Transcript The Physical Environment, I & II [Lectures 3, 4]

Chapter 2: The Physical
Environment
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 W. H. Freeman and
Company
Constraints and Solutions
Physical properties of the environment and of
biological materials constrain life, but also
provide solutions to many of its problems. For
example:
a constraint: blood and tissues of most vertebrates
freeze at temperatures above those found in polar
seas; how can fish living in such habitats survive?
a solution: increased blood and tissue levels of
glycerol lower freezing temperature without
disrupting functioning
(c) 2001 W. H. Freeman and
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Background
Living things have a purposeful
existence; their structures, physiology,
and behavior are directed toward
procuring energy and resources and
producing offspring. They:
depend on the physical world for:
energy from sunlight
nutrients from the soil and water
affect and alter the physical world
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function within limits
Company set by physical laws
Life Out of Balance
Life exists out of equilibrium with the
physical world and in a state of constant
tension with its physical surroundings:
consider the bird in flight, which expends
energy to counteract the force of gravity
consider the plant, which expends energy to
maintain high levels of scarce water and
nutrients in its tissues
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Water has many properties favorable
for the maintenance of life.
Water, an ideal life medium:
is abundant over most of earth’s surface
is an excellent solvent and medium for chemical
processes
allows for high concentrations of molecules
necessary for rapid chemical reactions
enables movements of organisms because of its
fluidity
(c) 2001 W. H. Freeman and
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Thermal Properties of
Water
Thermal properties:
liquid over broad range of temperatures
conducts heat rapidly
resists temperature changes because of its
heat capacity
resists changes in state:
freezing requires heat removal of 80 cal/g
evaporation requires heat addition of over 500
cal/g
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Water has other remarkable
thermal properties.
Most substances become denser as they cool.
Water also becomes denser, to a point, but:
reaches maximum density at 4oC, and expands as it
cools below that point
expands even further upon freezing
This property is of monumental importance to
life on earth:
bottoms of lakes and oceans prevented from freezing
floating layer of ice with covering of snow forms
protective, insulating surface
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The Buoyancy and
Viscosity of Water
 Density of water (800x that of air) means that water is
buoyant.
 Aquatic organisms achieve neutral density through:
reduction (bony fish) or elimination (sharks) of hard skeletal
components
use of gas-filled swim bladder (plants too!)
accumulation of lipids
 Water’s viscosity retards the movement of organisms
(some organisms are streamlined, others deploy
parachutes).
(c) 2001 W. H. Freeman and
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All natural waters contain
dissolved substances.
Water is a powerful solvent because of its
charge polarity.
Almost all substances dissolve to some extent in
water.
Nearly all water contains some dissolved
substances:
rainwater acquires dissolved gasses and trace
minerals
lakes and rivers contain 0.01-0.02% dissolved
minerals
oceans contain 3.4% dissolved minerals
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Fresh Versus Salt Water
Noteworthy differences in makeup of solutes:
salt water is rich in Na+, Cl-, Mg2+, SO42fresh water is rich in Ca2+, HCO3-, and SO42-
Solute loads of surface waters reflect bedrock
chemistry:
water of limestone areas is “hard” with substantial
Ca2+, HCO3water of granitic areas contains few mineral elements
Oceanic waters are saturated with respect to
Ca2+, but continue to accumulate Na+.
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Waters differ in contents
of essential nutrients.
N and P are among most the important
essential elements and are often
limiting:
typical fresh water N is 0.40 mg/L, while P
is about 0.01 mg/L (N>P).
typical salt water N is less than 0.01 mg/L,
while P is about 0.01-0.1 mg/L (P>N).
(c) 2001 W. H. Freeman and
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pH - the Concentration of
Hydrogen Ions
Normal pH range of surface waters is 6-9.
Acid rain can lower pH to as low as 4 in some
areas.
Acidity dissolves minerals
water in limestone areas is “hard” with substantial
Ca2+, HCO3most organisms regulate pH around neutrality;
adaptations to life out of balance with external
medium (high or low pH) are costly (it takes energy
to be different!)
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C and O are intimately involved
in energy transformations.
Compounds contain energy in their
chemical bonds:
energy is required to create bonds
energy is released when bonds are broken
Energy transformations proceed by
oxidation and reduction, often involving C:
oxidation removes electrons, releases
energy
reduction adds electrons, requiring energy
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Heterotrophs and
Autotrophs
Heterotrophs obtain their energy by
consuming organic (biological) sources of
carbon-rich food, which they oxidize.
Autotrophs obtain their energy from inorganic
sources, and use this energy to reduce carbon,
which they store for later use:
photoautotrophs obtain energy from light
chemoautotrophs obtain energy from oxidation of
inorganic compounds such as H2S, NH4+
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Photosynthesis and
Respiration
Think of photosynthesis and respiration as
complementary reactions which:
reduce carbon (photosynthesis):
energy + 6CO2 + 6H2O  C6H12O6 + 6O2
water is an electron donor (reducing agent)
oxidize carbon (respiration):
C6H12O6 + 6O2  energy + 6CO2 + 6H2O
oxygen is an electron acceptor (oxidizing agent)
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The Limited Availability of
Inorganic Carbon 1
Terrestrial plants have a difficult time acquiring
inorganic carbon:
carbon (as CO2) diffuses into leaf from atmosphere:
rate of diffusion of a gas is proportional to concentration
difference between external and internal media
atmosphere-to-plant difference in [CO2] is small
plant-to-atmosphere difference in [H2O] is great
bottom line: plants lose enormous amounts of water to the
atmosphere relative to carbon gained, at a rate of 500 g
water for each g of carbon
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The Limited Availability of
Inorganic Carbon 2
Aquatic plants have a more reliable source of
carbon than terrestrial plants. Here’s why:
at typical pH (6-9), solubility of CO2 in water is about
0.03% by volume
carbon is rapidly converted to HCO3- by:
CO2 + H2O  H2CO3  H+ + HCO3this process depletes dissolved CO2, allowing for more CO2
to enter the water, which in turn further enriches the HCO3pool, available for plant uptake
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Carbon dioxide diffuses
slowly through water.
Both CO2 and HCO3- diffuse slowly
through water.
A thin boundary layer (10-500 um)
adjacent to the plant surface becomes
carbon-depleted, and it forms a diffusion
barrier between the plant and C-rich
water beyond.
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Oxygen is scarce in water.
Oxygen is rather limited in water:
low solubility
limited diffusion
below limit of light penetration and in
sediments rich in organic matter, conditions
become anaerobic or anoxic
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Availability of Inorganic
Nutrients
After H, C, and O, elements required in greatest
quantity are N, P, S, K, Ca, Mg, and Fe.
Certain organisms require other elements:
diatoms require Si for their glassy cases
nitrogen-fixing bacteria require Mo as part of the key
enzyme in N assimilation
Terrestrial plants acquire most elements from
water in soil around roots:
availability varies with temperature, pH, presence of
other ions
P is particularly limiting in soils
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Light is the primary source of
energy for the biosphere.
A quick primer on light:
energy reaching earth from the sun covers a broad
spectrum of wavelengths:
visible light ranges from 400 nm (violet) to 700 nm (red)
shorter wavelength energy (<400 nm) is ultraviolet (UV)
longer wavelength energy (>700 nm) is infrared (IR)
energy content of light varies inversely with its
wavelength
the shorter the wavelength, the more energetic the light
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Ozone and Ultraviolet
Radiation
UV “light” has a high energy level and can
damage exposed cells and tissues.
Ozone in upper atmosphere absorbs strongly in
ultraviolet portion of electromagnetic spectrum.
Chlorofluorocarbons (formerly used as
propellants and refrigerants) react with and
chemically destroy ozone:
ozone “holes” appeared in the atmosphere
concern over this phenomenon led to strict controls
on CFCs and other substances depleting ozone
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Infrared Light and the
Greenhouse Effect 1
All objects, including the earth’s
surface, emit longwave (infrared)
radiation (IR).
Atmosphere is transparent to visible
light, which warms the earth’s surface.
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Infrared Light and the
Greenhouse Effect 2
Infrared light (IR) emitted by earth is
absorbed in part by atmosphere, which is
only partially transparent to IR.
Substances like carbon dioxide and
methane increase the absorptive capacity
of the atmosphere to IR, resulting in
atmospheric warming.
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Greenhouse Effect Summary
Greenhouse effect is essential to life on
earth (we would freeze without it), but
enhanced greenhouse effect (caused in
part by forest clearing and burning fossil
fuels) may lead to unwanted warming and
serious consequences!
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The Absorption Spectra of
Plants
Various substances (pigments) in plants
have different absorption spectra:
chlorophyll in plants absorbs red and violet
light, reflects green and blue
water absorbs strongly in red and IR,
scatters violet and blue, leaving green at
depth
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Algae and Light Quality
The quality of light is related to
photosynthetic adaptations in the ocean:
algae growing near the surface have
pigments like those in terrestrial plants
(absorb blue and red, reflect green)
algae growing at depth have specialized
pigments that enable them to use green light
more effectively
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Light Intensity
Ecologists measure PAR (photosynthetically
active radiation).
Total radiation is measured as radiant flux =
1,400 W/m2 above the atmosphere (solar
constant).
Radiant flux at earth’s surface is reduced by:
nighttime periods
low angle of incidence
atmospheric absorption and scattering
reflection from the surfaces of clouds
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The Thermal Environment
 Energy is gained and lost through various pathways:
radiation - all objects emit electromagnetic radiation and
receive this from sunlight and from other objects in the
environment
conduction - direct transfer of kinetic energy of heat to/from
objects in direct contact with one another
convection - direct transfer of kinetic energy of heat to/from
moving air and water
evaporation - heat loss as water is evaporated from
organism’s surface (2.43 kJ/g at 30oC)
change in heat content = metabolism - evaporation + radiation
+ conduction + convection
(c) 2001 W. H. Freeman and
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Organisms must cope with
temperature extremes.
Unlike birds and mammals, most organisms do
not regulate their body temperatures.
All organisms, regardless of ability to
thermoregulate, are subject to thermal
constraints:
most life processes occur within the temperature
range of liquid water, 0o-100oC
few living things survive temperatures in excess of
45oC
freezing is generally harmful to cells and tissues
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Tolerance of Heat
Most life processes are dependent on water in
its liquid state (0-100oC).
Typical upper limit for plants and animals is 45oC
(some cyanobacteria survive to 75oC and some
archaebacteria survive to 110oC).
High temperatures:
denature proteins
accelerate chemical processes
affect properties of lipids (including membranes)
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Tolerance of Freezing
Freezing disrupts life processes and ice crystals
can damage delicate cell structures.
Adaptations among organisms vary:
maintain internal temperature well above freezing
activate mechanisms that resist freezing
glycerol or glycoproteins lower freezing point effectively (the
“antifreeze” solution)
glycoproteins can also impede the development of ice
crystals, permitting “supercooling”
activate mechanisms that tolerate freezing
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Organisms use physical stimuli
to sense the environment.
To function in complex and changing
environments, organisms must:
sense and detect environmental change
(plants must sense changing seasons)
detect and locate objects (predators must
find food)
navigate the landscape (salmon must
recognize their home river to spawn)
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Sensing Electromagnetic
Radiation
 Many organisms rely on vision (detection of visible light
and other wavelengths):
light has high energy
light permits accurate location and resolution of targets
 Many variations in capabilities exist:
hawks have extreme visual acuity
insects and birds can perceive UV
insects can detect rapid movements
 Animals operating in dark surroundings may sense IR
(e.g., pit vipers utilize pit organs to sense prey).
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Sensing Sound
 Sounds are pressure waves created by movements,
impacts, vibrations.
 Directional sensitivity possible by comparing signals
received at two ears:
sensitivity is greatest when the distance between ears matches
wavelength (high-pitched sounds more useful to smaller
animals)
asymmetrical shapes of owls’ ears enable accurate pinpointing
of source
 Other examples:
bats echolocate using sound pulses they generate
whales communicate over long distances using low-frequency
sounds
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Sensing Odors
 Smell is the detection of molecules diffusing through
air or water.
 Odors differ from light and sound:
odors are difficult to localize
odors persist long after source has disappeared
 Moving “upstream” along a concentration gradient
can help localize the source of odor.
 Odors are the basis of much chemical
communication:
animals use odors to attract mates
plants use odors to attract pollinators
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Sensing Electrical Fields
Some aquatic animals specialize in using and
detecting electrical fields:
some fish create electric fields and sense distortions
caused by prey
paddlefish sense distortions caused by prey
other species use electrical signals to communicate
electric ray uses powerful currents to defend itself
and stun prey
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Sensing Physical Contact
Under conditions of poor visibility, catfish
use fins and barbels as sensitive touch
and taste receptors.
Physical contact is limited in its range, but
useful under many circumstances.
Touch can provide tremendous amount of
information regarding texture and
structure.
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Summary 1
Water is the basic medium for life. Its unique
properties both constrain and provide
opportunities for living things.
Biological energy transformations are based
largely on the chemistry of carbon and oxygen,
with photosynthesis and respiration representing
the most fundamental transformations of these
elements.
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Summary 2
Most of the energy for life comes from the sun
in the form of electromagnetic radiation.
Organisms have thermal budgets comprised of
metabolism, radiation, conduction, convection,
and evaporation.
Hot and cold environments pose special
problems for organisms, requiring unique
adaptations.
Organisms sense the physical environment via
many stimuli. (c) 2001 W. H. Freeman and
Company