PHOTOSYNTHESIS Photosynthesis is a process that involves transforming the energy from sunlight along with carbon dioxide and water to form sugar and oxygen. A simplified equation of the complete reaction of photosynthesis is shown: 6 CO H 2 O → C 6 H O 2 Not shown in the equation are two important components of the process: light energy and chlorophyll. If these two factors are omitted from the reaction, photosynthesis could not take place.

In plants, photosynthesis occurs within the leaf. Leaves provide a large surface area to capture the light energy from the sun and are situated at the top of branches in order to accomplish that task. They contain many stomata which allow gases to enter and exit the leaf.

In plants, photosynthesis occurs within the leaf. Leaves provide a large surface area to capture the light energy from the sun and are situated at the top of branches in order to accomplish that task. They contain many stomata which allow gases to enter and exit the leaf. In addition, leaves also contain a network of vascular bundles which transport needed materials such as water and minerals to the leaf.

Chloroplasts are double membrane organelles that are only found within eukaryotic cells. In addition, they also have their own DNA. The inner space contains a protein-rich, semi-liquid material called stroma. Within the stroma is a system of membrane bound sacs called thylakoids. Some thylakoids are arranged in stacks to form grana (singular granum). Adjacent grana are connected by unstacked thylakoids called lamellae. Photosynthesis takes place in the thylakoids and the stroma. Don't get confused by two similar terms: stoma and stroma. Remember that the stoma is the opening in the leaf needed for gas exchange. Stroma is the fluid inside of a chloroplast where photosynthesis takes place.

A thylakoid membrane contains chlorophyll, the light gathering pigment molecules, and the electron transport chain which are essential for photosynthesis. These materials are embedded within the thylakoid membrane. The interior of the thylakoid is filled with a watery substance called lumen

Thylakoid

Overall, there are three main stages of photosynthesis: Stage 1: Capturing Light Stage 2: Using light energy to produce ATP and NADPH Stage 3: Synthesis of organic compounds by using energy produced in stage 2. The process of photosynthesis takes place in two parts. The light reaction and the dark reaction also called the Calvin Cycle.

Light Reactions The light reaction of photosynthesis takes place in three phases: Photon of light is captured by chlorophyll and an electron becomes excited within the pigment. The excited electron is shuttled along a series of electron carrier molecules embedded in the membrane of the thylakoid. The final electron carrier is connected to a proton-pumping channel that transports H+ across the membrane. The movement of the proton across the membrane drives the chemiosmotic synthesis of ATP.

Excitation Electrons contained within a chlorophyll pigment are at their lowest energy level until a photon of light excites them. When the electron becomes excited, it gains energy and rises to a higher energy level. The electron is very unstable and before it returns to its ground state, it transfers its energy to a primary electron acceptor. In this redox reaction, the chlorophyll is oxidized and the primary electron acceptor is reduced.

There are two types of photosystems: Photosystem I (P700) and Photosystem II (P680). Millions of photosystems are embedded within the thylakoid membrane and their job is to provide enough energy to synthesize carbohydrates in the Calvin cycle. These photosystems are linked together in a two stage process.

Non-Cyclic Electron Flow Outlined below are the steps involved in the photosynthetic electron transport chain. Be sure to reference the diagram as you read through the steps. 1. Photon strikes photosystem II (PSII) and excites an electron of chlorophyll a. 2.The electron is captured by a primary electron acceptor called pheophytin. 3.Electron is transferred to a mobile carrier called plastoquinone (PQ). PQ also picks up two protons from the stroma. 4.At the oxygen evolving complex, photolysis occurs. A Z protein splits water into oxygen gas, hydrogen ions and electrons. The oxygen gas diffuses out of the membrane, the electrons replace the electron from the photosystem and the hydrogen ion remains in the lumen. Hydrogen ions accumulate within the lumen and generate an electrochemical gradient needed for ATP synthesis.

5.PQ then transfers the electrons to the cytochrome b 6-f complex and the two hydrogen ions it picked up are released into the lumen. These transfers are coupled with the pumping of two more hydrogen ions into the lumen by cytochrome b 6-f. 6.Electrons are then transferred to plastocyanin (PC) a mobile carrier which then transports the electrons to the photosystem I complex. In photosystem I, photons energize each electron and transfer them to ferrodoxin (Fd). 7.Next electrons move to ferredoxin- NADP-reductase (FNR). When two electrons are transferred to FNR, NADP is reduced to NADPH. NADPH is needed for the dark reactions of photosynthesis.

Non-Cyclic Electron Flow Outlined below are the steps involved in the photosynthetic electron transport chain. Be sure to reference the diagram as you read through the steps. 1. Photon strikes photosystem II (PSII) and excites an electron of chlorophyll a.

2.The electron is captured by a primary electron acceptor called pheophytin. 3.Electron is transferred to a mobile carrier called plastoquinone (PQ). PQ also picks up two protons from the stroma.

4.At the oxygen evolving complex, photolysis occurs. A Z protein splits water into oxygen gas, hydrogen ions and electrons. The oxygen gas diffuses out of the membrane, the electrons replace the electron from the photosystem and the hydrogen ion remains in the lumen. Hydrogen ions accumulate within the lumen and generate an electrochemical gradient needed for ATP synthesis.

5.PQ then transfers the electrons to the cytochrome b 6-f complex and the two hydrogen ions it picked up are released into the lumen. These transfers are coupled with the pumping of two more hydrogen ions into the lumen by cytochrome b 6-f.

6.Electrons are then transferred to plastocyanin (PC) a mobile carrier which then transports the electrons to the photosystem I complex. In photosystem I, photons energize each electron and transfer them to ferrodoxin (Fd).

7.Next electrons move to ferredoxin- NADP-reductase (FNR). When two electrons are transferred to FNR, NADP is reduced to NADPH. NADPH is needed for the dark reactions of photosynthesis.

Phase III: ATP Synthesis The hydrogen ions that accumulate in the lumen drive the photophosphoylation of ADP to ATP. The electrochemical gradient is used by ATP synthase to create ATP.

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The diagram below shows the electron flow and changes in energy that electrons experience as they progress down the chain. The products of non cyclic electron flow include ATP, NADPH and oxygen gas.

Cyclic Electron Flow Cyclic electron flow only uses photosystem I and is used as a way to generate ATP in eukaryotic plant cells. Photosynthetic prokaryotes use only cyclic photophosphorylation. This process is geared towards energy production rather than the synthesis of organic compounds. The steps involved in cyclic electron flow are summarized below. 1) Light energizes electrons at photosystem I. 2) Electron carriers move electrons only in the company of hydrogen ions, which are then transported across the membrane. The accumulation of hydrogen ions is used to drive the synthesis of ATP. 3) Electrons return to photosystem I with half of the energy they had when they left photosystem I.

In the cyclic electron flow, no NADPH is generated. Instead, cyclic electron flow generates a proton gradient for ATP synthesis. The end result of the light reactions of photosynthesis is the production of oxygen gas, NADPH and ATP. In order for these products to be produced, water, light energy, ADP, Pi, and NADP must be available. The next stage of photosynthesis, the dark reactions, will look at the process of synthesizing organic compounds.