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10/1 lecture

Photosynthesis Overview - Pigments and Chloroplasts

  • Eukaryotic microbes have chloroplasts, which are complex, membrane-bound organelles. They are characterized by an outer membrane and an inner membrane, which enclose a fluid-filled space called the stroma. Within the stroma lies an internal system of interconnected membranes called thylakoids.

  • Thylakoids are often arranged into stacks known as grana, where the light-absorbing photopigments are embedded.

  • Prokaryotic cells, lacking membrane-bound organelles, perform photosynthesis using specialized membrane systems such as chlorosomes (found in green sulfur bacteria) or invaginations of the plasma membrane, which house the necessary photopigments.

  • Photopigments

  • These molecules absorb solar energy at specific wavelengths, enabling the capture of light for photosynthesis. Different pigments absorb different regions of the light spectrum, allowing for more efficient and broader utilization of available light.

  • Chlorophyll a is the primary photosynthetic pigment, absorbing light most strongly in the blue-violet (approximately 430 nm) and red (approximately 662 nm) regions of the electromagnetic spectrum, while reflecting green light, which gives plants their characteristic color. Accessory pigments, such as chlorophyll b and carotenoids (e.g., carotenes and xanthophylls), absorb light at different wavelengths and then transfer this captured energy to chlorophyll a in the reaction center.

Light-Dependent Reactions of Photosynthesis

  • Basic Concept

  • These reactions are directly dependent on the presence of sunlight and convert light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). They occur in the thylakoid membranes of chloroplasts.

  • Process Description

  • Light energy is initially harvested by complexes of antenna molecules (containing accessory pigments) and funneled to a reaction center (a specialized chlorophyll a molecule). In Photosystem II, this reaction center is known as P680, referring to its optimal absorption wavelength of 680 nm.

  • When a photon of light strikes the reaction center chlorophyll (P680), an electron is excited to a higher energy level and is then immediately transferred to a primary electron acceptor, leaving P680 in an oxidized, electron-deficient state (P680^+).

  • Water's Role

  • To replenish the lost electron in P680^+ and maintain the electron flow, water molecules are split. This process, called photolysis, occurs on the lumen side of the thylakoid membrane.

  • The reaction is: 2H2O \rightarrow 4e^- + 4H^+ + O2

  • This photolysis provides electrons to Photosystem II (PS II), generates protons (H^+) that accumulate in the thylakoid lumen, and releases molecular oxygen (O_2) as a gaseous byproduct into the atmosphere.

  • Electron Transport Chain

  • The electrons from P680 are then passed sequentially down a series of electron carriers, forming an electron transport chain (ETC) embedded within the thylakoid membrane. This chain includes molecules like plastoquinone (PQ), the cytochrome b6f complex, and plastocyanin (PC).

  • As electrons move through specific protein complexes in the ETC (notably the cytochrome b6f complex), these complexes act as proton pumps, actively translocating protons (H^+) from the chloroplast stroma into the thylakoid lumen. This creates a substantial electrochemical proton gradient, with a high concentration of protons in the lumen.

  • ATP Synthesis

  • The significant accumulation of protons in the thylakoid lumen generates a strong proton motive force. Protons then flow back down their concentration gradient, from the lumen to the stroma, through the channel of ATP synthase, a transmembrane enzyme complex. This facilitated diffusion of protons powers the rotation of parts of the ATP synthase, which then drives the phosphorylation of ADP (adenosine diphosphate) and inorganic phosphate (Pi) to form ATP, a process called chemiosmotic photophosphorylation. This ATP provides energy for the Calvin Cycle.

Photosystems

  • Photosystem II (PS II)

  • Contains the reaction center P680 and initiates the non-cyclic (linear) electron flow of the light-dependent reactions.

  • It absorbs light energy, excites electrons in P680, and passes them to a primary acceptor. P680 then regains its electrons by splitting water molecules (H2O), releasing O2 and protons into the lumen and contributing electrons to the ETC.

  • Photosystem I (PS I)

  • Contains the reaction center P700, which has an absorption maximum of 700 nm. It absorbs light to re-energize electrons that have traveled from PS II via the first electron transport chain. These re-energized electrons move to a second, shorter electron transport chain.

  • In this second ETC, the electrons are used to reduce NADP^+ to NADPH. This reduction is catalyzed by the enzyme NADP^+ reductase, primarily in the stroma. NADPH is a crucial electron carrier, providing the reducing power necessary for the synthesis of sugars in the Calvin Cycle.

The Z Scheme

  • This is a conceptual diagram that illustrates the changes in redox potential (energy levels) of electron carriers during the light-dependent reactions. It gets its name from its 'Z' like shape.

  • The Z scheme depicts how electrons are initially excited in PS II (energy increase), then fall in energy as they pass through the first ETC (energy decrease to pump protons), are re-excited in PS I (energy again increases due to light absorption), and finally fall to a lower energy level to reduce NADP^+ to NADPH.

Light-Independent Reactions (Calvin Cycle)

  • Carbon Fixation

  • Also known as the Calvin-Benson Cycle, this metabolic pathway incorporates atmospheric carbon dioxide (CO_2) into pre-existing organic molecules. It primarily occurs in the chloroplast stroma.

  • This anabolic pathway the primary mechanism for carbon fixation in C_3 plants and occurs in three main phases: carboxylation, reduction, and regeneration.

  • Phases of the Calvin Cycle

  1. Carboxylation Phase

    • CO_2 enters plant cells through small pores called stomata and diffuses into the chloroplast stroma. Here, it is enzymatically fixed by binding to an existing five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), by the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).

    • This reaction forms an unstable six-carbon intermediate, which immediately hydrolyzes and splits into two molecules of 3-phosphoglycerate (PGA), a stable three-carbon compound. Rubisco is the most abundant enzyme on Earth.

  2. Reduction Phase

    • Each molecule of PGA is then phosphorylated by ATP (derived from the light-dependent reactions) to form 1,3-bisphosphoglycerate. This compound is subsequently reduced by NADPH (also from light-dependent reactions) to form glyceraldehyde-3-phosphate (G3P). For every 6 molecules of G3P produced, 6 ATP and 6 NADPH molecules are consumed per turn of the cycle to reduce 3 CO_2 molecules.

    • G3P is an energy-rich three-carbon sugar that serves as the building block for glucose and other organic molecules.

  3. Regeneration Phase

    • For every three turns of the Calvin cycle, which effectively fixes three CO_2 molecules into organic matter, six molecules of G3P are produced. Only one of these G3P molecules exits the cycle to be converted into glucose and other carbohydrates (like sucrose or starch). The remaining five G3P molecules are complexly rearranged, utilizing an additional 3 molecules of ATP, to regenerate three molecules of RuBP, allowing the cycle to continue. To synthesize one molecule of glucose (a 6-carbon sugar), the Calvin cycle must turn six times, consuming 18 ATP and 12 NADPH.

Anabolism and Energy Production

  • Role of ATP

  • ATP (adenosine triphosphate) serves as the universal energy currency for cells. Its hydrolysis to ADP + Pi (releasing 7.3 kcal/mol) provides the immediate energy required for anabolic processes – the synthesis of larger, complex organic molecules from smaller units. This includes macromolecule synthesis, active transport, and mechanical work.

  • Cellular metabolism constantly involves the hydrolysis of ATP, generated from catabolism (the breakdown of substances like glucose), to power anabolic reactions, facilitate cell growth, and maintain overall cellular functions.

Synthesis of Organic Molecules

  • Precursor Metabolites

  • These are small, intermediate molecules within metabolic pathways that can be converted into various larger macromolecules essential for cellular life. They are central to both catabolism and anabolism (amphibolic pathways).

  • For example, pyruvate (a three-carbon product of glycolysis) can be converted into acetyl-CoA for lipid synthesis (fatty acids and steroids), or it can enter biosynthetic pathways for the formation of certain amino acids necessary for protein synthesis.

  • G3P, directly produced from photosynthesis, can be readily used to synthesize glucose, which can then be polymerized into starch (storage) or cellulose (structural), or combined with fructose to form sucrose (transport sugar).

Construction of Peptidoglycan

  • Building Blocks

  • The synthesis of peptidoglycan, a unique polymer forming the rigid layer of bacterial cell walls, begins in the cytoplasm. UDP-glucose is converted to UDP-N-acetylglucosamine (UDP-NAG) and UDP-N-acetylmuramic acid (UDP-NAM), which are activated sugar units. These two modified sugars are the fundamental monomers.

  • The peptidoglycan structure consists of long chains of alternating \beta-1,4-linked N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars. Short peptide chains (typically 4-5 amino acids long) are attached to the NAM residues. These peptide side chains are then cross-linked to peptides on adjacent glycan strands to form a strong, mesh-like structure.

  • Role of Specific Enzymes

  • Enzymes such as glycosyltransferases (also called transglycosylases) are responsible for forming the \beta-1,4 glycosidic bonds between successive NAG and NAM units, elongating the glycan backbone.

  • Transpeptidases, also known as penicillin-binding proteins (PBPs), catalyze the crucial formation of peptide cross-links between adjacent peptidoglycan strands. This cross-linking provides the mechanical strength and rigidity to the cell wall. Flipase enzymes (specifically undecaprenyl phosphate flippase) are crucial for transporting the newly synthesized peptidoglycan precursors (lipid II) from the cytoplasm, across the cell membrane, to the periplasmic space (or outside the cell in Gram-positive bacteria) where they are incorporated into the existing cell wall.

  • Antibiotic Resistance

  • Many antibiotics target peptidoglycan synthesis. For instance, vancomycin inhibits peptidoglycan formation by binding tightly to the D-Ala-D-Ala terminus of the peptidoglycan precursor (lipid II). This binding sterically hinders the transpeptidases (PBPs) from accessing their substrate and catalyzing the necessary peptide cross-links, preventing cell wall assembly. This results in a weakened cell wall, making the bacterium susceptible to osmotic lysis.

Additional Pathways

  • Pentose Phosphate Pathway (PPP)

  • Also known as the hexose monophosphate shunt, this pathway is a metabolic route that runs parallel to glycolysis. It is crucial for generating several important products:

    • Ribulose-5-phosphate, which is a direct precursor for the synthesis of nucleotides (the building blocks of DNA and RNA) and other five-carbon sugars required for various biosynthetic processes (e.g., in coenzymes like FAD and NAD+).

    • NADPH, which is vital for two primary cellular functions: 1) supplying reducing power for reductive biosynthesis reactions (e.g., the synthesis of fatty acids, steroids, and cholesterol) and 2) protecting cells from oxidative stress by reducing oxidized glutathione, a key component of the cell's antioxidant defense system.

Conclusion

  • Photosynthesis, as the foundation of energy capture, and the broader scope of cellular metabolism, involving anabolic and catabolic interconversions, are fundamental processes for energy conversion, the synthesis of essential macromolecules, and maintaining cellular life.

  • A comprehensive understanding of these interconnected pathways, including their biochemical reactions, regulatory mechanisms, and enzymatic components, is vital for grasping overall cellular function, growth, and adaptation in diverse organisms, from microbes to complex eukaryotes.