Photosynthesis

Energy Transformations – TWO TYPES ATP (Adenosine Triphosphate): is the energy molecule used in all living things, so supplies must be constantly replenished.  Energy must be added to make ATP  Phosphorylation is the addition of a phosphate molecule to ATP  ADP + P + energy ---  ATP  Dephosphorylation the removal of a phosphate molecule from ATP gives off energy and is called  ATP ---  ADP + P + energy

Electron Transport System (ETS)  It is the means by which energy production occurs within cells.  Consists of a series of progressively stronger electron acceptors.  Each time an electron is transported, energy is either released or absorbed.  This is how most of the ATP in plants and animals is formed

Redox Reactions – Some simple Chemistry…   Oxidation – loss of electrons, producing a substance with a more positive charge.   Reduction – gain of electrons, producing a substance with a more negative ( or less positive) charge.

Photosynthesis  Photosynthesis is the most important chemical process on earth.  Photosynthesis harnesses sunlight energy and converts it into a form of energy (glucose and starch) that is useable by all living organisms.  The process also produces oxygen, which is essential for higher life forms.  CO2 (g) + H 2 O (l) + light  C 6 H 12 O 6(s) + O 2(g) + H 2 O (l) Sunlight energy is converted into chemical potential energy (glucose)

Pigments responsible for Photosynthesis A pigment is a compound that absorbs certain wavelengths of visible light, while reflecting others that give the pigment its specific color. Thus, chlorophyll, does not absorb green. Chlorophylls a and b and the carotenoids are photosynthetic pigments that absorb light

 A chlorophyll solution will absorb red and blue light while it transmits or reflects green light. Therefore, the light that reaches your eyes is green.  The carotenoids absorb blue and green light, so they are yellow, orange, and red in color.  Each photosynthetic pigment absorbs light of different colors. Having a variety of pigments enables a plant to use a greater percentage of the Sun’s light.

Site of Photosynthesis Photosynthesis occurs within the chloroplasts of plant cells, within the thylakoid membrane and the stroma.

Stages of Photosynthesis LIGHT REACTIONS– Thylakoid Membrane STAGE ONE : Solar energy is captured and transferred to the electrons, and water is split.  Photolysis – the splitting of water with light.  2H 2 O(l) + light ---  4H+(aq) + 4 e- + O 2 (g)

Photosystems  Within the chloroplasts, chlorophyll is found in clusters within the thylakoid membranes.  These clusters are called photosystems.  When light hits the photosystem, energy is absorbed and electrons are promoted to an electron acceptor at a higher energy level.  As the electrons fall down the electron transport chain, energy is released and can be used to make ATP.

STAGE TWO : Electron transfer and the production of ATP.  Light energy is captured by the pigments within the photosystems and is used to form more stable energy rich molecules, and to make ATP from ATP and phosphate.  This occurs by cyclic photophosphorylation and chemiosmosis.

CHEMIOSMOSIS  As H+ ions are pulled across the thylakoid membrane by the ETC, an concentration gradient of H+ ions is created.  H+ ions travel down the concentration gradient, across the membrane, through an ATP synthase complex.  As they do this, ATP is produced.

DARK REACTIONS – Stroma STAGE THREE : Calvin Benson cycle and carbon fixation.  The Calvin Benson cycle uses ATP, high energy electrons, NADPH and CO2 to produce glucose in the stroma.  Sunlight energy is not required.  RuBP, a 5 carbon sugar acts as a CO2 acceptor  The resulting 6 carbon sugar is split into two 3 carbon PGA molecules.  Using H from NADPH, PGA is converted to PGAL  PGAL can be used to produce glucose, starch, sucrose, glycerol or cellulose.  PGA can also be converted into amino acids or fatty acids.

Calvin-Benson Cycle The Calvin Benson cycle must turn THREE times to produce each PGA molecule, and SIX times to produce one glucose molecule. The plant cell can manufacture all necessary organic compounds from the products of photosynthesis. 6CO2(g) + 12H2O(l)  C6H12O6(s) + 6O2(g) + 6H2O(g)