Chapter 2: The Physical Environment

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Chapter 2 The Physical Environment

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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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

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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
• function within limits set by physical laws

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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 earths 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

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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
• Waters viscosity retards the movement of
  organisms (some organisms are streamlined, others
  deploy parachutes).

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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 (NgtP).
• typical salt water N is less than 0.01 mg/L,
  while P is about 0.01-0.1 mg/L (PgtN).

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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. Heres 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 HCO3- pool, 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 (lt400 nm) is
  ultraviolet (UV)
• longer wavelength energy (gt700 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 earths surface, emit
  longwave (infrared) radiation (IR).
• Atmosphere is transparent to visible light, which
  warms the earths 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 earths 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 organisms surface (2.43 kJ/g at 30oC)
• change in heat content metabolism - evaporation
  radiation
• conduction convection

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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.