Summary AQA A-level Biology Notes - Exchange and Transport Systems

Gas
Gas Exchange
Exchange
Organisms need to exchange substances like oxygen, nutrients, carbon dioxide, urea and heat
· Smaller animals have higher surface area : volume ratios
In single-celled organisms, substances can diffuse directly across cell membrane. Diffusion rate is quick




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because of the short diffusion pathway.
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Diffusion in multicellular animals is slow because some cells are deep within the body so there is a big
distance between them and the outside environment and larger animals have a low surface area to
volume ratio. This is why they need exchange organs.

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Heat also needs to be transported away as metabolic activity inside cells release heat. Body size and shape
affect heat exchange:
The rate of heat loss depends on surface area. More is lost the larger the surface area.
Animals with a compact shape have a small surface area relative to their volume unlike animals
that have a less compact shape.

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Organisms have behavioral and physiological adaptations to aid exchange:
Animals with high SA:V lose more water as it evaporates from their surface. Some small desert
animals have kidney structure adaptations so that they produce less urine.
Small animals in cold regions eat large amounts of high energy foods such as seeds and nuts
to support their high metabolic rates
Small mammals may have thick layers of fur to hibernate when weather gets really cold.
Larger organisms in hot regions find it hard to keep cool as their heat loss is slow. Elephants have
large ears to increase surface over while hippos spend much of the day in water.

Gas exchange surfaces are thin and have a large surface area

Insects
Insects have impermeable exoskeletons which protect them from predation and water loss
To prevent too much water loss, insects close their spiracles using muscles. They also have a waxy cuticle
and tiny hairs around spiracles to reduce evaporation.

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Spiracles open and close to let gases in and out




Dissociation curves:

Digestion

, Oxygen is used in respiration and carbon dioxide is produced.


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Air moves into tracheae and oxygen travels down concentration gradient towards cells




:
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Tracheae branch off into tracheoles which have thin, permeable walls and go to individual cells.
Carbon dioxide moves down its concentration gradient to spiracles where it is released




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Increase in carbon dioxide triggers abdominal pumping which squeezes tracheoles. Decreases
volume and increases pressure so air is forced out.


m
Insects use rhythmic abdominal movements to move air in and out of spiracles

Rest phase: tracheal fluid seeps into tracheoles and oxygen diffuses into fluid
down concentration gradient
Active phase: muscles contract and release tracheal fluid into cells. Oxygen
diffuses into cell down a concentration gradient

Adaptations:
Fluid filled ends of tracheoles: moves out during exercise so faster diffusion through air to gas exchange
surface and reduces diffusion pathway - can move out into surrounding cells increases surface area..
Ventilation by muscles: muscles within the insect can squeeze the trachea enabling movement of air in and
out of the insect - helps maintain concentration gradient
Tracheoles have thin surface to minimise diffusion pathway




:
Increased surface area: extensive network of trachea and tracheoles increase surface area exposed
for diffusion of gases
Tracheae provide tubes full of air so fast diffusion
Highly branched/large number of tracheoles so short diffusion distance to cells and large surface area

Fish
Issues: Gill arch
Lamellae




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Low concentration of oxygen in water so fish have special adaptations
⑳



Blood vessels
·
Waterproof scales Gill slit
Diffusion is slow in water
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Small surface area to volume ratio


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Filaments
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Water enters through mouth and passes out through gills
Gills are supported by gill arch and space between gill arches is called gill slit
Each gill is made of lots of thin gill filaments which have a big surface area

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Gill filaments are covered in folds called lamellae which increases surface area more
Lamellae have lots of blood capillaries and a thin surface layer of cells to increase diffusion




:
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Blood flows through lamellae in one direction and water in the opposite. This is called counter-current flow
It maintains a large concentration gradient along the length of the lamella. Concentration of oxygen in
water is always higher so as much oxygen diffuses as possible into blood

Plants
↳
Gas exchange in dicotyledonous plants occurs at surface of mesophyll cells.
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Mesophyll cells have large surface area due to air spaces
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Waxy cuticle helps reduce water loss as it is not permeable
Lower epidermis also has lots of stomata (large surface area)
Length of diffusion path is short as leaves are flat and thin Y




Stomata are pores that allow gases and water vapor in and out of the &
plant. They are surrounded by guard cells that can close them. When guard
cells are flaccid, stomata are closed. When turgid, stomata are open. Guard
cells have thick inelastic inner wall to expand when filled with water.
Stomata are closed at night. -

Veins of dicots and monocots are different in leaves
Xerophytes:
Small leaves or spines to reduce surface area for water loss. Also stops animals eating the plant to get




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water
Stomata sunk in pits that trap moist air around so reducing concentration gradient
Hairs on lower epidermis to trap water around stomata
Curled leaves with stomata inside to reduce air flow to them
Waxy, waterproof cuticles on leaves and stems to reduce evaporation
Reduced number of stomata so fewer places for water to escape

Fick’s Law

Raboy Diffusion & Suyace Area Concentration difference
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