1. Plants and other autotrophs are the producers of the biosphere

Transcription

1 1. Plants and other autotrophs are the producers of the biosphere Photosynthesis nourishes almost all of the living world directly or indirectly. All organisms require two basic kinds of organic compounds 1) those used for energy and 2) those used for carbon skeletons to use to make other molecules. Autotrophs can produce their own organic molecules from CO 2 and other inorganic raw materials obtained from the environment. Heterotrophs like ourselves can t do this trick. Except watch this sea slug 4 min.

2 Autotrophs can be separated by the source of energy that drives their metabolism. Photoautotrophs use light as the energy source. Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes. Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia. Chemoautotrophy is unique to prokaryotes. Fig. 9.1

3 Heterotrophs live on organic compounds produced by other organisms. These organisms are the consumers of the biosphere. The most obvious type of heterotrophs feed on plants and other animals. Other heterotrophs, decomposers, feed on dead organisms and on organic litter, like feces and fallen leaves. Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen, a byproduct of photosynthesis.

4 2. Chloroplasts are the sites of photosynthesis in plants Any green part of a plant has chloroplasts. There are about half a million chloroplasts per square millimeter of leaf surface. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts. Watch for 3 structural adaptations that enhance function very similar to what we saw in mitochondria.

5 Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. O 2 exits and CO 2 enters the leaf through microscopic pores, stomata, in the leaf. Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas. Fig. 10.2

6 Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana. Fig. 10.2

7 One of the first clues to the mechanism of photosynthesis came from the discovery that the O 2 given off by plants comes from H 2 O, not CO 2. C.B. van Niel proposed this hypothesis. In the bacteria that he was studying, hydrogen sulfide (H 2 S), not water, is used in photosynthesis. They produce yellow globules of sulfur as a waste. Van Niel proposed this reaction: CO 2 + 2H 2 S -> CH 2 O + H 2 O + 2S

8 He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis. CO 2 + 2H 2 O -> CH 2 O + H 2 O + O 2 Other scientists confirmed van Niel s hypothesis. They used 18 O, a heavy isotope, as a tracer. They could label either CO 2 or H 2 O. They found that plants gave off oxygen molecules containing the 18 O only when watered with the radioactive water. Essentially, plants split water molecules and the hydrogen is incorporated into sugar and the oxygen released to the atmosphere (where it will be inhaled and used in respiration).

9 Photosynthesis is a redox reaction. It reverses the direction of electron flow in respiration. Water is oxidized and its electrons are transferred with H + to CO 2, which is thus reduced to sugar. Light boosts the potential energy of electrons as they move from water to sugar. Fig. 10.3

10 2. Photosynthesis occurs in two steps: The light dependent reactions convert solar energy to chemical energy. The light independent reactions or Calvin cycle incorporates (fixes) CO 2 from the atmosphere into an organic molecule and uses energy from the light dependent reactions to reduce the new carbon piece to sugar.

11 2.A.2.d. Explain the products and reactants of the light-dependent reactions of photosynthesis in eukaryotes and the purpose of the reaction. 2.A.2.d.5. Explain how the products of the light reactions are connected to the production of carbohydrates from carbon dioxide in the Calvin cycle be sure to include where each occurs.

12 3. The light dependent reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look Light, like other forms of electromagnetic energy, travels in rhythmic waves. The distance between crests of electromagnetic waves is called the wavelength. Wavelengths of electromagnetic radiation range from less than a nanometer (gamma rays) to over a kilometer (radio waves).

13 The entire range of electromagnetic radiation is the electromagnetic spectrum. The most important segment for life is a narrow band between 380 to 750 nm, visible light. It is also the most abundant segment available. Fig. 10.5

14 When light meets matter, it may be reflected, transmitted, or absorbed. Different pigments absorb photons of different wavelengths. A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light. Fig. 10.6

15 A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light. It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted at each wavelength. An absorption spectrum plots a pigment s light absorption versus wavelength. Fig. 10.7

16 The light dependent reaction can perform work only with those wavelengths that are absorbed. In the thylakoid are several pigments that differ in their absorption spectrum. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. Other pigments with different structures have different absorption spectra. Fig. 10.8a

17 Here s why some leaves change color.

18 Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis. An action spectrum measures changes in some aspect of photosynthetic activity (for example, O 2 release) as the wavelength is varied. Fig. 10.8b

19 The action spectrum of photosynthesis was first demonstrated in 1883 through an elegant experiment by Thomas Engelmann. In this experiment, different segments of a filamentous alga were exposed to different wavelengths of light. Areas receiving wavelengths favorable to photosynthesis should produce excess O 2. Engelmann used the abundance of aerobic bacteria clustered along the alga as a measure of O 2 production. Neat, eh? Fig. 10.8c

20 2.A.2.d.1. During photosynthesis, describe the purpose of chlorophylls. 2.A.2.d.2. Describe the location and connection between Photosystems I and II.

21 Photons are absorbed by photosystems, clusters of pigment molecules in the thylakoid membranes. The energy of the photon is converted to the potential energy of an electron raised from its ground state to an excited state. In chlorophyll a and b, it is an electron from magnesium in the porphyrin ring that is excited. Look at the structure of chlorophyll.

22 Fig. 10.9

23 In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems. A photosystem acts like a light-gathering antenna complex consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules. Fig

24 When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center. Next to the reaction center is a primary electron acceptor, a molecule which removes an excited electron from the reaction center chlorophyll a. This starts the light dependent reactions. Each photosystem - reaction-center chlorophyll and primary electron acceptor surrounded by an antenna complex - functions in the chloroplast as a light-harvesting unit.

25 There are two types of photosystems. Photosystem I has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm. Photosystem II has a reaction center with a peak at 680nm. Which is more energetic, 680 or 700? The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center. These two photosystems work together to use light energy to generate ATP and NADPH.

26 Untested: Specific steps, names of enzymes and intermediates of the pathways for these processes are beyond the scope of the course and the AP Exam. Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam. Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and the names of the enzymes involved, are beyond the scope of the course and the AP Exam. The names of the specific electron carriers in the ETC are beyond the scope of the course and the AP Exam. No specific cofactors or coenzymes are within the scope of the course and the AP Exam

27 During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic. Noncyclic electron flow, the predominant route, produces both ATP and NADPH. Let s watch 1. When photosystem II absorbs light, an excited electron is captured by the primary electron acceptor, leaving the reaction center oxidized. For more detail, Watch 2. An enzyme extracts electrons from water and supplies them to the oxidized reaction center. This reaction, photolysis, splits water into two hydrogen ions and an oxygen atom, which combines with another to form O 2.

28 Mystery solved!!! The thing that grabs electrons from water causing the release of oxygen was a mystery for a good while. In 2006, Vittal Yachandra at Berkeley figured it out to be a complex of 4 manganese held by 5 oxygens and a calcium, all held in place by proteins in Photosystem II. Artificial reproduction of this could one day help solve the energy crisis.

29 3. Photoexcited electrons pass along an electron transport chain before ending up at an oxidized photosystem I reaction center. 4. As these electrons pass along the transport chain, their energy is harnessed to produce ATP. The mechanism of noncyclic photophosphorylation is similar to the process of oxidative phosphorylation.

30 2.A.2.d.3. Explain how an electrochemical gradient of hydrogen ions (protons) across the thykaloid membrane is established. 2.A.2.d.4. Describe how the formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from ADP and inorganic phosphate.

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33 5. At the bottom of this electron transport chain, the electrons fill an electron hole in an oxidized P700 center. 6. This hole is created when photons excite electrons on the photosystem I complex. The excited electrons are captured by a second primary electron acceptor which transmits them to a second electron transport chain. Ultimately, these electrons are passed from the transport chain to NADP +, creating NADPH. NADPH will carry the reducing power of these highenergy electrons to the Calvin cycle.

34 2.A.2.c. Explain how different energy-capturing processes use different types of final electron acceptors - NADP+ in photosynthesis - Oxygen in cellular respiration

35 Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow. Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and returns to the oxidized P700 chlorophyll. As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation. But they don t get passed to NADP, so no NADPH is made. Watch here This may sound like a bad thing, but wait

36 Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. However, the Calvin cycle consumes more ATP than NADPH. A little cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle. See, plants are pretty smart.

37 Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis. An electron transport chain pumps protons across a membrane as electrons are passed along a series of more electronegative carriers. This builds the proton-motive force in the form of an H + gradient across the thylakoid membrane. ATP synthase molecules (the lollipops) harness the proton-motive force to generate ATP as H + diffuses back across the membrane. Mitochondria transfer chemical energy from food molecules to ATP and chloroplasts transform light energy into the chemical energy of ATP.

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39 The proton gradient, or ph gradient, across the thylakoid membrane is substantial. When illuminated, the ph in the thylakoid space drops to about 5 and the ph in the stroma increases to about 8. How much difference in H+ concentration is that? The light-reaction machinery produces ATP and NADPH on the stroma side of the thylakoid. The structure of the chloroplast contributes to the efficiency of the light dependent reactions in three ways, similar to the mitochondrion and respiration. Can you explain two of them that we have encountered so far????

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41 Noncyclic electron flow pushes electrons from water, where they are at low potential energy, to NADPH, where they have high potential energy. This process also produces ATP. Oxygen is a byproduct. Cyclic electron flow converts light energy to chemical energy in the form of ATP.

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43 4. The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar: a closer look Like the Krebs Cycle, the Calvin cycle regenerates its starting material after molecules enter and leave the cycle. CO 2 enters the cycle and leaves as sugar. The energy of ATP and the reducing power of electrons carried by NADPH are used to make the sugar, and their energy is thus captured in its bonds. The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3- phosphate a.k.a. PGAL, or G3P.

44 Untested: Specific steps, names of enzymes and intermediates of the pathways for these processes are beyond the scope of the course and the AP Exam. Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam. Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and the names of the enzymes involved, are beyond the scope of the course and the AP Exam. The names of the specific electron carriers in the ETC are beyond the scope of the course and the AP Exam. No specific cofactors or coenzymes are within the scope of the course and the AP Exam

45 Each turn of the Calvin cycle fixes one carbon. For the net synthesis of one G3P/PGAL molecule, the cycle must take place three times, fixing three molecules of CO 2. To make one glucose molecule would require six cycles and the fixation of six CO 2 molecules.

46 The Calvin cycle has three phases. In the carbon fixation phase, each CO 2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). This is catalyzed by RuBP carboxylase or rubisco, the most abundant protein in the world. The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate, or PGA, per CO 2.

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48 Using energy from ATP and a pair of electrons from NADPH, the PGA is changed to G3P/PGAL. Had enough of this chemistry? Just wait

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50 If our goal was to produce one PGAL net, we would start with 3 CO 2 (3C) and three RuBP (15C). After fixation and reduction we would have six molecules of PGAL (18C). One of these six PGAL (3C) is a net gain of carbohydrate. This molecule can exit the cycle to be used by the plant cell. The other five (15C) must remain in the cycle to regenerate three RuBP (15C).

51 In the last phase, regeneration of the CO 2 acceptor (RuBP), these five TP/PGAL molecules are rearranged to form 3 RuBP molecules. To do this, the cycle must spend three more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next. (This is why more ATP than NADPH is needed).

52 Fig

53 The PGAL from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates, as well as lipids and parts of proteins and nucleic acids.

54 Back to my infatuation with themes Can you now explain a third way in which the structure of the chloroplast enhances its function? Hint like the other two examples you came up with a few slides back, this is also very similar to a way in which the mitochondrion s structure aided in it s function, but this has to do with the Calvin Cycle. The comparison with respiration should help you remember them, yes?

55 And how about this scientific method connection???? Limiting factors are things that can affect a process for better or worse (which is where the term limiting comes from). Just for fun, come up with three factors that could limit the rate of photosynthesis and explain why and how they have their effect.

56 Untested: Specific steps, names of enzymes and intermediates of the pathways for these processes are beyond the scope of the course and the AP Exam. Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam. Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and the names of the enzymes involved, are beyond the scope of the course and the AP Exam. The names of the specific electron carriers in the ETC are beyond the scope of the course and the AP Exam. No specific cofactors or coenzymes are within the scope of the course and the AP Exam

57 How about a little comic relief, you say? I happen to have just the thing - a video on C 4 plants!!! NOT on this test, but a good example of adaptation.

58 5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates One of the major problems facing terrestrial plants is dehydration. At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis. The stomata are not only the major route for gas exchange (CO 2 in and O 2 out), but also for the evaporative loss of water. On hot, dry days, plants close stomata to save water, but this means a CO 2 shortage for photosynthesis.

59 In most plants (C 3 plants) initial fixation of CO 2 occurs via rubisco and results in a three-carbon compound, GP/PGA). These plants include rice, wheat, and soybeans. When their stomata are closed on a hot, dry day, CO 2 levels inside the chloroplast drop as CO 2 is consumed in the Calvin cycle. At the same time, O 2 levels rise as the light reaction converts light to chemical energy. While rubisco normally accepts CO 2, when the O 2 /CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP.

60 When rubisco adds O 2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration. The two-carbon fragment is exported from the chloroplast and degraded to CO 2 by mitochondria and peroxisomes. Unlike normal respiration, this process produces no ATP, nor additional organic molecules. Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle, so no TP/PGAL (therefore no glucose) is made.

61 Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day. Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration. The C 4 plants fix CO 2 first in a four-carbon compound, not three carbon GP/PGA like C 3 plants. Several thousand plants, including sugercane and corn and crabgrass use this pathway.

62 C 4 plant s leaves have a different internal structure. mesophyll cells incorporate CO 2 into organic molecules. The mesophyll cells have the key enzyme PEP carboxylase, not rubisco. PEP carboxylase adds CO 2 to phosphoenolpyruvate (PEP) to form 4-carbon oxaloacetetate (where have you seen that before?) PEP carboxylase has a very high affinity for CO 2 and can fix CO 2 efficiently when rubisco cannot - on hot, dry days with the stomata closed. So there is not the competition between O 2 and CO 2 that leads to photorespiration with rubisco involved.

63 The mesophyll cells pump these four-carbon compounds into bundle-sheath cells. The bundle sheath cells strip a CO 2 from the fourcarbon compound and return the three-carbon remainder (your friend, pyruvate) to the mesophyll cells. The bundle sheath cells then use rubisco with this abundant supply of CO 2 to start the Calvin cycle. The Calvin cycle works just as it does in C 3 plants, the only difference is how CO 2 is delivered to it.

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65 In effect, the mesophyll cells pump CO 2 into the bundle sheath cells, keeping CO 2 levels high enough for rubisco to accept CO 2 and not O 2. Because the mesophyll cells don t have rubisco to grab O 2 instead of CO 2, and because they surround the bundle sheath cells, therefore blocking O 2 from getting to them, C 4 photosynthesis minimizes photorespiration and enhances sugar production. C 4 plants thrive in hot, dry regions with intense sunlight. This is why crabgrass grows better than St. Augustine grass (a C 3 plant) when it is hot and dry.

66 A second strategy to minimize photorespiration is found in succulent plants like cacti and pineapples. These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day, the opposite pattern of other plants. Temperatures are typically lower at night and humidity is higher, so water loss is minimized. During the night, these plants fix CO 2 into a variety of organic acids in mesophyll cells, kind of like C 3 plants. During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO 2 is released from the organic acids.

67 Both C 4 and CAM plants add CO 2 to organic intermediates before it enters the Calvin cycle, thus avoiding the photorespiration that happens when O 2 competes with CO 2 for the rubisco active site. In C 4 plants, carbon fixation and the Calvin cycle are spatially separated. In CAM plants, carbon fixation and the Calvin cycle are temporally separated. Both eventually use the Calvin cycle to incorporate light energy into the production of sugar.

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69 6. Photosynthesis is the biosphere s metabolic foundation: a review In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds. Fig

70 Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells. About 50% of the organic material is consumed as fuel for cellular respiration in plant mitochondria. Carbohydrate in the form of the disaccharide sucrose travels via the veins to nonphotosynthetic cells. There, it provides fuel for respiration and the raw materials for anabolic pathways including synthesis of proteins and lipids and building the extracellular polysaccharide cellulose.

71 Plants also store excess sugar by synthesizing starch. Some is stored as starch in chloroplasts or in storage cells in roots, tubers, seeds, and fruits. Heterotrophs, including humans, may completely or partially consume plants for fuel and raw materials. On a global scale, photosynthesis is the most important process to the welfare of life on Earth. Photosynthesis is your friend (until you have a test on it).

72 NOT on this test But we will study it later and it is definitely connected to what we are doing, so let s take a look. No extra charge

73 Introduction An ecosystem consists of all the organisms living in a community as well as all the abiotic factors with which they interact. The dynamics of an ecosystem involve two processes: energy flow and chemical cycling. Ecosystem ecologists view ecosystems as energy machines and matter processors. We can follow the transformation of energy by grouping the species in a community into trophic levels of feeding relationships.

74 1. Trophic relationships determine the routes of energy flow and chemical cycling in an ecosystem The autotrophs are the primary producers, and are usually photosynthetic (Photoautotrophs, which can be either?, or?, or?), but could be????? They use light energy to synthesize sugars and other organic compounds. Chemoautotrophs are the producers in some ecosystems, like deep sea vents (they are strictly prokaryotic).

75 Heterotrophs are at trophic levels above the primary producers and depend on their photosynthetic output. Decomposers, or detritivores, feed on dead organisms of all types, helping recycle nutrients. Fig. 54.1

76 An ecosystem s main decomposers are fungi and prokaryotes, which secrete enzymes that digest organic material and then absorb the breakdown products, defining them as saprotrophs. Fig. 54.2

77 3. The laws of physics and chemistry apply to ecosystems The law of conservation of energy applies to ecosystems. We can potentially trace all the energy from its solar input to its release as heat by organisms. The second law of thermodynamics allows us to measure the efficiency of the energy conversions.

78 Introduction The amount of light energy converted to chemical energy by an ecosystem s autotrophs in a given time period is called primary production, and is measured in the DRY mass of autotroph tissue made in a certain amount of time.

79 The Global Energy Budget Every day, Earth is bombarded by large amounts of solar radiation. Much of this radiation lands on the water and land that either reflect or absorb it. Of the visible light that reaches photosynthetic organisms, about only 1% is converted to chemical energy. Although this is a small amount, primary producers are capable of producing about 170 billion tons of organic material per year.

80 Gross and Net Primary Production. Total primary production is known as gross primary production (GPP). This is the amount of light energy that is converted into chemical energy. The net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R): NPP = GPP R

81 Q8 The net annual primary productivity of a particular wetland ecosystem is found to be 8,000 kcal/m2. If respiration by the aquatic producers is 12,000 kcal/m2per year, what is the gross annual primary productivity for this ecosystem, in kcal/m2 per year? Round to the nearest whole number.

82 Q8 NPP=GPP-R 8,000 = GPP 12,000 8, ,000= GPP 20,000=GPP

83 Primary production can be expressed in terms of energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time. This should not be confused with the total biomass of photosynthetic autotrophs present at a given time, called the standing crop.

84 Different ecosystems differ greatly in their production as well as in their contribution to the total production of the Earth. Fig. 54.3

85 Production in Freshwater Ecosystems. Solar radiation and temperature are closely linked to primary production in freshwater lakes. During the 1970s, sewage and fertilizer pollution added nutrients to lakes, which shifted many lakes from having phytoplankton communities to those dominated by diatoms and green algae.

86 This process is called eutrophication, and has undesirable impacts from a human perspective. Hey, how about explaining that to us.

87 3. In terrestrial ecosystems, temperature, moisture, and nutrients limit primary production Obviously, water availability varies among terrestrial ecosystems more than aquatic ones. On a large geographic scale, temperature and moisture are the key factors controlling primary production in ecosystems.

88 Introduction The amount of chemical energy in consumers food that is converted to their own new biomass during a given time period is called secondary production. In other words, how much of that double cheeseburger and fries actually becomes part of you is secondary production.

89 1. The efficiency of energy transfer between trophic levels is usually close to 10% Production Efficiency. One way to understand secondary production is to examine the process in individual organisms. Fig

90 Trophic Efficiency and Ecological Pyramids. Trophic efficiency is the percentage of production transferred from one trophic level to the next. Pyramids of production represent the multiplicative loss of energy from a food chain.

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93 Pyramids of biomass represent the ecological consequence of low trophic efficiencies. Most biomass pyramids narrow sharply from primary producers to top-level carnivores because energy transfers are inefficient (10%) Fig a

94 In some aquatic ecosystems, the pyramid is inverted. In this example, phytoplankton grow, reproduce, and are consumed rapidly. They have a short turnover time, which is a comparison of standing crop mass compared to production. Fig b

95 Pyramids of numbers show how the levels in the pyramids of biomass are proportional to the number of individuals present in each trophic level. Fig

96 The dynamics of energy through ecosystems have important implications for the human population. Fig

97 So save money and get your heart in shape Eating more plants makes a lot of sense. Animal rights activists would certainly agree. But Seems like there are always fringe groups no matter where you look. 4:30

98 About time for another test, eh? Here s one from the pep band