Reactions of Photosynthesis

Photosynthetic Equation Light

Inside a Chloroplast –Photosynthesis occurs in the chloroplasts Chloroplasts contain pouch-like photosynthetic membrane called thylakoids which are arranged in stacks called grana Thylakoids contain clusters of chlorophyll and other pigments/proteins able to capture sun’s light Stroma – space surrounding thylakoids

Two stages of photosynthesis –1. Light-dependent – takes place within thylakoid membranes Makes ATP and NADPH for Light-Independent Reaction –2. Light-independent – aka Calvin Cycle – takes place in the stroma – a region outside thylakoid membrane Makes glucose

NADPH – “the hot coal carrier” Sunlight excites electrons in chlorophyll and gain energy. A special “carrier” is needed – an electron carrier is a compound that can accept a pair of high-energy electrons and transfer them along with most of their energy to another molecule NADP+ - electron carrier (nicotinamide adenine dinucleotide phosphate) that accepts and holds 2 high-energy electrons and a hydrogen H+ - converts NADP+ into NADPH NADPH – then carries electrons to chemical reactions elsewhere in the cell for building a variety of molecules Summary: NADP+ accepts 2 electrons and H+, turning it into NADPH, and can now move those electrons elsewhere in the cell to help build molecules.

Chloroplast Light O2O2 Sugars CO 2 Light- Dependent Reactions Calvin Cycle NADPH ATP ADP + P NADP + Chloroplast Photosynthesis: An Overview

1. Light-dependent Reaction Occurs in the thylakoids Requires light to produce ATP and NADPH (used for Light- Independent Reaction) Produce oxygen gas and convert ADP and NADP+ into energy carriers ATP and NADPH Converts light energy into chemical energy (ATP and NADPH) What goes in: H2O Light What comes out: ATP NADPH O2

Steps in Light-dependent Reaction Use light and H2O for making ATP and NADPH for Calvin Cycle (aka Light- independent reaction) A) Pigments in Photosynthesis II absorb light (named II because it was discovered after Photosynthesis I) – light excites electrons and are passed along to the electron transport chain. Thylakoid membranes provide new electrons to chlorophyll to replace them. New electrons come from water molecules which are broken down into 2 electrons, 2 H+ ions, and 1 oxygen atom (oxygen eventually released) Summary: Light absorbed by pigments = excited electrons. Electrons passed to chain. H2O breaks down and replenishes electrons. O2 released, H+ kept in thlykoid to fuel ATP formation.

Steps in Light-dependent Reaction (continued) B) High-energy electrons move through electron transport chain from Photosynthesis II to Photosynthesis I. Energy from electrons used in the chain to transport H+ ions from stroma to thylakoid Summary: Electrons move down chain; move H+ into thylakoid (need H+ to fuel ATP formation)

Steps in Light-dependent Reaction (continued) C) Pigments in Photosynthesis I use energy from light to re- energize electrons. NADP+ then picks up these electrons and a H+ ion to become NADPH Summary: Pigments use sunlight to re- energize electrons; NADPH formed

Steps in Light-dependent Reaction (continued) D) Because H+ ions are released during water-splitting and electron transport, the inside of the thylakoid becomes positively charged and the outside negatively charged = energy for making ATP Summary: So many H+ inside thylakoid creates charge difference = make ATP

Steps in Light-dependent Reaction (continued) E) H+ ions pass through thylakoid membrane via a protein – ATP synthase – and binds ADP and a phosphate group to produce ATP Summary: ATP synthase needed to move H+ through membrane, making ADP bind to a phosphate, creating ATP

Hydrogen Ion Movement Photosystem II Inner Thylakoid Space Thylakoid Membrane Stroma ATP synthase Electron Transport Chain Photosystem IATP Formation Chloroplast Light-Dependent Reaction