Photosynthesis Process Overview

Overview

This lecture explains the process of photosynthesis, detailing how plants convert solar energy into glucose and oxygen through light-dependent and light-independent reactions within the chloroplast.

Light and Photosynthesis

• Sun emits energy as waves, with visible light (ROYGBIV) powering photosynthesis.
• White light is a combination of all visible colors, shown by a prism separating light into a rainbow.
• Colors are seen as reflected light; plants appear green because they reflect green and absorb other wavelengths.
• Photosynthesis is most efficient with blue (≈400 nm) and red (≈700 nm) light; green light is mostly reflected.
• The general formula for photosynthesis: carbon dioxide + water + sunlight → glucose + oxygen.

Photosynthetic Organisms and Structures

• Autotrophs (plants, algae, phytoplankton, kelp, cyanobacteria) perform photosynthesis.
• Photosynthesis is an endergonic reaction requiring input of solar energy.
• Leaf stomata are pores that exchange gases: intake CO₂ and release O₂.
• Chloroplasts are the cell structures where photosynthesis occurs.

Stages of Photosynthesis

Light Dependent Reactions

• Occur in thylakoid membranes within chloroplasts.
• Photosystems I and II contain chlorophyll for energy capture and transfer.
• Sunlight excites electrons in photosystem II, starting an electron transport chain.
• Water is split to replenish electrons, releasing oxygen as a byproduct.
• Energy moves hydrogen ions into the thylakoid, creating a concentration gradient.
• ATP synthase uses hydrogen ion flow to convert ADP and phosphate into ATP.
• NADPH is formed as a carrier molecule for hydrogen.
• Products: oxygen (waste), ATP, and NADPH (energy carriers); no glucose formed yet.

Light Independent Reactions (Calvin Cycle)

• Occur in the stroma of the chloroplast.
• RuBP (ribulose biphosphate) combines with CO₂ to make a short-lived 6-carbon compound.
• Energy from ATP and NADPH breaks this into two 3-carbon molecules (phosphoglycerate).
• Some 3-carbon compounds form glucose; others regenerate RuBP to continue the cycle.
• ATP and NADPH are recycled after use.

Key Terms & Definitions

• Photosynthesis — process converting solar energy, CO₂, and water into glucose and oxygen.
• Autotroph — organism that produces its own food via photosynthesis.
• Stomata — leaf pores that exchange gases.
• Chloroplast — organelle where photosynthesis happens.
• Thylakoid — membrane sac in chloroplast where light reactions occur.
• Granum — stack of thylakoids.
• Photosystem — protein complex containing chlorophyll for light absorption.
• ATP (Adenosine Triphosphate) — energy-carrying molecule made in light reactions.
• NADPH — hydrogen carrier molecule made in light reactions.
• RuBP (Ribulose Biphosphate) — 5-carbon compound involved in carbon fixation.
• Calvin Cycle — series of light-independent reactions generating glucose.

Action Items / Next Steps

• Review the photosynthesis formula and each step in the light-dependent and light-independent reactions.
• Be prepared to answer essay questions on photosynthesis in the next class.