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Chapter 6

Chapter 6 Online Textbook BIO120 2013.docx

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James Thomson

Chapter 6 9/22/2013 8:43:00 AM 6.0 Adaptations to the Physical Environment: Light, Energy, and Heat Chapter Concepts • Light is the primary source of energy for the biosphere • Plants capture the energy of sunlight by photosynthesis • Plants modify photosynthesis in environments with high water stress • Diffusion limits uptake of dissolved gases from water • Temperature limits the occurrence of life • Each organism functions best under a restricted range of temperatures • The thermal environment includes several avenues of heat gain and loss • Homeothermy increases metabolic rate and efficiency  The ability to counteract external physical forces distinguishes the living from the nonliving  Organisms transform energy to perform work  Ultimate source of energy  the sun 6.1 Plants modify photosynthesis in environments with high water stress  Because of self-limiting nature of C3photosynthesis as CO l2vels in leaves decrease, plants face serious limitations on their rate of photosynthesis, growth and reproduction o Solution: maintain high levels of CO i2 leaf cells o Do this by keeping stomates open to maintain free gas exchange o Lose water through transpiration through this process o enters plant cells because its concentration in the atmosphere is higher than its concentration in the cells, where it is continually used up by photosynthesis o drought-adapted plants evaporate a hundred or more grams of water from their leaves for every gram of carbon they assimilate  C 4hotosynthesis  To fix problem with photorespiration, many plants have adapted from C3 by adding a step to the initial assimilation of CO 2 o Call this ―C4photosynthesis‖ o CO is2first joined with a three-carbon molecule, phosphoenol pyruvate (PEP), to produce a four-carbon molecule, oxaloacetic acid (OAA): o CO + 2EP  OAA  Rxn. Catalyzed by PEP carboxylase (has high affinity for CO 2 o Pros:  allows CO 2o reach much higher concentrations within the bundle sheath cells than it could by diffusion from the atmosphere. At this higher CO co2centration, the Calvin–Benson cycle operates more efficiently  enzyme PEP carboxylase has a high affinity for CO , i2 can bind CO a2 a lower concentration in the cell, thereby allowing the stomates to remain closed longer and reduce water loss.  Carbon assimilation in CAM plants o Water-stressed environment plants use C4 system, but segregate CO as2imilation and the Calvin–Benson cycle between night and day o Crassulacean acid metabolism (CAM).  A photosynthetic pathway in which the initial assimilation of carbon into a four-carbon compound occurs at night; found in some succulent plants in arid habitats. o Open their stomates for gas exchange during cool desert nights o CAM plants assimilate CO int2 four-carbon OAA, which is converted to malic acid and stored at high concentrations in vacuoles within the mesophyll cells of the leaf o During day, stomates close, and stored organic acids are broken down to release CO to 2he Calvin–Benson cycle  Plants have spines and hairs to help adapt to heat and drought o Also helps trap moisture + reduce evaporation  Also can have waxy cuticle 6.2 Diffusion limits uptake of dissolved gases from water Carbon Dioxide  difficult to get enough carbon dioxide for aquatic plants o forms carbonic acid (H CO2) w3en in water   o bicarbonate ions provide a large reservoir of inorganic carbon in aquatic systems.  essential to have an abundance of inorganic carbon since carbon moves very slowly in water  boundary layer at the surface of an aquatic plant retards the exchange of gases between its leaves and the surrounding water.  When Bicarbonate enter cells: Oxygen  oxygen much less abundant in water o limits metabolism in aquatic animals  deeper the water, the less oxygen there is being produced by photosynthesis o habitats are anaerobic  no oxygen o or anoxic  lack of oxygen 6.3 Temperature limits the occurrence of life  most physiological occur between 0-100 degrees C  most plants
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