Korean, Edit

Chapter 4. Cell and Energy Metabolism

Recommended Reading: 【Biology】 Biology Index

1. Classification of Organisms Based on Energy Metabolism

2. ATP and Electron Carriers

3. Cellular Respiration

4. Photosynthesis

a. ATP Synthase

b. Glycolysis

c. Glucose Synthesis

d. Protein Degradation

e. Protein Synthesis

f. Lipid Degradation

g. Lipid Synthesis

h. Alcohol Breakdown and Hangover

1. Classification of Organisms Based on Energy Metabolism

⑴ Classification based on energy source

① Class 1. Phototrophs: Organisms that use light as an energy source. Divided into photoautotrophs and photoheterotrophs.

② Class 2. Chemotrophs: Organisms that use chemical energy as an energy source. Divided into chemoautotrophs and chemoheterotrophs.

○ Cyanobacteria O. limnetica uses H₂O as the initial electron donor when H₂S is absent, and H₂S when it is present.

○ For Cyanobacteria O. limnetica, the energy source is H₂S when it is used, otherwise it is light.

⑵ Classification based on carbon source

① Class 1. Autotrophs: Organisms with CO₂ as their carbon source. Divided into photoautotrophs and chemoautotrophs.

② Class 2. Heterotrophs: Organisms with organic compounds as their carbon source. Divided into photoheterotrophs and chemoheterotrophs.

⑶ Summary: When listed by energy source, carbon source,

① Photoautotrophs: Light + CO₂

② Photoheterotrophs: Light + Organic Compounds

③ Chemoautotrophs: Inorganic Substances + CO₂

④ Chemoheterotrophs: Organic Compounds + Organic Compounds

2. ATP and Electron Carriers

⑴ ATP (Adenosine Triphosphate)

① Causes conformational changes in proteins through phosphorylation to perform mechanical work, substance transport, and chemical reactions (coupled reaction molecules).

② Structure

○ Adenine + Ribose + 3 Phosphate Molecules

○ High-Energy phosphate bond: Due to electrostatic repulsion of negatively charged phosphate, it has high energy content and is unstable, thus easily hydrolyzed releasing a large amount of energy (7.3 kcal/mol).

○ ATP ⇄ ADP + P_i (phosphate), K = 1.5 × 10⁵

③ Types of Phosphorylation

○ Type 1. Substrate-level phosphorylation: Glycolysis

○ Type 2. Photophosphorylation: Bacteriorhodopsin

○ Type 3. Oxidative phosphorylation: Phosphorylation by the electron transport chain

⑵ NAD⁺ (Nicotinamide Adenine Dinucleotide): Coenzyme in oxidation reactions, involved in cellular respiration.

① Adenine + 2 Riboses + Nicotinamide Group

○ Derived from Niacin (Vitamin B₂ derivative)

② NAD⁺ + H₂ ⇄ NADH + H⁺

○ 1 molecule of NAD⁺ carries 1 hydrogen atom and 1 electron.

○ Thus, it carries 1 hydrogen ion and 2 electrons.

③ Mechanism: Nucleophilic aromatic addition reaction and hydride shift

Figure 1. The process of reducing NAD⁺ to NADH during glycerol metabolism

⑶ NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate): Coenzyme in anabolic reactions, involved in photosynthesis

① In liver cell cytoplasm, [NADPH] / [NADP⁺] is higher than [NADH] / [NAD⁺].

⑷ FAD (Flavin Adenine Dinucleotide)

① FAD + 2H ⇄ NADH₂

3. Cellular Respiration

⑴ Overview of cellular respiration

① External respiration: Breathing in O₂ and releasing CO₂ through the lungs.

② Internal Respiration: Tissue cells use O₂ for cellular respiration and release CO₂.

③ In cellular respiration, glucose oxidizes to CO₂, O₂ reduces to H₂O, synthesizing ATP.

⑵ Aerobic Respiration: O₂ is used as the final electron acceptor in the electron transport chain.

① Step 1. Glycolysis: An anaerobic process that does not require oxygen.

○ Overall Reaction: Glucose (6-C) + 2 ATP + 2NAD⁺ → 2 Pyruvate (3-C) + 4 ATP + 2 NADH

○ A series of 10 sequential reactions occurring in the cytosol.

○ CNADH in the cytoplasm utilizes shuttle molecules to move into the mitochondrial matrix.

○ Malate-aspartate shuttle: Cytoplasmic NADH → Mitochondrial matrix NADH

○ Glycerol-phosphate shuttle: Cytoplasmic NADH → Mitochondrial matrix FADH₂

○ The glycerol-3-phosphate shuttle allows cytoplasmic NADH to directly activate FADH2 without additional steps, enabling faster ATP production.

Figure 2. Shuttle systems located in the mitochondrial inner membrane

(A) Malate-aspartate shuttle, (B) Glycerol-phosphate shuttle

Figure 3. Overall process of glycolysis

② Step 2. TCA Cycle: Also known as the Citric Acid Cycle, Krebs Cycle, Tricarboxylic Acid Cycle, etc.

Figure 4. TCA Cycle

○ Inference of the overall reaction equation

○ Half oxidation reaction equation

○ Half reduction reaction equation

○ Incomplete reaction equation

○ Conclusion: 2C₃H₄O₃ + 6H₂O + 5O₂ + ADP + P_i → 6CO₂ + 10H₂O

○ Oxidation of pyruvate

○ Occurs in the inner membrane of mitochondria.

○ 2 Pyruvate (3-C) → 2 CO₂ + 2NADH + 2 Acetyl CoA (2-C)

○ Citric acid cycle

○ Occurs in the mitochondrial matrix.

○ Acetyl CoA → 2 CO₂ + 3 NADH + FADH₂ + GTP (by 8 enzymes)

○ GTP produced in the citric acid cycle is generally considered equivalent to ATP.

○ Oxaloacetate (OAA) continuously changes its components.

○ During the process of carbon dioxide release, C-C bonds are broken, and the energy released is used to produce NADH.

○ Memory Tip: Pyruvate - Acetyl CoA - Citrate - α-Ketoglutarate - Succinate - Fumarate - Malate - Oxaloacetate

③ Step 3. Electron Transport System (ETS) in Mitochondria

○ Complex I (NADH-Q Oxidoreductase): Releases 1 H⁺ as electrons pass through.

○ Complex II (Succinate Dehydrogenase)

○ Refers to membrane-bound protein FAD.

○ Does not pump H⁺ due to low energy.

○ Ubiquinone

○ A hydrophobic molecule in the quinone family, located inside the inner membrane.

○ Coenzyme A (related to Acetyl CoA)

○ Fat-soluble vitamin (the only lipid component)

○ Complex III (Cytochrome C Reductase, Cytochrome B): Releases 1 H⁺ as electrons pass through.

○ Cytochrome C

○ Attached to the inner membrane, facing the intermembrane space.

○ Leads to programmed cell death when released into the cytoplasm.

○ Complex IV (Cytochrome C Oxidase, Cytochrome A)

○ Releases 1 H⁺ as electrons pass through.

○ Transfers electrons to oxygen at the end, forming water.

○ ATP Synthase: H⁺ ions pass through a protein channel called ATP Synthase in the inner membrane, leading to ATP production.

Figure 5. Electron transport system in mitochondrial inner membrane

○ Redox potential of the electron transport chain

○ Reducing power: The tendency to reduce other substances. Low reducing power implies a tendency to reduce by itself.

○ Reduction potential: A higher standard reduction potential value for chemical species that tend to be reduced.

○ Gibbs free energy change: The more a chemical species prefers electrons, the more negative the change in free energy becomes.

○ Conclusion: As the reaction process progresses, reducing power decreases, reduction potential increases, and the free energy change ΔG becomes more negative.

○ For a chemical reaction to occur, ΔG ＜ 0, so ΔG must become more negative as the process progresses: ΔG_{earlier_step} + ΔG_{latter_step} ＜ 0

○ By using ΔG = -nFE, the relationship between reduction potential and ΔG can be easily understood.

Figure 6. Redox potential of the electron transport chain

④ Cellular respiration calculation

○ Yield 1. Approximately 1 NADH → 3 ATP, 1 FADH₂ → 2 ATP

○ Yield 2. For prokaryotes, fungi, algae, etc., the yield is lower: 1 NADH → 2.5 ATP, 1 FADH₂ → 1.5 ATP

○ 4H⁺ are released when electrons move through complexes I and III, and 2H⁺ are released when electrons move through complex IV.

○ Complex IV, being closer to ATP synthase, has a difficulty in structural change and therefore has a less yield.

○ 4H⁺ are required to synthesize 1 ATP.

○ ATP synthase makes one turn when 3H⁺ move.

○ H⁺ is consumed when pyruvate moves into the inner membrane.

○ NADH yield = Total H⁺ moved ÷ (4 H⁺ / 1 ATP) = (4 × 2 + 2) ÷ 4 = 2.5

○ FADH₂ yield = Total H+ moved ÷ (4 H⁺ / 1 ATP) = (4 × 1 + 2) ÷ 4 = 1.5

○ One molecule of glucose is converted into 2 pyruvate molecules through glycolysis, producing 2 ATP and 2 NADH.

○ 2 Pyruvates enter mitochondria, undergo oxidation and citric acid cycle, producing 2 ATP + 8 NADH + 2 FADH₂.

○ Tip: CO₂ release produces NADH.

○ Tip: Except for the conversion of malate to OAA, NADH formation accompanies CO₂ production.

○ Cytosolic NADH must pass through a shuttle in the mitochondrial inner membrane, and may be converted to FADH₂.

○ Liver, heart, kidneys: A total of 32 ATP produced (38 ATP, calculated with yield 1). Related to malate-aspartate shuttle.

○ Skeletal muscle, brain, etc.: A total of 30 ATP produced (36 ATP, calculated with yield 1). Related to glycerol phosphate shuttle.

○ Previously calculated as 32 ATP.

○ Skeletal muscle and brain use the glycerol phosphate shuttle for rapid energy despite lower efficiency.

○ In reality, 1 H⁺ is also used when free phosphate moves across the mitochondrial inner membrane.

⑤ Cellular respiration inhibitors

○ Complex I inhibitors: Rotenone, Barbiturates, Piericidin

○ Complex II inhibitors: Malonate

○ Complex III inhibitors: Antimycin

○ Complex IV inhibitors: CN^-, CO

○ ATP synthase inhibitors: Oligomycin

○ Uncoupling agents

○ DNP (2,4-Dinitrophenol), Valinomycin, Thermogenin (brown fat is related)

○ Uncoupling agents are a type of ionophore for H⁺ that rapidly decrease the proton concentration gradient, generating a lot of heat.

Table 1. Summary of inhibitors

⑥ Global warming and cellular respiration

○ Temperature increase accelerates the metabolic rate of organisms, speeding up growth, development, and reproduction.

○ Example: Acceleration of the life cycle of beetles (2 years → 1 year) → Destruction of spruce forests

⑶ Gluconeogenesis

⑷ Anaerobic respiration and fermentation

① Electron transfer interruption and continuation of glycolysis → Accumulation of pyruvate, NADH, depletion of NAD⁺ → Reaction stop

② Reason for anaerobic respiration: To supply NAD⁺ for glycolysis

○ Reduces pyruvate to lactate, ethyl alcohol, etc., while oxidizing NADH to NAD⁺.

○ A pair of glycolysis and ab anaerobic respiration produce 2 ATP.

③ Type 1. Lactic acid fermentation: Occurs in muscle cells, lactic acid bacteria, etc.

Figure 7. Lactic Acid Fermentation

○ Reaction 1. Glycolysis: Glucose → 2 Pyruvates

○ Reaction 2. Reduction of Pyruvate: 2 Pyruvates → 2 Lactic Acids

○ Lactic acid is decomposed in the liver.

○ During the reduction from pyruvate to lactic acid, lactate dehydrogenase (gene name: LDHA) is involved.

○ Warburg effect: Cancer cells prefer lactic acid fermentation over efficient cellular respiration.

○ The brain also performs lactic acid fermentation and is known to excrete lactate during rest.

④ Type 2. Alcohol fermentation: Occurs in yeast, etc.

○ Reaction 1. Glycolysis: Glucose → 2 Pyruvates

○ Reaction 2. Decarboxylation: 2 Pyruvates → 2 Acetaldehydes + 2CO₂

○ Reaction 3. Reduction of Acetaldehyde: 2 Acetaldehydes → 2 Ethanol

○ The decarboxylation reaction from pyruvate to acetaldehyde involves pyruvate carboxylase.

○ The reduction from acetaldehyde to ethanol involves alcohol dehydrogenase.

Figure 8. Alcohol fermentation

⑤ Type 3. Acetic acid fermentation: Oxygen is involved. Not a complete fermentation in the true sense.

Figure 9. Acetic acid fermentation

⑸ Other glucose catabolic pathways

① EMP (Embden-Meyerhof-Parnas) pathway: Glycolysis

○ Reaction that generates ATP.

○ Chemical reaction formula: C₆H₁₂O₆ + 2ADP + 2P_i + 2NAD⁺ → 2C₃H₄O₃ + 2ATP + 2NADH + 2H⁺

② TCA cycle

○ Reaction that generates ATP.

○ Chemical reaction formula: C₃H₄O₃ + 3H₂O + ADP + P_i + 4NAD⁺ + FAD → 3CO₂ + 4(NADH + H⁺) + FADH₂ + ATP

③ HMP (hexose monophosphate pathway)

○ Also known as PPP (pentose phosphate pathway), pentose shunt, Warburg-Dickens pathway.

○ Reaction to produce NADPH required for anabolic processes such as fatty acid synthesis.

○ Chemical reaction formula: C₆H₁₂O₆ + 12NADP⁺ + 3O₂ + ATP → 6CO₂ + 12NADPH + ADP + P_i

○ Glucose-6-phosphate can be completely reduced to carbon dioxide to generate 12 molecules of NADPH.

○ Sugars with different carbon numbers can be synthesized in this reaction.

○ The pentose sugars obtained from the breakdown of ingested nucleic acids are also metabolized through the HMP pathway.

○ Mechanism

Figure 10. HMP pathway mechanism

○ Oxidative phase: Generates NADPH.

○ Non-oxidative phase: Generates pentoses etc.

○ Reaction location

○ Animal cells: Cytoplasm

○ Plant cells: Chloroplasts, cytoplasm; dominant in chloroplasts.

○ Most active in the liver, mammary glands, and adrenal glands, accounting for over 60% of reducing power.

○ Basic role: Provides carbon skeletons needed for biosynthetic reactions and reduction power to support anabolic processes

○ Abundant of NADPH: Supplies the reducing power required for the biosynthesis of biomolecules.

○ Ribose-5-phosphate: Material for nucleotide bases, coenzymes, and a major precursor of histidine.

○ Ribulose-5-phosphate: Participates in the Krebs cycle.

○ Erythrose-4-phosphate: Material for aromatic amino acids.

○ Glyceraldehyde-3-phosphate: Participates in the EMP and TCA cycles.

④ ED (Entner-Doudoroff) pathway

○ Chemical reaction formula: C₆H₁₂O₆ + ADP + P_i → C₂H₅OH + 2CO₂ + ATP

○ Found in Zymomonas mobilis, Pseudomonas saccharophilia, etc.

○ Zymomonas mobilis: Gram-positive. Produces high concentrations of ethanol in a short time.

⑤ PK (phosphoketolase) pathway

○ Feature: Involvement of phosphateketolase.

○ Phosphateketolase: Breaks down into glyceraldehyde-3-phosphate, acetyl phosphate, and pentose phosphate.

⑹ Metabolism of carbohydrates, proteins, and lipids

① Overview

○ 1^st priority energy source: Carbohydrates

○ 2^nd priority energy source: Lipids

○ 3^rd priority energy source: Proteins. Since proteins have many unique functions, it’s a very dangerous situation if proteins are being broken down.

② Glycogen breakdown process

○ Almost all tissues can contain glycogen.

○ Glycogen storage tissues: Liver, muscles, kidneys. Glycogens exist in the cytoplasm in granular form.

○ Glycogen breakdown: Non-reducing end’s α 1→4 bond phosphorolysis enzyme → transferase → α 1→6 glucosidase hydrolase

Figure 11. Glycogen Breakdown Process

③ Protein breakdown

④ Protein synthesis

⑤ Lipid breakdown

⑥ Lipid synthesis

⑺ Tissue-specific glucose catabolism

① Liver

○ Absorbed by GLUT2.

○ Pathway 1. PPP

○ Pathway 2. TCA cycle

② Adipose tissue

○ GLUT4 is conditionally expressed in response to insulin.

○ Pathway 1. Glucose absorption → PPP pathway → DNA skeleton formation

○ Pathway 2. Glucose → Acetyl coA → TCA cycle

○ Pathway 3. Glucose → Fatty acids → Stored as triacylglycerol.

③ Muscle tissue

○ GLUT4 is conditionally expressed in response to insulin.

○ PPP pathway does not occur.

④ Brain

○ GLUT1, which is the most efficient glucose transporter, is expressed because the brain requires a high amount of glucose.

○ Glucose can pass through the BBB, and insulin does not affect glucose metabolism in the brain.

⑻ Response index

① P:O ratio

○ The ratio of P_i to oxygen. It represents the amount of ATP produced by oxidative phosphorylation when one oxygen atom is reduced to water.

○ 2O₂ + 2H⁺ → H₂O

② Respiratory quotient (RQ): Moles of produced carbon dioxide / Moles of consumed oxygen

③ Oxygen transfer rate (OTR): The rate at which oxygen in the air dissolves in solution.

④ Oxygen uptake rate (OUR): Oxygen respiration rate of microorganisms contained in a unit volume of culture medium.

4. Photosynthesis

⑴ Overview

① Photosynthetic organisms: Bacteria, Algae, Plants

○ Type 1. Bacteria: Purple bacteria, Yellow bacteria, Cyanobacteria

○ Type 1-1. Purple bacteria

○ Absorbs 870 nm wavelength. 1 photosystem.

○ Cyclic photophosphorylation only.

○ H₂S serves as electron donor.

○ Reverse electron transfer, because it cannot harvest light.

○ Type 1-2. Yellow bacteria

○ Absorbs 840 nm wavelength. 2 photosystems.

○ Cyclic photophosphorylation + primitive non-cyclic photophosphorylation

○ H₂S serves as electron donor.

○ Type 1-3. Cyanobacteria: Ancestor of chloroplasts.

○ Absorbs 670 nm wavelength. 2 photosystems.

○ Cyclic photophosphorylation + non-cyclic photophosphorylation

○ H₂O serves as electron donor.

○ Type 2. Algae

○ Red algae: Chlorophyll a, d

○ Diatoms: Chlorophyll a, c

○ Brown algae: Chlorophyll a, c

○ Green algae: Chlorophyll a, b

○ Type 3. Bryophytes, Ferns, Gymnosperms, Angiosperms

○ Bryophytes: Exchange substances through diffusion. Chlorophyll a, b

○ Ferns, Gymnosperms, Angiosperms: Chlorophyll a, b

○ Shade plants: Lack carotenoid pigments.

② Photosynthesis equation: 6 CO₂ + 12 H₂O + Light → Glucose (C₆H₁₂O₆)+ 6O₂ + 6H₂O (ΔG = 686 kcal / mol)

○ Achieves the opposite result of cellular respiration.

○ Tracing the photosynthesis equation

Figure 12. Experiment results using radioactive isotopes in photosynthesis

○ Niel’s research: Used yellow bacteria to prove that oxygen in photosynthesis comes from water.

○ Ruben’s research: Proved Niel’s hypothesis using radioactive isotope ¹⁸O.

③ Photosynthesis experiments

○ Engelmann’s experiment

○ Experiment on photosynthetic efficiency using Spirogyra and aerobic bacteria.

○ Chlorophyll absorbs light well at 400 ~ 500 nm and 600 ~ 700 nm wavelengths but not at 500 ~ 600 nm.

○ Thus, photosynthesis is efficient under blue-violet and red-orange light wavelengths.

○ Emerson experiment - Emerson red drop

○ Presented a graph of wavelength against quantum yield.

○ Quantum yield: A function of absorption spectrum and photosynthetic efficiency.

○ Absorption spectrum peaks at 680 nm and drops sharply after 700 nm: Because photosystem II is not functioning, resulting in a decrease in photosynthetic efficiency.

○ Emerson experiment - Emerson enhancement effect

○ Duysens’ experiment

○ Benson’s experiment

④ Principles of photosynthesis

○ CO₂ and O₂ are transported through stomata.

○ Light-dependent reactions

○ Utilizes light energy for electron transfer.

○ Reduction reactions: NADP⁺ → NADPH, ADP → ATP

○ Water decomposition, oxygen release

○ Light-independent reactions

○ Reduce CO₂ to produce C₆H₁₂O₆.

○ Oxidation reactions: NADPH → NADP⁺, ATP → ADP

⑤ Factors affecting photosynthesis

○ Law of the minimum

○ The growth of plants is governed by the scarcest essential nutrient, not by the abundant ones.

○ Proposed by the German botanist Justus von Liebig in 1840.

○ Dependent variable: CO₂

○ Independent variable 1. Light: Has a light saturation point

Figure 13. Light intensity and photosynthesis

○ Net photosynthetic amount: Visible photosynthesis amount.

○ Total photosynthetic amount: Total amount of CO₂ used in actual photosynthesis.

○ Respiratory amount = Total photosynthetic amount - Net photosynthetic amount

○ Compensation point (B): Light intensity where total photosynthesis amount equals respiratory amount.

○ Light saturation point (D): Light intensity where photosynthesis amount starts to maximize.

○ Sun plants have higher light saturation and compensation points as they are bigger and more metabolically active than shade plants.

○ Sun plants have carotenoids, hence a higher light saturation point.

○ Independent variable 2. Temperature: Has an optimal temperature.

⑵ Chloroplast

⑶ Pigments

① Photosynthetic pigment separation experiment: Paper chromatography (Toluene solvent)

② Chlorophyll: Divided into chlorophyll a and chlorophyll b. Absorbs red and blue light.

○ Porphyrin ring

○ Aromatic. Functions as a pigment.

○ Structure: Magnesium prosthetic group + tetrapyrrole + phytol

○ Hydrocarbon tail (phytol): Interacts with the hydrophobic parts of proteins (e.g., photosystems) located in the thylakoid membrane of chloroplasts.

○ Chlorophyll a: Methyl group

○ Chlorophyll b: Aldehyde group

③ Carotenoids: Accessory pigments. Absorb green light. Lower photosynthetic efficiency than chlorophyll. Present only in sun plants.

○ Function 1. Spectrum expansion

○ Function 2. Protecting photosynthetic machinery from intense light.

○ β-carotene: A type of carotenoid

○ Mainly absorbs light at 400 ~ 500 nm but also green light.

○ Can be converted into Vitamin A.

○ Retinal, a derivative of vitamin A, is used as a component of visual pigments in the rod and cone cells of the retina.

④ Xanthophyll

⑤ Phycobilin: Pigment molecules of purple bacteria (870 nm), yellow bacteria (840 nm), cyanobacteria (670 nm).

⑷ Photosystems

① Reaction center: The site in thylakoid membranes where light energy is converted into chemical energy.

○ Composed of 2 chlorophyll a, a primary electron acceptor, proteins, etc.

○ Photosystem I: Absorbs 700 nm wavelength. Exposed to the stroma and historically discovered first.

○ Photosystem II: Absorbs 680 nm wavelength. Hidden from the stroma and historically discovered later.

○ Chlorophyll a absorbs enough energy → Excited electron leaves its orbit → High-energy electron is transferred to the electron transport chain.

② Antenna complex (light-harvesting complex): Surrounds the reaction center, and contains chlorophyll a, chlorophyll b, carotenoids, xanthophylls, etc.

⑸ Light-dependent reactions: Occur in the thylakoid membrane, a disc-shaped membrane structure covered with chlorophyll molecules.

① 1^st. Antenna: Photons are absorbed by pigment molecules and transferred to adjacent pigment molecules by RET (resonance energy transfer).

○ RET: When two molecules are close enough, photons are transferred more directly rather than through fluorescence.

② 2^nd. Reaction center: Electrons move between pigment molecules until they reach P680 of Photosystem II or P700 of Photosystem I.

○ P680 shows maximum absorption at 680 nm, P700 at 700 nm.

③ 3^rd. P680 of Photosystem II oxidizes to P680⁺ and transfers to the primary acceptor.

○ The primary acceptor is also called pheophytin.

○ P680⁺ is the strongest oxidizing agent on Earth.

④ 4^th. P680 has a structure with Mn in the center of pheophytin; oxidized P680⁺ oxidizes H₂O to gain electrons, resulting in O₂ production.

○ Water photolysis occurs inside the thylakoid.

Figure 14. Pheophytin and water photolysis

⑤ 5^th. The electron moved to the first electron acceptor reaches P700 through PQ (plastoquinone), ATP synthase (cytochrome b0f complex, CF₀CF₁ complex), PC (plastocyanin).

○ In this process, 3H⁺ moves, generating 1 ATP.

○ ATP synthase consists of the CF₀ channel, which facilitates the facilitated diffusion of protons, and CF₁, which contains ATP synthase, creating ATP by chemiosmotic phosphorylation similar to mitochondria’s F₀F₁.

Figure 15. Structure of the CF0CF1 complex

⑥ 6^th. P700, having received electrons from ② and ⑤, becomes the excited P700*, which then transfers the electron to the primary electron acceptor.

○ The primary electron acceptor is also known as phylloquinone.

○ P700* is one of the strongest reductants on Earth.

⑦ 7^th. The electron is transferred from phylloquinone to ferredoxin.

⑧ 8^th - 1^st. Cyclic photophosphorylation: When the electron moves from ferredoxin to the cytochrome complex. 1 ATP is generated

○ Product: ATP

⑨ 8^th - 2^nd. Non-cyclic photophosphorylation: Ferredoxin transfers the electron to ferredoxin-NADP⁺.

○ Reduction of ferredoxin-NADP⁺ results in the production of 1 ATP + 1 NADPH.

○ Products: ATP, NADPH, O₂

○ NADP⁺ is the final electron acceptor.

Figure 16. Cyclic and non-cyclic photophosphorylation

Figure 17. Standard reduction potentials of the light reactions

⑨ Inhibitors

○ Photosystem I inhibitor: Paraquat

○ Photosystem II inhibitor: DCMU

⑹ Dark reactions (Calvin Cycle)

① Stroma: The matrix of chloroplasts. Site of dark reactions.

○ The reaction synthesizing sugar from glucose takes place in the cytosol.

② Calvin Cycle: Uses ATP and NADPH to reduce CO₂ and synthesize carbohydrates (12 NADPH, 18 ATP).

○ Part of the glucose neosynthesis process is proceeded: 3PGA → 1,3-BPG → G3P

Figure 18. Calvin Cycle

PGA is 3PG, DPGA is 1,3-BPG, PGAL is G3P

③ Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase)

○ Accounts for 50% of all proteins in leaves, and the most abundant protein on Earth.

○ Increased carbon dioxide → carboxylase activity

○ Increased oxygen → oxygenase activity

④ G3P: Becomes starch in the stroma or sugar in the cytosol of mesophyll cells.

⑤ Dark reaction enzymes function more efficiently under light conditions.

○ Rubisco

○ Light conditions → Alkalization of stroma → Increase in Rubisco activity

○ pH changes due to light reactions move Mg²⁺ stored in the thylakoid lumen to the stroma → Activates Rubisco.

⑺ C₃, C₄, and CAM plants

① C₃ plants: Common plants (e.g., roses, rice, and most plants in temperate climates)

○ First carbon fixation product: 3-PGA (3C)

○ Reactions at high temperatures

○ Guard cells: Regulate the opening and closing of stomata and transpiration.

○ Open stomata: Allows CO₂ intake, but also water loss.

○ Temperature ↑ → Stomata close → CO₂ deficiency → Decrease in photosynthesis rate

○ Temperature ↑ → Stomata close → CO₂ deficiency → Excess O₂ leads to photorespiration

② Photorespiration

Figure 19. Photorespiration

○ Rubisco can use either CO₂ or O₂ as substrates.

○ Rubisco acts as an oxygenase.

○ 1^st. RuBP + O₂ → 3PG + Glycolate (2C)

○ 2^nd. 3PG + Glycolate (2C) + ATP → RuBP + CO₂

○ Glycolate is decomposed, losing carbon sources.

○ Photorespiration involves chloroplasts, peroxisomes, and mitochondria.

○ Photorespiration uses energy, so cyclic photophosphorylation is dominant in photorespiratory conditions (high temperature).

○ Glycine and Serine are involved as intermediates.

③ C₄ plants: Tropical and subtropical plants (e.g., corn, sugarcane, sorghum, tropical plants)

○ Lack of Photosystem II: Mesophyll cells have both Photosystems I and II, but bundle-sheath cells only have Photosystem I (ref).

○ Only cyclic photophosphorylation, low compensation point, spatial separation of carbon fixation.

○ Krantz anatomy: Bundle-sheath cells surround the vascular bundle, and mesophyll cells surround them

○ 1^st. Stomata nearly closed. Mesophyll cells fix carbon to PEP, which has a high affinity for CO₂.

○ Mesophyll cells of C₄ plants lack chloroplasts.

○ 2^nd. PEP carboxylase binds CO₂ to PEP (3C) to produce oxaloacetate (OAA, 4C). First carbon fixation product.

○ PEP carboxylase has a higher CO₂ affinity than Rubisco.

○ PEP carboxylase does not exist in bundle-sheath cells.

○ 3^rd. Reduced malate (4C) moves to the interior (bundle-sheath cells), releasing CO₂.

○ 4^th. Rubisco in bundle sheath cells participates in the Calvin cycle with the supplied carbon dioxide and oxaloacetate in the stroma.

○ Rubisco does not exist in mesophyll cells.

○ 5^th. Malate releasing CO₂ generates NADPH again and produces pyruvate (3C): The reason why photosystem II is unnecessary.

○ 6^th. Pyruvate uses ATP to produce PEP.

○ 7^th. PEP moves to mesophyll cells.

○ Alternative pathway: Oxaloacetate passes through aspartate instead of malate.

④ CAM plants: Plants in extremely hot and dry environments, succulents (e.g., stonecrops, cacti, pineapple, and other desert plants)

○ Photosystem II exists, but its activity decreases under conditions where photorespiration occurs readily (ref).

○ Only cyclic photophosphorylation, low compensation point, carbon fixation temporally separated.

○ 1^st. During the day, stomata are completely closed to conserve water. Dark reactions occur but with lower efficiency.

○ 2^nd. At night, stomata open to produce malate from CO₂ in the C₄ cycle.

○ 2^nd - 1^st. PEP carboxylase binds CO₂ to PEP to produce oxaloacetate. The initial carbon fixation product.

○ 2^nd - 2^nd. Oxaloacetate is reduced to malate.

○ 3^rd. The produced malate is stored in the vacuole.

○ 4^th. During the day, the Calvin cycle operates using CO₂ obtained from the breakdown of malate.

○ 4^th - 1^st. Malate → CO₂ + NADPH + Pyruvate (3C)

○ 4^th - 2^nd. Pyruvate (3C) is converted to PEP using ATP or utilized in the TCA cycle to produce ATP.

○ 4^th - 3^rd. CO₂ moves to the stroma to participate in the Calvin cycle with oxaloacetate and Rubisco.

Figure 20. CAM Plants

⑤ Comparison of C₃, C₄, CAM Plants

Table 2. Comparison of C₃, C₄, and CAM Plants

⑻ Global warming and photosynthesis

① Temperature rise affects the relative proportions and distribution of C₃, C₄, CAM plants.

② Carbon dioxide is being released faster than current photosynthesis can remove.

③ 25% of the additional CO₂ in the atmosphere is due to deforestation (logging and fires) of tropical forests.

④ Reforestation helps; young trees have a faster rate of net photosynthesis than old trees.

Input: 2015.06.25 12:57

Edit: 2021.02.11 10:54