Photosynthesis: Light and Dark Reactions

• Photosynthesis is the process used by plants, algae, and some bacteria to convert light energy into chemical energy
• This chemical energy is stored in the form of carbohydrates, such as sugars, which are synthesized from carbon dioxide and water
• Oxygen is released as a byproduct
• Photosynthesis is essential for life on Earth, as it produces most of the oxygen in the atmosphere and is the primary source of energy for most ecosystems

Overall Equation of Photosynthesis

• 6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ + 6O₂
• Six molecules of carbon dioxide plus six molecules of water, in the presence of light energy, produce one molecule of glucose and six molecules of oxygen

Two Stages of Photosynthesis

• Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle)
• The light-dependent reactions convert light energy into chemical energy in the form of ATP and NADPH
• The light-independent reactions use ATP and NADPH to convert carbon dioxide into glucose

Light-Dependent Reactions

• The light-dependent reactions take place in the thylakoid membranes of the chloroplasts
• Chlorophyll and other pigment molecules absorb light energy, which excites electrons to higher energy levels
• These energized electrons are passed along an electron transport chain, releasing energy that is used to generate ATP through chemiosmosis
• Water molecules are split during this process, releasing oxygen as a byproduct and providing electrons to replenish the chlorophyll
• The final electron acceptor in the electron transport chain is NADP+, which is reduced to NADPH

Light Harvesting

• Photosynthetic pigments are molecules that absorb specific wavelengths of visible light
• Chlorophyll a is the primary photosynthetic pigment in plants and algae, absorbing blue-violet and red light
• Chlorophyll b and carotenoids are accessory pigments that absorb other wavelengths of light and transfer the energy to chlorophyll a
• Pigments are arranged in light-harvesting complexes (also called antenna complexes) that maximize light capture

Photosystems

• Photosystems are protein complexes that contain chlorophyll and other pigment molecules
• Photosystem II (PSII) and Photosystem I (PSI) are the two types of photosystems involved in the light-dependent reactions
• PSII absorbs light energy and uses it to split water molecules, releasing oxygen and electrons
• PSI absorbs light energy and uses it to energize electrons for the production of NADPH

Electron Transport Chain

• The electron transport chain (ETC) is a series of protein complexes that transfer electrons from PSII to PSI
• As electrons move through the ETC, they release energy that is used to pump protons (H+) into the thylakoid lumen, creating a proton gradient
• This proton gradient drives the synthesis of ATP by ATP synthase, a process called chemiosmosis

ATP Synthase

• ATP synthase is an enzyme that uses the proton gradient across the thylakoid membrane to generate ATP
• Protons flow down their concentration gradient from the thylakoid lumen into the stroma through ATP synthase, which provides the energy for ATP synthesis
• This process is called photophosphorylation

Light-Independent Reactions (Calvin Cycle)

• The light-independent reactions, also known as the Calvin cycle, take place in the stroma of the chloroplasts
• Carbon dioxide is fixed, meaning it is incorporated into an organic molecule
• The Calvin cycle uses the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into glucose
• The cycle involves three main phases: carbon fixation, reduction, and regeneration

Carbon Fixation

• Carbon fixation is the first step of the Calvin cycle
• Carbon dioxide is combined with a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)
• The resulting six-carbon molecule is unstable and immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA)

Reduction

• Reduction is the second phase of the Calvin cycle
• Each molecule of 3-PGA is phosphorylated by ATP and then reduced by NADPH, producing glyceraldehyde-3-phosphate (G3P)
• G3P is a three-carbon sugar that can be used to synthesize glucose and other organic molecules

Regeneration

• Regeneration is the third phase of the Calvin cycle
• Some of the G3P molecules are used to regenerate RuBP, which is needed to continue the cycle
• This process requires ATP

Photorespiration

• Photorespiration is a process that occurs when RuBisCO binds to oxygen instead of carbon dioxide
• This occurs when carbon dioxide levels are low and oxygen levels are high, such as on hot, dry days when plants close their stomata to conserve water
• Photorespiration consumes ATP and releases carbon dioxide, reducing the efficiency of photosynthesis

C4 Photosynthesis

• C4 photosynthesis is an adaptation that helps plants overcome photorespiration in hot, dry environments
• C4 plants use an enzyme called PEP carboxylase to fix carbon dioxide in mesophyll cells, forming a four-carbon molecule called oxaloacetate
• Oxaloacetate is converted to malate, which is transported to bundle sheath cells
• In the bundle sheath cells, malate is decarboxylated, releasing carbon dioxide that is then fixed by RuBisCO in the Calvin cycle
• This concentrates carbon dioxide in the bundle sheath cells, reducing photorespiration

CAM Photosynthesis

• Crassulacean acid metabolism (CAM) photosynthesis is another adaptation to hot, dry environments
• CAM plants open their stomata at night, allowing carbon dioxide to enter and be fixed into organic acids
• During the day, the stomata are closed to conserve water, and the organic acids are decarboxylated, releasing carbon dioxide that is then fixed by RuBisCO in the Calvin cycle
• This temporal separation of carbon fixation and the Calvin cycle minimizes water loss and photorespiration

Factors Affecting Photosynthesis

• Light intensity: As light intensity increases, the rate of photosynthesis increases until it reaches a saturation point
• Carbon dioxide concentration: As carbon dioxide concentration increases, the rate of photosynthesis increases until it reaches a saturation point
• Temperature: Photosynthesis has an optimal temperature range. If the temperature is too low or too high, the rate of photosynthesis will decrease
• Water availability: Water is essential for photosynthesis. If water is limited, the rate of photosynthesis will decrease

Explore the process of photosynthesis, where plants, algae, and bacteria convert light energy into chemical energy. Learn about the crucial light-dependent and light-independent reactions (Calvin cycle) that produce carbohydrates and release oxygen, vital for life on Earth and most ecosystems.