3.1 SURFACE AREA TO VOLUME RATIO
The relationship between the size of an organism, SA:V ratio
★ The larger the organism > the smaller the SA:V ratio > slower the metabolic rate/
rate of diffusion > unable to meet demands
Adaptations to compensate for the small SA:V ratio in multicellular organisms
1. Increase SA w/ gas exchange surfaces (foldings like villi, larger body parts like ears)
2. Specialised gas exchange systems
Why do multicellular organisms require specialised gas exchange surfaces and systems?
- Small SA:V ratio means the diffusion/travel distance of substances is longer and
substances cannot diffuse quick enough to meet the O2 volume demands
- To meet the higher demands of ATP needed for movement > higher rates of
respirations means higher demands for O2 and glucose
Volume = the O2 demand of the cell/organism
Surface area = how much O2 can get into organism in per unit time
PS 1.1 – Students could use agar blocks containing indicators to determine the effect of
surface area to volume ratio and concentration gradient on the diffusion of an acid or alkali.
MS 4.1 – Students could be given the dimensions of cells with different shapes from which to
calculate the surface area to volume ratios of these cells.
3.2 GAS EXCHANGE – insects, fish, humans, plants,
3 main features for efficient gas exchange
1. Large SA = folded membranes like the cristae in mitochondria = rate of diffusion
2. Short diffusion distances = one cell thick capillaries = rapid diffusion
3. Steep diffusion gradient maintained = good supply of blood, surrounded by dense
capillaries network + ventilation (CO2 out and O2 in) in alveoli
4. Partially permeable membrane
4 MAIN FACTORS TO KEEP IN MIND WHEN ANSWERING
1. Mention transport type
2. S.A
3. Diffusion distance
4. Diffusion gradient
5. How it affects rate of diffusion
SINGLE CELLED ORGANISM
- Large SA:V > no gas exchange system needed
,INSECTS – spiracles, tracheae, tracheoles
- CO2 moves out of the body by diffusing into blood and out through cuticles
(exoskeleton) on body surface + spiracles
3 MAIN PARTS OF ADAPTATION + 2 MECHANISMS
1. Spiracles – holes in tracheae that open and close by a valve allow in and out of gas
and water
a. More open when active for as increased O2 demand = water loss by diffusion
+ evaporation
b. Most closed, few open at rest to prevent water loss/ conserve water
= COMPROMISE between water loss and gas exchange
2. Trancheae – hollow, supported by rings of chitin >
3. Branches into MANY trancheoles
a. in large amounts + highly branched = large SA for gas exchange
b. one cell thick trancheoles walls = short d.d for O2
c. close to spiracles = short d.d
d. Trancheoles embedded within muscle fibres = short d.d
4. ABDOMINAL PUMPING (when active mostly)
1. Contraction of muscles decreases pressure in the tracheae, this draws air in and
volume of O2 increases, removes CO2 by ventilation as air flows from an area of
high > low pressure
2. maintains a steep diffusion gradient
5. MASS FLOW MECHANISM = muscles + tracheoles
1. When insect is at rest = fluid like water accumulates in trancheoles
2. When insect is highly active flying = aerobic respiration is unable to meet energy
demand from limited O2, muscle cells ANAEROBIC RESPIRES, producing lactic
acid as lactate ions
3. This decreases the water potential Ψ in muscle cells and water moves from the
tracheoles into muscle cells by osmosis, from a higher Ψ area to lower Ψ
4. Tracheoles now have a lower Ψ, with less water/fluid in them
a. Increased SA for gas exchange/ diffusion
b. O2 travels faster in gas than liquid = FASTER RATE OF DIFFUSION
, MINIMISING WATER LOSS from evaporation and diffusion = CONSERVE
- Insects have an exoskeleton that is WATERPROOF, IMPERMEABLE so diffusion out
of the body surface is impossible = conserves water
- small SA:V ratio!! Insects still big + multicellular keep in mind
- Spiracles open and close controlled by valves
General process of gas exchange in insects
1. Gases move in and out of tracheae through the spiracles
2. Abdominal pumping = Contraction of muscles in the tracheae draws air in and out?
3. Diffusion gradient + mass flow and allows O2 to diffuse into the body when Co2
diffuses out
FISH – gills and lamellae
Why can't fish bodies act as an exchange surface?
- O2 is relatively less soluble in water due to the higher density than air > transport of
O2 more difficult
- also waterproof, impermeable outer membrane
- Small SA:V ratio = system needed still big + multicellular keep in mind
2 MAIN ADAPTATIONS
1. Gills are supported by gill arches with two sides of stacked filaments that hold large
amount of lamellae
2. Lamellae are
a. one cell thick + flattened = short diffusion distance
b. Kept vertical upright in water = large SA
c. In large amounts on filaments = large SA
d. Covers dense network of capillaries = steep diffusion gradient +
e. COUNTERCURRENT for O2, CO2
i. Blood and water flow in opposite directions so a steep diffusion
gradient is maintained through the WHOLE GILL length ( low O2 conc
next to blood conc always)
The relationship between the size of an organism, SA:V ratio
★ The larger the organism > the smaller the SA:V ratio > slower the metabolic rate/
rate of diffusion > unable to meet demands
Adaptations to compensate for the small SA:V ratio in multicellular organisms
1. Increase SA w/ gas exchange surfaces (foldings like villi, larger body parts like ears)
2. Specialised gas exchange systems
Why do multicellular organisms require specialised gas exchange surfaces and systems?
- Small SA:V ratio means the diffusion/travel distance of substances is longer and
substances cannot diffuse quick enough to meet the O2 volume demands
- To meet the higher demands of ATP needed for movement > higher rates of
respirations means higher demands for O2 and glucose
Volume = the O2 demand of the cell/organism
Surface area = how much O2 can get into organism in per unit time
PS 1.1 – Students could use agar blocks containing indicators to determine the effect of
surface area to volume ratio and concentration gradient on the diffusion of an acid or alkali.
MS 4.1 – Students could be given the dimensions of cells with different shapes from which to
calculate the surface area to volume ratios of these cells.
3.2 GAS EXCHANGE – insects, fish, humans, plants,
3 main features for efficient gas exchange
1. Large SA = folded membranes like the cristae in mitochondria = rate of diffusion
2. Short diffusion distances = one cell thick capillaries = rapid diffusion
3. Steep diffusion gradient maintained = good supply of blood, surrounded by dense
capillaries network + ventilation (CO2 out and O2 in) in alveoli
4. Partially permeable membrane
4 MAIN FACTORS TO KEEP IN MIND WHEN ANSWERING
1. Mention transport type
2. S.A
3. Diffusion distance
4. Diffusion gradient
5. How it affects rate of diffusion
SINGLE CELLED ORGANISM
- Large SA:V > no gas exchange system needed
,INSECTS – spiracles, tracheae, tracheoles
- CO2 moves out of the body by diffusing into blood and out through cuticles
(exoskeleton) on body surface + spiracles
3 MAIN PARTS OF ADAPTATION + 2 MECHANISMS
1. Spiracles – holes in tracheae that open and close by a valve allow in and out of gas
and water
a. More open when active for as increased O2 demand = water loss by diffusion
+ evaporation
b. Most closed, few open at rest to prevent water loss/ conserve water
= COMPROMISE between water loss and gas exchange
2. Trancheae – hollow, supported by rings of chitin >
3. Branches into MANY trancheoles
a. in large amounts + highly branched = large SA for gas exchange
b. one cell thick trancheoles walls = short d.d for O2
c. close to spiracles = short d.d
d. Trancheoles embedded within muscle fibres = short d.d
4. ABDOMINAL PUMPING (when active mostly)
1. Contraction of muscles decreases pressure in the tracheae, this draws air in and
volume of O2 increases, removes CO2 by ventilation as air flows from an area of
high > low pressure
2. maintains a steep diffusion gradient
5. MASS FLOW MECHANISM = muscles + tracheoles
1. When insect is at rest = fluid like water accumulates in trancheoles
2. When insect is highly active flying = aerobic respiration is unable to meet energy
demand from limited O2, muscle cells ANAEROBIC RESPIRES, producing lactic
acid as lactate ions
3. This decreases the water potential Ψ in muscle cells and water moves from the
tracheoles into muscle cells by osmosis, from a higher Ψ area to lower Ψ
4. Tracheoles now have a lower Ψ, with less water/fluid in them
a. Increased SA for gas exchange/ diffusion
b. O2 travels faster in gas than liquid = FASTER RATE OF DIFFUSION
, MINIMISING WATER LOSS from evaporation and diffusion = CONSERVE
- Insects have an exoskeleton that is WATERPROOF, IMPERMEABLE so diffusion out
of the body surface is impossible = conserves water
- small SA:V ratio!! Insects still big + multicellular keep in mind
- Spiracles open and close controlled by valves
General process of gas exchange in insects
1. Gases move in and out of tracheae through the spiracles
2. Abdominal pumping = Contraction of muscles in the tracheae draws air in and out?
3. Diffusion gradient + mass flow and allows O2 to diffuse into the body when Co2
diffuses out
FISH – gills and lamellae
Why can't fish bodies act as an exchange surface?
- O2 is relatively less soluble in water due to the higher density than air > transport of
O2 more difficult
- also waterproof, impermeable outer membrane
- Small SA:V ratio = system needed still big + multicellular keep in mind
2 MAIN ADAPTATIONS
1. Gills are supported by gill arches with two sides of stacked filaments that hold large
amount of lamellae
2. Lamellae are
a. one cell thick + flattened = short diffusion distance
b. Kept vertical upright in water = large SA
c. In large amounts on filaments = large SA
d. Covers dense network of capillaries = steep diffusion gradient +
e. COUNTERCURRENT for O2, CO2
i. Blood and water flow in opposite directions so a steep diffusion
gradient is maintained through the WHOLE GILL length ( low O2 conc
next to blood conc always)
