Water moves down concentration gradient by diffusion. Water is more concentrated in freshwater environments than in the oceans.

1. Mary Nelodie E. Polido
BEED 4

2. • Water moves down concentration gradient by
diffusion. Water is more concentrated in freshwater
environments than in the oceans.
• Aquatic organisms can be viewed as an aqueous
solution bounded by a selectively permeable membrane
floating in an another aqueous solution
 Diffusion
 Osmosis
-Special case of diffusion -water movement across a
membrane.

3. • Salinity: concentration of dissolved salts
-salt water solution contains relatively less water than
fresh water
That means?
Water moves from area of less dissolved salts to more
dissolved salts
Water Concentration in Solutions

4. • Isosmotic – “balance”
• Hypoosmotic – “Low Concentration”
• Hyperosmotic – “High Concentration”

5. Organisms with body
fluids containing the same
concentration of water as
the external environment
are isosmotic.
• Isosmotic

6. Salts Water
Isosmotic
In an isosmotic aquatic organism, internal concentration of
water and salt equal their concentration in environment.
Salts and water diffuse at appropriately equal rates into
and out an isosmotic organism.

7. • Hypoosmotic
Organisms with body fluids
with a higher concentration of
water (lower solute
concentration) than the external
medium are hypoosmotic and
tend to lose water to the
environment.

8. Salts Water
Hypoosmotic
Compared to the environment, a Hypoosmotic aquatic organism
has a higher internal concentration of water and lower internal
concentration of salts.
Marine bony fish are strongly Hypoosmotic, thus need
to drink seawater for salt influx.

9. Those with body fluids with a
lower concentration of water
(higher solute concentration) than
the external medium are
hyperosmotic and are subject to
water flooding inward from the
environment.

10. Hyperosmotic
Compared to the environment, a hyperosmotic aquatic organism has a lower
internal concentration of water and a higher internal concentration of salts.
Salts Water
Hyperosmotic organisms that excrete excess internal water via
large amounts of dilute urine. Replace salts by absorbing sodium
and chloride at base of gill filaments and by ingesting food.

12. On land, water flows from the organism to the atmosphere at a
rate influenced by the vapor pressure deficit of the air surrounding
the organism. In the aquatic environment, water may flow either to
or from the organism, depending on the relative concentrations of
water and solutes in body fluids and the surrounding medium. But
here too, water flows down its concentration gradient.

13. As shown in the Picture, water
moving from the soil through a plant
and into the atmosphere flows down
a gradient of water potential.
Water in soils and plants moves
through the small pore spaces within
soils and within the small water-
conducting cells of plants.
Therefore, water potential in soils
and plants is determined by the
concentration gradient of water plus
other factors related to the
movement of water through these
small spaces.

14. Understanding water potential takes some patience,
but that patience will be paid off by a significant
improvement in understanding the water relations of
terrestrial plants.
We can define water potential as the capacity of
water to do work. Flowing water has the capacity to do
work such as turning the water wheel of an old-
fashioned water mill or the turbines of a hydroelectric
plant.

15. The capacity of water to do work depends upon
its free energy content. Water flows from positions of
higher to lower free energy. Under the influence of
gravity, water flows downhill from a position of higher
free energy, at the top of the hill, to a position of
lower free energy, at the bottom of the hill.

16. In the section "Water Movement in Aquatic
Environments,'' we saw that water flows down its
concentration gradient, from locations of higher water
concentration (hypoosmotic) to locations of lower water
concentration (hyperosmotic). The measurable "osmotic
pressure" generated by water flowing down these
concentration gradients shows that water flowing in
response to osmotic gradients has the capacity to do work.

17. We measure water potential, like vapor pressure deficit and
osmotic pressure, in pascals, usually megapascals (MPa = Pa x
106). By convention, water potential is represented by the symbol ψ
and the water potential of pure water is set at 0. If the water
potential of pure water is 0, then the water potential of a solution,
such as seawater, must be negative (i.e., < 0).

18. In nature, water potentials are
generally negative. must be so since
all water in nature, even rainwater,
contains some solute or occupies
spaces where matric forces are
significant. So, gradients of water
potential in nature are generally from
less negative to more negative water
potential. We can express the water
potential of a solution as:
ψ = ψ solutes
ψ solutes is the reduction in water
potential due to dissolved substances,
which is a negative number.

19. Within small spaces, such as the interior of a plant
cell or the pore spaces within soil, other forces, called
matric forces, are also at work. Matric forces are a
consequence of water's tendency to adhere to the walls
of containers such as cell walls or the soil particles lining
a soil pore. Matric forces lower water potential. The water
potential for fluids within plant cells is approximately:
Ψ plant = ψ solutes + ψ matric

20. In this expression, ψ matric is the reduction in water
potential due to matric forces within plant cells. At the
level of the whole plant, another force is generated as
water evaporates from the surfaces of leaves into the
atmosphere. Evaporation of water from the surfaces of
leaves generates a negative pressure, or tension, on the
column of water that extends from the leaf surface
through the plant all the way down to its roots.

21. So, the water potential of plant fluids is affected by solutes, matric forces, and
the negative pressures exerted by evaporation. Consequently, we can represent the
water potential of plant fluids as:
Ψ plant = ψ solutes + ψ matric + ψ pressure
ψ pressure is the reduction in water potential due to negative pressure created
by water evaporating from leaves.
Matric forces vary considerably from one soil to another, depending primarily
upon soil texture and pore size. Coarser soils, such as sands and loams, with larger
pore sizes exert lower matric forces, while fine clay soils, with smaller pore sizes,
exert higher matric forces. So, while clay soils can hold a higher quantity of water
compared to sandy soils, the higher matric forces within clay soils bind that water
more tightly. As long as the water potential of plant tissues is less than the water
potential of the soil, ψ plant< ψ soil, water flows from the soil to the plant.

22. The higher water potential of soil water compared to the
water potential of roots induces water to flow from the into plant
roots. As water enters roots from the surrounding soil, it joins a
column of water that extends from the roots through the water-
conducting cells, or xylem, of the stem to the leaves. Hydrogen
bonds between adjacent water molecules bind the water molecules
in this water column together.
Consequently, as water molecules at the upper end of this
column evaporate into the air at the surfaces of leaves, they exert a
tension, or negative pressure, on the entire water column. This
negative pressure further reduces the water potential of plant fluids
and helps power uptake of water by terrestrial plants.

23. In picture, water from the
soil, they soon deplete the water
held in the larger soil pore spaces,
leaving only water held in he smaller
pores. Within these smaller soil
pores matric forces are greater than
in the larger pores.
Consequently, as soil dries,
soil water potential becomes more
and more negative and the
remaining water becomes harder
and harder extract.