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Physiology of Organisms
Discuss how physiological constraints effect transport of water in different
The primary physiological constraint that effects transport of water is resistance,
both resistance to movement of water across cell membranes, and resistance to
movement of water through vessels in osmosis and bulk flow respectively. This
resistance must be overcome by various means in order for water to be
Resistance to movement across membranes by osmosis is overcome by the
mechanisms that create water potential gradients. This follows Fick’s first law of
diffusion, which states that flux goes from regions of high concentration to
regions of lower concentration, with a magnitude proportional to the
concentration gradient. In both plants and animals water is transported between
cells and from the intracellular fluid into the cells by osmosis; the movement of
water from an area of high water potential, to area of lower water potential
across partially permeable membrane. Water potential w = s + p, where s is
the solute potential, and p is the pressure potential. Movement of water down
its potential gradient involves transition to a position of lower free energy. The
creation of water potential gradients by active loading of is solutes used to
induce movement of water across membranes, overcoming the resistances to
this; for example isosmotic fluid reabsorption in the kidneys, which occurs in the
proximal tubule. Water is reabsorbed as Na+ is pumped over the basolateral
membrane of the cells by Na+/K+ ATPases, causing Na+ to move from the lumen
of the tubule into the epithelial cells down its concentration gradient. The solute
reabsorption results in a higher osmotic pressure (the pressure that has to be
applied to a concentrated solution in order to stop solvent moving into it from a
more dilute solution), or lower water potential, in the peritubular fluid, which
causes water to move by osmosis through leaky tight junctions, and through the
epithelial cells via aquaporin-1 channel proteins. Water is moved by a similar
mechanism involving pumping of solutes in plants. In the phloem, active sucrose
transporters create high concentrations of sucrose in the phloem. This produces
a lower water potential, therefore water follows the sucrose into the phloem to
create a pressure gradient that is used for transport of the phloem’s contents.
Pressure gradients are also utilised in overcoming resistance to the movement of
water on a larger scale than across cell membranes.
Bulk flow is a way in which water, and other fluids, are transported over large
distances in plants and animals, usually through specialised vessels. In bulk flow
water moves down its mechanical energy gradient. From Darcy’s law, flow of
water is defined by the equation: Flow = Δ Pressure/Resistance, meaning that
the rate of flow is proportional to the pressure gradient but inversely
proportional to resistance. The resistance is a physiological constraint.
Resistance to flow through vessels is described by the Poisseuille equation:
Resistance=(length of vessel x viscosity of fluid)/(internal radius of vessel)4. An
additional constraint is friction with the walls of the xylem and blood vessels. In
both plants and animals movement of water through the xylem and blood vessels
respectively is described by Darcy’s law and the resistance to this flow is
described by the Poisseuille equation. In animals, in order for blood containing
Physiology of Organisms
water to flow, a hydrostatic pressure gradient must be generated. In higher
animals such as mammals, birds, and fish this gradient is generated by the
pumping of the heart muscle. The flow of blood to organs through the blood
vessels is driven by the difference in pressure between the arteries that supply
the organ, a higher pressure created by contraction of the heart, and the veins
that drain it, which have a lower pressure. This is a positive pressure gradient
that moves water-containing blood through the vessels. In contrast to in the
blood, the driving force behind the movement of water through the xylem to
overcome resistance is negative pressure. In plants the hydrostatic pressure
gradient is generated by transpiration. Water evaporates and leaves the leaves
through the stomata, resulting in water present in the mesophyll cells inside the
leaf evaporating into the air spaces. The water in the xylem vessels are subject to
very low water potential, as a result of cohesive forces from hydrogen bonds
between water molecules in the vessel and those in the leaves, causing
transpiration tension. Additionally the adhesive forces between the water
molecules and the lignin lining the walls of the xylem contributes to the negative
tension in the plant. This tension draws water from the roots to the leaves
through the xylem, overcoming the resistance described in the Poisseuille
equation, in addition to gravitational forces and friction.
In conclusion, the physiological constraint of resistance to movement of water
across cell membranes by osmosis and through vessels by bulk flow results in
mechanisms to overcome these resistances and enable transport of water. These
mechanisms include the generation of water potential gradients from active
transport of solutes, and the generation of pressure potential gradients by the
pumping of the heart muscle in animals, and transpiration in plants. The
mechanisms of water transport into and out of cells in plants and animals by
osmosis are very similar, whereas bulk flow is more different; for example
transport of water in the blood uses a positive potential gradient, while in the
xylem a negative pressure potential it utilised.