Photosynthesis. Stage 1- Energy is captured from sunlight Stage 2 - Light energy is converted to...

Photosynthesis.

• Stage 1- Energy is captured from sunlight• Stage 2 - Light energy is converted to chemical energy, which is temporarily stored in ATP

and the energy carrier molecule NADPH

• Stage 3 - The chemical energy stored in ATP and NADPH powers the formation of organic compounds, using carbon dioxide, CO2

The chemical reactions that occur in the first and second stages of photosynthesis are called the “light reactions” while the third stage is called the “dark reaction”.

Stage 1: Energy is captured from sunlight.

We have talked about the chlorophyll molecule so far. However, there are two types of chlorophyll molecule; chlorophyll a and b.

There are also a second type of pigment molecules. They are called carotenoids. These pigments are responsible for the

yellow and orange colors we see in the fall and for the yellow and red fruits and vegetables we see at the grocery.

Carotinoids absorb wavelengths of light that are different from those of chlorophyll. Therefore, the plants can absorb more light energy during photosynthesis.

and carotinoid pigments

Molecular make up of Chlorophyll a and b

The electrons that leave the thylakoid membrane are replaced by the electrons from the splitting of the water molecule into H and O ions.

The oxygen molecule from the water is then released into the environment for us to breathe.

Remember:

1) Chlorophyll is the molecule which is the pigment that captures the energy from sunlight. This energy causes the water molecule to ‘split’ releasing an electron. It is the chlorophyll molecule which causes the leaf to look green.

2) The transfer of electrons from water takes place in the thylakoid membrane and this is where oxygen is released.

3) The granna are stacks of thylakoids in the chloroplast organelle.

4) The chloroplast is the organelle which you can see under the microscope and which is responsible for photosynthesis.

Chloroplast with stroma, thylakoid and granum.

Chloroplast is the organelle which captures sunlight and

turns it into carbohydrates and energy.

Chloroplast organelle

Stacks of granna

Stage 2: Light energy is converted to chemical energy

There are two types of electron transport chains both occur in the thylakoid membrane.

1. The first type is one that produces ATP from ADP.

2. The second is the one that produces NADPH from NADP+.

The first gains or loses a P molecule while the second gains or loses a H ion.

Both of these energy molecules are used in the third stage of photosynthesis. ( The dark reaction or Calvin cycle)

We have covered the first two stages of photosynthesis. They are both called the light-dependent stages.

The light-dependent stages can be summarized as follows:

1. Pigments in thylakoids absorb light

2. The electrons that are excited by light move through the electron transport chains in thylakoid membranes

5. H ions accumulate inside thylakoids that supply the energy to make ATP and NADPH

4. Oxygen atoms from water form to create oxygen we breathe.

3. These electrons are replaced by electrons from the water molecule, which is split by an enzyme.

The light reactions can be split into the followingspecific steps:

1. Light absorption & splitting of water.2. Production of ATP.3. Movement of electrons through electron acceptorsto power a hydrogen pump.4. Re-energizing electrons so they can produceNADPH

Stage 3: Energy is stored in organic compounds

The transfer of carbon dioxide from the atmosphere to organic compounds is called carbon dioxidecarbon dioxide fixationfixation.

This process is independent of light, therefore it is know as the “dark reactions” or light-independent reactions of photosynthesis.

The general process of photosynthesis involves 3 molecules of carbon dioxide with 3 molecules of

water to form a 3-carbon sugar and oxygen gas.

The reactants are carbon dioxide and water to form

the products: carbohydrates (starch) and oxygen gas.

The oxygen we breath comes from the molecule water.

3H2O + 3CO2 ----------> C3H6O3+ 3O2

The most common means by which CO2 is fixed is through the Calvin Cycle. However, there are other means to fix Carbon but this is the one we will study.

If you did not know, the Calvin Cycle was named after Melvin Calvin who discovered this means of CO2 fixation while a professor of chemistry at University of California at Berkeley in 1947 using Carbon 14. He received a Noble Prize for his discovery in 1961 and died in 1997.

Dr. Melvin Calvin is shown below in his “mad” scientist lab at UC Berkeley, circa

1952.

The Calvin Cycle: end result is a 3 carbon sugar (3 Ribulose 1,5 bisphosphate)

Points to remember about the Calvin Cycle:

1. The free energy of cleavage of ~P bonds of ATP, and reducing power of NADPH, are used to fix and reduce CO2 to form carbohydrate

2. Enzymes and intermediates of the Calvin Cycle are located in the chloroplast stroma

Ribulose Bisphosphate Carboxylase (RuBP Carboxylase) catalyzes CO2 fixation:

ribulose-1,5-bisphosphate + CO2

2 copies of 3-phosphoglycerate

A portion of the glyceraldehyde-3-phosphate is converted back to ribulose-1,5-bisphosphate, via reactions catalyzed by Triose Phosphate Isomerase, Aldolase, Fructose Bisphosphatase, Sedoheptulose Bisphosphatase, Transketolase, Epimerase, Ribose Phosphate Isomerase, and Phosphoribulokinase. Many of these enzymes are equivalent to enzymes of the cytosolic Glycolysis, Gluconeogenesis and Pentose Phosphate Pathways, but are separate gene products resident within the chloroplast stroma. (Enzymes of the other pathways listed are located in the cytosol.) The process is similar to the Pentose Phosphate Pathway running backwards .

• For three molecules of ribulose-1,5-bisphosphate (total of 15 C) that are carboxylated, cleaved, phosphorylated, reduced, and dephosphorylated, six molecules of glyceraldehyde-3-phosphate are produced (total of 18 C). 

Of these:

• One glyceraldehyde-3-phosphate (3 C) exits the pathway as product.  • Five of the 3-C molecules (total of 15 C) are recycled back into three 5-C molecules of ribulose-1,5-bisphosphate, (substrate for RuBisCO)

Factors regulating Calvin cycle :

1) Light activates, or dark inhibits, the Calvin Cycle

2)Alkaline pH activates stromal Calvin Cycle enzymes RuBP Carboxylase,

3) The light-activated shift of H+ into thylakoid disks is countered by Mg++ release from the thylakoids to the stroma. RuBP Carboxylase (in the stroma) requires Mg++ binding to carbamate at the active site .

Summary of Calvin Cycle

Omitting compounds that are regenerated:

3 CO2 + 9 ATP + 6 NADPH

glyceraldehyde-3-phosphate + 9 ADP +

8 Pi + 6 NADP+

The Beginning and the end of photosynthesis

• Glyceraldehyde-3-phosphate may be converted to other carbohydrates such as metabolites (e.g., fructose-6-phosphate and glucose-1-phosphate), energy stores (e.g., sucrose or starch), or cell wall constituents (e.g., cellulose).

• Glyceraldehyde-3-phosphate can also be utilized by plant cells as carbon source for synthesis of other compounds such as fatty acids and amino acids.

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