Lecture 3:The Physical
Environment
Pages 24-48
Constraints and Solutions
Physical properties of environment and 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
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
– function within limits set by physical laws
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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
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
Water has 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).
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
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
• The important essential elements and
often limiting are:
Nitrogen and Phosphorus
– 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).
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!)
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+
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)
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:
1. at typical pH (6-9), solubility of CO2 in water is
about 0.03% by volume
2. 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 HCO3- pool, available
for plant uptake
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.
Oxygen is scarce in water
• Oxygen is rather limited
in water:
– low solubility
– limited diffusion
– anaerobic or anoxic
conditions occur below
limit of light penetration
and in sediments rich in
organic matter
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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
Light
• A quick primer on light:
energy reaching earth from the sun covers a broad
spectrum of wavelengths:
– visible light ranges:
400 nm (violet) to 700 nm (red)
– ultraviolet (UV) shorter
wavelength energy (<400 nm)
– infrared (IR)
longer wavelength energy (>700 nm)
– energy content of
light varies inversely
with its wavelength
the shorter the wavelength,
the more energetic the light
Ozone and Ultraviolet Radiation
• UV “light” has a high energy level and can
damage exposed cells and tissues.
• Ozone in upper atmosphere absorbs most of the
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.
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.
• Carbon dioxide and
methane increase the
absorptive capacity of
the atmosphere to IR,
resulting in atmospheric
warming.
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! Climate Change
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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
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
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
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
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
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)
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
Sensing Odours
• Smell is the detection of molecules diffusing through air or water.
• Odours differ from light and sound:
– odours are difficult to localize
– odours persist long after source has disappeared
• Moving “upstream” along a concentration gradient can help
localize the source of odour.
• Odours are the basis of much
chemical communication:
- animals use odours to attract mates
- plants use odours to attract pollinators
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.
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.
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.
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