Ch. 5 Coping With Environmental Variation: Energy

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autotroph

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convert the energy of sunlight (photosynthetic organisms) or inorganic compounds (chemosynthetic archaea and bacteria) into chemical energy stored in the carbon-carbon bonds of organic compounds, typically carbohydrates

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heterotroph

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• organisms that obtain their energy by consuming energy-rich organic compounds made by other organisms
• include organisms that consume nonliving organic matter (detritivores) such as earthworms and fungi in the soil; organisms that consume living organisms, but do not necessarily kill them (parasites and herbivores) as well as consumers (predators) that capture and kill their food source
• also includes holoparasites

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holoparasites

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type of heterotroph that lost its photosynthetic function and thus obtains their energy by parasitizing other plants

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hemiparasites

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photosynthetic but obtain some of their energy, nutrients, and water from host plants

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chemosynthesis

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• process that uses energy from inorganic compounds to fix CO2 and produce carbohydrates
• earliest autotrophs were probably chemosynthetic bacteria or archaea that evolved when composition of atmosphere was different from today (low in oxygen, but rich in hydrogen, with significant amounts of CO2 and methane)

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chemosynthesis process

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(1) organism oxidizes (obtains electrons from) the inorganic substrate
(2) use the electrons to generate two high-energy compounds: ATP and NADPH
(3) use energy from ATP and NADPH to fix CO2 (take up carbon from gaseous CO2)
(4) fixed carbon is used to synthesize carbohydrates or other organic molecules

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Calvin cycle

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• pathway most commonly used to fix carbon
• catalyzed by several enzymes
• occurs in both chemosynthetic and photosynthetic organisms

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nitrifying bacteria

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in a two-step process, they convert ammonium into nitrite then oxidize it to nitrate

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photosynthesis

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• like chemosynthesis, it also involves conversion of CO2 into carbohydrates that are used for energy storage and biosynthesis
• has two major steps: light reactions and dark reactions

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light reactions

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first step which involves the harvesting of energy from sunlight which is then used to split water to provide electrons for generating ATP and NADPH

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dark reactions (Calvin cycle)

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• fixation of carbon and synthesis of sugars and subsequently carbohydrates
(1) energy from ATP and NADPH is used to fix carbon
(2) enzyme called rubisco catalyzes uptake of CO2 and synthesis of a 3-carbon compound: PGA
(3) PGA eventually converted into a 6-carbon sugar (usually glucose)

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light compensation

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when there is enough light that the plant's photosynthetic CO2 uptake is balanced by its CO2 loss by respiration, a plant is said to have reached the light compensation point

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as light level increases above the light compensation point, the photosynthetic rate...

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also increases

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light saturation

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photosynthetic rate levels off at a light saturation point which is reached when photosynthesis increases only slightly as light increases, which is typically at a level below full sunlight

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light response curve

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1. light compensation point occurs where photosynthetic CO2 uptake is balanced by CO2 loss by respiration
2. below light saturation point, photosynthesis is limited by light availability
3. light saturation point is reached when photosynthesis increases only slightly as light increases

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rubisco can catalyze two competing reactions...

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• ​one is a carboxylase reaction in which CO2 is taken up, leading to synthesis of sugars and release of O2 (i.e. photosynthesis)
• the other is an oxygenase reaction in which O2 is taken up, leading to breakdown of carbon compounds and release of CO2...this reaction is part of a process called photorespiration which results in a net loss of energy and is thus potentially detrimental for plants

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the balance between photosynthesis and photorespiration is related to two main factors...

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1. ratio of O2 to CO2 in atmosphere: as atmospheric concentration of CO2 decreases relative to that of O2, rate of photorespiration increases relative to rate of photosynthesis
2. temperature: photorespiration increases more rapidly at high temps than photosynthesis does

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closing of stomates

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• low water availability results in closure of the stomates, restricting entry of CO2 into leaves
• represents important trade-off for plant: keeping stomates open while tissues lose water can permanently impair physiological processes in the leaf, but closing stomates not only limits photosynthetic CO2 uptake, it also increases chances of light damage to the leaf

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temperature influences photosynthesis in two main ways...

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1. through its effects on rates of chemical reactions
2. by influencing structural integrity of membranes and enzymes

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higher amounts of nitrogen in leaves are correlated with...

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higher photosynthetic rates

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C4 pathway

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• reduces photorespiration
• well-known examples include corn, sugarcane, and sorghum
• CO2 initially taken up by an enzyme called PEPcase which has a greater capacity to take up CO2 than rubisco and does not take up O2
• PEPcase fixes CO2 and once CO2 is taken up, a four-carbon compound is synthesized and transported to bundle sheath where Calvin cycle occurs
• C4 plants can photosynthesize at higher rates than C3 plants in conditions that promote photorespiration
• C4 plants have lower transpiration rates because PEPcase can take up CO2 under lower CO2 concentrations that exist when stomates are not fully open

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CAM pathway

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• enhances water conservation
• separates CO2 uptake and Calvin cycle temporally
• open stomates at night...because air temps at night are cooler, humidity is higher which results in lower water potential gradient between leaf and air, so the plant loses less water by transpiration than it would during the day
• during night when stomates are open, CAM plants take up CO2 using PEPcase and incorporate it into a four-carbon organic acid
• during the day the organic acid is broken down, releasing CO2 to the Calvin cycle
• their high CO2 concentrations increase efficiency of photosynthesis and suppress photorespiration
• typically associated with arid and saline environments (deserts), some found in humid tropics
• epiphytes growing on branches of trees

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succulence

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photosynthetic rates in CAM plants are usually related to capacity of the plant to store the four-carbon organic acid, so many CAM plants are succulent, with thick, fleshy leaves or stems, which enhances their nighttime acid storage capacity

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heterotrophy and energy

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• heterotrophs consume energy-rich compounds from their environment and convert them into usable chemical energy, primarily ATP, by processes such as glycolysis, which breaks down carbohydrates
• energy gain from food depends on chemistry of the food
• most food consists of complex compounds that must be transformed into simpler compounds before they can be used as energy sources

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animal and plant tissues

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• animal tissues are generally more energy-rich than plant, fungal, or bacterial cells, which tend to have higher concentrations of fiber
• as a result, herbivores generally have to eat more food to get same benefit that carnivores do, but carnivores may expend more energy finding food than herbivores
• plants: low protein, high carbohydrates
• animals: high protein and fat, low carbohydrates

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most herbivores have digestive tracts that are...

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• longer than those of carnivores in order to cope with poor-quality food
• the longer length increases both food processing time and surface area for absorbing energy