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Photosystems and the Thylakoid

Photosystems and the Thylakoid. Photosynthesis – Chapter 6 Capturing Light Energy and The Electromagnetic Spectrum. Photosynthetic Pigments. The main pigment involved in the light reactions of photosynthesis is chlorophyll a.

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Photosystems and the Thylakoid

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  1. Photosystems and the Thylakoid Photosynthesis – Chapter 6 Capturing Light Energy and The Electromagnetic Spectrum

  2. Photosynthetic Pigments • The main pigment involved in the light reactions of photosynthesis is chlorophyll a. • Chlorophyll b, carotenoids, and xanthophylls are all accessory pigments. (aka antenna pigments)

  3. Accessory Pigments – Help maximize the amount of light energy absorbed! • Chlorophyll a, the main pigment of photosynthesis, absorbs light energy in the red and violet ranges of the spectrum. • The accessory pigments: chlorophyll b, carotenoids and xanthophylls absorb some of the remaining energies of light. (that were not absorbed by chlorophyll a)

  4. Photosystems • Photosystem = chlorophyll a surrounded by accessory pigments. By grouping the pigments together and having them work together, the maximum amount light can be absorbed. • Photosystems are embedded within the thylakoid membrane. • There are two different types of photosystems and their names are photosystem I and photosystem II.

  5. WHY are there two types of photosystems? • Remember, a photosystem essentially consists of chlorophyll a surrounded by accessory pigments. Photosystem I and Photosystem II contain similar kinds of of accessory pigments, but not the exact same accessory pigments. • Photosystem I and Photosystem II have different roles in the light dependent reactions of photosynthesis.

  6. How do pigments absorb (capture) the energy of light? • Each pigment absorbs a specific energy of light. • As the pigment absorbs the specific energy, the energy is used to move the pigment’s electrons to higher energy levels (excite the outer electrons). • Excited electrons LEAVE the pigment and travel from accessory pigments to chlorophyll a. • Chlorophyll a can then donate the excited electrons to electron carriers.

  7. Primary electron acceptors • REMEMBER – as pigments absorb specific energies of light, energy is used to raise the outer electrons to a higher energy level. (Excite the electrons) • Excited electrons LEAVE the individual pigments within the photosystems and go to chlorophyll a. • Chlorophyll a then gives the excited electrons to a molecule in the thylakoid membrane called the primary (first) electron acceptor.

  8. Electron Transport Chain • (Remember the flow of electrons: pigments to chlorophyll a to primary electron acceptor) • The primary electron acceptor donates the (high energy) excited electrons to the series of proteins known as the electron transport chain. • The electron transport chain is embedded within the thylakoid membrane.

  9. It all takes place in the chloroplast! • The pigments contained within photosystems (clusters of pigments), the primary electron acceptors, and the proteins of the electron transport chain are ALL LOCATED in the thylakoid of the chloroplast!

  10. Let’s review the structure of the chloroplast.

  11. Structure and Function • Outer Membrane – selectively permeable barrier. • Inner Membrane – selectively permeable barrier • Stroma – where light independent reactions take place • Grana – stacks of thylakoids • Thylakoid membrane– pigments, primary electron acceptors and proteins of the electron transport chain are all embedded there.

  12. Challenge Questions • Which energies of light are absorbed the MOST by chlorophyll a? • Answer: Red and violet range

  13. Challenge Question What does the diagram to the left represent? The diagram to the left is an absorption spectrum. An absorption spectrum is a graph that represents the energies of light that pigments absorb.

  14. Challenge Question • How do primary electron acceptors differ from proteins found in the electron transport chain? • Answer: Primary electron acceptors get their electrons directly from chlorophyll a. Once the primary electron acceptors get their electrons they then donate them to the proteins of the electron transport chain.

  15. Challenge Question • Where are primary electron acceptors and proteins of the electron transport chain located within the chloroplast? • Answer: They are embedded in the thylakoid membrane.

  16. Challenge Question • Completely explain the role of pigments in capturing the energy of light? • Answer: Each pigment absorbs a specific wavelength (color) of light energy that corresponds to the energy needed to excite their outermost electrons. As the energy is absorbed it is used to move the outer electrons to higher energy levels farther from the nucleus. These higher energy (excited) electrons then LEAVE and go from accessory pigments to chlorophyll a.

  17. Challenge Question • What is the benefit of clustering the pigments within photosystems? • Since each accessory pigment captures energies not captured by chlorophyll a, the maximum amount of energy can be absorbed. Also, the excited electrons of the accessory pigments can go directly to chlorophyll a then to the primary electron acceptor and then to the proteins of the electron transport chain.

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