How Thylakoids Drive Energy Conversion in Plants
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.
What are Thylakoids?
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.
Structure of Thylakoids
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.
Role of Thylakoids in Photochemical Reactions
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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The Function of Thylakoid in Chloroplast is Listed Below.
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.
ATP Synthesis
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.
Protein Synthesis
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 Microbial Plants
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.
FAQs on Thylakoids: Structure, Functions & Importance
1. What is a thylakoid in simple terms?
A thylakoid is a membrane-bound, sac-like compartment located inside the chloroplasts of plant cells and cyanobacteria. It is the primary site for the light-dependent reactions of photosynthesis. These sacs are often stacked on top of each other to form structures called grana.
2. What is the main function of a thylakoid?
The main function of the thylakoid is to carry out the light-dependent reactions of photosynthesis. Its membrane contains chlorophyll and other pigments that capture light energy from the sun. This energy is then used to split water molecules (photolysis), release oxygen, and produce energy-rich molecules like ATP and NADPH, which fuel the subsequent light-independent reactions.
3. How are thylakoids, grana, and stroma related within a chloroplast?
These three components work together inside a chloroplast:
- Thylakoids: Individual flattened sacs where light reactions occur.
- Grana (singular: granum): Stacks of thylakoids, which increase the surface area for efficient light absorption.
- Stroma: The fluid-filled space surrounding the grana. It contains enzymes for the light-independent reactions (Calvin cycle) and uses the ATP and NADPH produced by the thylakoids to create sugars.
In essence, thylakoids (in grana) capture light energy, and the stroma uses that energy to synthesise food.
4. What makes the thylakoid membrane uniquely suited for photosynthesis?
The thylakoid membrane is uniquely suited for photosynthesis because it embeds all the necessary machinery in a specific order. It contains photosystems (PS I and PS II), electron carriers, and ATP synthase enzymes. This arrangement creates an electron transport chain that efficiently converts light energy into chemical energy. Its lipid bilayer is also largely impermeable to protons, which is crucial for building the proton gradient needed for ATP synthesis.
5. Why is the thylakoid lumen important?
The thylakoid lumen, the space inside the thylakoid sac, is critical because it acts as a reservoir for protons (H+ ions). During the light reactions, protons are pumped into the lumen from the stroma. This accumulation creates a high concentration of protons, forming a proton motive force or electrochemical gradient. This gradient powers the ATP synthase enzyme, which generates ATP as protons flow back out into the stroma.
6. What would happen to photosynthesis if the thylakoid membrane became leaky to protons?
If the thylakoid membrane became leaky to protons, the process of photophosphorylation (ATP synthesis using light energy) would stop or be severely reduced. A leaky membrane would prevent the buildup of the essential proton gradient in the lumen. Without this gradient, the ATP synthase enzyme would have no power source, and the cell could not produce the ATP required for the Calvin cycle to fix carbon dioxide into sugars. Consequently, photosynthesis would fail.
7. Do prokaryotes like cyanobacteria have thylakoids?
Yes, photosynthetic prokaryotes like cyanobacteria have thylakoids, but they are not enclosed within chloroplasts (as prokaryotes lack membrane-bound organelles). Instead, their thylakoid membranes are located freely within the cytoplasm. These thylakoids contain the photosynthetic pigments and machinery needed to perform the light-dependent reactions, functioning analogously to those in plant chloroplasts.
8. What are the two types of thylakoids in a chloroplast?
There are two main types of thylakoids in a chloroplast, distinguished by their location and function:
- Granal Thylakoids: These are the disc-shaped membranes arranged in tight stacks to form the grana. They are rich in Photosystem II (PS II) and are the primary sites of water splitting and oxygen evolution.
- Stromal Thylakoids (or Stroma Lamellae): These are unstacked thylakoids that connect different grana. They are exposed to the stroma and are rich in Photosystem I (PS I) and ATP synthase.
