Photosynthesis: Light & Dark Reactions

• Photosynthesis is how plants, algae, and some bacteria transform light energy into chemical energy.
• The process of photosynthesis sustains life on Earth via the production of oxygen and organic compounds.

Overview of Photosynthesis

• Photosynthesis uses sunlight, water, and carbon dioxide to create carbohydrates (sugars) and oxygen.
• The equation: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
• Photosynthesis occurs in two stages: light-dependent reactions and light-independent reactions (Calvin cycle).

Light-Dependent Reactions

• Light-dependent reactions take place in the thylakoid membranes of chloroplasts.
• Chlorophyll and other pigments capture light energy and convert it into ATP and NADPH.
• Water molecules are split during photolysis and release oxygen.
• Photosystems II and I are involved in the light-dependent reactions.
• Electrons travel along an electron transport chain which creates a proton gradient that synthesizes ATP via chemiosmosis.
• ATP and NADPH are used in the light-independent reactions.

Light-Independent Reactions (Calvin Cycle)

• Light-independent reactions happen in the stroma of the chloroplasts.
• Carbon dioxide is captured and modified into glyceraldehyde-3-phosphate (G3P).
• The Calvin cycle's phases: carbon fixation, reduction, and regeneration.
• During carbon fixation, CO2 combines with ribulose-1,5-bisphosphate (RuBP), using the enzyme RuBisCO.
• During reduction, ATP and NADPH convert fixed carbon into G3P.
• During regeneration, RuBP is regenerated to continue the cycle.
• For every six CO2 molecules fixed, one molecule of glucose is produced.

Chloroplasts

• Photosynthesis takes place in chloroplasts, within plant cells.
• Chloroplasts have thylakoids in stacks called grana.
• The stroma is the fluid surrounding the thylakoids.
• Chlorophyll is in the thylakoid membranes.

Pigments

• Chlorophyll a, the main pigment, absorbs blue-violet and red light.
• Chlorophyll b and carotenoids broaden the range of light that can be used in photosynthesis.
• Pigments absorb some wavelengths of light, while reflecting or transmitting others, creating plant color.

Factors Affecting Photosynthesis

• Light intensity: Photosynthesis rate increases with light intensity until reaching a saturation point.
• Carbon dioxide concentration: Photosynthesis rate generally increases with CO2 concentration until a certain point.
• Temperature: Photosynthesis thrives in an optimal temperature range, decreasing if temperatures are too high or low.
• Water availability: Limited water can impede photosynthesis by closing stomata, which reduces CO2 entry into the leaf.
• Nutrient availability: Nutrients like nitrogen and magnesium are needed for chlorophyll synthesis and enzyme function.

Photorespiration

• Photorespiration occurs when RuBisCO binds oxygen instead of carbon dioxide.
• It reduces the effectiveness of photosynthesis, specifically in high temperature and low CO2 conditions.
• Photorespiration uses ATP and releases CO2.

C4 Photosynthesis

• C4 photosynthesis minimizes photorespiration in hot, dry environments.
• C4 plants initially fix CO2 into a four-carbon compound in mesophyll cells.
• This compound is transported to bundle sheath cells, where CO2 is released and enters the Calvin cycle.
• C4 photosynthesis concentrates CO2 around RuBisCO which reduces photorespiration.

CAM Photosynthesis

• CAM (Crassulacean Acid Metabolism) is an adaptation to arid conditions.
• CAM plants open their stomata at night to fix CO2, then store it as an organic acid.
• Stomata close during the day, and stored CO2 is released for use in the Calvin cycle.
• CAM photosynthesis helps plants conserve water by reducing transpiration during the hottest part of the day.

Significance of Photosynthesis

• Photosynthesis is the base of food chains, and provides energy and organic matter.
• It produces oxygen that is essential for aerobic respiration in animals.
• Photosynthesis regulates climate by removing carbon dioxide.
• Fossil fuels come from ancient photosynthesis.
• Photosynthesis is a key part of the global carbon cycle.

Adaptations for Photosynthesis

• Leaf structure maximizes light capture and gas exchange, with a large surface area and stomata for CO2 uptake.
• Specialized cells, like mesophyll cells, contain chloroplasts and perform photosynthesis.
• Vascular tissues move water and nutrients to the leaves and transport sugars.

Chemiosmosis

• Chemiosmosis involves proton movement across the thylakoid membrane.
• The electron transport chain pumps protons from the stroma into the thylakoid lumen.
• This creates a proton gradient.
• Protons flow down the gradient through ATP synthase, which synthesizes ATP.

Role of Water

• Water provides electrons to replace those lost by chlorophyll during light-dependent reactions.
• Oxygen is released as water splits during photolysis.
• Water maintains turgor pressure, which is required for stomatal opening and CO2 uptake.