Biology: Chap. 7/8

Chapter 8 & Photosynthesis Basics

• Material covered through chapter 7 is included in the quiz on Friday and the next exam.

• Next exam covers chapters 5 through 7.

Photosynthesis

• Divided into two types of reactions:

  • Light dependent reactions: Water is oxidized using energy from sunlight to produce oxygen.

    • NADP (a mobile electron carrier) is reduced to NADPH.

    • ADP is phosphorylated to ATP.

  • Light independent reactions: Carbon dioxide is reduced using the energy from the light-dependent reactions.

    • Carbon dioxide is reduced into a carbohydrate, glyceraldehyde 3-phosphate.

Chlorophyll and Photosystems

• Chlorophyll, the light-absorbing pigment, is located in the thylakoid of the chloroplast.

• Chlorophyll is organized into photosystems with a protein scaffold.

• Resonance energy transfer: Chlorophyll units become progressively excited, and the energy flows to a pair of chlorophyll called the reaction center.

• At the reaction center, the electron leaves, oxidizing the chlorophyll.

Photosystems I and II

• Two photosystems are used because the reduction of NADP into NADPH requires more energy than a single packet of sunlight provides.

• Photosystems are named in the order they were discovered, not the order they occur in.

• Photosystem II comes first, followed by photosystem I.

Thylakoid Membrane and Photosystem II

• Photosystem II absorbs a packet of sunlight, initiating resonance energy transfer.

• Energy flows to the reaction center, which is then oxidized.

• The electron is transferred to a primary electron acceptor (plastoquinone).

• Oxygen-generating complex: takes water, oxidizes it, and supplies the electron back to the reaction center.

  • Water is oxidized to form oxygen gas and hydrogen ions.

  • This process is the source of all atmospheric oxygen on the planet.

• The electron reduces a cluster of proteins (cytochromes).

• Cytochromes perform active transport, pumping hydrogen ions from the matrix into the thylakoid space, creating a hydrogen ion gradient.

Photosystem I and NADPH Production

• The electron pair is given a second packet of energy.

• Electron reduces chlorophyll in the reaction center of Photosystem I.

• Another packet of sunlight is absorbed, setting up resonance energy transfer.

• The electron is transferred to another electron carrier.

• NADP reductase: A protein reduces NADP into NADPH.

• NADPH is used in light-independent reactions.

ATP Synthase

• ATP synthase protein allows hydrogen ions to diffuse from high to low concentration, generating ATP from ADP and inorganic phosphate.

• ATP is primarily used for the reduction of carbon dioxide (light-independent reactions).

• Summary of light-dependent reactions: Two packets of sunlight are absorbed to produce NADPH and ATP.