Organisms exchange substances with their environment
Surface area to volume ratio
Single celled and small organisms
-have large SA:V
-can exchange substances over body using just diffusion especially if they have a low
metabolic rate
Larger organisms
-have small SA:V
-most cells are too far from exchange surfaces for diffusion alone to supply/remove materials
-specialised exchange systems have evolved to meet requirements
Specialised exchange surfaces
-large SA:V → increases rate of exchange
-thin so short diffusion distance → increases rate
-selectively permeable
-movement of environmental medium to maintain concentration/diffusion gradient
-transport system to ensure movement of internal medium to maintain concentration
gradient
Disadvantages of specialised exchange surfaces
-thin so easily damaged (fragile) and dehydrates quickly
→ solution: located inside organism
-needs way of moving external medium over exchange surface
→ solution: mechanism such as ventilation in lungs
Gas exchange in insects
Insects are too large for diffusion alone to meet their requirements
-their exchange system consists of an internal network of tubes which reduces evaporation
of water from exchange surface
-spiracles = tiny pores on body surface that allow air to enter and
leave tracheae (mostly closed to conserve water)
-tracheae = network of internal tubes throughout body of insect
→ supported by strengthening rings to prevent them from collapsing
→ divide into tracheoles (smaller permeable tubes)
,→every cell is short distance from a tracheole which are lined with a single layer of cells to
minimise diffusion pathway
Diffusion gradient
→ O diffuses from tracheoles to respiring cells
→O conc at end of tracheoles decreases creating a conc gradient
→ O diffuses from atmosphere down trachea and tracheoles
→ CO2 produced in repairing cells diffuses into tracheoles and creates a conc gradient in
the opposite direction (from tracheoles to trachea to air)
Mass transport
-mass transport system = system that moves materials from
exchange surfaces to cells where they are required
-rhythmic abdominal contractions squeeze tracheae which
enables mass movement of air in and out spiracles
Ends of tracheoles filled with water
-cells may respire anaerobically in periods of rapid activity → creates lactate which of soluble
(lowers water potential)
-water moves from tracheoles into cells by osmosis
-water decreases in volume
-final diffusion path is gas phase → gases move faster through air than water → faster
diffusion rate
Gas exchange in fish
-there is a lower concentration of oxygen in water than air
-water enters mouth and is passed over gills
-stacked gill filaments increase surface area
-covered by lamallae which further increases surface area
-lamallae has many capillaries and thin surface layer of cells
→ increases rate of diffusion
, Counter current flow
-water moves along lamallae in one direction and blood flows in an opposite direction
(counter current flow)
-maintains large concentration gradient between blood and water
-ensures conc of oxygen in water is always higher than that in blood
-water with lowest O conc is adjacent to most deoxygenated blood
When water enters gills:
-The concentration of O2 is always higher in H20
-Blood with a relatively high O2 concentration meets water also with its maximum O2
concentration
-Diffusion occurs from water → lamellae down a concentration gradient
As blood moves along lamellae:
-Blood with little oxygen meets water with most of its oxygen
removed -Diffusion occurs as the
concentration of O2 in water is still higher than in the blood
Advantages of counter current system
-diffusion gradient maintained along entire length of lamellae
-majority of available O from water is absorbed
Parallel flow - cartilaginous fish
-blood with high conc O would meet water with lowest conc
-diffusion down gradient
-equilibrium would be reached as water moved through gills
-diffusion gradient only maintained across half the length of
lamallae → blood absorbs less available oxygen from water
Surface area to volume ratio
Single celled and small organisms
-have large SA:V
-can exchange substances over body using just diffusion especially if they have a low
metabolic rate
Larger organisms
-have small SA:V
-most cells are too far from exchange surfaces for diffusion alone to supply/remove materials
-specialised exchange systems have evolved to meet requirements
Specialised exchange surfaces
-large SA:V → increases rate of exchange
-thin so short diffusion distance → increases rate
-selectively permeable
-movement of environmental medium to maintain concentration/diffusion gradient
-transport system to ensure movement of internal medium to maintain concentration
gradient
Disadvantages of specialised exchange surfaces
-thin so easily damaged (fragile) and dehydrates quickly
→ solution: located inside organism
-needs way of moving external medium over exchange surface
→ solution: mechanism such as ventilation in lungs
Gas exchange in insects
Insects are too large for diffusion alone to meet their requirements
-their exchange system consists of an internal network of tubes which reduces evaporation
of water from exchange surface
-spiracles = tiny pores on body surface that allow air to enter and
leave tracheae (mostly closed to conserve water)
-tracheae = network of internal tubes throughout body of insect
→ supported by strengthening rings to prevent them from collapsing
→ divide into tracheoles (smaller permeable tubes)
,→every cell is short distance from a tracheole which are lined with a single layer of cells to
minimise diffusion pathway
Diffusion gradient
→ O diffuses from tracheoles to respiring cells
→O conc at end of tracheoles decreases creating a conc gradient
→ O diffuses from atmosphere down trachea and tracheoles
→ CO2 produced in repairing cells diffuses into tracheoles and creates a conc gradient in
the opposite direction (from tracheoles to trachea to air)
Mass transport
-mass transport system = system that moves materials from
exchange surfaces to cells where they are required
-rhythmic abdominal contractions squeeze tracheae which
enables mass movement of air in and out spiracles
Ends of tracheoles filled with water
-cells may respire anaerobically in periods of rapid activity → creates lactate which of soluble
(lowers water potential)
-water moves from tracheoles into cells by osmosis
-water decreases in volume
-final diffusion path is gas phase → gases move faster through air than water → faster
diffusion rate
Gas exchange in fish
-there is a lower concentration of oxygen in water than air
-water enters mouth and is passed over gills
-stacked gill filaments increase surface area
-covered by lamallae which further increases surface area
-lamallae has many capillaries and thin surface layer of cells
→ increases rate of diffusion
, Counter current flow
-water moves along lamallae in one direction and blood flows in an opposite direction
(counter current flow)
-maintains large concentration gradient between blood and water
-ensures conc of oxygen in water is always higher than that in blood
-water with lowest O conc is adjacent to most deoxygenated blood
When water enters gills:
-The concentration of O2 is always higher in H20
-Blood with a relatively high O2 concentration meets water also with its maximum O2
concentration
-Diffusion occurs from water → lamellae down a concentration gradient
As blood moves along lamellae:
-Blood with little oxygen meets water with most of its oxygen
removed -Diffusion occurs as the
concentration of O2 in water is still higher than in the blood
Advantages of counter current system
-diffusion gradient maintained along entire length of lamellae
-majority of available O from water is absorbed
Parallel flow - cartilaginous fish
-blood with high conc O would meet water with lowest conc
-diffusion down gradient
-equilibrium would be reached as water moved through gills
-diffusion gradient only maintained across half the length of
lamallae → blood absorbs less available oxygen from water
