Unit 3 - Organisms exchange substances with their environment
Summary
Summary AQA A Level Biology - Unit 3 organisms Notes
1 view 0 purchase
Course
Unit 3 - Organisms exchange substances with their environment
Institution
AQA
Concise and detailed notes
Specification based
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.
Unit 3 - Organisms exchange substances with their environment
All documents for this subject (186)
Seller
Follow
22harmans
Content preview
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.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller 22harmans. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $8.48. You're not tied to anything after your purchase.
