Year 1
Biology Department
Exchange Surfaces
Name:
Teacher:
1
,Specialised system Simple diffusion Specialised arrangement of cells
The environment around the cells of multicellular organisms is called tissue fluid. The majority of cells are too
far from exchange surfaces for _______________ alone to supply their tissue fluid with the various materials
needed to keep its composition relatively constant. Organisms with a high metabolic rate will exchange more
materials and so require a _________ surface area to volume ratio.
Calculating surface area to volume ratio
Length of one edge Surface area of a 6 sided Volume of a 6 sided S.A. : Vol. ratio (surface
of a cube. (mm) cube (height x length x 6) cube (height x length x area / volume)
(mm2) width) (mm3)
1
2
3
4
5
6
As the cube size increases (or an organism gets bigger), the surface area to volume ratio decreases.
2
, Effect of surface area : volume ratio on diffusion rate
Objective Equipment/materials
• To calculate surface-area-to-volume ratio (SA:V) Ruler
• To show the effect of surface-area-to-volume ratio
on the diffusion rate of hydrochloric acid Agar blocks stained with pH indicator
Syringe
Safety
Stop clock
Razor blades are sharp – be careful not to cut yourself
HCl may damage eyes – wear safety goggles and lab Razor blade
coat to protect clothes
Procedure
1. Cut the agar into five different-sized cubes (from 2 mm to 10 mm) using a sharp razor.
2. Using the razor blade to lift the agar blocks, place each block into a test tube or beaker and label clearly
with the size of the block.
3. Using a syringe, immerse the blocks in 2 M HCl acid by covering each block with 2 cm3 of the acid.
4. Immediately start the stopwatch and time how long the pink colour takes to disappear and for each block
to turn completely colourless.
5. Record the results in a suitable table.
6. Draw a suitable graph to show the effect SA:Vol ratio has on the rate of diffusion.
Results
Size S.A. Volume S.A.:Volume Time taken Rate =
(mm) ratio (s) 1/time
2
4
6
8
10
Conclusion
Smaller blocks have larger surface area to volume ratio. Shorter diffusion distance to the centre so that HCl
diffuses more quickly in smaller blocks than larger blocks and so colour change is more rapid in smaller
blocks.
Controlled variables: temperature, no stirring of solutions (to avoid providing Kinetic Energy), concentration
and volume of acid added to the blocks.
The formula that helps to explain how diffusion is affected: diffusion = concentration gradient x surface area
Diffusion distance
To ensure results are valid, HCl should be delivered to the cube, and the syringe plunger depressed at the
same rate for each size of cube. To ensure reliability more cubes should be cut for each size investigated.
Limitations: Agar cubes are not animal cells, and therefore no membranes present which are selectively
permeable. Diffusion in living organisms involves many types of molecules.
The volume of an organism can be calculated by placing the organism in a bath of water and measuring the
volume of water that has been displaced. 3
, Importance of SA:Vol ratios
Single celled organism have a large SA:Vol which is adequate for molecules to move through the cell surface
membrane by diffusion and active transport. Multicellular organism have a small SA:Vol which is not adequate
to obtain sufficient molecules e.g oxygen that are required and therefore this is overcome by having
specialised gas exchange surfaces e.g. lungs, gills and tracheoles. Distance is too great in multicellular
organism, outer cells will use up the oxygen and nutrients before they reach the inner ones.
More active animals and mammals require more oxygen and glucose to supply energy for movement and to
maintain their body temperature.
Organisms which have a flattened shape such as a flatworm have an elongated thin body which increases
its surface area to volume ratio
Examples of molecules that need to be interchanged:
Example Mechanism
Respiratory gases e.g. carbon dioxide • Simple diffusion
and oxygen
Nutrients e.g.
• Glucose and amino acids • Polar glucose and amino acids by facilitated diffusion.
or active transport (when against a concentration
• Fatty acids, vitamins and gradient)
minerals • Non polar (fatty acids and some vitamins and minerals)
by simple diffusion.
Excretory products (urea and carbon • Simple diffusion
dioxide)
Features and their effect on a gas exchange surfaces
Feature Gas exchange
Increases diffusion rate
Short diffusion distance
Oxygen and carbon dioxide can diffuse freely through
phospholipid bilayer.
Maintain steep concentration/ diffusion gradient.
Maintain steep concentration / diffusion gradient.
4
Biology Department
Exchange Surfaces
Name:
Teacher:
1
,Specialised system Simple diffusion Specialised arrangement of cells
The environment around the cells of multicellular organisms is called tissue fluid. The majority of cells are too
far from exchange surfaces for _______________ alone to supply their tissue fluid with the various materials
needed to keep its composition relatively constant. Organisms with a high metabolic rate will exchange more
materials and so require a _________ surface area to volume ratio.
Calculating surface area to volume ratio
Length of one edge Surface area of a 6 sided Volume of a 6 sided S.A. : Vol. ratio (surface
of a cube. (mm) cube (height x length x 6) cube (height x length x area / volume)
(mm2) width) (mm3)
1
2
3
4
5
6
As the cube size increases (or an organism gets bigger), the surface area to volume ratio decreases.
2
, Effect of surface area : volume ratio on diffusion rate
Objective Equipment/materials
• To calculate surface-area-to-volume ratio (SA:V) Ruler
• To show the effect of surface-area-to-volume ratio
on the diffusion rate of hydrochloric acid Agar blocks stained with pH indicator
Syringe
Safety
Stop clock
Razor blades are sharp – be careful not to cut yourself
HCl may damage eyes – wear safety goggles and lab Razor blade
coat to protect clothes
Procedure
1. Cut the agar into five different-sized cubes (from 2 mm to 10 mm) using a sharp razor.
2. Using the razor blade to lift the agar blocks, place each block into a test tube or beaker and label clearly
with the size of the block.
3. Using a syringe, immerse the blocks in 2 M HCl acid by covering each block with 2 cm3 of the acid.
4. Immediately start the stopwatch and time how long the pink colour takes to disappear and for each block
to turn completely colourless.
5. Record the results in a suitable table.
6. Draw a suitable graph to show the effect SA:Vol ratio has on the rate of diffusion.
Results
Size S.A. Volume S.A.:Volume Time taken Rate =
(mm) ratio (s) 1/time
2
4
6
8
10
Conclusion
Smaller blocks have larger surface area to volume ratio. Shorter diffusion distance to the centre so that HCl
diffuses more quickly in smaller blocks than larger blocks and so colour change is more rapid in smaller
blocks.
Controlled variables: temperature, no stirring of solutions (to avoid providing Kinetic Energy), concentration
and volume of acid added to the blocks.
The formula that helps to explain how diffusion is affected: diffusion = concentration gradient x surface area
Diffusion distance
To ensure results are valid, HCl should be delivered to the cube, and the syringe plunger depressed at the
same rate for each size of cube. To ensure reliability more cubes should be cut for each size investigated.
Limitations: Agar cubes are not animal cells, and therefore no membranes present which are selectively
permeable. Diffusion in living organisms involves many types of molecules.
The volume of an organism can be calculated by placing the organism in a bath of water and measuring the
volume of water that has been displaced. 3
, Importance of SA:Vol ratios
Single celled organism have a large SA:Vol which is adequate for molecules to move through the cell surface
membrane by diffusion and active transport. Multicellular organism have a small SA:Vol which is not adequate
to obtain sufficient molecules e.g oxygen that are required and therefore this is overcome by having
specialised gas exchange surfaces e.g. lungs, gills and tracheoles. Distance is too great in multicellular
organism, outer cells will use up the oxygen and nutrients before they reach the inner ones.
More active animals and mammals require more oxygen and glucose to supply energy for movement and to
maintain their body temperature.
Organisms which have a flattened shape such as a flatworm have an elongated thin body which increases
its surface area to volume ratio
Examples of molecules that need to be interchanged:
Example Mechanism
Respiratory gases e.g. carbon dioxide • Simple diffusion
and oxygen
Nutrients e.g.
• Glucose and amino acids • Polar glucose and amino acids by facilitated diffusion.
or active transport (when against a concentration
• Fatty acids, vitamins and gradient)
minerals • Non polar (fatty acids and some vitamins and minerals)
by simple diffusion.
Excretory products (urea and carbon • Simple diffusion
dioxide)
Features and their effect on a gas exchange surfaces
Feature Gas exchange
Increases diffusion rate
Short diffusion distance
Oxygen and carbon dioxide can diffuse freely through
phospholipid bilayer.
Maintain steep concentration/ diffusion gradient.
Maintain steep concentration / diffusion gradient.
4
