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  • Created by: Laellex
  • Created on: 28-04-16 20:33
What do organisms need to do in order to survive?
Exchange materials between the external and internal environment
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Where does exchange take place?
At exchange surfaces
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What do smaller organisms have?
A large surface area to volume ratio
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What does this enable small organisms to do?
To exchange materials by simple diffusion
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What do larger organisms have?
A small surface area to volume ratio
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Why can't large organisms use diffusion to exchange materials?
It would be too slow to supply materials to cells in the extremities of the organism
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What do organisms have to exchange materials efficiently?
Specialised exchange surfaces
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What are some features of specialised exchange surfaces?
Thin, Large SA to V ratio, selectively permeable, movement of external environmental medium, transport system for movement of internal medium
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Specialised exchange surfaces being thin present what problem?
They are easily damaged and dehydrated
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How is this problem overcome?
They exist inside organisms
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Single-celled organisms exchange oxygen and carbon dioxide how?
By diffusion across their cell membrane
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Why is this possible?
They have a large SA to V ratio, and are covered only by a cell-surface membrane
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If a cell wall is present in a single-celled organism is this a barrier to diffusion?
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The large surface area of insects for exchange conflicts with what?
The conservation of water
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What do insects have in their cuticle?
Small openings called spiracles
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What do spiracles do to prevent water loss?
They close it by a valve most of the time, to prevent water vapour evaporating
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When are spiracles triggered to open?
When the carbon dioxide levels are getting too high
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What do spiracles connect to?
An internal network of tubes called trachea
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Where do tracheoles extend to?
Throughout the body, to all body tissues
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What does the tracheal system increase?
Surface area for increased exchange
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The system being internal benefits what?
The conservation of water
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Between the tracheole and body cells, the diffusion pathway is...
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Diffusion takes place in what phase, and why is this beneficial?
In the gas phase; it's faster than in liquid phase
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How do gases move in and out of the tracheole system?
Along a diffusion gradient, mass transport + the water filled ends of tracheoles
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The body cells respire, using up oxygen- creating what?
A concentration gradient
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What happens next?
Oxygen from the atmosphere diffuses along the tracheal system, to the body cells
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What can insects do to enable mass movement of gases in and out?
Contract their muscles, putting pressure on the trachea
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When lactate builds up in muscle cells what happens?
The muscle cells water potential lowers, and water is drawn from the tracheoles into them
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Why is air then drawn into the tracheoles?
Because the volume of water in them decrease
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What is the final diffusion in?
The gas phase
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What's a limitation of this system?
It relies mostly on diffusion, so it needs a short diffusion pathway. So insects have to small in size.
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What do fish have?
A small surface area to volume ratio, as they are relatively large
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What specialised exchange system do they have?
Gills + a circulatory system of blood
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Describe what gills are made of?
Gill filaments stacked in a pile, with lamellae at right angles
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The purpose of lamellae?
Increase the surface area, contains lots of capillaries and a thin surface layer of cells to speed difusion
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What is the counter current system in fish?
Water and blood flowing in opposite directions across the lamellae
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What does the counter-current system create?
A large concentration gradient between water and blood (oxygen content)
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Where is the oxygen concentration always higher?
In the water
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What does this enable?
As much oxygen as possible to diffuse from the water into the blood
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Is equilibrium ever reached?
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What's the benefit of equilibrium never being reached?
More oxygen can be exchanged
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Similarities in exchange of plants and insects?
No living cell is far from external air, diffusion occurs in gas phase
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So what does this say about gas exchange?
The diffusion pathway is short, so diffusion is rapid
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What makes diffusion even more rapid in leaves?
The air spaces inside have a large surface area, compared with the volume of the leaf
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What do stomata do?
Allow gases in and out
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What adaptations do leafs have for rapid diffusion?
Lots of stomata, lots of airspaces between mesophyll and stomata, mesophyll cells have a large surface area
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How does 'lots of stomata' aid fast diffusion?
No cell is far from one, so the diffusion pathway is short
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How does having 'lots of airspaces' aid fast diffusion?
So gases readily come into contact with mesophyll cells= rapid diffusion
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What do guard cells do?
Open and close the stomata
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Why are guard cells important?
They help conserve water vapour that evaporates through the stomata
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What do guard cells function enable?
A balance between the conservation of water and gas exchange
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When does the stomata open?
When the guard cells are full of water (turgid)
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When do the stomata close, why?
At times of drought- not a lot of water in guard cells, to prevent water loss
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Terrestrial organisms have conflict between what?
Efficient gas exchange and water conservation
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Why can insects become dehydrated?
Because water easily evaporates from their body surface
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Which features of good gas exchange conflict with preventing water loss?
Thin, permeable and large surface area
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What adaptations do insects have to prevent too much water loss?
Small surface area to volume ratio, waterproof coverings + Spiracles can close
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Why can't plant cells have small surface area to volume ratio; ideal for water conservation?
Because photosynthesis requires a large SA for light absorption + gas exchange
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How do plants reduce water loss?
Waterproof coverings (waxy cuticle) + able to close stomata e.g. during drought
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What are xerophytes?
Plants with a restricted supply of water, that have evolved adaptations to limit water loss through transpiration
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Without these adaptations what would happen to xerophytes?
They will desiccate and die
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How is water loss reduced?
Reducing the rate at which water is lost by evaporation
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Why do the leaves show the most modifications?
This is where water is mainly lost
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What 5 adaptations do xerophytes have?
Thick cuticle, hairy leaves, stomata in pits/grooves, small SA:V, rolled up leaves
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How much water loss still occurs with a thick waxy cuticle (waterproofing)?
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Therefore, the thicker the cuticle...
The less water loss
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How do hairy leaves reduce water loss?
It traps moist air next to the leafs surface, reducing the water potential gradient. So less water is lost by evaporation.
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How does the adaptation of stomata in pits and grooves help?
It helps to trap still moist air, next to the leaves surface reducing the water potential gradient
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The smaller the surface area to volume ratio, the...
Slower the rate of diffusion
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So what adaptations do xerophytes have to reduce their surface area to volume ratio- thus water loss?
Small and roughly circular cross-sections
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Where do many leaves have their stomata confined?
To the lower epidermis
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What happens when the leaves roll?
The lower epidermis is protected from the outside, helping to trap region of still moist air within the leaf
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What happens to this region?
It becomes saturated with water vapour so has a high water potential
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Why doesn't water vapour diffuse after the leaves rolled?
Because there's no concentration gradient between the inside and outside of the leaf
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Other cards in this set

Card 2


Where does exchange take place?


At exchange surfaces

Card 3


What do smaller organisms have?


Preview of the front of card 3

Card 4


What does this enable small organisms to do?


Preview of the front of card 4

Card 5


What do larger organisms have?


Preview of the front of card 5
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