The Reactions of Photosynthesis

The process of photosynthesis is quite complex; you will only have to know a simplified overview of the steps and stages. 2 Stages a.Light-dependent reaction (thylakoid) b.Carbon-fixation reaction (stroma)

Energy products from photosynthesis Photosynthesis produces different intermediate and final energy rich products through out its process ATP NADPH Glucose

1. ATP - Adenosine triphosphate Principal energy-supply molecule for cellular functions of all living cells (most important and USEABLE form of chemical energy) Provides an immediate source of energy (for growth and movement)

ATP is formed when an inorganic phosphate group (P) is attached to lower-energy ADP (Adenosine diphosphate) ATP ADPP+ Energy Input Energy output

Energy is released when an enzyme removes a phosphate group thus forming ADP

Molecules of ATP are broken down and regenerated thousands of times each day

2. NADPH - Nicotinamide dinucleotide phosphate NADPH – electron donor NADP + - electron acceptor NADPH contains STORED energy and RELEASES energy when it donates 2e - and one H + to become NADP +

3. Glucose Provides medium-term energy storage in most cells Transport molecule - glucose molecules are transferred to different parts of the plant for storage

An overview of Photosynthesis

Stage 1 - Capturing Solar Energy Chlorophyll (and other pigment molecules) are found in clusters embedded in the thylakoid membrane –these are called photosystems Chloroplasts have 2 photosystems (PS I and PS II) –named for the order in which they were discovered, not for their sequence in the photosynthetic process

Electrons in the chlorophyll molecule are ‘excited’ when they absorb a photon. This occurs within PSII

This high-energy electron leaves PSII and is passed through an electron transport chain Electron transport chain… a series of electron carrying molecules each has a progressively stronger attraction for electrons a small amount of energy is released at each step

As electrons pass through the ETC, they return to lower energy levels, become more stable and energy is released. Released energy makes ATP

Electrons that move through the ETC must be replaced……They are replaced during a process called photolysis Solar energy is used to split water into hydrogen ions (H + ) and oxygen gas Photolysis occurs in the thylakoid lumen Oxygen gas is released to the environment as a waste product!!! 2 H 2 O (l) + energy  4 H e - + O 2(g)

Stage 2: Electron Transfer and ATP synthesis

How does the transfer of electrons release energy? Oxidation - a reaction in which an atom or molecule loses electrons Reduction - a reaction in which an atom or molecule gains electrons When a molecule (such as NADP + ) gains electrons, it becomes more stable(NADPH) and releases energy

Pathway of other electrons Energy released when an electron passes through the ETC is used to “pull” hydrogen ions from the stroma into the lumen (B in diagram) The concentration of H + ions as well as electrical charge builds up within the lumen

Electrons have lost most of their original energy (received by light) by the time they reach PS I (electrons begin PS I inside the membrane) The electron again absorbs solar energy and reaches an excited state however… this electron does not enter an electron transport chain

The electrons are transferred to a molecule of NADP + Each NADP + accepts two electrons and an H + ion from its surroundings Newly formed NADPH molecules are used in the Calvin cycle in stage 3

Summary

Chemiosmosis Chemiosmosis is a process for synthesizing ATP using the energy of an electrochemical gradient and the ATP synthase enzyme

Recall - a high concentration of H + ions has built up in the lumen This creates an electrochemical gradient This gradient can be put to good use, as H + ions try to return to equilibrium

H + ions are able to escape from the lumen only through specialized protein complexes - ATP synthase complexes. As H + rush through the complex, they release energy This energy is used to combine ADP with a free phosphate group ATP is generated by the energy of the electrons from PSII

Chemiosmosis uses an electrochemical gradient and a special protein complex to generate ATP Lumen Stroma

Summary - Light dependent Reactions Water is consumed (split apart) ATP, NADPH, and O 2(g) are formed ATP and NADPH go on to be used in stage 3 - carbon fixation CO 2 and glucose are NOT involved in these stages!!