Lecture Notes-17 Biology 1004

North Arkansas College

Topic: Process of Photosynthesis

Photosynthesis- process which occurs in plants and algae and some bacteria.

1. Uses chlorophyll and or related pigments.
2. In this process, solar energy is converted to high energy compounds as ATP and NADPH.
3. These compounds are used to make energy rich chemical molecules. Primarily in form of carbohydrates.
4. The raw materials are carbon dioxide, water, and solar energy.
5. Water is split as it absorbs a packet of solar energy called photons.

This process is termed photolysis:

H₂O ---------H+ and O₂ is given off

6. Overall reaction scheme of photosynthesis:

6CO +12O ---------C H O + 6HO + 6O

7. The chloroplasts which trap solar energy chlorophyll A+B and auxiliary pigments are present.
8. Photosynthesis occurs in two key reactions:

1. Light-dependant
2. Light-independent

Light Dependant Reaction:

1. The chloroplasts trap solar energy
2. Solar energy splits water (photolysis)
3. Reactive centers of chlorophyll A is activated
4. Electrons as H+ are transferred to a series of carriers
5. Some of these electrons become so energized that they escape from
6. Chlorophyll and are then transferred to a second set of carrier molecules
7. In the form of NADP+ (oxidized) to form NADPH ( reduced)
8. This NADPH is eventually used to reduce carbon dioxide to form carbohydrates.
9. This less energized electrons form ATP and this supplies energy for remaining steps in photosynthesis.
10. This process involves two photo systems:

1. Photosystem I
2. Photosystem II

* Non Cyclic Pathways Produces 1 ATP and NADPH.

11. Auxiliary pigments used in photosynthesis:

1. Chlorophyll A
2. Chlorophyll B
3. Xanthophylls

Photosynthesis

1. 6 CO₂ & 12 H₂O - Solar Energy - C₆H₁₂O₆ & 6 O₂ & 6 H₂O.
2. Photosynthesis is like respiration; both are oxidation- reduction reactions.
3. Solar energy splits water to produce 2 H+ & O₂. The process of "splitting" is called photolysis. The packet of light energy used is called a photon. Light can act as a wave (wavelength of light) or a particle (photon).
4. This process occurs in the stoma of thyalkoids in chloroplast.
5. As chloroplast absorbs the photon of light energy and splits water, it uses 2 different types of photo systems (I & II - I is shorter; II is longer).
6. The most reactive form of chlorophyll is Chlorophyll A which absorbs red/violet light and reflects green light. Chlorophyll B is reactive and absorbs blue-green light and reflects yellow-pale blue. The electrons produced (H+) in Chlorophyll B are transferred to Chlorophyll A.
7. The electrons enter one of 2 electron pathways.

1. Cyclic Electron's Pathway - The electron energy from sunlight excites the electron's in the chlorophyll and is transferred to ADP & Pi - ADP ~ P (ATP). (Inorganic PO₄) It produces 1 ATP, and the non-excited electron falls back to ground state in chlorophyll.
2. Non-Cyclic Pathway - Similar to Cyclic Pathway in that energy from the sun is absorbed by chlorophyll to produce excited electrons that go make ATP through the Cyclic Pathway. But, a few electrons become so excited that they escape from the chlorophyll as H+ and combine with NADP+ forming NADPH. NADP+ - H+ = NADPH. The Non-Cyclic Pathway produces both ATP & NADPH. The NADPH is used in the Light Independent Reaction to reduce CO₂ and form carbohydrates as (CH₂O)n. All of the processes up to reduction of CO₂ take place during the Light Dependent Reaction (requires solar energy). At this point, we have O₂, ADP & NADPH. The O₂ is given off. The ATP & NADPH enter the Light Independent reaction.

1. Does not require light.
2. Uses the proton of NADPH to reduce CO₂.
3. It occurs as 3 key reactions:

1. Reduction of CO₂ to form a 6-C sugar.
2. The 6-C sugar is immediately split into 2 3-C compounds.
3. The 3-C compound is used to make glucose, other carbohydrates and other key molecules.

CO₂ & RuBP (5-C sugar) ------------------= 6-C sugar formed. RuBP is a 5-C sugar called ribulose biphosphate.

The 6-C sugar is split into 2 molecules of a 3-C compound called phosphoglyceric acid (PGA).

6-C sugar ------------------= (2) PGA molecules.

PGA is rapidly reduced to form 2 molecules of glyceraldehydes 3-phosphate (pGal).

(2) PGA ------------------= (2) pGal. For each 3 turns of the Calvin Cycle, it regenerates 5 molecules of RuBP to keep the cycle going.

1. Photosynthesis in most plants occurs in the mesophyll cells of leaves. There are at least 2 other key routes of photosynthesis that take place in various types of plants: (6-C) CO₂ & 5-C RuBP.

1. 4-C Plants - Sugar cane, corn, Bermuda grass, etc. are plants that grow in warm climates. They fix the CO₂ to a 3-C compound in the bundle sheath cells rather then the mesophyll cells of regular photosynthesis. The process forms a 4-C product (oxaloacetic acid). The 4-C unit is stored in the bundle sheath cells before release to the Calvin Cycle.
2. CAM Plants - These plants are succulent, flowering plants, found in sub-tropical and desert environments. Daytime is too hot for stomata to open, so stomata is kept closed until night. The CO₂ is combined with 3-C (phosphenol pyruvate) to form a 4-C intermediate (crassulineate acid) in the mesophyll. The next day, the CO₂ is released to the Calvin Cycle.

1. Greenhouse effect and global warming - Fossil fuels such as coal, gasoline, etc. release CO₂ into the atmosphere (as fuels burn). The CO₂ forms a layer that allows the sunlight to pass through, but traps the heat reflected from the Earth's surface. The Earth's temperature is increasing. A rise of the Earth's temperature of 2°C is enough to raise the ocean levels by 5-10 feet (This would place all the world's sea ports under water). Between 1925 and 1995, the Earth's temperature had risen an average of 1°C. Since 1995, this has increased another 0.5°C (accelerated rate).

In 1960, the CO₂ level in the atmosphere was 315 ppm. By 1995, this had increased to 360 ppm. 330 sq. miles of the Antarctica ice caps have melted.

2. Ozone layer - 0₃ forms layer at top of atmosphere and filters out UV light. UV light damages DNA causing mutations and cancer in animal cells. The ozone layer has been getting thinner, and holes have developed. Chlorinated, fluorinated hydrocarbons, etc. destroy ozone. These substances were banned in early 1970's.

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