Reactions of Photosynthesis

Photosynthetic Equation Light

Two stages of photosynthesis –1. Light Reactions – aka light-dependent reactions - takes place within thylakoid membranes Makes ATP and NADPH for Light-Independent Reaction –2. Calvin Cycle – aka light-independent reactions – 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 + from water- 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 + from water, 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 Reactions Calvin Cycle NADPH ATP ADP + P NADP + Chloroplast Photosynthesis: An Overview

1. Light Reactions (aka Light- Dependent Reaction) Occurs in the thylakoids Requires light to produce ATP and NADPH (used for Calvin Cycle) 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: H 2 O Light What comes out: ATP NADPH O 2

Steps in Light Reactions Use light and H 2 O for making ATP and NADPH for Calvin Cycle 1) Chlorophyll in Water-Splitting Photosystem absorb light – 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. H 2 O breaks down and replenishes electrons. O 2 released, H + kept in thylakoid to fuel ATP formation.

Steps in Light Reactions (continued) 2) High-energy electrons move through electron transport chain from Water-Splitting Photosystem to NADPH-Producing Photosystem. Energy from electrons used in the chain to transport H + ions from stroma to thylakoid to make ATP using ATP synthase protein Summary: Electrons move down chain; move H + into thylakoid to help fuel the process of making ATP using ATP Synthase protein

Steps in Light Reactions (continued) 3) Chlorophyll in uses energy from light to re-energize electrons. NADP + then picks up these electrons and a H + ion to become NADPH Summary: Chorophyll uses sunlight to re- energize electrons; NADPH formed

Light Reactions – Step 1 Primary electron acceptor Water-splitting photosystem Light H2OH2O 2 H  Reaction- center chlorophyll O2O2  2e2e – 2e2e – 1

Light Reactions – Step 2 H2OH2O O2O2 Primary electron acceptor Water-splitting photosystem Light H2OH2O 2 H  Reaction- center chlorophyll O2O2  Electron transport chain Energy to make ATP 2e2e – 2e2e – 12

Light Reactions – Step 3 H2OH2O O2O2 Primary electron acceptor Water-splitting photosystem Light H2OH2O 2 H  O2O2  Electron transport chain Energy to make ATP Primary electron acceptor 2e2e – Light NADPH-producing photosystem Reaction- center chlorophyll 2e2e NADPH NADP  2e2e – 2e2e – 123 –– Reaction- center chlorophyll

Light Reactions - Overall H2OH2O O2O2 Light H2OH2O Thylakoid membrane 2e2e – O2O2 ATP NADP  Light Stroma Inside thylakoid Photosystem Electron transport chain NADPH P HH ATP synthase To Calvin cycle HH Electron flow HH HH HH HH HH Thylakoid membrane ––

Light Reactions