Photosynthetic Conversion of Light

Photosynthetic Conversion of Light Biogradska gora by Snežana Trifunović (2007) http://en.wikipedia.org/wiki/File:Biogradska_suma.jpg ldh

Lesson Objective The student will describe the process of photosynthesis. Subobjective 1: The student will describe light-dependent reactions. Subobjective 2: The student will describe light-independent reactions.

Let's discuss photosynthesis. Pull out the tabs to learn more about what we will be  discussing in this lesson. Photosynthesis uses the energy of sunlight to convert  carbon dioxide and water into high energy sugars and  oxygen. Chemically, this is written: 6CO2 + 6H2O + energy C6H12O6 + 6O2 (Carbon Dioxide + Water + light energy --> sugar + oxygen) Sugar is the end product of photosynthesis and  oxygen is the byproduct. Organisms that use energy  to produce or synthesize complex organic molecules  (such as sugar) from inorganic molecules (such as  CO2) are called autotrophs. Those that use light as their energy source are called photoautotrophs. There are two sets of reactions involved in  photosynthesis: 1) Light-dependent reactions (or  light reactions) and 2) Light-independent  reactions (or dark reactions).

The diagram below shows the process of photosynthesis. A simplified diagram of photosynthesis by Daniel Mayer (2008) http://en.wikipedia.org/wiki/File:Simple_photosynthesis_overview.svg Light reactions (or  photophosphorylation)  produce oxygen, ATP  and NADPH. Dark reactions (or  the Calvin Cycle)  produce sugar, ADP  and NADP+.

Photosynthesis occurs in chloroplasts. Let's look  at the structure of a chloroplast. Click on each number to see the parts of the chloroplast. A diagram showing the simple structure of a chloroplast by Ollin (2009) http://en.wikipedia.org/wiki/File:Chloroplast_diagram.svg

Let's discuss the light-dependent reactions. This is the first stage of photosynthesis. Click on the  picture of chloroplasts below to learn more. Chloroplasts are organelles within plant cells. A typical  plant cell contains 10 to 100 chloroplasts. Chloroplasts are flat discs. They range from 2 to 10 μm in diameter and 1 to 2 μm thick. Chloroplasts contain stacks (grana) of thylakoids. Thylakoids are flat membrane bound discs.  Photosynthesis occurs on the thylakoid membrane. Photosynthesis occurs as a series of chemical reactions.

Let's look at the chemical reactions involved in the  light reactions. Click on the numbers below to learn more. Photosynthesis begins with the  sunlight exciting chlorophyll 680  to a higher energy state. It  loses an electron to a  pheophytin. Chlorophyll 680  gets a replacement electron by  splitting water. The electron passed to a pheophytin is then  passed to a quinone molecule precipitating the  beginning of an electron transport chain. This  chain creates the energy used to transform  ADP and an inorganic phosphate into ATP. 1 2 The ferredoxin carries the  electrons to the enzyme  ferredoxin NADP+ which then  reduces NADP+ to NADPH.  ATP and NADPH are both  energy storing molecules which  fuel the light-independent (or  dark) reactions. The end of this electron transport chain is  plastocyanin which transfers the electrons  to chlorophyll inside photosystem I (PSI).  The chlorophyll is excited to a higher  energy state by light where the electrons  are removed and transferred through an  intermediary to ferredoxin. 3 4 Z-scheme by Bensaccount (2005) http://en.wikipedia.org/wiki/File:Z-scheme.png

Let's discuss light-independent reactions. Click on the boxes to learn more. Light-independent reactions occur in the  stroma of the chloroplasts. Light-independent reactions are also known as carbon-fixing  reactions, dark reactions, and the Calvin cycle. There are three phases to the Calvin cycle: carbon fixation,  reduction reactions, and RuBP regeneration. These reactions occur only in the presence of  light but do not require light's energy to occur.

The Calvin cycle is pictured below. Click on each number below to see an explanation of the step depicted  in that portion of the cycle. After reading the explanation, click the  "Return to Home page" button to return to this page. 1 6 2 5 The colored dots  represent the following  atoms: Black - Carbon Red - Oxygen White - Hydrogen Pink - Phosphorus 4 3 Go to  Summary  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

Carbon dioxide is attached to ribulase-1-5-bisphosphate (RuBP),  a five carbon sugar compound, by the enzyme RuBisCO to form  an unstable intermediary six carbon compound. 1 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 We start with 18  carbon atoms (3 x 5  RuBP and 3 x 1  CO2). Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

Because this intermediary six carbon compound is unstable, it  breaks into two molecules of 3-phosphoglycerate (PGA). 2 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 We now have three  six-carbon molecules  for 18 carbon atoms. Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

One ATP and one NADPH are required to convert one molecule of  PGA to glyceraldehyde 3-phosphate (G3P), an energy rich  molecule. The ATP is reduced to ADP and the NADPH to NADP+. These are then used by the light-dependent reactions to create  more ATP and NADPH. 3 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 We still have three  six-carbon molecules  for 18 carbon atoms. Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

One of the G3P molecules leave the cycle. It may combine  with another G3P molecule to form a sugar molecule or it  may also be used to form starch, cellulose, or other  molecules needed by the plant. 4 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 One G3P molecule  leaves the cycle  leaving five three- carbon molecules in  the cycle or 15 carbon  atoms. Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

The remaining G3P molecules are recycled through various  intermediate steps back into Ribulose 5-phosphate. 5 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 We now have three  five-carbon  molecules or 15  carbon atoms. Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

One ATP is used per molecule to convert this back into  RuBP. This RuBP is now ready to accept another CO2 molecule and continue the cycle. 6 For simplicity, we will  assume that three CO2 molecules begin the  reaction with three RuBP  molecules. 1 6 2 5 4 3 We still have three  five-carbon  molecules or 15  carbon atoms. Return to  Home  page Overview of the Calvin Cycle pathway by Mike Jones (2010) http://en.wikipedia.org/wiki/File:Calvin-cycle4.svg

Let's summarize the light-independent reactions. Click on the boxes below. The light-independent reactions fix inorganic carbon from  CO2 into organic molecules used by the plant. The light-independent reactions consume ATP and  NADPH, which are produced by the light-dependent  reactions. The light-independent reactions produce ADP and  NADP+, which are consumed in the light-dependent reactions. The light-independent reactions consume RuBP and  then regenerate more RuBP so that the cycle can  continue.

The following website shows step-by-step, the  processes of both the light-dependent and light- independent reactions. http://www.botany.uwc.ac.za/ecotree/photosynthesis/photosynthesis1.htm

Photosynthetic Conversion of Light