Structure
and function of roots - absorption
of minerals - diffusion and active transport
In plants, most of the water and mineral ions
(e.g. magnesium ions or nitrate ions) are absorbed
by roots, these in turn will be translocated up through the stem to the rest
of the plant using the xylem tube system.
I'm concentrating on minerals
here, but remember plants need lots of water for their
internal transport systems (xylem and phloem cells),
photosynthesis and to keep cells turgid and support the
physical structure of the plant as a whole.
Whereas most of the plant consists of
green photosynthetic tissue, the roots are non-photosynthetic and pale in
colour.
Therefore sugars would be translocated
from photosynthetic tissues like leaves to non-photosynthetic tissues like
roots using the phloem tubes.
Root hairs are single-celled extensions to
epidermis cells in the root.
The root hair cells are connected to root cortex
cells, which in turn, are closely connected to xylem vessels which
transfer minerals and nutrients around the plant efficiently.
Root hair cells are well adapted for their
function:
large surface area for absorbing water
- which will also contain minerals,
they have no cuticle, but a thin membrane for
efficient absorption - short diffusion distance,
they have a thin cell wall, again reduces the
distance for water transfer in osmosis,
a large permanent vacuole to absorb and store
(temporarily) as much water as possible,
The
surface area of the
roots is increased by root hairs and the surface area of leaves is increased
by the flattened shape and internal air spaces.
Each branch of a root will have
millions of root hair cells creating a massive surface area to
absorb water and mineral ions.
The cells on the surface of a plant's
roots grow into long cells that shape into 'hairs' that stick out
into the soil.
The fine root hairs considerably
increase the surface area of the roots for absorbing water and minerals
and each branch of the root is covered millions of these microscopic
hairs.
Root hair cells are very
elongated combining into fine hair-like structures, which greatly increases
the surface area of contact with the soil through which most of the plant's water and mineral
intake are absorbed.
Osmosis and active transport
Reminder about water potential: The greater
the concentration of water, the more dilute the solution, the
greater the water potential. The lower the concentration of
water, the more concentrated the solution, the lesser the water
potential. Therefore in osmosis through a semi-permeable
membrane, water moves from a region of higher water potential, to a
region of lower water potential.
As long as the water
concentration is higher in the soil, the root hairs will naturally absorb
water by osmosis.
The water potential of the soil
is usually greater than that of the fluid in the root, so water will
be naturally be absorbed by osmosis and the large surface area
of the root cell hairs aid this.
However, the concentration of minerals in
the root hair cells is higher than in the moisture surrounding the roots,
and therefore an absorption problem.
This is because the plants cells would naturally lose essential mineral
ions by diffusion back into the soil moisture.
Not good, it means the root hair
cells can't use diffusion on its own to absorb minerals from the
soil, in fact, without active transport, mineral ions would move out
of the root hairs.
Therefore, active transport
systems must be used by the plant to counteract the natural direction of
diffusion from a high mineral concentration in the plant cells to a low
mineral concentration in the soil moisture.
The process requires energy from
respiration which powers the process by which the plant's root cells
can absorb minerals from the soil, even if they are only present in
a very dilute solution of the soil's moisture.
So, energy from respiration is
usually required to absorb minerals into the roots from the soil moisture by
working against the concentration gradient - active transport mechanism.
It has been shown
experimentally that when there is an increase in the uptake of
minerals by plants, there is an increase in active transport,
accompanied by an increase in respiration.
Examples of moving substances by
active transport from a low concentration to a higher concentration:
The concentration of nitrate
ions (NO3-)
is usually greater in the root hairs cells than the surrounding
soil water. Therefore the natural diffusion gradient for the
movement of nitrate ions is in the direction of root to soil,
but plants need the nitrogen in nitrate ions to synthesise
proteins. Therefore active transport is used to move nitrate
ions from the soil water into the root fluids against their
natural diffusion gradient. The active transport process
involves protein carrier molecules in the root cell membranes to
convey the nitrate ions into the root cells,.
The same argument applies to
magnesium ions which chlorophyll molecules need in
photosynthesis.
Active transport is much more
complicated than 'simple' osmotic water exchange in plant cells.
See
comparison of diffusion, osmosis
and active transport
