L03 Energy Flux in Ecology - Principles in Ecology PDF

Principles in Ecology PCB 4043 Energy flux Prof. Fahimipour Topics: Energy acquisition, metabolism, diet [email protected] D: 437 DW | B: 206 Sanson Last time (review growth forms!)  This time Organisms get energy from sunlight, from inorganic chemicals, or from...

Principles in Ecology PCB 4043 Energy flux Prof. Fahimipour Topics: Energy acquisition, metabolism, diet [email protected] D: 437 DW | B: 206 Sanson Last time (review growth forms!)  This time Organisms get energy from sunlight, from inorganic chemicals, or from eating organic compounds. Radiant and chemical energy captured by autotrophs is converted into stored energy in carbon–carbon bonds. Environmental constraints have resulted in the evolution of biochemical pathways that improve the efficiency of photosynthesis. Heterotrophs have adaptations for acquiring and assimilating energy efficiently from a variety of organic sources.  Reminder Homework 1 will be posted today and due on Canvas next Fri The homework will contain instructions, read them carefully There is a part of the assignment where I ask you to perform some simple data analysis. To do this, I will give you some computer code to run. You can run this code in a web browser, so no special equipment is necessary. Come to office hours if you get stuck.  Introduction Energy is the most basic requirement for all organisms. Without energy inputs, biological functioning ceases. Organisms use many mechanisms to obtain energy.  Keep in mind this lecture We want to understand the difference between autotrophy and heterotrophy; i.e., building energy compounds using external sources of energy versus consuming them from organic matter.  Keep in mind this lecture We want to understand the difference between autotrophy and heterotrophy; i.e., building energy compounds using external sources of energy versus consuming them from organic matter.  Sources of Energy Energy exists in many forms: – Radiant energy—sunlight – Chemical energy—stored in bonds of food molecules – Kinetic energy—movement of molecules; measured as temperature Chemical and radiant energy are captured by organisms for growth and maintenance. Kinetic energy influences temperature and rates of chemical reactions, thus the metabolic rates and energy demands of organisms.  Sources of Energy Autotrophs assimilate energy from sunlight (photosynthesis) or inorganic compounds (chemosynthesis), and convert it to chemical energy in bonds of organic molecules. Heterotrophs obtain energy by consuming organic compounds from other organisms (originally synthesized by autotrophs).  Sources of Energy Heterotrophs include: – Detritivores, such as earthworms and soil fungi, consume nonliving organic matter. – Parasites and herbivores consume live hosts, but do not necessarily kill them. – Predators and parasitoids capture and consume live prey animals.  Sources of Energy Some plants are holoparasites: they get energy from other plants (they are heterotrophs). – Dodder is a holoparasite that is an agricultural pest and can significantly reduce biomass in the host plant. Mistletoe is a hemiparasite—it is photosynthetic but gets nutrients, water, and some energy from the host plant. Figure 5.3 Plant Parasites  Sources of Energy Some animals act as autotrophs by acquiring or consuming photosynthetic organisms, or living in a close relationship with them called symbiosis. Some sea slugs have functional chloroplasts that are taken up from the algae that the slug eats. Autotrophy  Autotrophy Most autotrophs use photosynthesis: sunlight provides the energy to take up CO2 and synthesize organic compounds. Chemosynthesis: energy comes from inorganic compounds. In both, energy is stored in the carbon–carbon bonds of organic compounds. – Ecologists often use carbon as a measure of energy.  Autotrophy The earliest autotrophs were probably chemosynthetic bacteria or archaea. In chemosynthesis, organisms get electrons by oxidizing an inorganic substrate, which are used to generate high-energy ATP and NADPH. Energy in ATP and NADPH is then used to take up CO2 and make carbohydrates (fixation). CO2 is fixed in the Calvin cycle, catalyzed by enzymes; occurs in both chemosynthetic and photosynthetic organisms.  Autotrophy Chemosynthesizers include nitrifying bacteria. – They convert ammonium (NH4 +) to nitrite (NO2–), then oxidize it to nitrate (NO3–). – These conversions are important in the nitrogen cycle and plant nutrition. Sulfur bacteria occur in volcanic deposits, sulfur hot springs, and acid mine wastes. – They use H2S and HS– (hydrogen sulfide) as an energy source, producing elemental S. – Elemental S is then used as an electron source, producing SO42– (sulfate). Figure 5.5 Sulfur Deposits from Chemosynthetic Bacteria ­  Autotrophy Photosynthesis produces most of the biologically available energy on Earth. It is also responsible for the largest movements of CO2 between Earth and the atmosphere. Photosynthetic organisms include some archaea, bacteria, and protists [R], and most algae and plants.  Autotrophy Photosynthesis has two major steps: 1. Light-driven reactions—light energy is harvested and used to split water to provide electrons to make ATP and NADPH. 2. Carbon reactions—CO2 is fixed in the Calvin cycle, and carbohydrates are synthesized. (AI-generated image)  Autotrophy Light-driven reactions: Light harvesting—chlorophyll absorbs red and blue light and reflects green. Accessory pigments such as carotenoids help harvest light energy. The pigments are embedded in cell membranes or chloroplast membranes. 50 to 300 pigment molecules are grouped in antenna-like arrays. Figure 5.6 Absorption Spectra of Plant Photosynthetic Pigments  Autotrophy Pigments absorb energy from discrete units of light, called photons. The energy is used to split water and provide electrons. The electrons are passed to other molecules on the membranes, where they are used to synthesize ATP and NADPH.  Autotrophy Splitting of water generates O2. Evolution of photosynthesis led to the modern atmosphere with high O2 levels. This led to development of the ozone layer, and evolution of aerobic respiration in which O2 is an electron acceptor, facilitating great evolutionary changes for life on Earth.  Autotrophy Carbon reactions: A key enzyme in the Calvin cycle is rubisco, which catalyzes uptake of CO2 and synthesis of a 3-carbon compound. RubisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant enzyme on Earth (~4 billion yo). The net reaction of photosynthesis makes sugar from carbon dioxide: 6 CO 2 + 6 H 2O → C6 H12O 6 + 6 O 2  Autotrophy Photosynthetic rate determines the supply of energy and substrates for biosynthesis, which in turn influences growth and reproduction. Environmental controls on photosynthetic rate are thus an important topic in physiological ecology.  Quantifying photosynthesis Light response curves show the relationship between light levels and photosynthetic rate. Light compensation point: Where CO2 uptake is balanced by CO2 loss by respiration. Saturation point: When photosynthesis no longer increases as light increases. Figure 5.7 Plant Responses to Variations in Light Levels  Autotrophy Plants can acclimatize to changing light intensities with shifts in light saturation point. This may involve morphological changes such as thicker leaves and more chloroplasts. Density of light-harvesting pigments and enzyme concentrations may also be altered. Figure 5.8 Effects of Light Level on Leaf Structure  Autotrophy Some photosynthetic bacteria can grow in very low light levels. – A recently discovered form of chlorophyll absorbs light in the near- infrared region. – Allows cyanobacteria to grow at depth, underneath other photosynthetic cells that absorb red and blue. – Discovery of this pigment has implications for increasing the efficiency of photovoltaic panels used to generate electricity, which may help lower emissions of CO2.  Autotrophy Low water availability causes plants to close stomates, restricting CO2 uptake. – This is a trade-off: water conservation versus energy gain through photosynthesis and evaporative cooling. – Closing stomates also increases chance of light damage. – If the Calvin cycle is not operating, energy accumulates in light- harvesting arrays and can damage membranes. – Plants have various mechanisms to dissipate this energy as heat, including carotenoid pigments.  Autotrophy Temperature influences photosynthesis: – Affects rates of chemical reactions – Affects structure of membranes and enzymes Plants from different climate zones have enzymes with different optimal temperatures. Plants can also acclimatize by synthesizing different enzyme forms. Figure 5.9 Photosynthetic Responses to Temperature (Part 1) Figure 5.9 Photosynthetic Responses to Temperature (Part 2)  Autotrophy Nutrients affect photosynthesis: – Most nitrogen in plants is associated with rubisco and other photosynthetic enzymes. – Thus, higher N levels in a leaf are correlated with higher photosynthetic rates. – But supply of N is low relative to the demand for growth and metabolism. – Increasing N content of leaves increases risk of being eaten, as herbivores are often N-starved. – Plants must balance the competing demands of photosynthesis, growth, and protection from herbivores.  Focus on these things in the next section Understand the difference between photosynthesis and photorespiration and conditions where photorespiration is detrimental to plant growth. Summarize how adaptations associated with the C4 photosynthetic pathway minimize photorespiration, thereby enhancing photosynthesis rates. Describe how crassulacean acid metabolism reduces water loss relative to the C3 or C4 photosynthetic pathways.  Photosynthetic Pathways Three photosynthetic pathways: C3, C4, and CAM. In C3 plants, photorespiration lowers the efficiency of photosynthesis The enzyme rubisco can catalyze two competing reactions: – Carboxylase reaction: photosynthesis – Oxygenase reaction: O2 is taken up, carbon compounds are broken down, and CO2 is released (photorespiration). – Balance between the two reactions depends on temperature and relative amounts of atmospheric O2 and CO2. Figure 5.10 Influence of Oxygen Concentration on Photosynthesis  Photosynthetic Pathways Photorespiration increases as CO2 concentration decreases relative to O2, and temperature increases. Photorespiration results in net energy loss. Does photorespiration have any benefits? – An Arabidopsis plant with a mutation that knocks out photorespiration dies under normal light and CO2 conditions. – Idea: Photorespiration may protect plants from damage at high light levels. Figure 5.11 Does Photorespiration Protect Plants from Damage by Intense Light? (1)  Photosynthetic Pathways Altered tobacco plants with high rates of photorespiration show less light damage than plants with normal or lowered photorespiration rates (Kozaki and Takeba 1996). But photorespiration is not advantageous if CO2 is low and temperatures high. Such conditions existed 7 million years ago, when C4 photosynthesis appeared.  Photosynthetic Pathways The C4 photosynthetic pathway reduces photorespiration and evolved independently several times; involves biochemical and morphological specialization. Many grass species use this pathway, including corn, sugarcane, and sorghum. Also some eudicots. Figure 5.12 Plants with the C4 Photosynthetic Pathway