicellular organisms use a system to carry substances to and from cells.
Metabolic reactions also release heat energy.
-Heat energy is lost more easily in an organism which has a high SA:V ratio.
-Animals with a more compact shape have a lower SA:V ratio, minimising heat loss
from their surface.
-Animals with a less compact shape have a higher SA:V ratio, increasing heat loss
from their surface.
Exchange surfaces have:
● Large SA increasing rate of exchange.
● Very thin allowing a short diffusion path so materials can quickly diffuse across
the surface.
● A transport system to ensure the movement of the internal medium, i.e rich
blood supply to maintain concentration gradient.
● Movement of the environmental medium, i.e air to maintain a concentration
gradient.
● Selectively permeable to allow selected materials to cross
Rate of diffusion = SA X Difference in conc. / Length of diffusion path
,6.2 GAS EXCHANGE IN SINGLE-CELLED
ORGANISMS AND INSECTS
28 December 2020
17:50
SINGLE CELLULAR ORGANISMS
Have a large SA:V ratio, a thin cell-surface membrane and a short diffusion path.
They absorb and release gases by diffusion through their outer surface.
No need for a gas exchange system.
TRACHEAL SYSTEM OF AN INSECT
Air moves into the tracheae through spiracles.
● Tracheae have chitin rings and spiracles have valves.
Oxygen travels down a concentration gradient towards the cells.
The tracheae branches into tracheoles.
● These have thin and permeable walls which go to every individual cell.
Carbon dioxide from cells move down a concentration gradient, towards the
spiracles, into the atmosphere.
Insects contract and relax their abdomen to move air in and out of spiracles.
● Contracts, air pressure high, air pushed out.
● Relaxes, air pressure lower, air drawn in.
Gases move in and out the tracheal system:
-Along a diffusion concentration gradient.
● When cells respire, oxygen is used, so concentration at tracheole ends falls,
creating a concentration gradient.
- Oxygen diffuses from atmosphere, along tracheae and tracheoles, to cells.
● Carbon dioxide is produced by cells, so concentration increases at tracheole
ends, creating a concentration gradient.
- Carbon dioxide diffuses from the cells to the atmosphere.
-Mass transport.
● Contraction of muscles can squeeze the trachea, enabling mass movements of
air in and out.
- This speeds up the exchange of respiratory gases.
-Ends of tracheoles are filled with water.
● Muscle cells respire anaerobically during major activity, producing lactate.
● Lactate is soluble and lowers the water potential of the muscle cells.
● Water moves into the cells from the tracheoles by osmosis, which can have
gas dissolved.
, ● Water in the ends decrease in volume, drawing air further into the tracheoles.
● Final pathway is in a gas, therefore diffusion is more rapid.
Adaptations of tracheal system:
1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched/large number of tracheoles so short diffusion distance to cells;
3. Highly branched/large number of tracheoles so large surface area (for gas
exchange);
4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so
faster diffusion through the air to the gas exchange surface; OR Fluid in the end of
the tracheoles that moves out (into tissues) during exercise so larger surface area
(for gas exchange);
6. Body can be moved (by muscles) to move air so maintains diffusion/concentration
gradient for oxygen/carbon dioxide;
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