Overview
This lecture explains the process of photosynthesis, focusing on its chemical reactions, the structures involved, and the importance of photosynthesis in sustaining life on Earth.
Photosynthesis Basics
- Photosynthesis converts light energy from the sun into chemical energy in plants, algae, and some bacteria.
- Organisms use water and carbon dioxide to produce carbohydrates and release oxygen.
- Almost all life depends on the energy and organic molecules created by photosynthetic organisms.
Leaf and Chloroplast Structure
- Leaves contain mesophyll, epidermal cells, and vascular bundles; most photosynthesis occurs in chloroplast-rich mesophyll cells.
- Chloroplasts are organelles where photosynthesis takes place, containing thylakoids (membrane discs) and stroma (watery fluid).
- Carbon dioxide enters and oxygen exits leaves through stomata; organic molecules are transported by vascular bundles.
Stages of Photosynthesis
- Photosynthesis involves two main reactions: the light reactions and the Calvin cycle.
- Light reactions occur in thylakoid membranes; the Calvin cycle takes place in the stroma.
Light Reactions
- Light reactions use light energy to split water, releasing oxygen and moving electrons through photosystem II and I.
- Electrons are transported by the electron transport chain, producing ATP and NADPH.
- ATP synthase uses a proton gradient to form ATP from ADP and phosphate.
- NADPH and ATP store energy for use in the Calvin cycle.
Calvin Cycle
- The Calvin cycle consists of three phases: fixation (COâ‚‚ binds to RuBP), reduction (3-PGA to G3P using NADPH and ATP), and regeneration (G3P to RuBP).
- Enzyme rubisco catalyzes the fixation of carbon.
- One G3P molecule is produced per three COâ‚‚ molecules and can be used for other organic molecules.
- Other G3P molecules regenerate RuBP; cycle relies on energy from light reactions.
Importance of Photosynthesis
- Photosynthesis produces carbohydrates that fuel plant growth and provide oxygen for the atmosphere.
- Plants store energy as starch or transport sucrose to other parts of the plant.
- Globally, photosynthesis generates approximately 150 billion metric tons of carbohydrate per year.
Key Terms & Definitions
- Photosynthesis — process of converting light energy to chemical energy in plants, algae, and some bacteria.
- Chloroplast — plant cell organelle where photosynthesis occurs.
- Thylakoid — membrane-bound disc in chloroplasts where light reactions take place.
- Stroma — fluid surrounding thylakoids in a chloroplast.
- Stomata — pores in leaf surfaces for gas exchange.
- Photosystem — protein-pigment complex that absorbs light and transfers electrons.
- ATP (Adenosine Triphosphate) — energy-carrying molecule produced during light reactions.
- NADPH — reduced electron carrier produced during light reactions.
- Calvin Cycle — set of reactions that use ATP and NADPH to fix carbon and synthesize sugars.
- Rubisco — enzyme that catalyzes carbon fixation in the Calvin cycle.
- G3P (Glyceraldehyde-3-phosphate) — three-carbon sugar produced by the Calvin cycle.
Action Items / Next Steps
- Review the stages of the light reactions and Calvin cycle.
- Study the structure and function of chloroplast components.
- Prepare notes on the role of photosynthesis in the global carbon and oxygen cycles.