Biology

Chapter 7

Photosynthesis

Concepts/Ideas/Facts:

1. all organisms/cells need energy for life processes
2. energy is releases when ATP reacts to form ADP or AMP
3. photosystem II the first to be discovered
4. CO₂ is incorporated into organic compounds

Definitions:

1. Photosynthesis – this first stage of energy conversion is a process which converts radiant solar energy (light) into chemical energy stored in the bonds of organic compounds, usually carbohydrates.  Basically photosynthesis stores energy in chemical compounds.               CO₂ + H₂O + light energy à C₆H₁₂O₆ (sugar) + O₂
2. Respiration – this second stage passage of energy from the sun to cells is the releases of energy from food.  A type of intermediary metabolism, it is the releases of chemical energy (made by photosynthesis) for cellular use, which occurs in all cells.  Respiration basically releases energy stored in chemical compounds.

      C₆H₁₂O₆ (sugar)  +  6O₂  à  6CO₂  +  6H₂O  + energy

3. Biochemical Pathway – complex series of chemical reactions, which can produce products that are reused.
4. Photosystem – a unit of several hundred chlorophyll molecules and associated carrier molecules.
5. Adenosine Triphosphate (ATP) – cell energy molecule consisting of adenine, which contains a nitrogen, ribose (sugar) with a five-carbon ring structure, and a phosphate group, which contains phosphorous (P).  Adenosine bonded to three phosphate groups is called ATP.
6. Adenine – molecule consisting of ribose bonded to adenosine.
7. Adenosine Monophosphate (AMP) – consists of adenosine bonded to one phosphate group
8. Adenosine Diphosphate (ADP) – consists of adenosine bonded to two phosphate groups
9. ATPase – enzyme which breaks phosphorous (P) bonds releasing energy an creating ADP or AMP.  The breaking of the third P bond creates ADT, breaking the second P bond makes AMP.
10. ATP synthetase – enzyme that catalyzes ADP during phosphorylation to form ATP
11. Phosphorylation – the transfer of a phosphate group from one molecule to another along with the transfer of energy.  This is the process in the formation of ATP from ADP.
12. Oxidative Phosphorylation – the formation of ATP from ADP and inorganic phosphorous which takes place in the electron-transport chain of the mitochondria
13. Photophosphorylation – the production of ATP using the energy of sunlight
14. Light-Dependent Reactions – photosynthetic series of reactions requiring light.  The energy yield from the light-dependent reactions is stored in the molecules of NADPH and ATP formed by photophosphorylation.  Light-dependent reactions convert light energy to chemical energy and take place in the thylakoid.
15. Light-Independent Reactions (Dark Reactions) – photosynthetic series of reactions not requiring light that takes place in the stroma.  NADPH and ATP produced in the light-dependent reactions are used to reduce carbon dioxide to organic carbon by means of the Calvin Cycle.  Light-independent reactions are chemical reactions that convert carbon dioxide and other compounds into glucose.
16. Calvin Cycle (C₃ Cycle) – the series of reaction that occur in the stroma of chloroplasts that reduce carbon and synthesize sugar.  These are light-independent reactions (dark reactions).
17. Electron Transport Chain (hydrogen transport system) – sunlight converted to electrical energy causes the passage of electrons from a high energy level to a lower energy level by electron carriers.  Energy is released.
18. Chemiosmosis – diffusion of chemicals through a membrane, resulting in ATP formation.  Electron transport within the thylakoid triggers chemiosmosis.
19. Carbon Fixation – process by which organic compounds are formed from inorganic compounds through carbon-carbon bonding.
20. PGA (3-phosphoglycerate) – a three carbon molecule, one of the resultant molecules of the Calvin Cycle
21. RuBp (ribulose 1,5-bisphosphate) – five-carbon sugar with two phosphate groups that combines with carbon dioxide at the beginning of the Calvin Cycle forming two molecules of PGA.  RuBP is catalyzed by the enzyme RuBP carboxylase (Rubisco)
22. RuBP carboxylase/oxygenase (Rubisco) – the enzyme that catalyzes RuBP in the Calvin Cycle.  Rubisco is found in the chloroplasts and uses CO₂ as well as O₂ when necessary.  Rubisco (Ribulose Bisphosphate Carboxylase) is the most abundant protein in the world.
23. Glyceraldehyde 3-Phosphate (G3P) – immediate product of the Calvin Cycle which is the primary molecule transported from the chloroplast to the ground substance of the cell.  This molecule is later converted into glucose, sucrose, and starch.
24. NAPD⁺ (nicotine adenine dinucleotide phosphate) – carrier compound in the Calvin Cycle that carries Hydrogen ions (H⁺).
25. NAPDH – formed when a electron from photosystem I and an hydrogen ion (from water) are added to NAPD⁺ .
26. PEP (phosphoenolpyruvate) – compound, which is the immediate precursor of pyruvic acid in glycolysis.
27. Glycolysis – metabolic pathway by which glucose is anaerobically degraded to pyruvic acid.
28. PGAL (phosphoglyceraldehyde) – formed by the addition of a phosphate group from ATP and a hydrogen ion from NADPH to a PGA molecule.
29. Oxidation – is the loss of electrons
30. Reduction – is the gain of electrons

Structure of Chloroplasts: chloroplasts produce ATP and NADPH using the energy of sunlight.  Chloroplasts have a deep resemblance to certain prokaryotes, which suggests that chloroplasts originated as intracellular symbionts in primitive eukaryotic cells.

1. Pigment –light absorbing compounds
2. Carotenoids – red, orange, and yellow fat-soluble pigments found in all chloroplast and cynobacteria that capture light energy but have to transfer the energy to chlorophyll a.
3. Plastid – organelle where food or pigments are stored.
4. Chlorophyll – pigment that absorbs light principally in the violet and blue wavelengths of light and reflects green light, therefore appearing green.  Chlorophyll can convert light energy to chemical energy only when the chlorophyll molecules are associated with certain proteins and embedded in the specialized membranes of the thylakoids.
5. Chloroplasts – three membrane plastid, the site of light-dependent and light independent reactions.
1. inner membrane -  similar to a cell membrane consisting of a double-lipid layer (bilayer) embedded with proteins such as ATP synthetase.  It is formed or arranged into tiny stacks (thylakoids) called grana.
2. middle membrane – middle space
3. outer membrane – permeable outer protective membrane
6. Intermembrane Space – middle space between the outer membrane layer and the inner membrane layer.
7. Grana (Granum) – stacks of disk-like thylakoids embedded with chlorophyll and carotenoid pigments.  The shape of grana provides a large surface are afor light absorption and the diffusion of photosynthetic sproducts arcross the thylakoid membrane
8. Thylakoids – elaborate system of membranes in the form of flattened sacs that traverse the stroma.  Thylakoids of various grana are interconnected by other thylakoids, called stroma thylakoids or lamellae.  Light-dependent reactions take place in the thylakoid and it is the place of light absorption.
9. Stroma – the protein-rich ground substance or “soup” of a chloroplast, which surrounds the grana.  Light-independent reactions occur in the stroma.
10. Lamellae (stroma thylakoids) – bridges that traverse the stroma and interconnect the grana.
11. Chlorophyll a – the essential chlorophyll for photosynthetic eukaryotes and the cynobacteria , it absorbs more blue light and less red light than chlorophyll b.
12. Chlorophyll b – an accessory pigment in vascular plants, bryophytes, and green algae that serves to broaden the range of light that can be used in photosynthesis.  When chlorophyll b absorbs light, the energy is transferred to chlorophyll a, which then transforms it into chemical energy.
13. Chlorophyll c– takes the place of chlorophyll b in brown algae

Light to Energy:

1. the first step in the conversion of light energy to chemical energy is the absorption of light.  This is accomplished by pigments, any substance that absorbs visible light.
2. the temporary boosting of electrons to a higher energy level
3. electrons returned to a lower energy level. There are three consequences to this:
1. the energy may be dissipated as heat
2. the energy may be reemitted almost instantaneously as light energy of longer wavelengths (fluorescence)
3. the energy may be captured by the formation of chemical bonds (photosynthesis)

Stages of Photosynthesis:

1. light absorption or energy-capturing by chlorophyll
2. light-dependent reactions
3. light-independent reactions

Stages of Light-Dependent Reactions:

A.   Electron Transport (first stage): converts sunlight to electrical energy, which is a flow of electrons.

1. begins in photosystem II
2. located in the thylakoid
3. light causes electrons to leave chlorophyll
4. electrons move to electron carriers along a chain of carrier molecules to a lower energy level in photosystem I
5. energy is released

B.   Chemiosmosis (stage 2): diffusion of chemicals through a membrane, resulting in ATP formation.

1. a concentration gradient of hydrogen ions (H⁺) forms across the thylakoid membrane triggered by electron transport.
2. ions move across the thylakoid to the stroma creating electrochemical energy
3. triggering of the phosphorylation of ADP to ATP
4. ATP in stroma is ready for use for carbon fixation in photosystems II

C.   Photosystem I – located in the thylakoids, it uses chlorophyll a and produces NADPH, an energy carrier needed in the Light-Independent Reaction

Stages of Light-Independent Reactions (Calvin Cycle): the forming of organic compounds by using energy stored during light-dependent reactions in the bonds of NADPH and ATP.  There are three phases to the light-independent reactions, collectively called the Calvin Cycle:

A.      Carbon Fixation – the first step of the light-independent reactions in which inorganic one-carbon molecules are bonded or “fixed” into organic three-carbon and five-carbon intermediate molecules and eventually form six-carbon molecules.  Carbon from carbon dioxide is "fixed" into a larger carbohydrate causing three pathways to occur: C3 carbon fixation (the most common), C4 carbon fixation, and CAM (Crassulacean Acid Metabolism). C3 fixation occurs as the first step of the Calvin-Benson cycle in all plants.

B.       Reduction Reactions – gaining of electrons

C.      Ribulose 1,5-biphosphate (RuBP) regeneration.

D.      Calvin Cycle (C₃ Cycle) – chemical pathway that takes place in the stroma of the thylakoid that reduces carbon and synthesizes sugar

a.       Step 1 - CO₂ diffused into the stroma from the cytosol combines with a 5-carbon carbohydrate (RuBP) producing a 6-carbon molecule that splits into a pair of 3-carbon molecules known as PGA (three-carbon phosphoglyceric acid).

b.      Step 2 – PGA is converted into another 3-carbon molecule PGAL in a two-part process.  First PGA receives a phosphate group from ATP and then that compound gets a proton and then releases a phosphate group producing PGAL as well as ADP, and NADP⁺

c.       Step 3 – PGA is converted back into RUBP releasing energy and starting the cycle over again and releases some PGAL molecules to leave the cycle to make organic compounds

C₃ Plants – plants that exclusively use the Calvin cycle to fix carbon (using the 3-carbon compounds)

C₄ Pathway – plants that use 4-carbon compounds to fix carbon.  Usually plants that have low CO₂ levels and high O₂ levels.  These plants restrict the lose of water. (plants such as corn, sugar cane, and crabgrass)

Cam Pathway – plants that open their stomata at night and close them during the day such as cacti and pineapples.  These plants loose even less water than C₃ or C₄ plants

The Rate of Photosynthesis:

1.      Increase in light intensity increases photosynthesis until all electrons are excited  then a plateau is reached

2.      Increase  in CO₂  levels increases photosynthesis until a plateau is reached

3.      Increase in temperature increases photosynthesis until a point is reached in which temperature adversely affects photosynthesis

Photosystem I – located in thylakoid

Photosystem II – located in the thylakoid, it produces Oxygen and an electron used in P II

Light absorption à electron transport (or chemiosmosis) à Calvin Cycle (carbon fixation) à carbohydrates

Sunà radiant energy(light) à photosynthesis by autotrophs à carbohydrates (stored chemical energy) à respiration à ATP (usable chemical energy à cell metabolism

RESPIRATION:

                                                                                   à CO₂                                                          ß O₂

Sugar à glycolysis  à  pyruvate à Kreb’s Cycle  à NADH  à Oxydation Phosphorylation à H₂O

               (in cytosol)                                                  à ATP                       (in cristae)               ==> ATP

PHOTOSYNTHESIS:

                                                                              ß CO₂                                            à O₂

Sugar ß sucrose synthesis ß triose ß Calvin Cycle ß NADPH ß light reactions + P ß H₂O

                (cytosol)                                   (stroma)      ß ATP                                           < == LIGHT

Respiration:
CH₂O + O₂ → CO₂ + H₂O + ATP
Photosynthesis:
CO₂ + H₂O + light → CH₂O + O₂

VERY GOOD DIAGRAM AND EXPLANATION OF PHOTOSYNTHESIS/CHLOROPHYLL WEBSITE

http://www.ftexploring.com/photosyn/chloroplast.html

Calvin Cycle Web Site

www.msu.edu/~smithe44/calvin_cycle_process.htm