Pathways that Harvest and Store Chemical Energy

Pathways that Harvest and Store Chemical Energy
6 Pathways that Harvest and Store Chemical Energy

Chapter 6 Pathways that Harvest and Store Chemical Energy
Key Concepts 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

Chapter 6 Pathways that Harvest and Store Chemical Energy
6.4 Catabolic and Anabolic Pathways Are Integrated 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates

Chapter 6 Opening Question
Why does fresh air inhibit the formation of alcohol by yeast cells?

Chemical energy available to do work is termed free energy (G).
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is stored in chemical bonds and can be released and transformed by metabolic pathways. Chemical energy available to do work is termed free energy (G).

Five principles govern metabolic pathways:
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Five principles govern metabolic pathways: Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. Each reaction is catalyzed by a specific enzyme. Most metabolic pathways are similar in all organisms.

Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism
In eukaryotes, many metabolic pathways occur inside specific organelles. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

In cells, energy-transforming reactions are often coupled:
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism In cells, energy-transforming reactions are often coupled: An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction. Two coupling molecules are the coenzymes ATP and NADH.

Adenosine triphosphate (ATP) is a kind of “energy currency” in cells.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Adenosine triphosphate (ATP) is a kind of “energy currency” in cells. Energy released by exergonic reactions is stored in the bonds of ATP. When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

Figure 6.1 The Concept of Coupling Reactions
Figure 6.1 The Concept of Coupling Reactions Some exergonic cellular reactions are coupled with the formation of ATP from ADP and Pi (an endergonic reaction). The cell can later couple the (exergonic) hydrolysis of ATP with endergonic cellular processes.

Figure 6.2 ATP Figure 6.2 ATP ATP is built by the addition of terminal phosphate groups onto the nucleoside adenosine.

ATP + H2O ADP + Pi + free energy
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Hydrolysis of ATP is exergonic: ATP + H2O ADP + Pi + free energy ΔG is about –7.3 kcal/mol

Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism
The free energy of the bond between phosphate groups is much higher than the energy of the O—H bond that forms after hydrolysis.

Reduction is the gain of one or more electrons.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy can also be transferred by the transfer of electrons in reduction–oxidation, or redox reactions. Reduction is the gain of one or more electrons. Oxidation is the loss of one or more electrons.

Oxidation and reduction always occur together.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Oxidation and reduction always occur together.

When a molecule loses a hydrogen atom, it becomes oxidized.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism It is also useful to think of oxidation and reduction in terms of gain or loss of hydrogen atoms: Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–). When a molecule loses a hydrogen atom, it becomes oxidized.

Energy is transferred in a redox reaction.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism The more reduced a molecule is, the more energy is stored in its bonds. Energy is transferred in a redox reaction. Energy in the reducing agent is transferred to the reduced product.

Figure 6.3 Oxidation, Reduction, and Energy
Figure 6.3 Oxidation, Reduction, and Energy The more oxidized a carbon atom is, the less free energy it has.

Coenzyme NAD is a key electron carrier in redox reactions.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Coenzyme NAD is a key electron carrier in redox reactions. NAD+ (oxidized form) NADH (reduced form)

Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 1)
Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (A) NAD+ is an important electron acceptor in redox reactions, and its reduced form, NADH, is an important energy intermediary in cells. The unshaded portion of the molecule (left) remains unchanged by the redox reaction. (B) Coupling of redox reactions using NAD+/NADH.

NAD+ + H+ + 2 e– NADH NADH + H+ + ½ O2 NAD+ + H2O
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Reduction of NAD+ is highly endergonic: NAD+ + H+ + 2 e– NADH Oxidation of NADH is highly exergonic: NADH + H+ + ½ O NAD+ + H2O

Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 2)
Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (A) NAD+ is an important electron acceptor in redox reactions, and its reduced form, NADH, is an important energy intermediary in cells. The unshaded portion of the molecule (left) remains unchanged by the redox reaction. (B) Coupling of redox reactions using NAD+/NADH.

Energy for anabolic processes is supplied by ATP.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH. Energy for anabolic processes is supplied by ATP. Most energy-releasing reactions produce NADH, but most energy-consuming reactions require ATP. Oxidative phosphorylation transfers energy from NADH to ATP.

Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy
Cellular respiration: the set of metabolic reactions used by cells to harvest energy from food A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2. The oxidation occurs in a series of small steps, allowing the cell to harvest about 34% of the energy released.

Figure 6.5 Energy Metabolism Occurs in Small Steps
Figure 6.5 Energy Metabolism Occurs in Small Steps (A) In living systems, glucose is oxidized via a series of steps, releasing small amounts of energy that can be efficiently trapped by coenzymes. (B) Glucose that is burned releases its energy as heat in one big step.

Glycolysis—glucose is converted to pyruvate.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Catabolism of glucose under aerobic conditions (in the presence of O2), occurs in three linked biochemical pathways: Glycolysis—glucose is converted to pyruvate. Pyruvate oxidation—pyruvate is oxidized to acetyl CoA and CO2. Citric acid cycle—acetyl CoA is oxidized to CO2. 26

Figure 6.6 Energy-Releasing Metabolic Pathways
Figure 6.6 Energy-Releasing Metabolic Pathways The catabolism of glucose under aerobic conditions occurs in three sequential metabolic pathways: glycolysis, pyruvate oxidation, and the citric acid cycle. The reduced coenzymes are then oxidized by the respiratory chain, and ATP is made.

Takes place in the cytosol Final products:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Glycolysis Ten reactions Takes place in the cytosol Final products: 2 molecules of pyruvate (pyruvic acid) 2 molecules of ATP 2 molecules of NADH

Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 1)
Figure 6.7 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, energy is released to form ATP and NADH.

Steps 6 and 7 are examples of reactions that occur repeatedly in metabolic pathways:

Oxidation–reduction (step 6): exergonic; glyceraldehyde 3-phosphate is oxidized and energy is trapped via reduction of NAD+ to NADH. Substrate-level phosphorylation (step 7): also exergonic; energy released transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP. 33

Occurs in mitochondria in eukaryotes.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Pyruvate Oxidation Occurs in mitochondria in eukaryotes. Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA) to form acetyl CoA. NAD+ is reduced to NADH.

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Occurs in mitochondria in eukaryotes
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Citric Acid Cycle Eight reactions Occurs in mitochondria in eukaryotes Operates twice for every glucose molecule that enters glycolysis Starts with Acetyl CoA; acetyl group is oxidized to two CO2 Oxaloacetate is regenerated in the last step

Figure 6.8 The Citric Acid Cycle
Figure 6.8 The Citric Acid Cycle Also called the Krebs cycle for its discoverer, Hans Krebs, the citric acid cycle involves eight steps and fully oxidizes acetyl CoA to CO2.

Final reaction of citric acid cycle:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Final reaction of citric acid cycle:

Cells transfer energy from NADH and FADH2 to ATP by oxidative phosphorylation: NADH oxidation is used to actively transport protons (H+) across the inner mitochondrial membrane, resulting in a proton gradient. Diffusion of protons back across the membrane then drives the synthesis of ATP.

When NADH is reoxidized to NAD+, O2 is reduced to H2O: NADH + H+ + ½ O NAD+ + H2O This occurs in a series of redox electron carriers, called the respiratory chain, embedded in the inner membrane of the mitochondrion. 40

Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria
Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria As electrons pass through the protein complexes of the respiratory chain, protons are pumped from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

Electron transport: electrons from the oxidation of NADH and FADH2 pass from one carrier to the next in the chain. The oxidation reactions are exergonic, energy released is used to actively transport H+ ions across the membrane. 42

Oxidation is always coupled with reduction.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Oxidation is always coupled with reduction. When NADH is oxidized to NAD+, the reduction reaction is the formation of water from O2. 2 H+ + 2 e– + ½ O H2O The key role of O2 in cells is to act as an electron acceptor and become reduced. 43

ATP synthase uses the H+ gradient to drive synthesis of ATP by chemiosmosis: Chemiosmosis: Movement of ions across a semipermeable barrier from a region of higher concentration to a region of lower concentration. ATP synthase converts the potential energy of the proton gradient into chemical energy in ATP.

Figure Chemiosmosis Figure Chemiosmosis (A) If a cell can generate a proton (H+) gradient across a membrane, the potential energy resulting from the concentration gradient can be used by a membrane-spanning enzyme to make ATP. (B) ATP synthase has a membrane-embedded channel for H+ diffusion and a motor that turns, releasing some energy to produce ATP.

ATP synthase is a molecular motor with two subunits:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase is a molecular motor with two subunits: F0 is a transmembrane domain that functions as the H+ channel. F1 has six subunits. As protons pass through F0, it rotates, causing part of the F1 unit to rotate. ADP and Pi bind to active sites that become exposed on the F1 unit as it rotates, and ATP is made.

ATP synthase structure is similar in all organisms.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase structure is similar in all organisms. In prokaryotes, the proton gradient is set up across the cell membrane. In eukaryotes, chemiosmosis occurs in mitochondria and chloroplasts. The mechanism of chemiosmosis is similar in almost all forms of life.

Chemiosmosis can be demonstrated experimentally.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Chemiosmosis can be demonstrated experimentally. A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube. ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present.

Figure 6.11 An Experiment Demonstrates the Chemiosmotic Mechanism
Figure 6.11 An Experiment Demonstrates the Chemiosmotic Mechanism The chemiosmosis hypothesis was a bold departure from the conventional scientific thinking of the time. It required an intact compartment separated by a membrane. Could a proton gradient drive the synthesis of ATP? The first experiments to answer this question used chloroplasts, plant organelles that use the same mechanism as mitochondria to synthesize ATP.a [a A. T. Jagendorf and E. Uribe Proceedings of the National Academy of Sciences USA 55: 170–177.]

Some bacteria and archaea use other electron acceptors.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy About 32 molecules of ATP are produced for each fully oxidized glucose. The role of O2: most of the ATP is formed by oxidative phosphorylation, which is due to the reoxidation of NADH. Some bacteria and archaea use other electron acceptors. Geobacter metallireducens can use iron (Fe3+) or uranium, making it potentially useful in environmental cleanup.

Under anaerobic conditions, NADH is reoxidized by fermentation.
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Under anaerobic conditions, NADH is reoxidized by fermentation. There are many different types of fermentation, but all operate to regenerate NAD+. The overall yield of ATP is only two—the ATP made in glycolysis.

Lactic acid fermentation: End product is lactic acid (lactate).
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Lactic acid fermentation: End product is lactic acid (lactate). NADH is used to reduce pyruvate to lactic acid, regenerating NAD+. Occurs in many microorganisms and complex organisms, including vertebrate muscle during exercise when O2 can not be delivered to the muscle fast enough.

Figure 6.12 Fermentation (Part 1)
Figure Fermentation (A) In lactic acid fermentation, NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+ to keep glycolysis operating. (B) In alcoholic fermentation, pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, again regenerating NAD+ for glycolysis.

Alcoholic fermentation: End product is ethyl alcohol (ethanol).
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Alcoholic fermentation: End product is ethyl alcohol (ethanol). Pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD+. Occurs in certain yeasts and some plant cells under anaerobic conditions.

Figure 6.12 Fermentation (Part 2)
Figure Fermentation (A) In lactic acid fermentation, NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+ to keep glycolysis operating. (B) In alcoholic fermentation, pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, again regenerating NAD+ for glycolysis.

Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Metabolic pathways are linked. There is an interchange of molecules into and out of the pathways for synthesis and breakdown. Carbon skeletons (molecules with covalently linked carbon atoms) can enter catabolic or anabolic pathways. These relationships comprise a metabolic system.

Figure 6.13 Relationships among the Major Metabolic Pathways of the Cell
Figure Relationships among the Major Metabolic Pathways of the Cell Note the central positions of glycolysis and the citric acid cycle in this system of metabolic pathways. Also note that many of the pathways can operate essentially in reverse.

Catabolism: Polysaccharides are hydrolyzed to glucose, which enters glycolysis. Lipids break down to fatty acids and glycerol. Fatty acids can be converted to acetyl CoA. Proteins are hydrolyzed to amino acids that can feed into glycolysis or the citric acid cycle.

Anabolism: Many catabolic pathways can operate in reverse. Gluconeogenesis—citric acid cycle and glycolysis intermediates can be reduced to form glucose. Acetyl CoA can be used to form fatty acids. Some citric acid cycle intermediates can form nucleic acids.

Amounts of different molecules are maintained at fairly constant levels in the metabolic pool. This is accomplished by regulation of enzymes— allosteric regulation and feedback inhibition. Enzymes can also be regulated by altering the transcription of genes that encode the enzymes. This is slower than feedback inhibition.

ATP and reduced coenzymes link catabolism, anabolism, and photosynthesis. Cellular respiration and photosynthesis are linked by their reactants and products and by the energy “currency” of ATP and reduced coenzymes.

In cellular respiration glucose is oxidized: glucose + 6 O CO2 + 6 H2O + chemical energy In photosynthesis, light energy is converted to chemical energy: CO2 + H2O + light energy carbohydrates + O2

Figure 6.14 ATP, Reduced Coenzymes, and Metabolism
Figure ATP, Reduced Coenzymes, and Metabolism The major pathways of energy metabolism, cellular respiration and photosynthesis, are related by their use of energy-transferring substrates, ATP and reduced coenzymes. Note that the net result of these pathways is to convert light energy into chemical energy to fuel the processes of life.

Photosynthesis (anabolic) involves two pathways:
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis (anabolic) involves two pathways: Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH). Carbon-fixation reactions use the ATP and NADPH to produce carbohydrates.

Figure 6.15 An Overview of Photosynthesis
Figure An Overview of Photosynthesis Photosynthesis consists of two pathways: the light reactions and the carbon-fixation reactions. In eukaryotes, these occur in the chloroplast.

Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy
Light is a form of electromagnetic radiation; it is propagated as a wave but also behaves as particles (photons). The amount of energy in the radiation is inversely proportional to its wavelength.

Figure 6.16 The Electromagnetic Spectrum
Figure The Electromagnetic Spectrum The portion of the electromagnetic spectrum that is visible to humans as light is shown in detail at the right.

Photons can be absorbed by specific receptor molecules, which are raised to an excited state (higher energy). 68

Pigments: molecules that absorb wavelengths in the visible spectrum
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Pigments: molecules that absorb wavelengths in the visible spectrum Chlorophyll absorbs blue and red light; the remaining light is mostly green. Absorption spectrum—plot of light energy absorbed against wavelength. Action spectrum—plot of the biological activity of an organism against wavelength.

Figure 6.17 Absorption and Action Spectra
Figure Absorption and Action Spectra The absorption spectrum of the purified pigment chlorophyll a from the aquatic plant Anacharis is similar to the action spectrum, obtained when different wavelengths of light are shone on the intact plant and the rate of photosynthesis is measured. In the thicker leaves of land plants, the action spectra show less of a dip in the green region (500–650 nm).

In plants, two chlorophylls absorb light energy— chlorophyll a and chlorophyll b. Accessory pigments absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls.

Figure 6.18 The Molecular Structure of Chlorophyll
Figure The Molecular Structure of Chlorophyll Chlorophyll consists of a complex ring structure (green) with a magnesium ion at its center, plus a hydrocarbon “tail.” The tail anchors chlorophyll molecules to integral membrane proteins in the thylakoid membrane. Chlorophyll a and chlorophyll b are identical except for the replacement of a methyl group (—CH3) with an aldehyde group (—CHO), shown on the upper right side of the ring structure.

A photosystem spans the thylakoid membrane in the chloroplast
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The pigments are arranged into light- harvesting complexes, or antenna systems. A photosystem spans the thylakoid membrane in the chloroplast It consists of multiple antenna systems surrounding a reaction center.

Figure 6.19 Photosystem Organization
Figure Photosystem Organization (A) The molecular structure of a single light-harvesting complex shows the polypeptide in brown with three helices that span the thylakoid membrane. Pigment molecules (carotenoids and chlorophylls a and b) are bound to the polypeptide. (B) Light-harvesting complexes are organized into large photosystems that span the thylakoid membrane and have a centrally placed reaction center. The light-harvesting chlorophylls absorb light and pass the energy on to a chlorophyll in the reaction center.

Chl* + acceptor Chl+ + acceptor –
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl+. The acceptor molecule is reduced. Chl* + acceptor Chl+ + acceptor – The reaction center has converted light energy into chemical energy.

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The final acceptor is NADP+, which gets reduced:
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The electron acceptor is the first carrier in an electron transport system in the thylakoid membrane. The final acceptor is NADP+, which gets reduced: NADP+ + H+ + 2 e– NADPH ATP is produced chemiosmotically during electron transport (photophosphorylation).

Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems
Figure Noncyclic Electron Transport Uses Two Photosystems As chlorophyll molecules in the reaction centers of photosystems I and II absorb light energy, they pass electrons into a series of redox reactions, ultimately producing NADPH and ATP. The term “Z scheme” describes the path (blue arrows) of electrons as they travel through the two photosystems. In this scheme the vertical positions represent the energy levels of the molecules in the electron transport system.

Two photosystems: Photosystem I absorbs light energy at 700 nm and passes an excited electron to NADP+, reducing it to NADPH. Photosystem II absorbs light energy at 680 nm, oxidizes water, and initiates ATP production.

2 Chl* + H2O 2 Chl + 2 H+ + ½ O2 Photosystem II
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem II When Chl* gives up an electron, it is unstable and grabs an electron from H2O, which splits the H—O—H bonds. 2 Chl* + H2O Chl + 2 H+ + ½ O2

A proton gradient is generated and used by ATP synthase to make ATP.
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The excited (energetic) electron is passed through a series of thylakoid membrane-bound carriers to a final acceptor at a lower energy level. A proton gradient is generated and used by ATP synthase to make ATP.

This electron ends up reducing NADP+ to NADPH.
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem I When Chl* gives up an electron, it grabs another electron from the last carrier in the transport system of Photosystem II. This electron ends up reducing NADP+ to NADPH.

ATP is needed for carbon-fixation pathways. The noncyclic light reactions would not provide enough ATP. Cyclic electron transport uses only photosystem I and produces only ATP. An electron is passed from an excited chlorophyll, through the electron transport chain, and recycles back to the same chlorophyll.

Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP
Figure Cyclic Electron Transport Traps Light Energy as ATP Cyclic electron transport produces ATP but no NADPH.

Occurs in the stroma of the chloroplast.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates Calvin cycle: the energy in ATP and NADPH is used to “fix” CO2 in reduced form in carbohydrates Occurs in the stroma of the chloroplast. Each reaction is catalyzed by a specific enzyme. The cycle is composed of three distinct processes.

Figure 6.22 The Calvin Cycle
Figure The Calvin Cycle The Calvin cycle uses the ATP and NADPH generated in the light reactions to produce G3P from CO2. The G3P is used as a starting material for the production of glucose and other carbohydrates. Six turns of the cycle are needed to produce one molecule of the hexose glucose.

CO2 is added to ribulose 1,5-bisphosphate (RuBP).
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 1. Fixation of CO2: CO2 is added to ribulose 1,5-bisphosphate (RuBP). Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction. A 6-carbon molecule results, which quickly splits into two 3-carbon molecules: 3-phosphoglycerate (3PG)

Figure 6.23 RuBP Is the Carbon Dioxide Acceptor
Figure RuBP Is the Carbon Dioxide Acceptor The enzyme rubisco adds CO2 to the five-carbon compound RuBP. The resulting six-carbon compound immediately splits into two molecules of 3PG.

2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).

3. The CO2 acceptor RuBP is regenerated from G3P.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 3. The CO2 acceptor RuBP is regenerated from G3P. Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose). When glucose accumulates, it is linked to form starch, a storage carbohydrate.

Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates
The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth. Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction. Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.

Answer to Opening Question
Pasteur’s findings: Catabolism of the beet sugar is a cellular process, so living yeast cells must be present. In air (O2), yeasts use aerobic metabolism to fully oxidize glucose to CO2. Without air, yeasts use alcoholic fermentation, producing ethanol, less CO2, and less energy (slower growth).

Figure 6.24 Products of Glucose Metabolism
Figure Products of Glucose Metabolism Beer, wine, and bread are all made using fermentation reactions in yeast cells.

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