c4.2 energy transfer
C4.2 Transfers of Energy and Matter
Guiding Questions:
Reason for matter recycling but not energy: Matter (e.g., carbon, nitrogen, phosphorus) consists of physical elements that can be rearranged and reused within an ecosystem through biogeochemical cycles. Energy, however, follows the laws of thermodynamics. As energy is transferred through trophic levels, a significant portion is converted into heat (unusable energy) and dissipated into the environment, adhering to the second law of thermodynamics (entropy increases). Therefore, energy flows unidirectionally and requires continuous input, primarily from the sun.
Replacement of lost energy: The energy lost by organisms within an ecosystem is continuously replaced by new energy entering the system, predominantly from sunlight. Photoautotrophs capture light energy and convert it into chemical energy stored in organic compounds, which then flows through the food chain.
Linking Questions:
Energy transformation and biological processes: The transformation of energy from one form to another is essential for all biological processes. For example, light energy is transformed into chemical energy during photosynthesis, allowing the synthesis of organic molecules. This chemical energy is then transformed into a usable form, ATP, through cellular respiration, which powers metabolic activities such as growth, movement, and reproduction. Without these transformations, life cannot be sustained.
Direct and indirect consequences of rising carbon dioxide levels:
Direct consequences: Increased rates of photosynthesis in some plant species (carbon fertilization effect), and ocean acidification due to increased dissolution of CO2 in seawater, forming carbonic acid(H2CO_3).
Indirect consequences: Global warming, changes in global climate patterns (e.g., more extreme weather events), sea-level rise due to thermal expansion and melting ice, shifts in species distribution, changes in agricultural yields, and increased frequency of wildfires.
C4.2.1 Ecosystems as Open Systems
Define ecosystem: An ecosystem is a community of living organisms (biotic components) interacting with their non-living physical environment (abiotic components).
Compare open and closed systems:
Open system: Exchanges both matter and energy with its surroundings.
Closed system: Exchanges energy but not matter with its surroundings.
State that ecosystems are open systems: Ecosystems are open systems because they continuously exchange both matter (e.g., water, nutrients, gases) and energy (e.g., sunlight, heat) with their external environment.
C4.1.2 Sunlight as the Principal Source of Energy
State that sunlight is the primary energy source for most ecosystems: The sun is the ultimate source of energy for almost all ecosystems on Earth, providing light energy utilized by photoautotrophs.
Outline an example of an exception of sunlight as the principal energy source in most ecosystems: Deep-sea hydrothermal vent ecosystems are an exception. They derive energy primarily from chemosynthesis, where certain microorganisms (chemoautotrophs) convert chemical energy from the oxidation of inorganic compounds (e.g., hydrogen sulfide, methane) into organic matter, rather than using sunlight.
C4.2.3 Flow of Chemical Energy Through Food Chains
Define food chain: A food chain is a linear sequence showing how energy is transferred from one organism to another through feeding.
Identify producers and consumers in a food chain:
Producers (autotrophs): Organisms that produce their own food using an external energy source (e.g., plants, algae).
Consumers (heterotrophs): Organisms that obtain energy by feeding on other organisms.
Primary consumers (herbivores): Feed on producers.
Secondary consumers (carnivores/omnivores): Feed on primary consumers.
Tertiary consumers (carnivores/omnivores): Feed on secondary consumers.
Identify the apex predator in a food chain: The apex predator is the organism at the top of the food chain, with no natural predators.
State what is indicated by the arrow in a food chain: The arrow in a food chain indicates the direction of energy flow (e.g., grass
ightarrow rabbit means energy flows from grass to rabbit).
C4.2.4 Construction of Food Chains and Food Webs
Draw a food chain, labeling the producer, primary consumer, secondary consumer and tertiary consumer:
Sun
ightarrow Grass (Producer)
ightarrow Grasshopper (Primary Consumer)
ightarrow Frog (Secondary Consumer)
ightarrow Snake (Tertiary Consumer)
ightarrow Hawk (Apex Predator)
State the reason why food webs are better representations of trophic relationships in an ecological community than a food chain: Food webs are more comprehensive and realistic representations because most organisms have varied diets and feed on multiple species. A food web consists of multiple interconnected food chains, showing the complex feeding relationships and energy flows within an entire community, whereas a food chain simplifies these relationships to a single pathway.
C4.2.5 Supply of Energy to Decomposers
Outline the role of decomposers in a food web: Decomposers (detritivores and saprotrophs) break down dead organic matter (detritus) and waste products from all trophic levels. This process recycles essential nutrients back into the ecosystem, making them available for producers, thereby completing the nutrient cycle. They obtain energy from the carbon compounds in this dead organic matter.
List examples of decomposers, including both detritivores and saprotrophs:
Detritivores: Earthworms, millipedes, woodlice, maggots.
Saprotrophs: Bacteria, fungi (mushrooms, molds).
C4.2.6 Autotrophs
Define autotroph: Autotrophs are organisms that can produce their own food from simple inorganic substances (like carbon dioxide and water) using external energy sources (like light or chemical reactions).
Define carbon fixation: Carbon fixation is the process by which inorganic carbon (carbon dioxide) is converted into organic carbon compounds (e.g., glucose) by living organisms.
State the reason why autotrophs must “fix” carbon: Autotrophs must fix carbon because they need to convert inorganic carbon dioxide into usable organic carbon compounds (sugars, proteins, lipids) to build their own biomass and store chemical energy for their metabolic processes.
List the two primary uses of energy in autotrophs:
Synthesis of complex organic molecules (e.g., sugars, proteins, lipids, nucleic acids) from simpler inorganic precursors.
Powering metabolic processes and growth, including maintaining cell structure and active transport.
C4.2.7 Photoautotrophs and Chemoautotrophs
Compare the energy sources used by photoautotrophs and chemoautotrophs:
Photoautotrophs: Use light energy from the sun to synthesize organic compounds (photosynthesis).
Chemoautotrophs: Use chemical energy released from the oxidation of inorganic substances (e.g., hydrogen sulfide, ammonia, iron) to synthesize organic compounds (chemosynthesis).
List examples of photoautotrophs: Plants, algae (e.g., kelp, phytoplankton), cyanobacteria.
List examples of chemoautotrophs: Nitrifying bacteria, sulfur bacteria, iron-oxidizing bacteria, methanogens (some archaea).
Outline how oxidation reactions serve as a source of energy in iron-oxidizing bacteria: Iron-oxidizing bacteria obtain energy by oxidizing ferrous iron (Fe^{2+}) to ferric iron (Fe^{3+}) in their environment. This chemical reaction releases energy, which the bacteria then use to drive the synthesis of organic molecules from carbon dioxide, similar to how photoautotrophs use light energy.
C4.2.8 Heterotrophs
Define heterotroph: Heterotrophs are organisms that cannot produce their own food and must obtain energy and carbon by consuming organic compounds from other organisms (autotrophs or other heterotrophs).
Outline the functions of digestion, assimilation and synthesis of carbon compounds in heterotrophs:
Digestion: The process of breaking down complex organic molecules obtained from food into simpler, smaller molecules (e.g., proteins into amino acids, carbohydrates into monosaccharides) that can be absorbed by the body.
Assimilation: The absorption and incorporation of digested simple molecules into the cells and tissues of the organism. These molecules are then used for energy production or as building blocks.
Synthesis of carbon compounds: Using assimilated simple organic molecules, heterotrophs synthesize their own required complex carbon compounds (e.g., structural proteins, enzymes, nucleic acids, fats) for growth, repair, and other metabolic functions.
C4.2.9 Release of Energy by Oxidation in Cell Respiration
State that both autotrophs and heterotrophs perform cellular respiration to produce ATP: Both autotrophic and heterotrophic organisms carry out cellular respiration to break down organic compounds and release chemical energy, which is then used to synthesize ATP (adenosine triphosphate), the primary energy currency of the cell.
Describe cellular respiration as an oxidation reaction: Cellular respiration is fundamentally an oxidation reaction where organic compounds (like glucose) are oxidized (lose electrons/hydrogen atoms), and oxygen (in aerobic respiration) is reduced (gains electrons/hydrogen atoms). This transfer of electrons releases energy, which is harnessed to produce ATP. For example, in glucose oxidation: C6H{12}O6 + 6O2
ightarrow 6CO2 + 6H2O + ext{Energy (ATP + heat)}. Glucose loses hydrogen (is oxidized), and oxygen gains hydrogen (is reduced).
C4.2.10 Classification of Organisms into Trophic Levels
Define trophic level: A trophic level is the position an organism occupies in a food chain, indicating its feeding relationship and how it obtains energy.
Identify the trophic level of an organism in a food chain:
Producer: First trophic level (e.g., plants).
Primary Consumer: Second trophic level (herbivores, e.g., deer).
Secondary Consumer: Third trophic level (carnivores/omnivores feeding on herbivores, e.g., wolf feeding on deer).
Tertiary Consumer: Fourth trophic level (carnivores/omnivores feeding on secondary consumers, e.g., bear feeding on wolf).
State that many organisms have a varied diet and occupy different trophic levels in different food chains: An organism like an omnivore can eat both plants and animals. For example, a bear eating berries is a primary consumer, but a bear eating a fish is a secondary or tertiary consumer, depending on what the fish ate.
C4.2.11 Construction of Energy Pyramids
State the unit used for communicating the energy in each trophic level of a food chain: The unit used is typically kilojoules per square meter per year (kJm^{-2}yr^{-1}) or Joules per square meter per year (Jm^{-2}yr^{-1}), representing the rate of energy flow over time within a given area.
Describe the shape of a pyramid of energy: An energy pyramid always has a broad base representing the high amount of energy at the producer (first) trophic level. Each successive trophic level above it is narrower, showing a progressive decrease in the amount of energy available, resulting in a pyramidal shape.
Draw a pyramid of energy given data for an ecosystem: To draw a pyramid of energy, you would create stacked horizontal bars. The lowest and widest bar represents the energy at the producer level. The next, narrower bar represents the primary consumers, followed by even narrower bars for secondary and tertiary consumers. The length or area of each bar is proportional to the energy content (kJm^{-2}yr^{-1}) at that trophic level. For example:
Producers: 100,000 kJm^{-2}yr^{-1}
Primary Consumers: 10,000 kJm^{-2}yr^{-1}
Secondary Consumers: 1,000 kJm^{-2}yr^{-1}
Tertiary Consumers: 100 kJm^{-2}yr^{-1}
C4.2.12 Reductions in Energy Availability
Outline three reasons why the amount of energy decreases at higher trophic levels:
Not all biomass is consumed: Not every part of an organism is eaten by the next trophic level (e.g., bones, fur, roots).
Energy loss through metabolic processes: Organisms at each trophic level use a significant portion of the acquired energy for their own metabolic activities, such as respiration, movement, growth, and reproduction. This energy is released as heat and lost to the environment.
Incomplete assimilation: Not all consumed food is digested and absorbed; some energy remains in undigested waste products (e.g., feces) which are egested rather than assimilated.
State the average amount of energy passed through each trophic level in a food chain: On average, only about 10% of the energy from one trophic level is transferred to the next higher trophic level. The remaining 90% is lost, primarily as heat or in unconsumed/undigested biomass.
C4.2.13 Heat Loss to the Environment
Describe the reasons why heat created by living organisms is eventually lost from the ecosystem: All living organisms release heat as a byproduct of metabolic reactions, particularly cellular respiration, due to the inefficiencies of energy transfer (Second Law of Thermodynamics). This heat energy continuously dissipates into the surrounding environment and eventually radiates into space. Ecosystems are open systems, allowing for this constant loss of heat.
C4.2.14 Restrictions on the Number of Trophic Levels
State that at each successive trophic level there are few organisms and less biomass: As energy is lost at each step, higher trophic levels support fewer individuals and represent a smaller total biomass compared to the trophic levels below them.
Explain why there is a limited number of trophic levels in an ecosystem: Due to the significant energy loss (approximately 90%) at each trophic transfer, there is insufficient energy available to support a large population or biomass beyond typically three to five trophic levels. If there were too many trophic levels, the amount of energy remaining for the highest level would be too small to sustain a viable population.
C4.2.15 Primary Production
Define biomass: Biomass refers to the total mass of living organisms (both plants and animals) in a given area or ecosystem, typically expressed as dry weight per unit area or volume.
Define gross and net primary production:
Gross Primary Production (GPP): The total amount of organic matter (energy) produced by autotrophs through photosynthesis (or chemosynthesis) in a specific area and time period.
Net Primary Production (NPP): The amount of organic matter (energy) remaining after autotrophs have used some of the GPP for their own respiration (Rp). It represents the energy available to heterotrophs. NPP = GPP - Rp
State the unit of primary production: The unit of primary production is typically expressed as grams of carbon per square meter per year (gCm^{-2}yr^{-1}) or as energy per unit area per unit time (e.g., kJm^{-2}yr^{-1}).
Outline why different biomes will vary in their capacity to accumulate biomass: Different biomes vary significantly in their capacity to accumulate biomass due to differences in key limiting factors affecting primary productivity:
Temperature: Higher temperatures generally increase metabolic rates and growth, up to an optimum.
Water availability: Ample water is crucial for photosynthesis and transport.
Light intensity: Higher light intensity supports higher photosynthetic rates.
Nutrient availability: The presence of essential mineral nutrients (e.g., nitrates, phosphates) in the soil or water is vital for plant growth.
Growing season length: Longer growing seasons allow for more extended periods of biomass accumulation.
C4.2.16 Secondary Production
Define secondary production: Secondary production is the rate at which heterotrophs convert the energy from their food into their own new biomass (growth and reproduction) over a given period.
Explain why secondary production is lower than primary production in an ecosystem: Secondary production is significantly lower than primary production because only a fraction of primary production is consumed by primary consumers, and then a large portion of the ingested energy is lost at each trophic level. Energy is lost through unconsumed biomass, metabolic processes (respiration/heat), and waste products (egestion). Therefore, the energy available for heterotrophs to convert into their own biomass is always less than the total energy produced by autotrophs.
C4.2.17 Constructing Carbon Cycle Diagrams
Define sink, pool and flux as related to the carbon cycle:
Pool (or reservoir): A component of the carbon cycle where carbon is stored (e.g., atmosphere, oceans, biomass, fossil fuels).
Sink: A pool that absorbs more carbon than it releases (e.g., growing forests, oceans absorbing atmospheric CO_2).
Flux: The transfer or movement of carbon between different pools in the carbon cycle (e.g., photosynthesis, respiration, combustion, diffusion).
Draw a diagram of the carbon cycle through a terrestrial ecosystem; include processes of diffusion, photosynthesis, feeding and respiration:
(Atmospheric CO_2 pool):
Flux 1 (Photosynthesis): Autotrophs (plants) take CO_2 from the atmosphere.
(Biomass pool - Plants/Autotrophs): Carbon is incorporated into organic compounds.
Flux 2 (Feeding): Heterotrophs (animals) consume plants, transferring carbon.
(Biomass pool - Animals/Heterotrophs): Carbon is incorporated into animal organic compounds.
Flux 3 (Respiration): Both autotrophs and heterotrophs release CO_2 back into the atmosphere through cellular respiration.
Flux 4 (Death/Waste): Carbon from dead organisms and waste products enters the soil.
(Decomposers/Soil organic matter pool): Decomposers break down organic matter.
Flux 5 (Decomposition/Respiration): Decomposers release CO_2 back to the atmosphere through respiration.
Flux 6 (Diffusion/Exchange): CO_2 exchanges between the atmosphere and bodies of water (like lakes/rivers within a terrestrial context).
C4.2.18 Ecosystems as Carbon Sinks and Carbon Sources
State the conditions under which an ecosystem is a carbon sink: An ecosystem acts as a carbon sink when the rate of carbon uptake (e.g., through photosynthesis) exceeds the rate of carbon release (e.g., through respiration, decomposition, combustion). This leads to a net accumulation of carbon within the ecosystem's biomass or soil.
State the conditions under which an ecosystem is a carbon source: An ecosystem acts as a carbon source when the rate of carbon release exceeds the rate of carbon uptake. This often occurs due to disturbances like deforestation, fires, or decomposition in wetlands where organic matter releases CO_2 or methane.
Define sequestration in relation to a carbon sink: Carbon sequestration is the long-term storage of carbon in carbon sinks, which can be natural (e.g., forests, oceans, soil) or artificial. It is a process aimed at reducing atmospheric CO_2 levels by capturing and storing carbon to mitigate climate change.
C4.2.19 Release of Carbon Dioxide during Combustion
Define combustion: Combustion is a rapid chemical process (oxidation) that involves the reaction of a fuel with an oxidant (usually oxygen), producing heat and light. In the context of the carbon cycle, it typically refers to the burning of organic matter or fossil fuels.
State the reactants and products of a combustion reaction:
Reactants: Fuel (organic carbon compound) + Oxygen (O_2)
Products: Carbon dioxide (CO2) + Water (H2O) + Energy (heat and light)
State sources of fuel for a combustion reaction: Biomass (wood, agricultural waste), peat, coal, oil, natural gas.
Outline formation of peat, coal, oil, natural gas and biomass:
Biomass: Organic matter derived from living or recently living organisms (e.g., plants, animal waste). Readily available through current biological processes.
Peat: Formed from partially decomposed plant matter accumulating in waterlogged, acidic conditions (e.g., bogs) where oxygen is limited, preventing full decay. This is an early stage of coal formation.
Coal: Formed over millions of years from extensive deposits of plant material (often peat) buried under sedimentary rock. Under intense heat and pressure, peat is transformed into various grades of coal.
Oil (Petroleum): Formed from the remains of marine organisms (plankton, algae) that settled on the seafloor, mixed with sediment, and were buried. Over millions of years, under heat and pressure, the organic matter was converted into liquid hydrocarbons.
Natural Gas: Often found alongside oil deposits, formed from the same marine organic matter under similar conditions of heat and pressure but at higher temperatures, converting hydrocarbons into gaseous forms (primarily methane).
C4.2.20 Analysis of the Keeling Curve
Sketch a graph of the annual fluctuation in atmospheric carbon dioxide concentration: The Keeling Curve shows an overall increasing trend, superimposed with a regular annual zigzag pattern. The CO_2 concentration dips in the Northern Hemisphere's summer and peaks in late winter/early spring.
Explain the annual fluctuation in atmospheric carbon dioxide concentration in terms of photosynthesis and respiration:
Decrease during Northern Hemisphere summer: During the Northern Hemisphere's summer growing season (spring to autumn), terrestrial plants undergo widespread photosynthesis, absorbing large amounts of atmospheric CO2, causing CO2 levels to decrease globally.
Increase during Northern Hemisphere winter: In winter, photosynthesis decreases significantly (deciduous trees lose leaves, less sunlight), while respiration by plants, animals, and decomposers continues to release CO2 into the atmosphere, leading to a net increase in CO2 levels until spring.
State the long-term trend depicted in the Keeling curve: The Keeling Curve shows a consistent and continuous long-term increase in atmospheric carbon dioxide concentration since measurements began in 1958.
Explain the reason for the long term trend depicted in the Keeling curve: The long-term upward trend in atmospheric CO2 is primarily attributed to anthropogenic activities, especially the combustion of fossil fuels (coal, oil, natural gas) and deforestation. These activities release large quantities of stored carbon into the atmosphere as CO2 at a rate exceeding the capacity of natural sinks (oceans, forests) to absorb it.
C4.2.21 Dependence of Aerobic Respiration on Atmospheric Oxygen
State the source of atmospheric oxygen: The primary source of atmospheric oxygen (O_2) is photosynthesis carried out by photoautotrophs (plants, algae, cyanobacteria).
Explain the interdependence of aerobic respiration and photosynthesis: Aerobic respiration and photosynthesis are highly interdependent processes that form a vital cycle:
Photosynthesis: Utilizes atmospheric CO2 (a product of respiration) and sunlight to produce organic compounds (food) and releases O2 (a reactant for respiration) into the atmosphere.
Aerobic Respiration: Utilizes O2 (a product of photosynthesis) to break down organic compounds (food produced by photosynthesis) and releases CO2 (a reactant for photosynthesis) and water back into the environment, along with ATP energy.
They are essentially reverse processes that facilitate the cycling of carbon and oxygen and the flow of energy necessary for life.
Outline how the annual flux of CO2 is estimated, including the unit: The annual flux of CO2 into and out of segments of the carbon cycle (e.g., terrestrial ecosystems, oceans) is estimated by measuring the change in carbon stored in these pools over time, or by directly measuring CO2 exchange rates (CO2 uptake/release) using instruments like eddy covariance towers. These measurements are integrated over large areas and time scales using models. The unit for annual flux is typically gigatonnes of carbon per year (GtC/yr) or petagrams of carbon per year (PgC/yr).
C4.2.22 Recycling of All Chemical Elements
State that chemical elements can be recycled but energy can not: Chemical elements, such as carbon, nitrogen, phosphorus, and hydrogen, exist in finite quantities on Earth and are continuously recycled through biogeochemical cycles. Energy, however, flows through ecosystems in one direction and is ultimately lost as heat, requiring constant replenishment from an external source (the sun).
List elements required by living organisms that must be cycled through ecosystems: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Sulfur (S). Also, various trace elements like Potassium (K), Calcium (Ca), Magnesium (Mg), Iron (Fe).
Outline the generalized flow of nutrients between the abiotic, autotrophic and heterotrophic components of an ecosystem:
Abiotic Pool: Essential nutrients exist in inorganic forms in the non-living environment (e.g., CO_2 in atmosphere, nitrates in soil, phosphates in rocks).
Autotrophic Uptake: Autotrophs (producers) absorb inorganic nutrients from the abiotic environment (e.g., plants take up nitrates and phosphates from soil, CO_2 from atmosphere) and convert them into organic compounds.
Heterotrophic Transfer: Heterotrophs (consumers) obtain these organic nutrients by feeding on autotrophs or other heterotrophs. Nutrients are incorporated into their biomass.
Return to Abiotic Pool (Decomposition): When organisms (autotrophs or heterotrophs) die, or produce waste products, decomposers (detritivores and saprotrophs) break down the organic matter. This decomposition process converts organic nutrients back into their inorganic (abiotic) forms, which are then released into the soil, water, or atmosphere, making them available again for autotrophic uptake.
State the role of decomposers in nutrient cycles: Decomposers are crucial for nutrient cycling. They break down dead organic matter and waste products, releasing inorganic nutrients (e.g., nitrates, phosphates, CO_2) back into the abiotic environment (soil, water, atmosphere). This completes the cycle