Environmental Engineering Reference
In-Depth Information
centuries for some seeds, but once exposed to water and
oxygen, the process of cellular respiration and growth
begin. On the other hand, once a leaf is removed from a
plant, it is isolated from its water source and will dry out
according to its transpiration characteristics.
Plants need water to survive but cannot simply abstract it
from the humidity in air in most cases. It may appear as a
dilemma that plants require water but limit uptake to only
one main area, the roots. Whereas all of the other parts of the
plant are designed to minimize the loss of water, the roots are
designed to maximize the gain of water. Plant structures
have specifically evolved to interact with water above and
below ground; in the latter, roots can interact with the water
stored in the pore spaces of sediment, where the water is less
likely to be removed by evaporation. For roots to interact
with water it must be present in the soil in the shallow root
zone or deeper, such as groundwater.
Unlike the capability of plants to react to light sources
from great distances, roots cannot react to the presence of
moisture more than a few millimeters away from each root.
In this sense, roots do not grow toward water but expand and
lengthen until water is reached. After a root encounters
water, root hairs grow quickly until the water is depleted.
If a root encounters no water, or if water has been removed
and is not recharged, the root hairs become desiccated and
die. This is why phreatophytes that have root masses in the
capillary fringe or water table are able to tolerate drought
conditions even though shallower sediments dry out.
directly enter our bodies because the distance across which
diffusion would have to occur would be too great. It has been
calculated that for oxygen to diffuse from our head to our
toes would require 100 years. With the advent of lungs and a
circulatory system connected to every cell, however, oxygen
diffuses rapidly into the capillaries in the lungs, where the
cell thickness is about 1.5 microns. This relation between
diffusion and distance is one of the reasons that plants have
thin leaves and thin root hairs, in order to facilitate steep
concentration gradients of essential reactants over a short
diffusional distance. Life simply could not exist without
diffusion.
On an individual cell-by-cell basis, the diffusive move-
ment of water molecules in response to a water-concentration
gradient across a selectively permeable membrane is called
osmosis (from the Greek,
osmos
, for push). Water in contact
with a root cell that contains solutes essentially enters
the cell to lower the water concentration inside and
will continue to diffuse until equilibrium conditions are
established. The greater the solute concentration across
such a membrane relative to pure water, the greater the
tendency is for water to cross that membrane, and the higher
the osmotic potential. Conversely, the higher the osmotic
potential of a solution, the higher the tendency for water
transport into the solution. As the water moves to the place
of higher solute concentration, the pressure will increase if in
a fixed volume.
An alternative definition of water transport into cells
relates to the minimum osmotic pressure applied to a solu-
tion of water and solute to keep it from gaining more water.
Here, osmotic potential is operationally defined as the nega-
tive of the osmotic pressure. Hence, if a solution is placed in
contact with pure water separated by a membrane, the
greater the tendency for water to be transported across
the membrane to equalize water concentrations. The higher
the osmotic pressure is, the lower, or more negative, is the
osmotic potential. These measurements are done with an
osmometer, derived from a much earlier simple one made
from a pig's bladder by the French physician Joachim Henri
Dutrochet in the early 1800s. These experiments further
illustrate that osmosis is essentially simple diffusion but
through a semipermeable membrane.
Polar water can enter individual cells by diffusion
because of the water molecule's small size and weight.
Unlike the solute molecules that enter the cell under the
direct regulation imposed by the physical structure of the
cell membrane, water enters and exits the cell with no
apparent regulation or discrimination by the cell membrane
and its semipermeable structure. This is similar to how a
GORE-TEX
®
jacket prevents liquid water from entering
from the outside but allows water vapor to exit to the outside.
Diffusion is a slow process, even for a small molecule like
water, but if a plant can grow a large enough volume of
3.4.1 Diffusion and Osmosis
In general, all life can be defined as a dynamic steady-state
condition of the acquisition, processing, and excretion of
various chemicals and substances with the surrounding envi-
ronment. These processes occur on a cellular level continu-
ally until death, and, in most cases, are driven by diffusion.
Diffusion is the process in which molecules of a sub-
stance tend toward an even distribution throughout a given
volume. Movement is in the direction of concentration
gradients. If a drop of food coloring is placed in still water,
the molecules of pigment will bump into each other at a
greater probability in the center of the drop. This random
interaction propels the other food-coloring particles away
from the center. As the particles leave the center, there is less
probability that adjacent food-coloring particles will inter-
act. Once an equilibrium distribution has been reached, each
particle will have the same probability of interaction with
other particles. It follows, then, that the rate of diffusion of a
particular particle is proportional to the original concentra-
tion of the particle.
Diffusion is a scale dependent process. Take for example
the human need for oxygen. Atmospheric oxygen cannot
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