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Chapter 2: The Physical Environment Robert E. Ricklefs The Economy of Nature, Fifth Edition 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:

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chapter 2 the physical environment

Chapter 2: The Physical Environment

Robert E. Ricklefs

The Economy of Nature, Fifth Edition

(c) 2001 W. H. Freeman and Company

constraints and solutions
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 Company

background
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

(c) 2001 W. H. Freeman and Company

life out of balance
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

(c) 2001 W. H. Freeman and Company

water has many properties favorable for the maintenance of life
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 Company

thermal properties of water
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

(c) 2001 W. H. Freeman and Company

water has other remarkable thermal properties
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

(c) 2001 W. H. Freeman and Company

the buoyancy and viscosity of water
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 Company

all natural waters contain dissolved substances
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

(c) 2001 W. H. Freeman and Company

fresh versus salt water
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+.

(c) 2001 W. H. Freeman and Company

waters differ in contents of essential nutrients
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 Company

ph the concentration of hydrogen ions
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) 2001 W. H. Freeman and Company

c and o are intimately involved in energy transformations
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

(c) 2001 W. H. Freeman and Company

heterotrophs and autotrophs
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+

(c) 2001 W. H. Freeman and Company

photosynthesis and respiration
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)

(c) 2001 W. H. Freeman and Company

the limited availability of inorganic carbon 1
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

(c) 2001 W. H. Freeman and Company

the limited availability of inorganic carbon 2
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 HCO3- pool, available for plant uptake

(c) 2001 W. H. Freeman and Company

carbon dioxide diffuses slowly through water
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.

(c) 2001 W. H. Freeman and Company

oxygen is scarce in water
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

(c) 2001 W. H. Freeman and Company

availability of inorganic nutrients
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

(c) 2001 W. H. Freeman and Company

light is the primary source of energy for the biosphere
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

(c) 2001 W. H. Freeman and Company

ozone and ultraviolet radiation
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

(c) 2001 W. H. Freeman and Company

infrared light and the greenhouse effect 1
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.

(c) 2001 W. H. Freeman and Company

infrared light and the greenhouse effect 2
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.

(c) 2001 W. H. Freeman and Company

greenhouse effect summary
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!

(c) 2001 W. H. Freeman and Company

the absorption spectra of plants
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

(c) 2001 W. H. Freeman and Company

algae and light quality
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

(c) 2001 W. H. Freeman and Company

light intensity
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

(c) 2001 W. H. Freeman and Company

the thermal environment
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 Company

organisms must cope with temperature extremes
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

(c) 2001 W. H. Freeman and Company

tolerance of heat
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)

(c) 2001 W. H. Freeman and Company

tolerance of freezing
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

(c) 2001 W. H. Freeman and Company

organisms use physical stimuli to sense the environment
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)

(c) 2001 W. H. Freeman and Company

sensing electromagnetic radiation
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).

(c) 2001 W. H. Freeman and Company

sensing sound
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

(c) 2001 W. H. Freeman and Company

sensing odors
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

(c) 2001 W. H. Freeman and Company

sensing electrical fields
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

(c) 2001 W. H. Freeman and Company

sensing physical contact
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.

(c) 2001 W. H. Freeman and Company

summary 1
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.

(c) 2001 W. H. Freeman and Company

summary 2
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