Photosynthesis 6 2 Reactions of Photosynthesis Photosynthesis Purpose

Photosynthesis 6. 2 Reactions of Photosynthesis

Photosynthesis �Purpose: to use photons from sunlight to create glucose - solar energy converted to usable chemical energy �Occurs in the thylakoid membranes within chloroplasts �Requires 6 molecules of CO 2 and 6 molecules of H 2 O �During photosynthesis, many complex chemical reactions form intermediates and final energy-rich molecules - ATP: principal energy-supply molecule for living cells (immediate) - NADPH: electron donor in energy transfers

ATP and NADPH �Used by ALL living things as an immediate source of energy for cellular functions �Formed by addition of phosphate group (Pi) to a molecule of lower energy ADP. �NADP+ accepts one hydrogen atom and 2 electrons to form NADPH. - these electrons can then be donated to other molecules and NADPH becomes NADP+ again.

Reactions �Stage 1: capturing solar energy and transferring it to electrons �Stage 2: using captured solar energy to make ATP and to transfer high-energy electrons to NADP+; creates NADPH (electron carrier) �Stage 3: energy stored in ATP and electrons carried by NADPH used to form glucose from CO 2 �Stage 1 & 2: light dependent reactions requiring chlorophyll �Stage 3: light independent Calvin Cycle forming glucose (due to carbon fixation)

Capturing Solar Energy (Stage 1) �Photosystems: clusters of chlorophyll and pigment molecules - found on thylakoid membranes - 2 distinct but interconnected photosystems (I & II) �Electrons in chlorophyll capture and absorb photons - electron now has high amount of energy (excited state) �High-energy electrons move along electron transport chain - NEED TO BE REPLACED! �Photolysis: solar energy used to split H 2 O into H+ and O 2 gas - occurs in thylakoid lumen - 2 H 2 O molecules consumed for every 4 electrons

Electron Transfer & ATP Synthesis (Stage 2) �Electron Transport Chain: excited electrons are passed along the chain, slowly releasing energy in each step - some of this energy is captured to make ATP - electrons eventually rejoin H+ to form new

Electron Transfer & ATP Synthesis (Stage 2) � Photolysis: splits water, electrons move into Photosystem II � Electrons then passed along ETC toward the inside of the thylakoid membrane. - releases energy - draws H+ ions across membrane toward lumen - concentration of H+ ions in lumen increases creating buildup of positive charge � Electrons enter Photosystem I - replace electrons energized by light � Energized electrons move through chemical complexes to NADP+ - accepts 2 high- energy electrons and an H+ ion; becomes NADPH - NADPH used in light-independent reaction (Calvin

Chemiosmosis �H+ ions pulled across membrane into lumen - creates concentration gradient; H+ cannot escape unless through ATP synthase complexes - movement through releases energy �Combines ADP with Pi. . creates. . ATP! �Recall: energy stored in H+ ion gradient derived from energy of electrons energized in Photosystem II

Review Stage 2 Light-dependent reactions: �Consume water �Form ATP, NADPH, oxygen �ATP and NADPH used in carbon fixation - light-independent Calvin Cycle (Stage 3)

Calvin Cycle & Carbon Fixation (Stage 3) �Final stage of photosynthesis - results in formation of high-energy organic molecules from CO 2 (carbon fixation) �Calvin Cycle: - occurs in the STROMA of chloroplasts - CO 2 must be readily available -utilizes both ATP and high-energy electrons on NADPH from light-dependent reactions - makes G 3 P (or PGAL), a sugar used to create glucose

Calvin Cycle & Carbon Fixation (Stage 3) Steps of Calvin Cycle: �must cycle 6 times for 1 glucose to be produced �Atmospheric CO 2 diffuses into chloroplast - carbon joins 5 -carbon sugar Ru. BP (ribulose biphosphate) - forms unstable 6 -carbon sugar �Splits into TWO 3 -carbon sugars PGA (phosphoglyceric acid) �PGA’s use energy of ATP to strip H from NADPH - makes a stable 3 -carbon organic compound (G 3 P or PGAL) and water �PGAL (G 3 P): some goes on to make glucose, rest converted to Ru. BP to continue cycle

PGAL (G 3 P) �The PGAL that goes on to make glucose must be united with another molecule of PGAL in order for glucose to be formed. �PGAL therefore has 3 important functions: - used for energy to fuel the light independent reaction - some can be converted to glucose for energy storage - a portion is used to replenish Ru. BP and drive the cycle