Transport in Cells
Diffusion
Diffusion is the net movement of particles form a high concentration to a low concentration,
down the concentration gradient, through a partially permeable membrane. If there is a
steep concentration gradient then the rate of diffusion is fast. If there is a shallow
concentration gradient the rate of diffusion is slow.
Lungs
Our lungs are structured to never reach an equilibrium, this is because:
the tiny capillaries allow one red blood cell
we’re constantly breathing
blood is constantly pumped
So, it maintains a steep concentration gradient.
SA:V
length of sides = 1
surface area of one side = (1x1) = 1
total surface area = (1x6) = 6
= 6/1 = 6
= 6:1
Having a large surface area to volume ratio means a faster rate of diffusion. The bigger the
organism the smaller the SA:V. The more complex an organism is, the more it relies on a
specialised exchange system.
Factors Affecting Diffusion
Diffusion is greater when there is:
a large surface area
small distance to travel
a steep concentration gradient
An efficient gas exchange surface will have:
a large surface area
a thin membrane, so short diffusion distance
be organised and maintain a favourable concentration gradient, moving away diffusing
substances e.g. efficient blood supply and ventilating gas exchange system
However, the structure of the gas exchange system depends on:
, size of organism
habitat in land or water
metabolic demands: high, moderate or low
Examples of Adaptions
Lungs have lots of alveoli, thin cell membranes and a constant blood flow.
Roots ensure instantly water is transported to the xylem, root hair cells are thin and they
have a large surface area.
Leaves have lots of stomata, air spaces between cells, are wide and flat and thin and have
lots of chloroplasts.
Fish live in water which has a lower concentration of oxygen in the air and is denser. They
have specialised exchange organs called gills, which are composed of thousands of
filaments. Each filament is covered in feathery fill lamellae, each one is a few cells thick and
containing lots of blood vessels.
many gill filaments and lamellae – increases the surface area to volume ratio
gill filaments only one cell thick – a short diffusion path
each lamella has capillaries – maintains a constant diffusion gradient
The small intestine is very long and the lining is folded into tiny projections called villi which
helps absorb the end products of digestion e.g. glycerol and amino acids:
villi on small intestine and micro villi on each one so a large surface area, so more
absorption of nutrients
the epithelium is thin (one cell thick) so short diffusion distance, which is more efficient
a good capillary network and the stomach muscle churns food so there is a constant
steep concentration gradient which never equilibrates
Osmosis
The movement of water from a high concentration of water to a low concentration of
water. The movement of water from a dilute solution to a concentrated solution.
Osmosis is only the movement of water, it’s passive (no energy required) and always over a
partially permeable membrane. Only water can travel across a partially permeable
membrane in a solution as it is small enough.
Diffusion
Diffusion is the net movement of particles form a high concentration to a low concentration,
down the concentration gradient, through a partially permeable membrane. If there is a
steep concentration gradient then the rate of diffusion is fast. If there is a shallow
concentration gradient the rate of diffusion is slow.
Lungs
Our lungs are structured to never reach an equilibrium, this is because:
the tiny capillaries allow one red blood cell
we’re constantly breathing
blood is constantly pumped
So, it maintains a steep concentration gradient.
SA:V
length of sides = 1
surface area of one side = (1x1) = 1
total surface area = (1x6) = 6
= 6/1 = 6
= 6:1
Having a large surface area to volume ratio means a faster rate of diffusion. The bigger the
organism the smaller the SA:V. The more complex an organism is, the more it relies on a
specialised exchange system.
Factors Affecting Diffusion
Diffusion is greater when there is:
a large surface area
small distance to travel
a steep concentration gradient
An efficient gas exchange surface will have:
a large surface area
a thin membrane, so short diffusion distance
be organised and maintain a favourable concentration gradient, moving away diffusing
substances e.g. efficient blood supply and ventilating gas exchange system
However, the structure of the gas exchange system depends on:
, size of organism
habitat in land or water
metabolic demands: high, moderate or low
Examples of Adaptions
Lungs have lots of alveoli, thin cell membranes and a constant blood flow.
Roots ensure instantly water is transported to the xylem, root hair cells are thin and they
have a large surface area.
Leaves have lots of stomata, air spaces between cells, are wide and flat and thin and have
lots of chloroplasts.
Fish live in water which has a lower concentration of oxygen in the air and is denser. They
have specialised exchange organs called gills, which are composed of thousands of
filaments. Each filament is covered in feathery fill lamellae, each one is a few cells thick and
containing lots of blood vessels.
many gill filaments and lamellae – increases the surface area to volume ratio
gill filaments only one cell thick – a short diffusion path
each lamella has capillaries – maintains a constant diffusion gradient
The small intestine is very long and the lining is folded into tiny projections called villi which
helps absorb the end products of digestion e.g. glycerol and amino acids:
villi on small intestine and micro villi on each one so a large surface area, so more
absorption of nutrients
the epithelium is thin (one cell thick) so short diffusion distance, which is more efficient
a good capillary network and the stomach muscle churns food so there is a constant
steep concentration gradient which never equilibrates
Osmosis
The movement of water from a high concentration of water to a low concentration of
water. The movement of water from a dilute solution to a concentrated solution.
Osmosis is only the movement of water, it’s passive (no energy required) and always over a
partially permeable membrane. Only water can travel across a partially permeable
membrane in a solution as it is small enough.
