chapter 3: adaptation to aquatic and terrestrial environments robert e. ricklefs the economy of...
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Chapter 3: Adaptation to Aquatic and Terrestrial Environments
Robert E. RicklefsThe Economy of Nature, Fifth Edition
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Chapter Overview - Basics
The physical world both provides the context for life and constrains its existence.
A world of environmental factors... resources: water, minerals and food items conditions: temperature and relative humidity
Most factors have extremely wide ranges: each type of organism is typically adapted to
a narrow range of each factor
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Chapter Overview - Regulation
Organisms typically contrast with their external environments: internal conditions are maintained +/- constant fluxes of heat and substances must be
regulated but organisms are open systems...
resources must be acquiredwastes must be eliminated
How do organisms accomplish this?
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Chapter Overview - Bottom Line
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
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What’s next?
This chapter examines 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 (causes surface tension) surfaces (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
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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
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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
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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 (carbohydrates and proteins) cannot readily leave the cell
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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 deathdesert 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)
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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 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 cuticlerecess stomates in pits, sometimes also hair-filled
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Plants obtain mineral nutrients from soil water.
Nutrients must move from the soil solution into cells of root hairs… a nutrient element moves passively (via
diffusion) into root when its concentration in soil water exceeds that of root cells
when nutrient concentration in soil water is lower than that in roots, active uptake (energy-demanding) is essential
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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
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Plant Mineral Nutrition - a Case Study in Patchiness
Distributions of nutrients in soils is highly patchy (heterogeneous) - how does such patchiness affect plant mineral nutrition? ragweed and pokeweed plants, when grown
in monoculture, performed best when soil nutrients were patchy instead of homogeneous
when these plants were grown together, advantage of patchy nutrients disappeared
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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
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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
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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)
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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 lessenedC3 approach is biochemically more energy-efficient
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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
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Balancing Salt and Water
Osmotic regulation is not just a problem for plants
Aquatic animals are rarely in equilibrium with their surroundings: fresh-water fish are hyperosmotic
(internal salt concentration higher than that of medium)
marine fish are hypo-osmotic (internal salt concentration lower than that of medium)
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Ion retention is critical to freshwater organisms.
Freshwater fish must eliminate excess water and selectively retain dissolved ions: they gain water by osmosis they eliminate excess water in their urine their kidneys selectively retain dissolved
ions active uptake of ions via gills is also
important
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Water retention is critical to marine organisms.
Saltwater fish must retain water and excrete excess ions: they tend to lose water to surrounding sea
water and must drink to replace this excess salt must be excreted from gills and
kidneys 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
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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
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Animals excrete excess nitrogen.
Carnivorous animals acquire excess nitrogen from their high-protein diet: excess nitrogen must be eliminated:
aquatic animals eliminate nitrogen as ammoniaterrestrial 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
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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 activityseeking cool microclimatesmigrating 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
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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 urineproducing nearly dry fecesminimizing evaporative losses from lungs
kangaroo rats avoid desert heat by:venturing above ground only at nightremaining in cool, humid burrow by day
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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 temperaturessome poikilotherms can regulate internal
temperatures behaviorally, and are thus considered ectotherms, while homeotherms are endotherms
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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
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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 brainsome moths and bees undergo pre-flight
warm-uphummingbirds may reduce body temperature
at night (torpor)
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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
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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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Conservation and Countercurrents
Countercurrent fluxes can also assist in conservation of heat; here are two examples: birds of cold regions conserve heat through
countercurrent circulation of blood in legswarm arterial blood moves toward feetcooler venous blood returns to body coreheat from arterial blood transferred to venous blood
returns to core instead of being lost to environment
kangaroo rats use countercurrent process to reduce loss of moisture in exhaled air
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Each organism functions best under a restricted range of conditions.
Organisms function best in a relatively narrow range of conditions, the optimum: optimum is a result of natural selection for
biochemical properties of enzymes and lipids, as well as internal structures, body form, etc.
such specialization precludes efficient function across wide ranges of conditions, which would be expensive and compromise optimal function
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Compensation is possible.
Many organisms accommodate to predictable environmental changes through their ability to “tailor” various attributes to prevailing conditions: rainbow trout are capable of producing two
forms of the enzyme, acetylcholine esterase:winter form has highest substrate affinity between
0 and 10oCsummer form has highest substrate affinity
between 15 and 20oC
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Adaptation is the key to under-standing success of organisms.
Organisms living in different environments function equally well under their constraints: Antarctic and tropical fish both swim actively!
Acclimatization permits some degree of adjustment to changing conditions: rainbow trout example rapid adjustment of O2 transport capabilities to
changing partial pressure of O2 with elevation in vertebrates, including humans
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Summary
The mechanisms by which organisms interact with their physical environment help us understand why organisms are specialized to narrow ranges of conditions and how adaptations of morphology and physiology are associated with certain conditions.
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