Question 4(1 point) Recall our comparison of photochemistry between a photoreceptor and a photosystem. What is one way a photosystem is different from a photoreceptor? A. It is only in a photosystem where a pigment molecule is specifically bound to a protein. B. It is only in a photosystem where light absorption triggers a change in shape of a protein. C. It is only in a photosystem where light absorption excites an electron. D. It is only in a photosystem where light absorption triggers the oxidation (loss of an e-) of pigment. √ Saved

The correct answer is **D. It is only in a photosystem where light absorption triggers the oxidation (loss of an e-) of pigment.**<br /><br />Here's why:<br /><br />* **Photosystems:** The key function of a photosystem is to convert light energy into chemical energy through a process called photoinduced charge separation. This means that when a pigment molecule in the photosystem absorbs light, an electron is excited and then transferred to another molecule (an electron acceptor), effectively oxidizing the original pigment. This initiates the electron transport chain, which ultimately leads to ATP synthesis.<br /><br />* **Photoreceptors:** Photoreceptors also use pigments to absorb light, but their primary function is to detect light and initiate a signaling cascade. While light absorption *does* cause a change in the photoreceptor's pigment shape (which subsequently leads to changes in protein conformation), the pigment itself is not oxidized. The energy from light absorption is used to trigger a conformational change in the protein, which then initiates a signaling pathway. The excited electron is not transferred to an electron acceptor as it is in a photosystem.<br /><br />Let's look at why the other options are incorrect:<br /><br />* **A:** Pigments are bound to proteins in *both* photosystems and photoreceptors. This binding is crucial for the proper function of both.<br />* **B:** Light absorption triggers a change in protein shape in *both* photosystems and photoreceptors. This conformational change is central to how they function.<br />* **C:** Light absorption excites an electron in *both* photosystems and photoreceptors. The difference lies in what happens *after* the electron is excited.<br />