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Lecture 5

Lecture 5 - Coping with Environmental Variation - Energy

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Biology 2483A
Hugh Henry

LECTURE 5: COPING WITH ENVIRONMENTAL VARIATION: ENERGY  Autotrophs – assimilate radiant energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthesis) o The energy is converted into chemical energy stored in the bonds of organic molecules  Heterotrophs – obtain their energy by consuming organic compounds from other organisms o This energy originated with organic compounds synthetized by autotrophs  Some heterotrophs consume non-living organic matter  Parasites and herbivores consume live hosts, but do not necessarily kill them  Predators capture and consume live prey animals  Some plants are holoparasites – they have no photosynthetic pigments and get energy from other plants (heterotrophs) o Dodder is a holoparasite that is an agricultural pest and can significantly reduce biomass in the host plant o Very extreme draw down in terms of energy from the host  Mistletoe is a hemiparasite – it is photosynthetic, but obtains nutrients, water, and some of its energy from the host plant  Sea slugs have functional chloroplasts that are taken up from the algae that the slug eats o Assimilates the chloroplast of algae into its own tissues allowing it to photosynthesize  Photosynthesis - (most autotrophs): sunlight provides the energy to take up CO an2 synthesize organic compounds  Chemosynthesis (chemolithotrophy): energy from inorganic compounds is used to produce carbohydrates. o Chemosynthesis is important in nutrient cycling bacteria, and in some ecosystems such as hydrothermal vent communities  Most of the biologically available energy on Earth is derived from photosynthesis  Photosynthetic organisms include some archaea, bacteria, and protists, and most algae and plants Photosynthesis  Photosynthesis has two major steps: 1. Light reaction—light is harvested and used to split water and provide electrons to make ATP and NADPH 2. Dark reaction—CO is 2ixed in the Calvin cycle, and carbohydrates are synthesized  Photosynthetic rate determines the supply of energy, which in turn influences growth and reproduction  Environmental controls on photosynthetic rate are an important topic in physiological ecology  Light response curves show the influence of light levels on photosynthetic rate.  Light compensation point – where CO upta2e is balanced by CO 2oss by respiration o Amount of light where respiratory cost of the plant is balanced out by photosynthesis – no net photosynthesis  Saturation point – when photosynthesis no longer increases as light increases  Plants can acclimatize to changing light intensities with shifts in light response curves  Shifts in light saturation point involve morphological and physiological changes  Leaves at high light intensity may have thicker leaves and more chloroplasts  Water availability influences CO2supply in terrestrial plants  Low water availability causes stomates to close, restricting C2 uptake  This is a trade-off: Water conservation versus energy gain  Closing stomates increases chance of light damage: If the Calvin cycle isn’t operating, energy accumulates in the light-harvesting arrays and can damage membranes.  Plants have various mechanisms to dissipate this energy, including carotenoids  Plants from different climate zones have enzyme forms with different optimal temperatures that allow them to operate in that climate  Plants can acclimatize by synthesizing different enzyme forms  Nutrients can also affect photosynthesis: o Most nitrogen in plants is associated with rubisco and other photosynthetic enzymes o Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates  But nitrogen supply is low, relative to demand for growth and metabolism  Increasing nitrogen content of leaves increases the risk that herbivores will eat them, as plant- eating animals are also nitrogen-starved  Some metabolic processes decrease photosynthetic efficiency  Rubisco can catalyze two competing reactions: o Carboxylase reaction: photosynthesis o Oxygenase reaction: O is2taken up, carbon compounds are broken down, and CO is 2 released (photorespiration)  Uses energy, and causes loss of carbon compounds – not good from a plant’s perspective  Does photorespiration have any benefits? o Experiments with Arabidopsis thaliana plants with a mutation that knocks out photorespiration:  These plants die under normal light and CO conditions. 2 o Hypothesis: Photorespiration may protect plants from damage at high light levels. o Altered tobacco plants with high rates of photorespiration showed less light damage than plants with normal or lowered photorespiration rates (Kozaki and Takeba 1996)  But photorespiration is not advantageous if CO 2s low and temperatures high.  Such conditions existed 7 million years ago, when C 4hotosynthesis first appeared.  The C photosynthetic pathway reduces photorespiration, and evolved independently several 4 times  Many grass species use this pathway, including corn, sugarcane, and sorghum. It involves biochemical and morphological specialization. 
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