Lecture 4: Adaptation to
Aquatic and Terrestrial
Environments
Pages 49-72
Background
• It is important for us to understand the
mechanisms organisms use to interact with
their environment.
• This understanding may lead to insights:
– why organisms are specialized
– why organisms have specific geographic
distributions
– why certain adaptations are associated with
certain environments
Background
• We examine adaptations by considering
various challenges facing organisms, for
example:
– how do plants acquire water and nutrients from
soils and transport these?
– how do plants carry out photosynthesis under
varied environmental conditions?
– how do plants and animals cope
with extremes of temperature,
water stress, and salinity?
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Availability of Soil Water
• Water molecules are attracted to:
– each other which causes surface tension
– surfaces which causes capillary action
• When a soil is saturated and excess
(gravitational) water drains:
– remaining water exists as thin films around soil
particles (mineral and organic)
– the greater the area of such particles (as in clayey
soils), the more water
the soil retains
All soil water molecules are not equal.
• It’s all a matter of physical attraction...
– the closer a water molecule is to a soil particle, the
greater the force with which it is attracted
– this force is the matric potential of the soil, contributing
to the overall water potential
– matric potentials (units are MPa or atm)
are considered increasingly negative
as they represent greater attractive
forces
It’s all a matter of potential...
• Soil water potential is:
– usually dominated by matric forces
– determined as the force required to remove the
most loosely bound water molecules
• Typical “benchmark” values are:
– -0.1 atm (field capacity)
– -15 atm (wilting point)
– -100 atm (exceedingly dry soil)
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Plants obtain water from the soil.
• How do water molecules move?
– in the direction of more negative potential
– across most biological membranes
• Why does water move from the soil
into plant roots?
– water potential in cells of the root hairs is more negative
than that in the soil
– negative potential in root cells is generated mostly by
solutes -- osmotic potential
Membranes are selectively leaky.
• Can solutes exit root cells as readily as
water enters?
– no, internal and external concentrations would
equilibrate and osmotic potential gradient would
disappear
– cell membranes are semipermeable; large
molecular weight solutes cannot readily leave
the cell (carbohydrates and proteins)
So why does water move into roots?
Internal (cellular) osmotic potential is more
negative than external (soil) matric potential,
up to a point:
• root hair cells with 0.7 molar concentration of solutes
maintain inward flux of water against a soil matric potential
as low as -15 atm:
• as soil becomes drier, water flux ceases and may
reverse, leading to wilting and death
• desert plants may obtain water to soil matric
potentials as low as -60 atm (high solute conc.)
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Moving Water from Roots to Leaves
• Once water is in root cells, then what?
– water moving to the top of any plant must
overcome tremendous forces caused by
gravity and friction in conducting elements
(xylem):
• opposing force is generated by evaporation of water
from leaf cells to atmosphere (transpiration)
• water potential of air is typically highly negative
(potential of dry air at 20 oC is -1,332 atm)
• force generated in leaves is transmitted to
roots - water is drawn to the top of the
plant (tension-cohesion theory)
Water vapour diffuses out of the stomates
H2O evaporates from mesophyll cells
Tension pulls water column upward and
outward in the xylem of veins in leaves…
…in the stem
…and in the root
Water molecules form a cohesive
column in the xylem
Figure 3.4
Water moves into the root by
osmosis and then into xylem
Adaptations to Arid Environments 1
• Most water exits the plant as water vapor
through leaf openings called stomates:
– plants of arid regions must conserve limited
water while still acquiring CO2 from the
atmosphere (also via stomates) - a dilemma!
• potential gradient for CO2 entering the plant is
substantially less than that for water exiting the plant
• heat increases the differential between internal and
external water potentials, making matters worse
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Adaptations to Arid Environments 2
• Numerous structural adaptations address
challenges facing plants of arid regions by:
• reducing heat loading:
– increase surface area for convective heat
dissipation
– increase reflectivity and boundary layer effect with
dense hairs and spines
• reducing evaporative losses:
– protect surfaces with thick, waxy cuticle
– recess stomates in pits, sometimes
also hair-filled
Figure 3.6 Spines and hairs help plants adapt
to heat and drought
Plants obtain mineral nutrients from
soil water.
• Nutrients must move from the soil
solution into cells of root hairs…
– Diffusion - a nutrient element moves
passively into root when its concentration
in soil water exceeds that of root cells
– Active uptake is essential when nutrient
concentration in soil water is lower than
that in roots (this is energy-demanding)
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Other Plant Strategies for Obtaining
Nutrients
• Enlist partners!
– many plants have intimate associations
(symbioses) with fungi -- fungal partners
enhance mineral absorption
• Regulate growth!
– plants of nutrient-poor soils typically:
• grow slowly, maintain leaves for multiple growing
seasons (evergreenness), and store surplus
• shift growth toward more root and less shoot
Photosynthesis varies with levels of light.
• Photosynthetic rate is a
function of light intensity
(proportional to light intensity
at low light levels, leveling off
at high levels):
– in dim light, plants fail to offset
respiratory losses with
photosynthetic gains
– as light intensity increases, a
break-even point (losses offset
by gains) is reached, called
compensation point
– at saturation point, further
increase in light level does not
stimulate further photosynthesis
Plants modify photosynthesis in
stressful environments.
• Fixation of atmospheric carbon into glucose
(dark reactions of photosynthesis) is
accomplished by Calvin cycle:
– first step involves synthesis of two 3-carbon
molecules (PGA) from RuBP and CO2:
CO2 + RuBP → 2PGA
– enzyme accomplishing this is RuBP carboxylase...
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C3 Photosynthesis
• C3 plants depend solely on
Calvin Cycle for
photosynthetic CO2 fixation.
• C3 plants have certain
disadvantages:
– RuBP carboxylase has low
affinity for its substrate, CO2
– RuBP carboxylase also
catalyzes the oxidation of
PGA when leaf [CO2] low and
[O2] high, especially at high
temperatures
C4 Photosynthesis
• C4 plants add an
additional carboxylation
step to the Calvin cycle:
CO2 + PEP → OAA
– carbon is fixed to OAA in
mesophyll cells, then
shuttled to bundle
sheath cells where CO2
is unloaded for use in
Calvin cycle
– PEP regenerated in
bundle sheath cells is
reused (shuttled back to
mesophyll)
Advantages of C4 Photosynthesis
Biochemical and anatomical features lead to
photosynthetic advantages:
• Calvin cycle isolated from high O2 levels while
supplied with high levels of CO2 - leads to much more
efficient operation
• PEP carboxylase has high affinity for CO2, thus
permitting plant to obtain CO2 while increasing
stomatal resistance to water loss
• these advantages come at an energy cost, but are
especially helpful under conditions of high light, high
temperature and water stress
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Photosynthesis in Hot/Arid
Environments
• C4 photosynthesis favored as environmental
conditions become increasingly hot/arid:
– latitudinal gradients quite conspicuous: C4 plants
become much more common in transect from
polar regions toward equatorial regions
– but, C3 species are favored in cooler, moister
habitats because:
• disadvantages of C3 photosynthesis are lessened
• C3 approach is biochemically more energy-efficient
Carbon Assimilation in CAM Plants
• Some plants (succulents
in several families) add a
temporal “twist” to C4
process...
– CO2 is acquired at night
when evaporative demand
is lowest
– carbon from CO2 is stored in
4-C organic acids (such as
OAA)
– stored carbon is used by
Calvin cycle during daylight
hours when energy is
available for dark reactions
Balancing Salt and Water
• Osmotic regulation is not just a problem for
plants
• Aquatic animals are rarely in equilibrium with
their surroundings:
– When the internal salt concentration is higher than
that of the medium – like in fresh-water fish they
are called hyperosmotic
– When the internal salt concentration is lower than
that of the medium like marine fish - they are
called hypo-osmotic
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Ion retention is critical to freshwater
organisms.
Freshwater fish must
eliminate excess water
and selectively retain
dissolved ions:
1. they gain water by
osmosis
2. they eliminate excess
water in their urine
3. their kidneys selectively
retain dissolved ions
4. active uptake of ions via
gills is also important
Water retention is critical to marine organisms.
Saltwater fish must retain
water and excrete excess
ions:
1. they tend to lose water to
surrounding sea water and
must drink to replace this
2. excess salt must be
excreted from gills and
kidneys
3. some fish (sharks and rays)
raise osmotic potential of
blood by retaining waste
nitrogen as urea -- their high
internal osmotic potential
matches that of seawater
Water and Salt Balance in Terrestrial Plants
• Plants take up excessive salts
along with water, especially in
saline soils.
– plants must actively pump salts
back into soil
• In coastal mudflats, mangroves
must acquire water while
excluding salts. They:
– establish high root osmotic
concentrations to maintain water
movement into root
– exclude salts at the roots and also
excrete excessive salts from
specialized leaf glands
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Water and Salt Balance in
Terrestrial Animals
• Terrestrial animals must eliminate excess salts
acquired in diet:
– copious amounts of water can serve to flush excess
salts in more humid climates
– where water is scarce, other options exist:
* desert mammals produce highly
concentrated urine
* birds and reptiles eliminate
excess salts via salt glands
Animals excrete excess nitrogen.
• Carnivorous animals acquire excess
nitrogen from their high-protein diet:
– excess nitrogen must be eliminated:
• aquatic animals eliminate nitrogen as
ammonia
• terrestrial animals cannot afford copious amounts of
water necessary for elimination of ammonia
– mammals excrete urea
– birds and reptiles excrete uric acid, which can be
eliminated with very little water
Conserving Water in Hot
Environments 1
• Animals of deserts may experience
environmental temperatures in excess
of body temperature:
– evaporative cooling is an option, but water
is scarce
– animals may also avoid high temperatures
by:
• reducing activity
• seeking cool microclimates
• migrating seasonally to cooler climates
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Conserving Water in Hot
Environments 2
• Desert plants reduce heat loading in
several ways already discussed. Plants
may, in addition:
– orient leaves to minimize
solar gain
– shed leaves and become inactive during
stressful periods
The Kangaroo Rat - a Desert Specialist
• These small desert rodents
perform well in a nearly
waterless and extremely hot
setting.
– kangaroo rats conserve water
by:
• producing concentrated urine
• producing nearly dry feces
• minimizing evaporative losses
from lungs
– kangaroo rats avoid desert heat
by:
• venturing above ground only at
night
• remaining in cool, humid burrow
by day
Organisms maintain a constant
internal environment.
• An organism’s ability to maintain constant
internal conditions in the face of a varying
environment is called homeostasis:
– homeostatic systems consist of sensors,
effectors, and a condition maintained constant
– all homeostatic systems employ negative
feedback -- when the system deviates from set
point, various responses are activated to return
system to set point
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Temperature Regulation: an
Example of Homeostasis
• Principal classes of regulation:
– homeotherms (warm-blooded animals) maintain relatively constant internal
temperatures
– poikilotherms (cold-blooded animals) –
tend to conform to external temperatures
some poikilotherms can regulate internal
temperatures behaviorally, and are thus
considered ectotherms, while homeotherms are
endotherms
Homeostasis is costly.
• As the difference between internal and
external conditions increases, the cost
of maintaining constant internal
conditions increases dramatically:
– in homeotherms, the metabolic rate
required to maintain temperature is directly
proportional to the difference between
ambient and internal temperatures
Limits to Homeothermy
• Homeotherms are limited in the extent to
which they can maintain conditions different
from those in their surroundings:
– beyond some level of difference between ambient
and internal, organism’s capacity to return internal
conditions to norm is exceeded
– available energy may also be limiting,
because regulation requires substantial
energy output
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Partial Homeostasis
• Some animals (and plants!)
may only be homeothermic
at certain times or in certain tissues…
– pythons maintain high temperatures when incubating
eggs
– large fish may warm muscles or brain
– some moths and bees undergo pre-flight warm-up
– hummingbirds may reduce body temperature at night
This is called torpor.
Delivering Oxygen to Tissues
• Oxidative metabolism releases energy.
• Low O2 may thus limit metabolic activity:
– animals have arrived at various means of
delivering O2 to tissues:
• tiny aquatic organisms (<2 mm) may rely on diffusive
transport of O2
• insects use tracheae to deliver O2
• other animals have blood circulatory systems that
employ proteins (e.g., hemoglobin) to bind oxygen
Countercurrent Circulation
• Opposing fluxes of fluids can lead to efficient
transfer of heat and substances:
– countercurrent circulation offsets tendency for
equilibration (and stagnation)
– some examples:
• in gills of fish, fluxes of blood and water are opposed,
ensuring large O2 gradient and thus rapid flux of O2 into
blood across entire gill structure
• similar arrangement of air and blood flow in the lungs of
birds supports high rate of O2 delivery
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Figure 3.23
Heat transfer
from warm artery
to cool vein
Summary
• The mechanisms by which
organisms interact with their
physical environment help
us understand why
organisms are specialized
to narrow ranges of
conditions (optimum) and
how adaptations of
morphology and physiology
are associated with certain
conditions.
Sample midterm exam questions
1. Which of the following properties of water is most important in
preventing the bottoms of large bodies of water (lakes and oceans) from
freezing solid?
a) water conducts heat rapidly.
b) water is most dense at 4oC.
c) water is capable of dissolving a wide array of substances.
d) freezing of water requires the removal of 180 times as much heat as that
needed to lower the temperatures of the same quantity of water by 1oC.
2. How do plants growing in deserts and salty environments obtain water from
the soil?
a) by greatly expanding the surface area of their root systems.
b) by actively pumping water molecules from soil into their roots.
c) by increasing the concentrations of solutes in their root cells.
d) by decreasing the concentrations of solutes in their root cells.
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