Korean, Edit

Chapter 4. Cells 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. Relevant Process

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

② Category 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 Category 1. Autotrophs : Organisms with CO₂ as their carbon source. Divided into photoautotrophs and chemoautotrophs

② Category 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 phosphate (negatively charged), 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 : Relevant Process etc.

○ Type 2. Photophosphorylation : Bacteriorhodopsin etc.

○ 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)

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. Process of NAD+ reducing to NADH in glycerol metabolism

⑶ NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate) : Coenzyme in assimilation 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 using O2 for cellular respiration and releasing CO₂

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

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

① Step 1. Relevant Process (Glycolysis) : An anaerobic process that does not require oxygen

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

○ Occurs in the cell cytoplasm through 10 consecutive reactions

○ Cytoplasmic NADH is transferred to mitochondrial matrix using shuttle molecules

○ Malate-Aspartate Shuttle : Cytoplasmic NADH → Mitochondrial Matrix NADH

○ Glycerol-Phosphate Shuttle : Cytoplasmic NADH → Mitochondrial Matrix FADH₂

○ Glycerol-Phosphate Shuttle directly activates FADH₂ from cytoplasmic NADH, speeding up 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 the relevant process

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

○ Oxidation Half-Reaction Equation

○ Reduction Half-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 equated to ATP

○ Oxaloacetate (OAA) continuously changes its components

○ Energy from breaking C-C bonds during CO₂ release is used to produce NADH

○ Memory Tip: Pyruvate - Citrate - Isocitrate - α-Ketoglutarate - Succinyl CoA - 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 (Coenzyme Q)

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

○ Related to Coenzyme A (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, generating ATP

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 be reduced

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

○ Free Energy Change: The more a chemical species likes electrons, the more negative the free energy change

○ 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

○ Understanding the trend of reducing power and ΔG is easier using ΔG = -nFE

Figure. 6. Redox Potential of the Electron Transport Chain

④ Cellular Respiration Calculation

○ Value 1: Approximately 1 NADH → 3 ATP, 1 FADH₂ → 2 ATP

○ Value 2: For prokaryotes, fungi, algae, etc., the value 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+ through complex IV

○ Complex IV, being closer to ATP synthase, has a difficult structural change and therefore 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 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 glucose molecule produces 2 pyruvates, 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, CO₂ production accompanies NADH formation

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

○ Liver, Heart, Kidneys: Total 32 ATP produced (38 ATP, calculated with value 1), Malate-Aspartate Shuttle

○ Skeletal Muscle, Brain, etc.: Total 30 ATP produced (36 ATP, calculated with value 1), 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 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 the Process → Accumulation of Pyruvate, NADH, Depletion of NAD+ → Reaction Stoppage

② Reason for Anaerobic Respiration: To supply NAD+ for the process

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

○ A pair of these processes and 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. Process : Glucose → 2 Pyruvate

○ Reaction 2. Reduction of Pyruvate : 2 Pyruvate → 2 Lactic Acid

○ 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. Process : Glucose → 2 Pyruvate

○ Reaction 2. Decarboxylation : 2 Pyruvate → 2 Acetaldehyde + 2CO₂

○ Reaction 3. Reduction of Acetaldehyde : 2 Acetaldehyde → 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 : Process

○ 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

○ Mechanism : See above

③ HMP (hexose monophosphate pathway)

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

○ Reaction to produce NADPH required for assimilation 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

○ 5-carbon 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 and reducing power needed for biosynthetic reactions

○ Large amounts of NADPH : Provide reducing power necessary 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 phosphoketolase

○ Phosphoketolase : Breaks down into glyceraldehyde-3-phosphate, acetyl phosphate, 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, exist in the cytoplasm in granular form

○ Glycogen breakdown : Non-reducing end’s α 1→4 bonds by glycogen phosphorylase → transferase → α 1→6 glucosidase hydrolysis

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

○ Expression of conditionally expressed GLUT4 by 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

○ Expression of conditionally expressed GLUT4 by insulin

○ PPP pathway does not occur

④ Brain

○ Expression of GLUT1, which transports glucose best : Brain needs a lot of glucose

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

⑻ Response indices

① P : O ratio

○ Ratio of P_i to oxygen, the amount of ATP generated per oxygen atom reduced to water by oxidative phosphorylation

○ 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, Green sulfur bacteria, Cyanobacteria

○ Type 1-1. Purple bacteria

○ Absorbs 870 nm wavelength. 1 photosystem

○ Cyclic photophosphorylation only

○ H2S as electron donor

○ Reverse electron transfer ( ∵ Cannot harvest light)

○ Type 1-2. Green sulfur bacteria

○ Absorbs 840 nm wavelength. 2 photosystems

○ Cyclic photophosphorylation + primitive non-cyclic photophosphorylation

○ H₂S as electron donor

○ Type 1-3. Cyanobacteria : Ancestor of chloroplasts

○ Absorbs 670 nm wavelength. 2 photosystems

○ Cyclic photophosphorylation + non-cyclic photophosphorylation

○ H₂O 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

○ Van Niel’s research : Used green sulfur bacteria to prove that oxygen in photosynthesis comes from water

○ Ruben’s research : Proved Van 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 : Due to Photosystem II not functioning, reducing 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 splitting, 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

○ Apparent photosynthetic rate : Visible photosynthesis rate

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

○ Respiratory rate = Total photosynthetic rate - Apparent photosynthetic rate

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

○ Light saturation point(D) : Light intensity where photosynthesis rate 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 coordinating group + tetrapyrrole (pyrrole) + phytol

○ Hydrocarbon tail(phytol) : Interacts with the hydrophobic parts of proteins (like 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), Green sulfur 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, contains chlorophyll a, chlorophyll b, carotenoids, xanthophylls, etc.

⑸ Light-dependent reactions : Occur on the disc-shaped membrane structures covered with chlorophyll molecules in the thylakoid(thylakoid) membrane.

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 (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 in the world

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: 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, the most abundant protein on Earth

○ CO₂ ↑ → Carboxylase activity, O₂ ↑ → 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 temperate plants)

○ 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

○ 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: Specifically, 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 ( 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

○ PEP carboxylase (PEP carboxykinase)

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

○ 4^th. Bundle-sheath cell Rubisco uses the supplied CO₂ and oxaloacetate in the stroma for the Calvin cycle

○ Rubisco does not exist in mesophyll cells

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

○ 6^th. Pyruvate uses ATP to produce PEP

○ 7^th. PEP moves to mesophyll cells

○ Alternative pathway: Oxaloacetate converted to malate via aspartate

④ CAM plants: Plants in extremely hot and dry environments, succulents (e.g., jade plants, cacti, pineapple, 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

○ PEP carboxylase (PEP carboxykinase)

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

○ 4^th. During the day, malate is decomposed to operate the Calvin cycle with the CO₂ obtained

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

○ 4^th - 2^nd. Pyruvate (3C) is used to produce ATP or used 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, 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