exchange Flashcards
what are the two different environments
we have both external and internal environments
the internal environment within an organism and within its cells is different form the external environment
what must organisms do in order to survive
to survive, organisms transfer materials between the two environments
where does this transfer of materials take place
this transfer of material takes place at exchange surfaces and always involves crossing cell plasma membranes ( including membranes within internal organelle of cells )
what is the environment around the cells called
the environment around the cells of multicellular organisms is called tissue fluid
why can’t diffusion be the only source of transport for materials
the majority of cells are too far from exchange surfaces for diffusion alone to supply or remove their tissue fluid with the various materials needed to keep its composition relatively constant
therefore, once absorbed, materials are rapidly distributed to the tissue fluid and the waste products returned to the exchange surface for removal - this involves a mass transport system
It is this mass transport system that maintains the diffusion gradient that bring materials to and from cell - surface membranes
how will size and metabolic rate of an organism affect the amount of each material that is exchanged
organisms with a high metabolic rate exchange more materials and so require a larger surface area compared to the volume ratio
this reflects the type of exchange surface and transport system that evolved to meet the requirements of each organisms
what are some examples of things that need to be interchanged between an organism and its environment
- respiratory gases (oxygen and carbon dioxide)
- nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
- excretory products (urea and carbon dioxide)
- heat
except for heat, these exchanges can take place in two ways:
- passively by diffusion and osmosis
- actively by active transport
what must happen in order for exchange to be effective
for exchange to be effective,
the exchange surfaces of the organism must be large compared with its volume
what is the surface area of small organisms like
small organisms have a surface area that is large enough, compared with their volume
to allow efficient exchange across their body surface
what happens to the organism as it gets larger
as an organisms become larger, their volume increases at a larger rate than their surface area
because of this, simple diffusion can only meet the needs of relatively inactive organisms
even if the outer surface could supply enough of a substance, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport
what have organisms done to increase the surface to volume ratio ( to reduce the volume relative to the surface area)
organisms have evolved one or more of the following feature:
- a flattened shape so that no cell is ever far from the surface (e.g. flatwom or a leaf)
- specialised exchange surfaces with large area to increase the surface area to volume ratio (e.g. lungs in mammals, gills in fish)
what are some of the features of exchange surfaces
To allow effective transfer of materials across specialised exchange surfaces by diffusion or active transport, exchange surfaces show the following characteristics:
- large surface area relative to the volume of the organism which increases the rate of exchange
- very thin so that the diffusion distance is short and therefore across the exchange surface rapidly
- selectively permeable to allow selected materials to cross
- movement of the environment medium, e.g. air to maintain a diffusion gradient
- a transport system to ensure the movement of the internal medium, e.g. blood, in order to maintain a diffusion gradient
what is diffusion proportional to
diffusion ∝ surface area x differences in concentration/ length of diffusion path
why is the exchanged surfaces being thin a disadvantage
being thin, specialised exchange surfaces are easily damaged and dehydrated
This is why an exchange surface is located inside the body
Since the exchange surfaces is located inside the body,
the organisms needs to have a means of moving the external medium over the surface, e.g. a means of ventilating the lungs of a mammal
how is gas exchange in a single celled organism
single celled organisms are small and therefore have a large area to volume ratio
Oxygen is absorbed by diffusion across body surface, which is only covered by a cell surface membrane
CO2 (from respiration) diffuses out across their body surface
furthermore, living cells are surrounded by a cell wale - single celled organisms do not have this additional barrier therefore, it has a short pathway of diffusion
describe gas exchange in insects
insects have evolved an internal network of tubes called trachea
The trachea is supported by strenghthend rings to prevent them from collapsing. The trachea is divided into smaller dead - end tubes called tracheoles
The tracheoles extend throughout all the body tissue of the insect
In this way, atmospheric air with the oxygen it contains, is brought directly to the respiratory tissues, as there is a short diffusion pathway from a tracheole to any body cell
Gas enters and leaves the trachea through tiny pores, called spiracles on the body surface
what are the three ways that respiritory gases move in and out of the tracheal system
- Along a diffusion gradient
- mass transport
- the end of the tracheoles are filled with water
how do insects maintain a concentration gradient
when cells are respiring, oxygen is used and so its concentration towards the ends of the tracheoles falls
This creates a concentration gradient that causes gaseous oxygen to diffuse from the atmosphere along the trachea and tracheous to the cells
CO2 is produced by cells during repiration
This creates a concentration gradient in the opposite direction and causes the CO2 to diffuse along the tracheoles and tracheae from the cells to the atmosphere
As diffusion in the air is much more rapid than in water, respiratory gases are changed quickly by this method
how does mass transport speed up the exchange of respiratory gases/abdominal pumping
the contraction of muscles in the abdomen in insects can squeeze the trachea enabling mass movement of air in and out
This further speeds up the exchange of respiratory gases
how does the ends of the tracheoles being filled with water speed up the rate of gas exchange
during periods of major activity, exercise for example, the muscle cells around the tracheoles carry out some anaerobic respiration
This produces lactate, which is soluble and lowers the water potential of the muscle cells
Water therefore moves into the cells from the ends of the tracheoles decreases in volume and doing so draws air further into them
This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid
This increases the rate at which air is moved in the trachea but leads to greater water evaporation/ loss
why must the spiracles control when they open and close
gas enters and leaves the trachea through tiny pores, called spiracles on the body surface
The spiracles may be open and closed by a valve
When the spiracles are open, water vapour can evaporate from the insect
Therefore, much time, insets keep their spiracles closed to prevent waters and they open periodically
what are the limitations of the tracheal system
The tracheal system is efficient but:
- it relies mostly on diffusion to exchange gases between the environment and cells
For diffusion to be effective, the pathway needs to be short which is why insects are small
As a result, the length of the diffusion pathway limits the size that insects can attain
why do fish need specialised exchange surfaces to exchange gases
fish are waterproof, and therefore a gas - tight outer covering
They are also relatively large, they also have a small surface area to volume ratio
Their body surface is therefore not adequete to supply and remove their respiratory gases and so, like insects and humans, they have evolved a specialised internal gas exchange surface: the gills
what is the structure of the gills
the gills are located within the body of the fish head
the gills are made up of filaments
The gill filaments are stacked up in a pile , rather like the pages in a book - increases the surface area of the gill
From the gill lamellae sit at right angles increasing SA
what is the passage of water in a fish
water is taken in through the mouth and forced over the gills and out through an opening on each side of the body
what is the flow of water in relation to the flow of blood
The flow of water over the gill lamelae and the flow of blood over the gill lamellae are in opposite directions
This is known as a countercurrent flow
what is the essential feature of the countercurrent exchange
this that flow water over goes over the gill lamellae do so in opposite directions of the blood
This arrangement means that:
1. blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen
Therefore diffusion of oxygen from the water to the blood takes place
- blood with little oxygen in it meets water which has the most, but not all, of its oxygen removed.
Again, diffusion of oxygen from the water to blood
As a result, a diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamealle
In this way, about 80% of the oxygen available in the water is absorbed into the blood of the fish
what would the uptake of water into the blood would be if the the flow was in the same direction
if the flow of water had been parallel ( in the same direction), the diffusion gradient would only be maintained across part of the length of the gill lamelae and 50% of the available oxygen would be absorbed by the blood
do plants undergo respiration
like animal cells, all plant cells require oxygen and carbon dioxide during respiration
what is the difference between plants and animals in gas exchange
some plant cells carry out photosynthesis
during photosynthesis plant cells take in carbon dioxide and produce oxygen
At times the gases produced in one process can be used for the other
The benefits of this is that it reduces gas exchange with external air
at times, the gases produced in one process can be used for the other, what does this depend on
This depends on the balance between the rates of the photosynthesis and respiration:
- when photosynthesis is taking place, although some CO2 comes from respiration of the cells, most of it is obtained from the external air. In the same way, some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant
2) when photosynthesis is not occurring, e.g. in the dark, oxygen diffuses into the leaf because it is constantly being used by cells during respiration. In the same way, CO2 produced during respiration diffuses out
what are the similarities between plant and insects gas exchange
- no living cells is far from external air, and therefore a source of oxygen and carbon dioxide
- diffusion takes place in the gas spaces (air) which makes it more rapid than if it were in water
Overall, therefore, there is a short, fast diffusion pathway
In addition, the air spaces inside a leaf have a very large surface area compared with the volume of living tissue
what are some adaptions of leaves for efficient gas exchange
most gaseous exchange occurs in the leaves, which show the following adaptations for rapid diffusion:
- many small pores, called stomata, and so no cell is far from a stomata and therefore the diffusion pathway is short
- numerous interconnecting air - spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells
- large surface area of mesophyll cells for rapid diffusion
what is the stomata
the stomata are minature pores that occur mainly, but not exclusively, on the leaves especially the underside
