2. Q Why do multi cellular organisms need mass transport systems (3)
Diffusion 22/07/2019
Q Why do multi cellular organisms need mass transport systems (3)
1.1 Understand why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).
I can describe limitations of diffusion as a means of transport in large organisms in terms of surface area: volume ratio.
I understand why large organisms require a mass transport system to supply materials and remove waste efficiently

3. Q Why do multi cellular organisms need mass transport systems (3)
Diffusion is slower over larger distances.
Rate of diffusion would be too slow to carry raw materials from specialised cells to other body cells
And to effectively remove metabolic waste from these body cells.

4. Why do multicellular organisms need transport systems ?
All cells need energy
Deliver glucose and oxygen and remove waste products.
Single celled organisms=diffusion directly from external environment (rate is quick due to short distance).

5. Quick iGCSE recap:
Diffusion= The passive movement of particles (atoms ions or molecules) down a concentration gradient, or from regions of higher to regions of lower concentration.
(iGCSE) Factors affecting diffusion:
Distance
Concentration gradient
Temperature
Particle size (a smaller particle at a given temperature moves faster than a larger particle)

6. Diffusion of oxygen into a cell9
The concentration of
oxygen molecules is
greater outside the cell
than inside
So the oxygen molecules
diffuse into the cell
The concentration of carbon dioxide is greater inside the cell than outside. Carbon dioxide molecules will diffuse out of the cell.

7. Because the cell is using up oxygen, the
concentration of oxygen inside the cell is
always lower then the concentration
outside.
The diffusion gradient is maintained
The oxygen is used in respiration. Carbon dioxide is produced by respiration. The concentration of carbon dioxide inside the cell increases, so carbon dioxide diffuses out of the cell.
So oxygen continues to diffuse in

8. Single-celled organisms are rarely more tan 1mm in diameter13
In a single-celled organism (such as Amoeba) the distance is so small that diffusion is rapid enough for the cell’s needs
maximum
distance
is 0.1 mm
oxygen
Single-celled organisms are rarely more tan 1mm in diameter
Bacteria range from microns (1-10 µ )
carbon dioxide
Compared with its bulk, a cell like this has a huge surface area through which diffusion can occur
( 1 µ is one thousandth of a millimetre)

9. Inside the bodies of large animals, diffusion of oxygen into their cells is rapid enough
For transport across the whole body, diffusion
would be much too slow
Large organisms have evolved mass transport
Systems that carry oxygen from outside the body to
the cells inside

10. Using oxygen as an example
Inside the bodies of large animals, diffusion of oxygen into their cells is rapid enough
For transport across the whole body, diffusion would be much too slow.-we will look in more detail at why later
Large organisms have evolved mass transport
Systems carry raw materials from specialised exchange organs body to the cells inside

11. Mammals = circulatory system.
Mass transport systems: carry raw materials from specialised exchange organs to body cells, and to remove metabolic waste.
Mammals = circulatory system.
Heart = pump; cells in tissues and organs get nutrients and oxygen from the blood and dispose of metabolic waste back into the blood.
Metabolism: the chemical processes that occur within a living organism in order to maintain life.

12. Earthworm 16 diffusion takes place through the thin skin of the worm
CO2 diffuses
out
O2 diffuses in
Section through
worm’s skin
0.04mm
the blood vessels
absorb the O2 and
carry it to the body

13. In mammals, birds, reptiles and amphibia, 22
In mammals, birds, reptiles and amphibia,
oxygen and carbon dioxide are exchanged by
diffusion in the lungs
In fish, this exchange of gases takes place by
diffusion through the gills
The oxygen dissolved in the water diffuses into
the blood vessels in the gills.

14. Leaf 26 O2 and CO2 diffuse through pores in the epidermis the ‘veins’
bring water
The leaf needs carbon dioxide for photosynthesis and oxygen for respiration.
These gases diffuse through pores in the leaf’s epidermis and enter the air spaces between cells. They then diffuse through the cell wall into the cytoplasm.
In a thin leaf, the
diffusion distance
is short
O2 and CO2 diffuse
into the spaces between cells

15. Low surface area to volume ratios
Large organisms diffusion too slow (rate) to supply cells tissue and organs with substance they need and remove waste substances due to
large distances.
Low surface area to volume ratios
So we have mass transport systems.
Little = high ratio
Large = low ratio

16. Activity text book pg 60-61 (blue shaded box)
As the size of an organisms increases
Surface area increases by a factor of 4
Volume increases by a factor of 8
So SA:V ratio halves.