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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