The plant cell organelles are quite complex as they function as a remarkable site for synthesis or any other metabolic biochemical reactions occurring in the cells. One such cell organelle present in the green tissues of plants is the chloroplast. A significant location in this cell organelle is thylakoid. In this article, we will study what is thylakoid, its structural features, and its functions.
As mentioned earlier, thylakoids are the significant portions of the chloroplasts of the green tissues in leaves and cyanobacteria that produce their own food via photosynthesis. They are sheet-like structures bounded by membranes containing chlorophyll. These are the prime locations where sunlight energy is trapped.
The word ‘Thylakoid’ has been derived from the Greek word ‘Thylakos’. It means a pouch or a sac. The ending ‘oid’ added to this word means ‘sac or pouch like’. This is the location where the photochemical reactions for photosynthesis take place. It is also known as lamellae. This term can also refer to the portion of thylakoids that connects with grana.
As we all know, thylakoids remain submerged in the cytoplasmic material or space inside the chloroplasts. This space is called the stroma. It contains enzymes, ribosomes, along with the DNA material of this cell organelle. The thylakoid structure comprises a simple thylakoid membrane creating an empty tubular space inside called thylakoid lumen.
These thylakoids are stacked to form a cumulative structure called a granum. A granum looks like a stack of coins. These units are interconnected by thylakoids in the chloroplasts of higher plants. These connections are called stroma thylakoids. Each chloroplast has 10 to 100 such grana inside connected by the stroma thylakoids creating tunnels for communication and exchange of biochemical substances. The biochemical structure of the grana and stroma thylakoids vary in terms of the protein constituents of the membrane.
This part of the chloroplast participates in the photochemical process of, photosynthesis. It contains photosynthetic pigments such as chlorophyll A, zeaxanthin, β-carotene, echinenone, myxoxanthophyll, etc to trap the energy of sunlight and provide the energy for carrying the other biochemical reactions involved in photosynthesis. In fact, thylakoids form the prime sites for carrying light-dependent or photochemical reactions.
The reason for the formation of so many thylakoids inside a chloroplast is to increase the surface area-volume ratio in order to provide more space and efficiency to carry photosynthetic reactions that depend on sunlight. The internal pH of this section is 4 whereas the stroma has a pH of 8. It happens due to the pumping of protons (H+) inside the lumen of thylakoids. Refer to the thylakoid membrane diagram to understand the structural features and correlate its features with the respective functions.
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Photolysis of Water
If we look carefully into this term, we can understand the dissociation of the water molecules takes place in presence of solar energy. It happens inside the thylakoids (lumen) where solar energy is used to split water molecules and produce electrons for the transport chains. Protons are then pumped inside the thylakoid lumen for creating a proton gradient. In this process, oxygen is produced as a byproduct.
Electron Transport Chain
The electrons produced during the photolysis of water are delivered to the electron transport chains of the photosystems. There are two photosystems present in this area. Photosystem I utilize light to reduce NADP+ to form H+ and NADPH. Photosystem II uses solar energy for the oxidation of water molecules to form O2 along with protons (H+) and electrons. Both these systems have antenna complexes that can trap sunlight of different wavelengths.
Both these systems inside the lumen covered by the thylakoid membrane are capable of producing ATP. It is the energy currency of almost all living beings used in different biological processes. Similarly, this energy currency is used for various other photosynthetic processes.
It is produced using the ATP synthase enzyme which resembles the mitochondrial ATPase. These energy molecules are produced by this enzyme integrated into the thylakoid membrane. The ATPs produced are then used for conducting light-independent photosynthetic reactions.
The thylakoid lumen contains a set of proteins that are used for photosynthesis, protein processing, redox reactions, metabolism, and the defence of plant cells. For example, plastocyanin is used for electron transportation from cytochrome to Photosystem I.
Thylakoids in unicellular plants such as bacteria and algae are not stacked to form grana. They remain un-stacked and scattered inside the chloroplasts of the eukaryotes such as algae. On the other hand, prokaryotes do not have chloroplasts. The entire cell of a prokaryote (bacteria) acts as a thylakoid.
These cells have thylakoid membranes surrounding bacterial DNA, carboxysomes, and cytoplasm. This membrane functions in the same way a eukaryotic thylakoid does.
This is all about thylakoids present in the eukaryotic and prokaryotic cells. The prime thylakoid function is to offer a controlled site for carrying the light-dependent photosynthetic reactions and to generate ATP for conducting other reactions of photosynthesis. Refer to a detailed diagram of this part of the chloroplast and understand how it functions to carry the most important metabolic reactions for plants.
1. Why prokaryotic cells do not have chloroplasts?
Prokaryotic cells are primitive cells that do not have a well-defined nucleus and cell organelles. These organisms generally exist in the form of unicellular beings. The single-cell acts as the entire body and has very few functions to perform. In most cases, the primitive cell organelles perform multiple functions and contribute to the fewer requirements of resources for survival.
2. Why does a eukaryotic cell have proper cell organelles?
Eukaryotic cells are advanced in terms of the cell organelles present inside the cytoplasm. These organelles have evolved based on evolution to carry on specific functions to support life. Moreover, the eukaryotic cells of multicellular organisms are significant parts of tissues and organs. It means their functions will be defined based on the functions of an organ or tissue.