Photosynthesis

Cell Energy: Photosynthesis

·        Living systems must utilize energy to function.  This energy is found through chemical reactions (formation and breaking of chemical bonds)

·        In order for energy to be used, it must be converted to chemical energy from other forms.  Plants do this through the process of Photosynthesis, the conversion of light energy to chemical energy (in the form of sugars/carbohydrates).

·        Plants do this by converting CO₂ & H₂O into C₆H₁₂O₆ & O₂ in the presence of light according to the following formula:

·        Photosynthesis has 3 basic requirements:

1.      Raw Materials including CO₂ & H₂O and …

2.     Sunlight

·        Requires light in the appropriate wavelength to provide energy for the process

·        Light is categorized according to wavelength with shorter wavelength = greater energy.

·        Visible light exists within the 700nm - 400nm range in the following order:

Roy G. Biv

1.       Red

2.     Orange

3.     Yellow

4.     Green

5.     Blue

6.     Indigo

7.     Violet

·        Above violet exists the Ultraviolet, and below the visible red spectrum exists the Infared.

The EM spectrum

3.     Pigments

·        Photosynthesis requires the presence of certain Photoreceptive pigments. The function of these pigments is to absorb light energy

·        The primary pigment of photosynthesis is called Chlorophyll.

·        There are two types of chlorophyll; Chlorophyll-a & Chlorophyll-b they differ slightly in what wavelengths of light they absorb.

·        Both forms of chlorophylls absorb light at the ends of the visible spectrum (Blues & Reds), therefor it appears green, giving most plants their green color.

·        Many Plants have accessory pigments (eg. Caroteine or xanthophylls) which allow them to capture extra wavelengths of light

Photosynthesis occurs in two phases:

1. The light reactions:

·        Occur on the thylakoids of the chloroplast

·        Function to provide ATP and NADPH for the "Dark" reactions

·        Occur only in the presence of sunlight

2. The Dark reactions

·        Take the energy molecules to "fix" organic carbon molecules (CO₂), into sugars.

·        Occurs continually

·        Occurs in the stroma of the chloroplast

The chloroplast

Overview of Photosynthesis

The Light Reactions:

·        Two Types

·        Non-cyclic Photophosphoryllation

1.      Light is absorbed by Photosystems (2) located on the Thylakoid Membranes of chloroplasts. Photosystem II absorbs a photon of light which releases an electron [e^-] from a water molecule which is split during the process.

2.     This high energy electron passed through a series of chemical compounds located in the Chloroplasts' Thylakoid membrane

3.     As the e^- passes through the chain, it gives its energy to a molecule of ADP (and a phosphate) forming a molecule of ATP.

4.     This e^- is returned to another photosystem, Photosystem I, where it again receives energy from a photon of light.

5.     This electron then yields its energy to NADP⁺ and a H⁺ ion (obtained from the splitting of H₂O in step 1), to form a molecule of NADPH.

Mechanics of Noncyclic photophosphoryllation

·        Cyclic Photophosphoryllation:

·        Only uses photosystem I

·        Electron is repeatedly excited and passed through the electron chain producing ATP only.

·        Does not require splitting of H₂O to function as it does not make NADPH

 Photosynthesis: The Dark Reactions

1.    The Dark Reactions

·        Are misnamed because the occur during light AND dark.  They are called the light reactions because the do NOT require light to function.

·        Occur within the Lumen (open space) of the stroma of chloroplast.

·        Here energy stored in ATP and NADPH from the light reactions are used to convert CO₂ to C₆H₁₂O₆ (Glucose).

·        This occurs in a process known as the Calvin Cycle.  It is an ongoing and cyclical process involving many steps.

The Calvin Cycle

These Steps are as follows:

1.      CO₂ is brought into the system and joins with a 5-Carbon molecule.  This splits to form 2 C₃ molecules.

2.     2 ATP molecules release their Pi groups to form 2 ADP molecules and thus give energy to the C₃ molecules.*

3.     2 molecules of NADPH release their Hydrogens to form 2NADP⁺ molecules.  This once again releases energy to the C₃ molecules, and forms 2 molecules of PGAL (Phospho glyceraldehyde).

4.     The 2 PGAL molecules are removed from the cycle.  These can be joined to form Glucose or other needed molecules.

5.     Remaining carbon compounds are acted on by many enzymes and move through several intermediate compounds.  This results in another C₅ molecule which can combine with CO₂ and repeat the cycle

*Note: NADP+ and ADP molecules are returned an reused in the light reactions