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Photosynthesis

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

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Photosynthesis

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  1. Photosynthesis 6.2 Reactions of Photosynthesis

  2. 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 CO2 and 6 molecules of H2O • 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- glucose: energy storage for later use by cells

  3. 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.

  4. 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 CO2 • Stage 1 & 2: light dependent reactions requiring chlorophyll • Stage 3: light independent Calvin Cycle forming glucose (due to carbon fixation)

  5. 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 H2O into H+ and O2 gas - occurs in thylakoid lumen- 2 H2O molecules consumed for every 4 electrons transferred

  6. 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 compounds

  7. 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 Cycle)

  8. 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

  9. 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)

  10. Calvin Cycle & Carbon Fixation (Stage 3) • Final stage of photosynthesis- results in formation of high-energy organic molecules from CO2 (carbon fixation) • Calvin Cycle:- occurs in the STROMA of chloroplasts- CO2 must be readily available-utilizes both ATP and high-energy electrons on NADPH from light-dependent reactions- makes G3P (or PGAL), a sugar used to create glucose

  11. Calvin Cycle & Carbon Fixation (Stage 3) Steps of Calvin Cycle: • must cycle 6 times for 1 glucose to be produced • Atmospheric CO2 diffuses into chloroplast- carbon joins 5-carbon sugar RuBP (ribulosebiphosphate)- 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 (G3P or PGAL) and water • PGAL (G3P): some goes on to make glucose, rest converted to RuBP to continue cycle

  12. PGAL (G3P) • 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 RuBP and drive the cycle

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