Exchange across mass surfaces Flashcards
What is tissue fluid?
The environment around the cells of multicellular organisms
What does there need to be for exchange to be effective?
The exchange surface must be large compared with its organism
What happens as organisms become larger?
Their volume increases at a faster rate than their surface area
What kind of organisms can simple diffusion meet the needs of?
Relatively inactive organisms
Single celled organisms
What kind of features have organisms evolved?
- A flattened shape so that no cell is ever far from the surface (e.g. leaf)
- Specialised exchange surfaces with large areas to increase the surface area ti volume ratio
Give one characteristic of exchange surfaces
A large surface area to volume ratio
Give one characteristic of exchange surfaces
Very thin so that the diffusion distance is short and so materials cross the exchange surface rapidly
Give one characteristic of exchange surfaces
Selectively permeable to allow selected materials to cross
Give one characteristic of exchange surfaces
Movement of the environmental medium(e.g. air) to maintain a diffusion gradient
Give one characteristic of exchange surfaces
A transport system to ensure the movement of the internal medium (e.g. blood) to maintain a diffusion gradient
What is diffusion proportional to?
(Surface area x difference in concentration) / length of diffusion path
What can happen to specialised exchange surfaces, given that they are thin?
They are easily damaged and dehydrated
How are specialised exchange surfaces prevented from getting damaged and dehydrated?
They are located inside an organism
Where an exchange surface is located in the body, what does the organism need to have a means of?
A means of moving the external medium over the surface (e.g. a means of ventilating the lungs in a mammal)
Why is diffusion in plants quicker than in water?
Diffusion takes place in the gas phase (air) which makes it more rapid than if in water
In plants, do all living cells have a source of CO2 and O2?
Yes as no living cell is far from the external air
Inside plants, how are air spaces adapted for quick diffusion?
They have a very large surface area compared with the volume of living tissue
Is there a specific transport system for gas?
No - gases simply move in and out of the plant through diffusion
What adaptations do leaves show for rapid diffusion/gaseous exchange?
- Many small pores, stomata, and so no cell is far from a stoma so diffusion pathway short
- Numerous interconnecting air spaces throughout the mesophyll so gases can readily come in contact with mesophyll cells
- Large surface area of mesophyll cells for rapid diffusion
What are stomata?
Minute pores that occur mainly (but not exclusively) on the leaves, especially the underside
What is each stoma surrounded by?
Guard cells
- These can open and close the stomatal pore
- In this way, they control the rate of gaseous exchange
How do terrestrial organisms lose water?
By evaporation
How have plants evolved to balance the conflicting needs of gas exchange and control of water loss?
They do this by closing stomata at times when water loss would be excessive
How do terrestrial plants reduce water loss?
They have a waterproof covering over parts of the leaves and the ability to close stomata when necessary
What are xerophytes?
- Plants that are adapted to living in areas where water is in short supply
- They have evolved a range of other adaptations
What kind of modifications do leaves have to reduce water loss?
- A thick cuticle
- Rolling up of leaves
- Hairy leaves
- Stomata in pits or grooves
- A reduced surface area to volume ratio of the leaves
How does a thick cuticle reduce water loss in leaves?
- Although the waxy cuticle on leaves forms a waterproof barrier, up to 10% of water loss can still occur via this route
- Thicker the cuticle, less water can escape by this means
How does the rolling up of leaves reduce water loss?
- Most leaves have their stomata largely on the lower epidermis
- The rolling of leaves protects the lower epidermis from the outside
- This helps to trap a region of still air within the rolled leaf
- This region becomes saturated with water vapour and also has a high water potential
- No water potential gradient between the inside and outside of leaf so no water loss
How do hairy leaves reduce water loss?
- A thick layer of hairs, especially on lower epidermis, traps still, moist air next to leaf surface
- Water potential gradient between inside and outside of leaf is reduces so less water lost by evaporation
How do stomata in pits or grooves reduce water loss?
These trap still, moist air next to leaf and reduce water potential gradient
How does a reduced surface area to volume ratio of leaves reduce water loss?
- By having leaves that are small and roughly circular in cross section, rate of water loss can be considerably reduced
- This reduction in SA is balanced against need for a sufficient area of photosynthesis to meet requirements of plants
What have insects evolved for gas exchange?
An internal network of tubes called tracheae
What are tracheae supported by?
Strengthened rings - to prevent from collapsing
What do tracheae divide into?
Smaller dead-end tubes called tracheoles
What do tracheoles extend throughout?
All the body tissues of the insect
Is atmospheric air brought directly to the respiring tissues in an insect?
Yes as there is a short diffusion pathway from a tracheal to any body cell?
How do respiratory gases move in and out of the tracheal system?
Along a diffusion gradient
Mass transport
The ends of the tracheoles are filled with water
How do respiratory gases move in and out of the tracheal system via a diffusion gradient?
- As cells respire, O2 is used up to its concentration towards the end of the tracheoles fall
- This creates a diffusion gradient
- Gaseous oxygen from atmosphere moves in along gradient
- CO2 is the same but in the opposite way
How do respiratory gases move in and out of the tracheal system via mass transport?
- The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out
- This further speeds up the exchange of respiratory gases
How do respiratory gases move in and out of the tracheal system via (the ends of the tracheole are filled with water)?
- During times of major activity, muscle cells around tracheole carry out anaerobic respiration
- This produces lactae, which is soluble and lowers water potential of cells
- Water moves into cells from tracheoles by osmosis
- Water in ends of tracheoles decrease in volume which draws air further into them
- So final diffusion pathway is in a gas rather than liquid phase so diffusion is quicker
What does (the ends of the tracheole are filled with water) mean for the rate of exchange of respiratory gases?
It increases the rate at which air is moved in to the tracheoles but leads to greater water evaporation
How do gases enter and leave the tracheae?
Through tiny pores, spiracles, on the body surface
How are spiracles opened and closed?
By a valve
What happens when the spiracles are open in terms of water?
Water vapour can evaporate from the insect
What is the opening and closing of spiracles like in insects?
- For most of the time, spiracles are closed to prevent water loss
- Periodically, insects open their spiracles to allow gas exchange
What are the limitations of the tracheal system?
Relies mostly on diffusion to exchange gases between environment and cells
How does exchange of gases in insects limit their size?
For exchange to be effective, diffusion pathway needs to be short so insects are a small size
As a result, the length of the diffusion pathway limits the size that insects can attain
What adaptations of efficient gas exchange conflict with the need to conserve water in insects?
A thin, permeable surface with a large area
Give one adaptation insects have evolved that reduce water loss in terms of size?
Small surface area to volume to minimise area over which water is lost
Give one adaptation insects have evolved that reduce water loss in terms of body surfaces?
- Waterproof coverings over their body surface
- This covering is a rigid outer Skeleton of chitin that is covered with a waterproof cuticle
Give one adaptation insects have evolved that reduce water loss in terms of gas exchange?
- Spiracles can be closed to reduce water loss
- This conflicts with the need for oxygen and so occurs largely when the insect is at rest
Do fish have an outer covering?
Yes
What kind of outer covering do fish have?
A waterproof, gas-tight one
Why do fish need a specialised gas exchange system?
They have a small SA to volume ratio so their body surface is not adequate enough to supply and remove their respiratory gases
Where are gills located in a fish?
Within the body, behind the head
What are gills made up of?
Gill filaments
How are the gill filaments arranged?
They are stacked up in a pile
What are at right angles to the gill filaments and how do they increase the rate of gas exchange?
Gill lamellae - they increase the SA of the gills
They are thin and so there is a short diffusion distance
They have an extensive network of capillaries
How does water travel through a fish?
- Taken in through the mouth
- Forced over the gills and out through an opening on each side of the body
What is the countercurrent flow?
The flow of water over the gill lamellae and the flow of blood within them are in opposite directions
What would happen if water and blood flowed in the same direction?
Far less gas exchange would take place
How does oxygenated blood interact with the water flowing over it?
- Oxygenated blood meets water, which has its maximum concentration of oxygen
- So diffusion of oxygen from the water to the blood takes place
How does deoxygenated blood interact with the water flowing over it?
- Blood with little oxygen in it meets water which has had almost all of its oxygen removed
- So diffusion of the remaining oxygen from the water to the blood takes place
What happens as a result of the countercurrent flow?
A diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae
How much of the oxygen available in the water is absorbed into the blood of the fish?
About 80%
How much of the oxygen available in the water is absorbed into the blood of the fish if they flow in the same direction?
About 50%
What is a dicotyledonous plant?
A regular plant
