Cellular Organelles and Photosynthesis Mechanisms
Ribosomes and Protein Synthesis
The primary function of ribosomes is to synthesize proteins.
Rough Endoplasmic Reticulum (RER)
The RER consists of a set of flattened saccules, tubules, and cisterns arranged concentrically near the nucleus. Its functions include:
- Protein Storage: Proteins formed in the ribosomes are introduced into this organelle and transported through tubules.
- Glycosylation: Enzymes and proteins remain inactive until specific sugars are added.
- Nuclear Membrane Formation: The double nuclear membrane is reformed after cell division from this organelle.
- Chemical Storage: It serves as a storage site for various cellular substances.
Smooth Endoplasmic Reticulum (SER)
The SER has a structure similar to the RER but lacks ribosomes and is located further from the nucleus. Its primary function is the synthesis of lipid bilayers, which are essential for creating and maintaining cell membranes.
Light-Dependent Reactions of Photosynthesis
The light-dependent phase consists of cyclic and acyclic stages. The acyclic phase involves the following steps:
1. Photoexcitation of Chlorophyll P680
Light energy reaches Photosystem II, causing the release of two electrons from chlorophyll, which are then transported to plastoquinone.
2. Photolysis of Water
The two electrons lost by chlorophyll are recovered through the rupture of water molecules: H2O → 2H+ + 2e- + ½O2. Protons accumulate in the intratilacoidal space, while oxygen diffuses out of the chloroplast.
3. Electron Transport in the Thylakoid Membrane
Electrons captured by plastoquinone are transported through redox components, generating energy to pump protons from the stroma into the intratilacoidal space, creating an acidic environment.
4. Photoexcitation of P700
Electrons reach the P700 chlorophyll in Photosystem I. Photons excite the chlorophyll, causing it to lose electrons.
5. Photo-reduction
The electrons from P700 are transported to membrane components, including ferredoxin, which reduces NADP+.
6. Photophosphorylation
ATP is formed from ADP and inorganic phosphorus in the stroma, catalyzed by the enzyme ATP synthase.