Photosynthesis 2024-2025 - Copy.pdf

LU2: BIOCHEMISTRY, CELL STRUCTURE & FUNCTION: Photosynthesis Mohamad Fhaizal bin Mohamad Bukhori [email protected] 012-3942055 Pejabat Akademik 2  LEARNING OBJEC...

LU2: BIOCHEMISTRY, CELL STRUCTURE & FUNCTION: Photosynthesis Mohamad Fhaizal bin Mohamad Bukhori [email protected] 012-3942055 Pejabat Akademik 2  LEARNING OBJECTIVES 1. Explain the basic principle of light reaction process and Calvin cycle (C3). 2. Explain the basic mechanism of alternative of CO2 fixation pathways (C4 and CAM pathways) in photosynthesis. 3. Describe the basic principle of the differences between C3 and C4 plants. 4. Describe the basic principle of the limiting factors in photosynthesis. Photosynthesis Greek: [photo] = “light”; [synthesis] = “putting together”, “composition” A chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight. Oxford’s Dictionary of Biology CHLOROPLAST ▪ Photosynthesis occurs within light-absorbing organelles called chloroplast. ▪ Their green colour is from chlorophyll, a light-absorbing pigments. ▪ In most plants, the leaves have the most chloroplasts (≈ 500,000 per mm2 of leaf surface). ▪ Major location of photosynthesis. CHLOROPLAST ▪ Concentrated in the interior cells of leaves. ▪ CO2 enters and O2 exits through tiny pores called stomata. ▪ Double-layer membrane envelope. ▪ Inner membrane encloses a compartment filled with thick fluid called stroma. CHLOROPLAST ▪ Suspended in the stroma are interconnected membranous sacs called thylakoids. Concentrated in stacks called grana. ▪ The chlorophyll molecules that capture light energy are built into the thylakoid membranes. ▪ Why do the thylakoids are in stacks? Stacks of discs structure aids its function by providing a large surface area for the reactions of photosynthesis. Overall Equation for Photosynthesis  LIGHT REACTION PROCESS ▪ Chlorophyll in the thylakoid membranes absorbs solar energy, then converted to the chemical energy; i. ATP (Adenosine Triphosphate) ii. NADPH (Nicotinamide Adenine Dinucleotide Phosphate) ▪ Light drives e- from H2O to NADP+ (the oxidized form of the carrier) to form NADPH (the reduced form of the carrier). ▪ H2O is split, providing a source of e- & giving off O2 as a by- product.  CALVIN CYCLE ▪ The cycle uses the products of the ‘light reaction’ to power the production of sugar from CO2 ▪ ATP generated by the light reaction provides the high- energy for sugar synthesis. ▪ NADPH produced by the light reaction provides the high-energy e- for the reduction of CO2 to glucose. ▪ Calvin cycle depends indirectly on light to produce sugar. LIGHT REACTION PROCESS: Converting Solar Energy to Chemical Energy ▪ The sun emits energy in a broad spectrum of electromagnetic radiation. ▪ Light and other electromagnetic waves are composed of individuals packets of energy called photons. ▪ The energy of a photon corresponds to its wavelength; i. Short (very energetic) ii. Long (lower energy) ▪ When a specific wavelength of light strikes an object, it can be either absorbed, reflected or transmitted. ▪ Absorbed light can heat up the object or drive biological processes. ▪ Reflected or transmitted light can reach the observer’s eyes, perceived as the colour of the object. ▪ Chloroplast contains various pigments molecules that absorbs different light wavelength; i. Chlorophyll a strongly absorbs violet & red. ii. Chlorophyll b absorbing blue and red-orange. iii. Pigment molecules (accessory pigments) that absorbs additional wavelength of lights and transfer them to chlorophyll. ▪ E.g., Carotenoids (xanthophylls, carotenes). Carotenoids absorb blue and green light. LIGHT REACTION PROCESS: Association with the Thylakoid Membranes ▪ Thylakoid membranes contain many photosystems. ▪ Consisting of a cluster of chlorophyll and accessory pigments molecules surrounded by various proteins. ▪ Two photosystems work together during light reaction; i. Photosystem II (PS II) ii. Photosystem I (PS I) ▪ Each type of photosystem has a different electron transport chain (ETC) located adjacent to it. ▪ Each ETC consists of a series of e- carrier molecules embedded in the thylakoid membrane. ▪ Within the thylakoid membrane, the overall path of e- is as follows; PS II → ETC II → PS I → ETC I → NADP+ 1. Light energy ejects e- from PS II. 1.1 The energy hops from one pigment to the next until it is funneled into the reaction center. 1.2 When it reaches the reaction center, the energy from light boosts an e- from one of the reaction center chlorophyll to the primary e- acceptor which captures the energized e-. Reaction Center 2. The PS II pulls for replacement e- from water molecules (H2 O), which breaks apart into O2 and H ions. 2.1 The O2 leave the cell as O2 2.2 H ions contribute to the H+ gradient that drives ATP synthesis. 2.3 For every 2 molecules of water that are split, 1 molecule of oxygen gas is produced. 3. The e- enter an ETC II in the thylakoid membrane. 4. Energy lost by the e- as they move through the ETC II causes H ions to be pumped from the stroma into the thylakoid compartment. 4.1 H ion gradient forms across the thylakoid membrane. 4.2 This will be used to generate ATP. 5. Meanwhile, light has also been striking the pigment molecules of PS I, as in PS II. 5.1 Light energy ejects e- from PS I. Replacement e- come from an ETC II. 6. The e- move through ETC I , then combine with NADP+ and H+, so NADPH (energy-carrier molecule) forms. 6.1 Picks up two energetic e-, along with one H+ 7. H ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane. 7.1 H ion flow causes ATP synthases to attach Pi to ADP, so ATP forms in the stroma. CALVIN CYCLE (C3 Pathway): Converting Chemical Energy to be Stored in Sugar Molecules ▪ Captures CO2 ▪ The ATP and NADPH synthesized during the light reaction are dissolved in the stroma. ▪ These energy carriers powered the synthesis of a simple, 3- C sugar (glyceraldehyde-3-phosphate or G3P) from CO2. ▪ The cycle is catalyzed by enzymes dissolved in stroma. ▪ For every 3 CO2 molecules captured by Calvin cycle, 1 molecule of G3P is produced. ▪ It is a cycle because it begins and ends with the same molecule, a 5-C sugar called ribulose biphosphate (RuBP). ▪ RuBP is recycled continuously. ▪ Calvin cycle is best understood if it divided into 3 parts (phases); i. Fixation ii. Reduction iii. Regeneration Rubisco 1) Fixation Phase (Fixation of CO2) ▪ The cycle begins when CO2 reacts with RuBP. ▪ This phase fixes C and produces molecules of 3- phosphoglycerate. ▪ The cycle uses the enzyme rubisco to combine each CO2 molecule with an RuBP molecule. Rubisco 2) Reduction Phase (Reduction of 3-phosphoglycerate to G3P) ▪ 3-Phosphoglycerate is phosphorylated by ATP, then reduced by electrons from NADPH. ▪ The product is the phosphorylated sugar, glyceraldehyde-3- phosphate (G3P). ▪ Some of the G3P that is synthesized is drawn off to manufacture glucose and fructose. Rubisco 3) Regeneration Phase (The regeneration of RuBP from G3P) ▪ 5 of the 6 G3P molecules are used to regenerate the RuBP necessary to repeat the cycle. ▪ The remaining 1 G3P molecule, which is the product of photosynthesis, exits the cycle. Rubisco ▪ Most of these are then used to form sucrose (a dissacharide storage molecule consisting of glucose linked to a fructose). OR ▪ Linked together in long chains to form starch (another storage molecule). OR ▪ Cellulose (a major component of plant cell walls). ▪ Most of the glucose synthesis from G3P and the subsequent complex molecules synthesis from glucose occurs outside the chloroplast. Rubisco  CALVIN CYCLE Summary 1. Fixation ▪ 3 RuBP + 3 CO2 6 3-Phosphoglycerate 2. Reduction ▪ 6 3-Phosphoglycerate + 6 ATP + 6 NADPH 5 G3P to Step 3 1 G3P to glucose/fructose 3. Regeneration ▪ 5 G3P + 3 ATP 3 RuBP (to Step 1) The Plant’s Dilemma: Compromising between obtaining adequate light and CO2 and Minimizing water loss through evaporation Plant leaf structure… ▪ Most leaves have a; i. large surface area for intercepting light. ii. a waterproof cuticle to reduce evaporation. iii. dynamic stomata. ▪ When there’s adequate water, the stomata open letting in CO2 and vice versa. ▪ Closing the stomata reduces evaporation but prevents CO2 intake for photosynthesis. Plant leaf structure… ▪ In hot, dry conditions, stomata remains closed much of the time. ▪ As a result, the amount of CO2 decreases and O2 increases. ▪ Unfortunately, rubisco is not very selective. Either CO2 or O2 can bind to the active site of rubisco and combine with RuBP ▪ When O2 (rather than CO2) combined with RuBP, a wasteful process called photorespiration occurs. So, what’s wrong with Photorespiration? ▪ Prevents the Calvin cycle from synthesizing sugar. – Effectively derailing photosynthesis. ▪ Plants (particularly fragile seedling) may die under these circumstances. – Unable to capture enough energy to meet their metabolic rate. The fact that, ▪ Reaction with CO2 during photosynthesis – RuBP + CO2 2 3-phosphoglycerate (used in Calvin Cycle) ▪ Reaction with O2 during photorespiration – RuBP + O2 1 3-phosphoglycerate (used in Calvin Cycle) + 1 2-phosphoglycolate (when processed, CO2 is released, and ATP is used) ▪ In hot, dry climates, alternative modes of incorporating C from CO2 have evolved in some plants. ▪ Allowing them to save water without shutting down photosynthesis. ▪ These plant species are categorized as either; i. C4 plants OR ii. CAM plants (Crassulacean Acid Metabolism) ▪ Both pathways involve several additional steps and consume more ATP than C3 plants. C4 Plants (Hatch-slack Pathway) ▪ Capture C and synthesize sugar in different cells. ▪ Mesophyll cells of C4 plants fix C using an enzyme called PEP carboxylase. – Highly selective for CO2 over O2 ▪ PEP carboxylase catalyzes a reaction between CO2 and a 3-C molecule called phosphoenolpyruvate (PEP). – Produces 4-C molecule oxaloacetate ▪ Oxaloacetate is rapidly converted into another 4-C molecule, malate which diffuses from the mesophyll cells into bundle sheath cells. – Malate effectively acts as a shuttle for CO2 ▪ Malate is broken down in the bundle sheath cells, forming the 3-C molecule pyruvate and releasing CO2 – Creates high CO2 concentration in the bundle sheath cells (10 x higher than atmospheric CO2). ▪ Then CO2 are fixed to generate sugar with little competition from O 2. ▪ The pyruvate is then actively transported back to the mesophyll cells. – More ATP energy is used to convert back into PEP, allowing the cycle to continue.  CHARACTERISTICS C3 PLANTS C4 PLANTS Usually, temperate Usually, tropical & Climatic adaptation regions subtropical regions CO2 fixation Produces 3-C molecule Produces 4-C molecule Photosynthetic cycle Mesophyll cells Bundle sheath cells site Elevated high CO2 concentration Regulated by diffusion concentrations Stomatal behaviour Open for longer periods Open for shorter periods Water usage efficiency Not very efficient Very efficient CO2 saturation High saturation Low saturation requirement Light intensity Low intensity High intensity Crassulacean Acid Metabolism (CAM) Plants ▪ Capture C and synthesize sugar at different times; i. C fixation at night ii. Sugar synthesis at day ▪ In contrast to C4 plants, CAM plants perform both activities in the same mesophyll cells. ▪ At night, when temperature are cooler and humidity is higher, the stomata of CAM plants open. ▪ Allowing CO2 to diffuse in, captured in mesophyll cell using the C4 pathway. ▪ The resulting oxaloacetate is converted to malate, which is then shuttled into the central vacuole (stored as malic acid) until daytime. ▪ During the day, when stomata are closed to conserve water, the malic acid leave the vacuole and ATP re-enters the cytoplasm as malate. ▪ The malate is broken down, forming pyruvate and releasing CO2 which enters the Calvin cycle to produce sugar. ▪ The pyruvate is then regenerated into PEP using ATP energy. Sugarcane Pineapple 1 1 C4 CO2 CO2 CAM Mesophyll Organic Organic Night cell acid acid CO2 2 CO2 2 Bundle- Day sheath Calvin Calvin cell Cycle Cycle Sugar Sugar (a) Spatial separation (b) Temporal separation of steps of steps Example of C3, C4 and CAM plants FACTORS THAT INFLUENCE PHOTOSYNTHESIS i. Light intensity, duration, and wavelength ii. Temperature iii. Water availability iv. Chlorophyll concentration v. Carbon dioxide concentration vi. Size and number of leaves vii. Numbers of stoma  FACTORS THAT INFLUENCE PHOTOSYNTHESIS The initial correlation between rate of photosynthesis and any factor is linear. At a certain stage, the reaction rate will stabilize. This is because one factor which is in shortest supply has become the limiting factor and prevents the rate from increasing. Rate-limiting step = step that is proceeding at the lowest rate a particular time.  Light Intensity ▪ An increase in the light intensity will increase the rate of photosynthesis until other limiting factors set in. ▪ When there are no limiting factors, the rate of photosynthesis is proportional to the light intensity. ▪ At low range of light intensity, when light intensity is a limiting factor, the rate of photosynthesis is also low. ▪ At high intensity range, light intensity becomes non-limiting. Some other factors become limiting.  Temperature ▪ An increase in temperature will increase the rate of photosynthesis. ▪ The rate of photosynthesis decreases when the temperature exceeds the optimum temperature (35° C). ▪ At low light intensity, any increase in temperature has no effect. The rate-limiting steps is in the light-dependent reaction. ▪ At high light intensity, temperature becomes the limiting factor. The rate limiting-steps is in the Calvin cycle. The enzyme becomes denatured and inactive.  CO2 Concentration ▪ The rate of photosynthesis is directly proportional to the CO2 concentration where light intensity and temperature are not limiting. ▪ At low and medium CO2 concentration, the rate-limiting step is the C fixation in the Calvin cycle. ▪ At high CO2 concentration, some other factors such as light intensity or temperature becomes limiting. ▪ In nature, CO2 concentration is usually the limiting factor.