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Chapter 3

Ch3 Adaptations to the Physical Environment.docx

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Spencer Barrett

chapter 3: ADAPTATIONS TO THE PHYSICAL ENVIRONMENT  when air temp. approach the max. tolerable body temp., animals dissipate heat only by evaporating water from their skin and respiratory surfaces  in hot deserts, water is scarce, evaporative cooling is costly  animals become less active, seek cool microclimates, and sometimes undertake seasonal migrations to cooler regions  kangaroo rats: venture out only at night, during the day stay in their cool humid burrows  ground squirrels: remain active during the day, and their body temperature rises; before their temp. goes too high, they go to their cool burrows to lose heat w/o losing water  they shuttle back and forth b/w their burrows and the surface  camels: allow body temp. to rise during the day  their large bodies give them an advantage  w/ increasing size, the surface area of an animal (across it which it absorbs heat and intercepts solar radiation) increases less rapidly than the animal’s volume (which is the bulk that heats up)  thus, the camel heats up slowly during the day and dumps the excess heat at night to its cooler surroundings  living organisms transform NRG to perform work  sunlight is the ultimate source of NRG for most life processes  plants use this NRG by photosynthesis, producing high-NRG bonds of organic molecules which forms the basis of the food chain  sunlight is also the ultimate source of thermal NRG  creates suitable condition for life Light is the Primary Source of Energy  plants, algae, and some bacteria absorb sunlight and assimilate its NRG by photosynthesis (but not all the light striking the earth’s surface can be used this way)  light consists of a spectrum of wavelengths (nm; spectrum of electromagnetic radiation)  ultraviolet (UV)/gamma rays: shortest  visible light: suitable for photosynthesis; 400nm (violet) to 700nm (red); photosynthetically active region PARR)  infrared (IR)/radio waves: longest, perceived as heat  sunlight is packaged in photons (small, particle-like units of NRG)  their NRG varies inversely w/ their wavelength  photons making up shorter-wl blue light vibrate more rapidly, have a higher NRG level/light intensity than those that make up longer-wl red light  a small fraction of solar radiation is converted into biological production through photosynthesis  irradiance is the intensity of the light of all wavelengths impinging on a surface (watts/square m)  is diminished by nighttime periods, light reflection from clouds, absorption of light by the atm. before it even reaches earth  albedo is the proportion of light that is reflected by a particular surface  snow and clouds 80%-90%  sand, dry soils, deserts 20%-30%  savannas, meadows, crops 20%  forests, water surfaces <10%  avg. albedo of earth 30%  this reflected light is potential NRG lost to the eart Light Absorption Spectra of Plants  the visible portion of the spectrum used for photosynthesis is the portion that has the highest irradiance  leaves contain pigments (contained in the chloroplasts) that absorb light to harness NRG  chlorophyll: captures light NRG in the light rxns of photosynthesis  absorbs red and violet light, reflects green and blue  carotenoids (esp. carotenes and xanthophylls): accessory pigments; the pass light NRG to cholorphyll to begin the sequence of rxns in photosynthesis  absorbs blue and green light, reflect yellow and orange wl of the spectrum  complement the absorption spectrum of chlorophyll  water absorbs visible light weakly  transparency of a glass of water is deceptive (appears colourless in small quantities)  absorbs/scatters light enough to limit the depth of the photic zone  absorbs longer (red) wl more strongly than shorter ones  IR rays disappear within the topmost meter of water  shortest visible wavelengths (violet and blue) scatter  both fail to penetrate deeply, thus green light predominates w/ increasing depth  algae living near surface of oceans (ex. green sea lettuce Ulva) have pigments resembling terre. plants that absorb blue and red and reflect green light  deep-water red alga Porphyra has additional pigments enable it to use green light eff. Photosynthesis  photons of light react with pigments (to which the NRG is transferred)  NRG is transferred by photosynthetic organisms into chemical NRG in high-NRG bonds of organic molecules  reduces carbon from CO in t2e process  6 CO 2 6H O 2 photons  C H O +66 12 6 2 The Light Reactions  pigments (ex. chlorophyll) absorbs photons, releases e-  these e- are passed along a chain of rxns to produce ATP and NADPH (both high-NRG)  cell uses the contained NRG to reduce C and produce glucose (C H O ) 6 12 6  chlorophyll molecules regain e- by taking single e- from H O mole2ules, producing O as 2 a waste produce C 3hotosynthesis  conversion of CO at2m into a three-carbon sugar  a single molecule of CO co2bines with a five-carbon sugar (ribulose biphosphate, RuBP)  produces 2 molecules of glyceraldehydes 3-phosphate (G3P)  this stage is one part of the light rxns  CO 2 RuBP  2 G3P  product is a three-carbon compound, thus this pathway is called C photosynthesis C3photosynthesis  then, 2 molecules of G3P enter the Calvin-Benson cycle  regenerates 1 RuBP molecule while making 1 reduced carbon atom available to synthesize glucose and other organic compounds  occurs in mesophyll cells of leaves  RuBP carboxylase-oxidase (Rubisco) is the enzyme responsible for the assimilation of C  has low affinity for CO 2  thus, at low conc. of CO in2mesophyll cells, plants assimilate carbon inefficiently  to assimilate eff., plants pack mesophylls w/ large amounts of Rubisco (makes up 30% of dry weight of some species)  binds O 2nd CO (pa2t. under high O and lo2 CO conc., and2esp. at high leaf temp.)  reverses light rxns when Rubisco binds O inste2d of CO 2  2 G3P  CO + R2BP  resembles respiration (uses O and produces CO ; requires ARP and NADPH from light 2 2 rxns) o photorespiration  the tendency of Rubisco to undergo this rxn (which partially undoes what it accomplishes when it assimilates C) makes photosynthesis inefficient and self- limiting  photorespiration is a wasteful and counterproductive process, and C assimilation therefore tends to inhibit itself as levels of CO decline in the leaf tissue 2 Modification of Photosynthesis in Environments with High Water Stress  plants must maintain high CO conc2 in leaf cells  some keep stomates open to allow free gas exchange b/w atm.  but transpiration leads to water loss  detrimental in hot/dry envmts  CO enters plants through diffusion (even though CO conc. is only 0.038% in atm; but 2 2 atm-to-plant difference is much less than plant-to-atm)  the atm. diff drives water out of plants too C Photosynthesis 4  has additional step to the initial assimilation of CO in 2 phot3synthesis  CO 2s first joined w/ a three-carbon phosphoenol pyruvate (PEP), producing a four- carbon oxaloacetic acid (OAA)  CO 2 PEP  OAA  rxn is catalyzed by the enzyme PEP carboxylase (has high affinity for CO ) 2  occurs in mesophyll cells  photosynthesis (in the Calvin-Benson cycle) occurs in bundle-sheath cells (which surround the leaf veins)  to get carbon from mesophyll cells into bundle sheath cells:  the plant converts OAA into malic acid which diffuses into the bundle sheath cell  there, another enzyme breaks it down producing CO and pyruv2te (a 3-C compound)  CO 2s then used in the light rxns to make G3P which enters the Calvin-Benson cycle  the pyruvate is converted back to PEP which moves back into mesophyll cells  this strategy solves the problem of photorespiration by allowing CO to reach2much higher conc. within the bundle sheath cells than it could by diffusion from the atm.  high CO c2nc. = efficient Calvin-Benson cycle  PEP carboxylase (high aff. for CO ) can bind to CO at lower conc. in the cell 2 2  allows stomates to remain closed  2 disadvantages:  less leaf tissue devoted to photosynthesis  some NRG produced by light rxns is used up in the C carbon4assimilation reactions  C plants are more efficient, thus are favoured in cooler climates with abundant soil 3 water  C 4lants make up most of our crops (highly productive during hot growing seasons) CAM Plants  succulent plants in water-stressed envmts (ex. cacti, pineapples)  use same biochemical pathway as C plants 4ut segregate CO assimilation2and the Calvin-Benson cycle b/w night and day  discovery was made in plants of the family Crassulaceae (the stonecrop family) w/ initial assimilation and storage of CO as 4-C organic acids (malic acid and OAA) 2  thus, this photosynthetic pathway is called crassucrassulacean acid metabolism CAM  during the night:  open stomates (transpiration is minimal during the cool desert night)  assimilate CO i2to 4-C OAA, which is converted to malic acid  stored in high conc. in the vacuoles within the mesophyll cells of the leaf  the enzyme for CO assi2ilation works best at the cool temp. at night  during the day:  close stomates  stored organic acids broken down to release CO to the C2lvin-Benson cycle  enzyme (w/ high temp. optimum; to promote daytime photosynthesis) regulates the regeneration of PEP from pyruvate following the release of CO 2  CAM photosynthesis has high water use efficiencies Structural Adaptations to Control Water Loss  physiological modifications to reduce transpiration across surfaces, reduce heat loads, enable tolerance to high temp.  dense hairs and spines protect surfaces from direct sunlight (which would cause heating up and water loss)  also produce a still boundary layer of air (traps moisture and reduces evaporation) boundary layer  thick boundary layers retard heat loss too, thus hair-covered plants are prevalent in arid envmts that are cool  boundary layer also forms on flat surfaces of leaves but are broken up by air turbulence at leaf edges  plants in hot deserts reduce heat loads by having finely divided leaves  large ratio of edge to surface area (breaks boundary layers)  some desert plants have no leaves (made into protective thorns)  rely on stems for photosynthesis  reduce transpiration by having a thick waxy cuticle (impervious to water)  OR recess stomates in deep pits (filled with hairs; slows air mvmt, and traps water) Diffusion Limits Uptake of Dissolved Gases from Water Carbon Dioxide  CO 2onc. in water is same as atm.  when dissolved in water, most CO form c2rbonic acid (H CO ) 2 3  depending on acidity of the water, H CO mol2cul3s release H ions to form bicarbonate bicarbonate - - ions (HCO ; -ithin pH 6-9, most common form) or carbonate ions (CO ) 3  as HCO f3rms, CO is re2oved from soln and more of it can enter from atm.  CO + H O  H CO  H + HCO + - 2 2 2 3 3  process continues until CO con2. is more than 100 times that of the air  inorganic carbon IS abundant, but rate of supply is important  carbon moves extremely slowly through water (diffuses-through unstirred water 10 000 times more slowly than in air; the larger HCO ions d3ffuse even slower)  every surface of an aquatic plant, alga, or microbe is surrounded by a boundary layer of unstirred water through which C must diffuse -  thus, despite high conc. of HCO , p3otosynthesis is limited by C availability  once inside plant cells, HCO ca3 be used directly as a source of C (not as eff. as CO ) 2 -
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