3.1.1a/b Diffusion & Exchange Surfaces Flashcards
What is diffusion
The net random movement of particles from an area of higher concentration to lower concentration, down a concentration gradient, until an equilibrium is reached
Why is diffusion usually enough to satisfy the needs of a single-celled organism
- Metabolic activity is usually low
- Diffusion distante is short
- SA:V is high due to small size
Surface area of a sphere
4πr²
Volume of a sphere
4/3 πr^3
Relationship between size of organism and its SA:V
As the size of the organism increases, the SA:V decreases
Diffusion issues for organisms with a small SA:V (multicellular organisms)
- diffusion distance is too great
- rate of exchange is not fast enough & quantities exchanged are not large enough to meet metabolic demands
4 features that effective exchange surfaces have in common
- Large SA
- Thin layers
- Good blood supply
- Ventilation
Why does large SA make exchange surfaces more effective
This provides the area needed for exchange & overcomes the limitations of the SA:V ratio of larger organisms
EXAMPLE of exchange surfaces with a large SA
ROOT HAIR CELLS
- Cells on plant roots grow into long ‘hair-like’ extensions. Each branch of a root will be covered by millions of these microscopic hairs
- This increases SA, helping increase rate of absorption of water by osmosis & mineral ions by active transport
Why do thin layers make exchange surfaces more effective
This means the distance that substances have to diffuse across are short, making the process faster & efficient.
EXAMPLE of exchange surfaces that have thin layers
ALVEOLI
- These are the gas exchange surface in lungs
- Each alveolus is made from a single layer of thin flat cells called alveolar epithelium
- Oxygen diffuses out of the alveolar space into blood, CO2 diffuse in opposite direction
- The thin alveolar epithelium helps to decrease distance over which O2 & CO2 diffusion takes place, increasing rate of diffusion
Why does good blood supply make exchange surfaces more effective
Having a good blood supply ensures substances are constantly delivered to & removed from the exchange surface.
This maintains a steep concentration gradient for diffusion.
The steeper the concentration gradient, the faster diffusion takes place.
EXAMPLE of exchange surfaces with good blood supply
ALVEOLI
- Alveoli are surrounded by a large capillary network, giving each alveolus its own blood supply. The blood constantly takes oxygen away from the alveoli & brings more CO2
FISH GILLS
- Gills contain a large network of capillaries - keeps them well-supplied w blood.
Why does ventilation make exchange surfaces more effective
For gases, a ventilation system helps maintain concentration gradients & makes the process more efficient.
Eg the alveoli & gills of a fish where ventilation means a flow of water carrying dissolved gases
EXAMPLE of exchange surfaces with ventilation
ALVEOLI
- The lungs are ventilated so the air in each alveolus is constantly replaced
- This helps to maintain concentration gradients of O2 & CO2
FISH GILLS
- Gills are well-ventilated - fresh water constantly passes over them.
- This helps to maintain a concentration gradient of O2 - increasing rate at which O2 diffuses into blood
How to calculate SA of cube
- find area of one side
- multiply the area of one side by 6 (6 faces)
if all faces are not of equal area, calculate the area of all individual faces and add them up
How to calculate V of cube
length x width x height
Why do organisms need to exchange substances with their environment
- Cells need to take in substances like oxygen & glucose for aerobic respiration & other metabolic reactions
- Also need to excrete waste products from these reactions - like CO2 & urea
See pg70 for eg of SA:V ratio
Why do single-celled organisms need exchange surfaces
An organism needs to:
- supply every one of its cells w substances like glucose & oxygen
- remove waste products from every cell
In single-celled organisms, these substances can diffuse directly into or out of the cell across the cell surface membrane. The diffusion rate is quick bc of the small distances the substances have to travel
Why do multicellular organisms need exchange surfaces
In multicellular organisms, diffusion across the outer membrane is too slow for several reasons:
_1._Some cells are deep within the body - theres a big distance between them & the outside env
_2._Larger animals have a low SA:V ratio - its difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface
_3._Multicellular organisms have a higher metabolic rate than single-celled, so they use up oxygen & glucose faster
Therefore they need exchange surfaces
