Courses: BIO111-General College Biology I: Chapter 10 Outline

Note: There are some figures available by clicking on the link below. You should learn these figures carefully. In addition, you should be familiar with the figures in your text.

What is Photosynthesis?
1. General: For almost all organisms (exception of those that are chemoautotrophs and organisms that live in complete darkness) the photosynthesis reactions were the most important development in the evolution of eukaryotic cells. We, as heterotrophs, rely on the photosynthesizers to supply us with the reduced carbon molecules that we call food. We use this food to power our own metabolic reactions.
2. Respiration: In the process of respiration, organisms convert the sugars produced by the photosynthetic reactions to produce ATP (cell fuel). Never forget that plants are capable of photosynthesis, but must also be able to metabolically catabolize the sugars that they themselves produce.
3. Autotrophs vs. Heterotrophs: The autotrophs are the “self feeders” that produce the organic molecules that enter metabolic reaction for the production of ATP. Each year the photosynthetic organisms produce an incredible amount of sugars in the form of photosynthetic products. This gross primary productivity is not all available to heterotrophs…why?
An overview
1. Stomata and Basic Products
2. 6CO₂ + 12H₂O ----> C₆H₁₂O₆ + 6O₂ + 6H₂O (take careful notes)
3. Sulphur Bacteria: Be aware that some organisms can undergo anabolic photosynthetic reactions involving H₂S instead of H₂O. These bacteria produce sulphur instead of oxygen as an end product.
4. Two Pathways of Photosynthesis: We will look at the two stages of photosynthesis:
  1. The light reaction is broken down into two components, cyclic and non- cyclic photophosphorylation. These reactions produce reduced molecules, ATP and NADPH respectively that represent chemical energy yield from light energy.
  2. The dark reaction: The high energy products of the light reactions are used in the dark reaction to reduce CO₂ to sugars.
What is Light?
1. Photons
2. Wavelengths
3. Visible Light
How is Light Trapped?
1. Pigments: A molecule that can absorb wavelengths in the visible light spectrum is called a pigment. Different pigment molecules absorb different wavelengths depending on their chemical configuration. Examine the absorption spectra for the most common photosynthetic pigments used in photosynthesis. Why are most plants green? Why does a black car look black? Why is a red pair of trousers red? Why is it best to drive a white car in Phoenix?
2. Ground State vs Excited State.
3. Action Spectrum: What is the difference between action and absorption spectra?
4. Chlorophyll Pigments: a and b
5. Accessory Pigments: Carotenoids, Xanthophyllus, and Phycobilins.
6. Antennae Systems and Reaction Centers: Light energy can be utilized at a maximum by plants that have a wide variety of pigments that absorb many pigments. These pigments are concentrated in an antennae system that directs photon energy to the molecules that absorb the highest wavelength; this molecules is a specific chlorophyll a molecules is a specific chlorophyll a molecule that absorbs in the range from 680nm - 700nm.
7. Chlorophyll a is energized by light, and can lose an electron in its high ground state. It becomes oxidized as it loses its high energy electron.
The Chloroplast
1. Thylakoid Membrane and Thylakoid Space
2. Stroma
The Light Reaction
1. Non-cyclic photophosphorylation
  Reactants:
  Molecule of water
  Four photons
  One molecule each ADP and NAD
  Energy
  Products:
  O₂
  One molecule each of NADPH and ATP
2. Cyclic Photophosphorylation: As you saw in the last section, non-cyclic photophosphorylation produces equal quantities of ATP and NADPH. Some plants have the ability to switch to cyclic phosphorylation, still producing ATP, but no NADPH. Compare Figures in your text comparing Cyclic and Non-Cyclic Photophosphorylation.
3. ATP production. Both Cyclic and Non-cyclic phosphorylation produces ATP by chemiosmosis. The high energy electrons produced by the Antennae systems can undergo exergonic reactions as they are passed along the membrane proteins of the thylakoid. These exergonic reactions allow for the pumping of H+ ions from the stroma to the thylakoid space. As the hydrogen ion (proton) concentration gradient builds, protons move by diffusion through ATP Syntases on the membrane, generating ATP. Make sure you understand this process.
4. Why two cycles? Note that both cycles perform chemiosmosis for the Production of ATP. Non-cyclic photophosphorylation utilizes P680, while cyclic utilizes p700. Very few ancient photosynthetic bacteria still use only cyclic photophosphorylation (with no production of O₂), while more derived photosynthesizers such as plants and algae use the cycle cycle to produce ATP when NADH levels get too high.
  Note: Okay folks… at this point you should be comfortable with the oxidized/reduced terminology. For example…water is oxidized by photons and the photolysis of water replenishes the electron missing from the antennae complex. This electron can be energized to its excited state and act as a reducing agent.
The Dark Reaction (Calvin-Benson Cycle):
1. Take place only in the light.
2. Enzyme-mediated reactions occur in the stroma of the chloroplast
3. Rubisco and RuBP
4. Photosynthetic Products (sugar phosphates and other organics)
Rubisco and the Evil Photorespiration
1. Rubisco and Specificity
2. Oxygen and its Effects
3. Glycolate and the Mitochondria
4. Ways Around the Problem:
  1. C4 Plants: e.g., corn
    1. PEP Carboxylase mediates the reaction of PEP (3C) to Oxaloacetate (4C). No affinity for oxygen means no photorespiration. Dark reaction proceeds in the same manner when a carbon dioxide molecule is liberated from the 4C molecule. See Figs.
    2. Note cell anatomy difference.
  2. CAM (Crassulacean Acid Metabolism): Similar to C3 and C4?
5. Don’t forget that plants need oxygen!!! Why? Also, why is it obvious that plants must photosynthesize more than they respire?