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Unit 3 - organisms exchange substances with their environments.
Includes diagrams to make it easier to understand
Follows specification order
Recently written hence suitable for new/current specification AQA.
ORGANISMS EXCHANGE SUBSTANCES WITH THEIR ENVIRONMENT:
6.1 – EXCHANGE BETWEEN ORGANISMS AND THEIR ENVIRONMENT:
- External environment is different from the internal environment found within an organism and
within its cells.
- To survive, organisms transfer materials between the 2 environments.
This transfer takes place at exchange surfaces and always involves crossing cell plasma
membranes.
- Tisue fluid = Environment around cells of multicellular organisms.
- Mass transport system maintains diffusion gradients that bring materials to and from the cell-
surface membranes.
- Size and metabolic rate of an organism will affect the amount of each material that is
exchanged.
E.g. organisms with high metabolic rate exchange more materials and so require a larger SA:V
ratio.
- Examples of things that need to be interchanged between organism and its environment
include:
Respiratory gases (Oxygen and CO2).
Nutrients (Glucose, fatty acids, amino acids, vitamins, minerals).
Excretory products (urea, CO2).
Heat.
- These exchanges can take place in 2 ways:
1- Passively (no metabolic energy required), by diffusion and osmosis.
2- Actively (metabolic energy required), by active transport.
Surface Area To Volume Ratio:
- Exchange takes place at surface of an organism, but materials absorbed are used by cells that
make most of its volume up.
- For exchange to be effective.
Exchange surface of organism must be larger than its volume.
- Small organisms have high SA:V ratio, to allow efficient exchange across their body surface.
, SA bigger than V.
- Large organisms have lower SA:V ratio. Volume increases at faster rate than their SA.
- Because of this simple diffusion of substances across outer surface can only meet the needs of
relatively inactive organisms.
- Even if outer surface could supply enough of a substance, it would take too long for it to reach
the middle of the organism if diffusion was the only method of transport.
Organisms have evolved these features:
1- Flattened shape so that no cell is ever far from the surface.
(E.g. Flatworm or Leaf).
2- Specialised exchange surfaces with large areas to increase the SA:V ratio.
(E.g. lungs in mammals, gills in fish).
Features Of Specialised Exchange Surfaces:
Exchange surfaces show the following characteristics:
, - large SA:V of organism = increases rate of exchange.
- Very thin = diffusion distance is short and therefore materials cross the exchange surface
rapidly.
- Selectively permeable to allow selected materials across.
- Movement of environmental medium.
E.g. air, to maintain a diffusion gradient.
- A transport system to ensure movement of internal medium e.g. blood, in order to maintain a
diffusion gradient.
Fick’s Law:
- Being thin, specialised exchange surfaces are easily damaged and dehydrated.
Therefore they’re often located inside an organism.
6.2 – GAS EXCHANGE IN SINGLE-CELLED ORGANISMS AND INSECTS:
Gas Exchange In Single-celled Organisms:
- Single celled organisms are small so have a large SA:V ratio.
- Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-
surface membrane.
- In the same way CO2 from respiration diffuses out across their body surface.
- Single celled organism has no cell wall, so there’s no barrier to diffusion.
Gas Exchange In Insects:
- Insects have evolved mechanisms to conserve water.
- The increase in SA required for gas exchange conflicts with conserving water because, water will
evaporate from it.
- For gas exchange, insects have an internal network of tubes called tracheae.
- Tracheae are supported by strengthened rings (of cartilage) to prevent them from collapsing.
- Tracheae divide into smaller dead-end tubes called tracheoles.
- Tracheoles extend throughout all the body tissues of the insect.
- In this way, Oxygen from air is brought directly to respiring tissues, as there’s a short diffusion
pathway from a tracheole to any body cell.
Respiratory gases move in/out of the tracheal system in 3 ways:
, 1- ALONG A DIFFUSION GRADIENT:
- When cells are respiring, oxygen is being used up so its concentration towards the ends of the
tracheoles falls.
- This creates a diffusion gradient that causes gaseous oxygen to diffuse from atmosphere along
the tracheae and tracheoles to cells.
- CO2 is produced by cells during respiration.
- This creates diffusion gradient in opposite direction.
- Causing gaseous CO2 to diffuse along the tracheoles and tracheae from cells to atmosphere.
- As diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly by
this method.
2- MASS TRANSPORT:
- Contraction of muscles in insects can squeeze the trachea enabling mass movements of air
in/out.
- This further speeds up the exchange of respiratory gases.
3- THE ENDS OF THE TRACHEOLES ARE FILLED WITH WATER:
- During major activities muscle cells around tracheoles respire carry out some anaerobic
respiration.
- This produces lactate.
Which is soluble and lowers water potential of muscle cells.
- Water therefore moves into cells from tracheoles by osmosis.
- Water in the ends of the tracheoles decreases in volume, so draws air further into them.
- Meaning, the final diffusion pathway is in gas rather than a liquid phase.
Therefore diffusion is more rapid.
- This increases rate at which air is moved in the tracheoles but leads to greater water
evaporation.
- Gases enter/leave tracheae through spiracles (tiny pores) on the body surface.
- Spiracles may be opened/closed by a valve.
- When spiracles are open, water vapour can evaporate from the insect.
- Insects keep their spiracles closed to prevent water loss (advantage).
- Periodically they open spiracles to allow gas exchange.
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