Understanding Photosynthesis

• Photosynthesis converts light energy into chemical energy, fueling organism activities through cellular respiration.
• Carbohydrate molecules, such as sugars, store this chemical energy; they are synthesized from carbon dioxide and water.
• Plants, algae, and many bacteria species perform photosynthesis.
• Photoautotrophs create their own food through photosynthesis.
• Photosynthesis maintains Earth's atmospheric oxygen and provides most of the energy needed for life.

Overall Reaction

• The overall photosynthesis equation is 6CO2 + 6H2O + Light energy → C6H12O6 + 6O2.
• The equation represents the net process, which occurs in multiple steps.

Two Stages of Photosynthesis

• Photosynthesis has two main stages: light-dependent reactions and light-independent reactions (Calvin cycle).

Light-Dependent Reactions

• Light-dependent reactions occur in the thylakoid membranes of the chloroplast.
• Chloroplasts, organelles within plant and algae cells, perform photosynthesis.
• Thylakoids are flattened sacs inside chloroplasts, arranged in stacks called grana.
• Chlorophyll and other pigment molecules in the thylakoid membranes absorb light energy.
• Water molecules (H2O) split into protons (H+), electrons, and oxygen (O2) using light energy.
• Oxygen is released as a byproduct.
• Electrons pass along an electron transport chain, releasing energy to pump protons (H+) into the thylakoid lumen, to create a proton gradient
• ATP is generated through chemiosmosis (photophosphorylation) using the proton gradient's potential energy.
• NADP+ reduces to NADPH, another energy-carrying molecule.

Light-Independent Reactions (Calvin Cycle)

• Light-independent reactions (Calvin cycle), occur in the stroma of the chloroplast.
• The stroma is the fluid-filled space surrounding the thylakoids.
• ATP and NADPH from light-dependent reactions fix carbon dioxide (CO2) and produce glucose (C6H12O6) during the Calvin cycle.
• Carbon fixation involves incorporating CO2 into an organic molecule.
• The Calvin cycle's phases include carbon fixation, reduction, and regeneration.
• During carbon fixation, CO2 is added to ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon compound, which splits into two molecules of 3-phosphoglycerate (3-PGA).
• During the reduction phase, ATP and NADPH convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
• Some G3P produces glucose and other organic molecules, while the rest regenerates RuBP.
• ATP is required for RuBP regeneration.

Pigments

• Pigments absorb certain light wavelengths and reflect others.
• Chlorophyll is the primary photosynthesis pigment.
• Chlorophyll absorbs red and blue light and reflects green light, hence plants' green appearance.
• Chlorophyll types include chlorophyll a and chlorophyll b.
• Carotenoids absorb blue and green light and reflect yellow and orange light.
• Carotenoids play a role in light harvesting and photoprotection.

Factors Affecting Photosynthesis

• Light intensity: Photosynthesis rate generally rises with light intensity until saturation.
• Carbon dioxide concentration: Photosynthesis rate generally rises with carbon dioxide concentration until saturation.
• Temperature: Photosynthesis has an optimal temperature range; rates decrease if it is too low or high.
• Water availability: Limited water decreases photosynthesis rate.

Photorespiration

• Photorespiration occurs when RuBisCO binds to oxygen (O2) instead of carbon dioxide (CO2).
• The process consumes energy and releases carbon dioxide, reducing photosynthesis efficiency.
• Photorespiration is more likely at high temperatures and low carbon dioxide levels.

Adaptations to Minimize Photorespiration

• C4 and CAM plants have evolved adaptations to minimize photorespiration.
• C4 plants:
  • Use a different enzyme to fix carbon dioxide in mesophyll cells, forming a four-carbon compound.
  • This compound is transported to bundle sheath cells, where it is broken down to release carbon dioxide, which is then fixed by RuBisCO in the Calvin cycle.
  • This concentrates carbon dioxide in bundle sheath cells, reducing photorespiration.
• CAM plants:
  • Open stomata at night and close them during the day.
  • Take up carbon dioxide at night and fix it into organic acids.
  • Break down organic acids during the day to release carbon dioxide, then fixed with RuBisCO in the Calvin cycle.
  • This conserves water and reduces photorespiration in hot, dry environments.

Importance of Photosynthesis

• Photosynthesis is the foundation of most food chains and food webs on Earth.
• Heterotrophic organisms need the energy and organic molecules it provides to survive.
• Photosynthesis produces oxygen, essential for aerobic respiration.
• Photosynthesis regulates Earth's climate by removing atmospheric carbon dioxide.

Explore the process of photosynthesis, used by plants and organisms to convert light energy into chemical energy. Learn about the two main stages: light-dependent reactions and the light-independent reactions. Discover how photosynthetic organisms, or photoautotrophs, create their own food and maintain Earth's oxygen.