Module 3: Exchange Surfaces & Breathing Flashcards
Why do organisms need exchange surfaces?
All organisms need to be able to take up simple substances from the environment e.g. Oxygen.
They also need to get rid of waste products e.g. CO2.
How do very large and very small organisms differ in the way they exchange substances?
Very small organisms (singe-celled organisms) are able to exchange nutrients, gases and other waste across their surface membrane.
Very large organisms (multicellular) require exchange surfaces.
Why do very small organisms not require exchange surfaces and large organisms do?
Small organisms have a very small surface-area:volume ratio and very low demands for nutrients and oxygen.
Large organisms have a very small surface area: volume ratio. This means that their outer surface is not large enough to enable oxygen to exchange fast enough into the body to keep all the cells alive.
Large organisms are also very active which means that they have very high demands for nutrients and oxygen etc.
What is an exchange surface?
A specialised area that makes it easier for a substance to travel from one side of the surface to another.
Examples of exchange surfaces
alveoli, vili of small intestine, root hair cells
What are features of a good exchange surface?
1) Large surface area- more space for molecules to pass through
2) Thin walls- shorter diffusion pathway
3) Fresh supply of molecules on one side- keeps conc high
4) Removal of molecules on other side- keep conc low
5) Permeable to exchange molecules
SA:VOL
SA:VOL= surface area (cm2) / volume (cm3)
Larger organisms have a smaller surface-area to volume ratio
What is gaseous exchange?
The movement of gas by diffusion between an organism and its environment across a barrier. e.g. alveoli
The function of the lungs
The function of the lungs is for ventilation for gaseous exchange.
Air is breathed into the lungs. Oxygen passes from the air in the alveoli into the capillaries and is used for aerobic respiration.
Veins carry deoxygenated blood to the lungs which contain CO2 which is breathed out.
Ventilation
Breathing
Inspiration (breathing in)
1) Diaphragm contracts, flattens and moves downwards.
2) Intercostal muscles contract- ribs move up and out.
3) This increases the volume in the thorax and lungs.
4) This reduces the pressure in the thorax and lungs below atmospheric pressure.
5) Air moves down a pressure gradient into the lungs
Expiration (breathing out)
1) Diaphragm contracts and moves upwards.
2) Intercostal muscles contract- ribs move down and in
3) This decreases the volume in the thorax and lungs
4) This increases the pressure in the thorax and lungs above atmospheric pressure
5) Air moves down a pressure gradient out of the lungs
Why are the lungs good for gaseous exchange?
- Millions of alveoli provide a large S.A. for more space for O2 and CO2 to diffuse across (NOT SA:VOL).
- Thin barrier- 2 cells thick, walls of alveoli and capillaries are made of squamous epithelial cells. Alveoli and capillary only one cell thick.
- Plasma membranes of the capillaries and alveoli are fully Permeable to O2 and CO2.
Maintaining the conc gradient in the lungs
SUMMARY:
good blood supply - many capillaries ‐ to carry dissolved gases to and from the alveoli
ventilation caused by intercostal muscles and diaphragm ‐ to refresh the air in the alveoli to keep conc of O2 in the alveoli high.
Surfactant
Alveoli are lined with a thin water layer. As we breathe out the water evaporates and leaves the lungs
The cohesion between water molecules would cause the alveoli to collapse.
A compound called surfactant produced in the alveoli lines the alveoli. This reduces the cohesion to prevent alveoli collapsing.