3.1 Surface area to volume ratio
● Surface area to volume ratio decreases with increasing organism size
● Cube SA = 6 a2
● Cube vol = a3
● Sphere SA = 4 π r2
● Sphere vol = 4/3 π r3
● Exchange surfaces need a large surface area to maximize space for diffusion, a thin
barrier to minimize diffusion distance, a way to maintain concentration gradient so
diffusion rate can be maximized
● Larger organisms have transport systems that allow for a higher concentration
gradient to be maintained. They also allow substances to move closer to cells that
need to exchange them
3.2 Gas exchange
Single-celled organisms:
● Are small so have a large surface area to volume ratio
● Allows for oxygen to be absorbed by diffusion across their cell surface membrane
Insects:
● Oxygen moves into the insect by small openings called spiracles, then diffuses
down the concentration gradient into tracheae which branch off into tracheoles
○ Large number of tracheoles so large surface area and ensures no cell is far
away from one, reducing diffusion distance
○ Tracheoles have thin walls to minimize diffusion distance
● Fluid at the ends of tracheoles: during intense exercise, anaerobic respiration takes
place which produces lactate
○ Lactate lowers the water potential of muscle cells, causing the fluid to move
from the tracheoles into cells by osmosis
○ Fluid at the ends of tracheoles decreases in volume, so faster diffusion as
oxygen can directly diffuse through air to the tracheole walls, rather than
dissolving in liquid first
● Rhythmic abdominal movements: moves air in and out of tracheae to maintain a
concentration gradient
● Adaptations to control water loss:
○ Spiracles can close to reduce water loss by evaporation
○ Spiracles have hairs around them to trap a layer of moist air around them,
reducing the water potential gradient
○ Waterproof, waxy cuticle covering their surface
Fish:
● Water enters the fish through its mouth and flows over the gills, made of numerous
gill filaments covered in gill lamellae
, ○ Many gill lamellae so large surface area, thin layer of surface cells to
minimize diffusion distance
● Counter-current flow:
○ Blood in the lamellae and water flow in opposite directions to each other
○ Maintains steep concentration gradient between the water and blood where
the concentration of oxygen is always higher in water than in blood
○ Concentration gradient maintained throughout the length of the gill
Leaves:
● Mesophyll cells have a large surface area and numerous interconnecting air spaces
between them so that gases can readily come into contact with them
● Many stomata to ensure no cell is far away from one, reducing diffusion distance
● Xerophytic plants are adapted to dry conditions and control water loss by their:
○ Thick, waxy cuticle: reduces evaporation
○ Stomata: pitted, traps a layer of moist air around them, reducing the water
potential gradient between inside and outside of the leaf, meaning less water
diffuses out
○ Curled leaves: stomata on the inside traps a layer of moist air within the leaf
Humans:
● Oxygen moves down the trachea, bronchi, bronchioles and alveoli, down a pressure
gradient
● Then diffuses across the alveolar epithelium then capillary epithelium into the blood,
down a concentration gradient
● Alveoli: many so large surface area, alveolar epithelium is one cell thick to minimize
diffusion distance, constant flow of blood to maintain concentration gradient
● Inspiration (inhalation):
○ External intercostal muscles contract, ribcage moves up and out
○ Diaphragm contracts and flattens
○ Thoracic cavity volume increases, pressure decreases, drawing air in
● Expiration (exhalation):
○ External intercostal muscles relax, ribcage moves down and in
○ Diagram relaxes and curves
○ Thoracic cavity volume decreases, pressure increases, pushes air out
● Surface area to volume ratio decreases with increasing organism size
● Cube SA = 6 a2
● Cube vol = a3
● Sphere SA = 4 π r2
● Sphere vol = 4/3 π r3
● Exchange surfaces need a large surface area to maximize space for diffusion, a thin
barrier to minimize diffusion distance, a way to maintain concentration gradient so
diffusion rate can be maximized
● Larger organisms have transport systems that allow for a higher concentration
gradient to be maintained. They also allow substances to move closer to cells that
need to exchange them
3.2 Gas exchange
Single-celled organisms:
● Are small so have a large surface area to volume ratio
● Allows for oxygen to be absorbed by diffusion across their cell surface membrane
Insects:
● Oxygen moves into the insect by small openings called spiracles, then diffuses
down the concentration gradient into tracheae which branch off into tracheoles
○ Large number of tracheoles so large surface area and ensures no cell is far
away from one, reducing diffusion distance
○ Tracheoles have thin walls to minimize diffusion distance
● Fluid at the ends of tracheoles: during intense exercise, anaerobic respiration takes
place which produces lactate
○ Lactate lowers the water potential of muscle cells, causing the fluid to move
from the tracheoles into cells by osmosis
○ Fluid at the ends of tracheoles decreases in volume, so faster diffusion as
oxygen can directly diffuse through air to the tracheole walls, rather than
dissolving in liquid first
● Rhythmic abdominal movements: moves air in and out of tracheae to maintain a
concentration gradient
● Adaptations to control water loss:
○ Spiracles can close to reduce water loss by evaporation
○ Spiracles have hairs around them to trap a layer of moist air around them,
reducing the water potential gradient
○ Waterproof, waxy cuticle covering their surface
Fish:
● Water enters the fish through its mouth and flows over the gills, made of numerous
gill filaments covered in gill lamellae
, ○ Many gill lamellae so large surface area, thin layer of surface cells to
minimize diffusion distance
● Counter-current flow:
○ Blood in the lamellae and water flow in opposite directions to each other
○ Maintains steep concentration gradient between the water and blood where
the concentration of oxygen is always higher in water than in blood
○ Concentration gradient maintained throughout the length of the gill
Leaves:
● Mesophyll cells have a large surface area and numerous interconnecting air spaces
between them so that gases can readily come into contact with them
● Many stomata to ensure no cell is far away from one, reducing diffusion distance
● Xerophytic plants are adapted to dry conditions and control water loss by their:
○ Thick, waxy cuticle: reduces evaporation
○ Stomata: pitted, traps a layer of moist air around them, reducing the water
potential gradient between inside and outside of the leaf, meaning less water
diffuses out
○ Curled leaves: stomata on the inside traps a layer of moist air within the leaf
Humans:
● Oxygen moves down the trachea, bronchi, bronchioles and alveoli, down a pressure
gradient
● Then diffuses across the alveolar epithelium then capillary epithelium into the blood,
down a concentration gradient
● Alveoli: many so large surface area, alveolar epithelium is one cell thick to minimize
diffusion distance, constant flow of blood to maintain concentration gradient
● Inspiration (inhalation):
○ External intercostal muscles contract, ribcage moves up and out
○ Diaphragm contracts and flattens
○ Thoracic cavity volume increases, pressure decreases, drawing air in
● Expiration (exhalation):
○ External intercostal muscles relax, ribcage moves down and in
○ Diagram relaxes and curves
○ Thoracic cavity volume decreases, pressure increases, pushes air out
