You need to explain how this happens having seen that in osmosis only water
passes through a partially permeable membrane.
However, cells cannot just rely on diffusion and osmosis for
the input (e.g. nutrients) and output (e.g. waste) of substances.
What is 'active transport'
Active transport differs from spontaneous diffusion where
there is a net transfer of substances down a concentration gradient in two
(i) The transport of materials works against the
concentration gradient - opposing the normal diffusion direction.
(ii) Active transport requires a net energy input
involving other molecular systems to transport material through a
membrane against the concentration gradient.
The mechanisms of active transport are complex
but the process involves protein carrier molecules in cell
membranes to convey the nitrate ions into the root cells,.
Active transport is the movement of
particles across a membrane against a concentration gradient.
So, sometimes, substances are absorbed by cells against a
concentration gradient - a net transfer against the normal diffusion
gradient action is required.
This means transfer occurs in
the opposite direction to the natural direction of the diffusion gradient.
e.g. active transport enables cells
to absorb ions from very dilute solutions.
BUT, this movement of chemicals across a
cell membrane against a natural diffusion gradient, requires the use of energy from respiration
(via ATP molecules) and the
overall process is called active transport.
Remember that absorption by
diffusion down the concentration gradient through membranes doesn't require
energy from respiration.
Active transport is very complicated
BUT, 'basically' it uses transport molecules
and ions within the membrane structure and the process powered by ATP
By this means soluble nutrient molecules/ions can pass through
the membrane into cells.
Examples of active transport
The villi in the small intestine absorb
glucose and other nutrients from the gut and transfer them into the
The diagram illustrates the movement of
molecules (green spheres) being moved through the membrane of the gut from
the gut into the bloodstream, in the opposite
direction to the natural diffusion gradient.
(The blue circles represent water
molecules - solvent medium.)
The red circles represent the
relatively large red blood cells, which are too large to get through the
membrane, so staying in the bloodstream, to be joined by nutrient
molecules (green circles) and ions via active transport.
Active transport is required to
absorb nutrients (green circles) like amino acids, sugars like glucose etc. from the gut
when the concentration in the gut is lower than their concentrations in the
blood supply, and a healthy body requires these nutrients all the time.
If the concentrations of
nutrients (e.g. sugars, amino acids) in the gut is higher than that in the blood stream, then the
nutrients will naturally diffuse into the blood stream because of the
direction of the concentration gradient (more concentrated ==> less
If the concentration gradient
flow is in the direction of the blood stream (higher) to the gut (lower), then
respiration powered active transport must be used to work against the
natural diffusion flow.
So active transport enables the gut
to move nutrients like into the blood even though the natural
concentration gradient (diffusion gradient) is the wrong way round.
Glucose can be transferred into the
blood stream, even if its concentration is higher in the blood stream,
and so conveyed to cells for respiration.
Other examples where active
transport is essential in animals
Cells in the kidney reabsorb
sodium ions from urine - sodium ions are needed for many biochemical
processes in the body.
Fish in seawater have cells in
the gills than can transfer salt back into the more salty sea water.
Similarly, crocodiles have
salt glands in their tongue that can transfer excess salt from
their bodies back into the water.
These are two good animal
examples of adaptations to their environment.
The thyroid glands have cells
that can concentrate iodine, against the diffusion gradient, to make
the important hormone thyroxine.
For more on the gut and other
Surfaces for the exchange of substances in
Active transport in plants
is used in the absorption of nitrates and other ions by
It is essential plant roots can absorb vital nutrients
for the health and growth of a plant.
For details see
Transport and gas exchange in plants,
transpiration, absorption of nutrients etc.