Unit 3 - Organisms exchange substances with their environment
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Summary AQA Biology Unit 3: Organisms exchange substances with their environment. full notes
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Unit 3 - Organisms exchange substances with their environment
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AQA
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AQA Biology A Level Student Book
full notes hitting all areas of specification for unit 3 of AQA A Level biology
consisting from gas exchange to mass transport
with diagrams and definitions
highlighted key information
Unit 3 - Organisms exchange substances with their environment
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3.3 organisms exchange substances with their
environment
6 exchange
The larger an organism's size, the smaller its surface area to volume ratio.
Larger animals have adaptations such as protrusions from the cell, such as in root hair cells,
or balloon- like alveoli. The larger the SA:V the higher the metabolic rate in the organism.
Specialised exchange surfaces have characteristics of:
- Large SA relative to the V of the organism
- Very thin so rate of diffusion is shorter
- Selectively permeable membrane
Single celled organisms have a larger surface area to volume ratio, and so can rely on
diffusion to move into and out of them. Organisms can move to different areas of oxygen to
improve the concentration gradient. Eg amoeba.
Many insects have spiracles - containing valves to allow air in and out by diffusion - along
their thorax. The insect's trachea is supported by strengthened rings to stop them from
collapsing .The trachea divides into smaller dead-end tubes called tracheoles. These extend
throughout all the body tissue of the insect. gases move in and out through diffusion, mass
transport as a result of muscle contraction and as a result of volume changes in the
tracheoles.
spiracles>trachea>tracheoles
In hot, dry conditions water can be lost rapidly via evaporation from the spiracles. The valves
are closed to reduce this.
Fish have a waterproof, and therefore gas tight outer covering.
The gills of a fish have
- A large large surface area of lamellae and filaments when in water
- Rich blood supply by mass flow to the gills
- Concentration gradient of oxygen along the whole length of the lamella by a counter
current principle
Counter current principle:
Blood in the capillaries flows in the opposite direction to the water flowing over them
By allowing the blood to flow in the opposite direction to the water, an oxygen
concentration gradient is maintained, where the oxygen concentration in the water is
always higher than this in the blood
This maximises the amount of oxygen that can diffuse into the bloodstream and prevents
oxygen diffusing back into the water again once the blood is oxygen rich
Adaptations of xerophytes:
Plants that grow in dry habitats. They have a adaptation to reduce water evaporation from
leaves whilst enabling gas exchange and photosynthesis
1
, ● Thicker cuticle
● Reduced leaf surface area
● Sunken and fewer stomata
● Rolled leaves
In plants leaves have small holes called stomata;
- Allow gases to pass in and out of pores in the leaf surface
- Gases diffuse due to concentration gradient between the inside and outside of the
cell
- Stomata can close to reduce water loss
Mesophyll:
- Differentiated into columnar palisade cells which have many chloroplasts and
irregularly shaped spongy parenchyma cells
- Large moist surfaces to absorb oxygen and carbon dioxide, facilitating diffusion
- Concentration gradient formed as gases are absorbed or released
Human gas exchange system
→ Air is pulled into the lungs through the
trachea. The trachea divides into two bronchi,
then into bronchioles until they reach a dead
end at the alveoli sacs.
Gas exchange between the blood in capillaries
takes place in the alveoli sacs
Alveoli are adapted
- Large surface area
- Moist to aid diffusion of gases
- Rich blood supply to maintain a concentration gradient
- The alveolar epithelium and the capillaries are very thin so the diffusion distance
between air in the alveoli and red blood cells in the capillaries is short
Human ventilation
inspiration expiration
-external intercostal muscles contract and -internal intercostal muscles contract and
the internal ones relax, resulting in the the external ones relax, resulting in the
moving of the ribs upwards moving of the ribs downwards
-the diaphragm contracts and flattens. -the diaphragm relaxes and raises upwards.
-the effect of the intercostal muscles and -the effect of this causes the volume in the
diaphragm cause the volume in the thorax thorax to decrease, thus increasing the
2
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