Thylakoid membrane function, structure, and definition

The thylakoid membrane function as the site that captures sunlight that is needed for photosynthesis. The structure of the thylakoid helps in accomplishing this role.

Table of Contents

Thylakoid Definition

A thylakoid is a sheet-like membrane-bound organelle in chloroplasts and cyanobacteria that is the site of light-dependent photosynthesis reactions

What are thylakoids?

Thylakoids are membrane-bound compartments found within chloroplasts and cyanobacteria. They are the location for light-dependent reactions in photosynthesis.

Thylakoids are made up of a thylakoid membrane that surrounds a thylakoid lumen. It is the location where chlorophyll is found, which is used to absorb light and used in biochemical reactions. Thylakoid is derived from the Greek word thylakos, which means pouch or sac. “Thylakoid” means “pouch-like” with the -oid suffix.

Thylakoids are also known as lamellae, though this term can refer to the portion of a thylakoid that connects grana.

Thylakoid membrane function

  1. Water photolysis
  2. Electron transport chains
  3. Chemiosmosis
  4. ATP generation

The thylakoids are the site for light-dependent reactions in photosynthesis. Therefore, the functions of the thylakoids membrane include light-driven water oxidation and oxygen evolution, proton pumping across thylakoid membranes coupled with the electron transport chain of the photosystems, and cytochrome complex. Another function of the thylakoid is ATP synthesis by the ATP synthase using the generated proton gradient.

Water photolysis

The first step in the function of the thylakoid membrane is water photolysis, which occurs on the thylakoid membrane’s lumen site. In this step, light energy is used to reduce or split water. This reaction generates electrons, which are required by electron transport chains and protons. The electrons are pumped into the lumen to create a proton gradient. The proton gradient created across the thylakoid membrane produces oxygen. Despite the fact that oxygen is required for cellular respiration, the gas produced by this reaction is released into the atmosphere.

Electron transport chains

The electrons produced by photolysis are directed to the photosystems of the electron transport chains. The photosystems contain an antenna complex that collects light at various wavelengths by using chlorophyll and related pigments. The localization of the thylakoid membrane electron transport chains ensures that the energy released can be used to establish an H+ ion gradient.

Photosystem I converts NADP+ to NADPH + H+ using light energy and is involved in both noncyclic and cyclic electron transport. The energized electron is passed down a chain in cyclic mode, eventually returning it (in its base state) to the chlorophyll that energized it.

Photosystem II is only active in noncyclic transport and uses light energy to oxidize water molecules, producing electrons (e), protons (H+), and molecular oxygen (O2). Electrons are not conserved in this system, but rather enter from oxidized 2H2O (O2 + 4 H+ + 4 e) and exit with NADP+ when it is finally reduced to NADPH.

During photosynthesis, two types of electron transport are used namely Noncyclic electron transport, also known as noncyclic photophosphorylation, which generates NADPH + H+ and ATP. The other type of electron transport used is cyclic electron transport or cyclic photophosphorylation which generates only ATP.

The noncyclic electron flow is dependent on both photosystems, whereas the cyclic electron flow is dependent on only photosystem I.

Chemiosmosis

The establishment of chemiosmotic potential is a critical function of the thylakoid membrane and its associated photosystems. The carriers in the electron transport chain actively transport protons from the thylakoid membrane and stroma to the lumen by utilizing some of the electron’s energy. During the process of photosynthesis, the lumen becomes acidic, with a pH as low as 4, compared to the stroma’s pH of 8. Chemiosmosis in thylakoid membrane represents a 10,000-fold gradient in proton concentration across the thylakoid membrane.

The source of the proton gradient in thylakoid membrane

Protons in the lumen are derived from three primary sources.

  • In the lumen, photosystem II oxidizes water to oxygen, protons, and electrons.
  • During non-cyclic electron transport, the transfer of electrons from photosystem II to plastoquinone consumes two protons from the stroma. When the reduced plastoquinol is oxidized by the cytochrome b6f protein complex on the lumen side of the thylakoid membrane, they are released into the lumen. Electrons from the plastoquinone pool pass through the cytochrome b6f complex. This type of integral membrane assembly is similar to cytochrome bc1.
  • In the process of cyclic electron transport, ferredoxin reduces plastoquinone, which transfers two protons from the stroma to the lumen.

The consumption of protons in the stroma to produce NADPH from NADP+ at the NADP reductase also contributes to the proton gradient.

ATP generation

The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that of mitochondria, and the required energy is derived from the proton motive force (PMF). The PMF is the product of a proton chemical potential (as determined by the proton concentration gradient) and a transmembrane electrical potential (given by charge separation across the membrane). Unlike mitochondrial inner membranes, which have a significantly higher membrane potential due to charge separation, thylakoid membranes do not have a charge gradient.

To compensate for the lack of a charge gradient, the 10,000 fold gradient in proton concentration across the thylakoid membrane is much higher than the 10 fold gradient across the inner membrane of mitochondria. The resulting chemiosmotic potential between lumen and stroma is sufficient to drive ATP synthesis via ATP synthase. ADP + Pi are combined into ATP as protons travel back down the gradient through channels in ATP synthase. The light dependent reactions in the thylakoid membrane are thus linked to the synthesis of ATP via the proton gradient.

Thylakoid membrane structure

  • Membrane
  • Lumen
  • Granum and stroma lamellae

The above-mentioned make up the structure of thylakoid membrane.

This is a diagram of the thylakoid membrane showing its various structures.
The thylakoid membrane diagram labeled above shows the structures found in the membrane.

Membrane

The membrane is the site of the light-dependent reactions of photosynthesis, with photosynthetic pigments embedded directly in it. The membrane consists of an alternating pattern of dark and light bands, each measuring 1 nanometre. The thylakoid membrane has lipid bilayer like acidic lipids. These lipids are also found in cyanobacteria, and other photosynthetic bacteria and play a role in the functional integrity of photosystems.

Higher plant thylakoid membranes are primarily made up of phospholipids and galactolipids that are arranged asymmetrically along and across the membranes. Thylakoid membranes are richer in galactolipids than phospholipids, and they are primarily composed of hexagonal phase II lipids that form monogalacotosyl diglyceride lipids. Despite their distinct composition, plant thylakoid membranes have been shown to be primarily made of lipid-bilayer that is dynamically organized.

The lipids that make up the thylakoid membranes, which are rich in high-fluidity linolenic acid, are synthesized in a complex pathway that involves the exchange of lipid precursors between the endoplasmic reticulum and the inner membrane of the plastid envelope and are then transported from the inner membrane to the thylakoids via vesicles.

Lumen

The thylakoid lumen is a continuous aqueous phase that is surrounded by the thylakoid membrane. It is essential for photophosphorylation during photosynthesis. Protons are pumped across the thylakoid membrane into the lumen during the light-dependent reaction, making it acidic in the region of a pH of 4.

Granum and stroma lamellae

Thylakoids in higher plants are organized into a granum-stroma membrane assembly. A granum (plural grana) is a thylakoid disc stack. Chloroplasts can have between 10 and 100 grana. Grana contribute to the high surface area to volume ratio of chloroplasts.

Stroma thylakoids, also known as intergranal thylakoids or lamellae, connect grana. The distinguishing factor between grana thylakoids and stroma thylakoids is their protein composition.

At the granal interface, the stroma lamellae are organized in wide sheets perpendicular to the grana stack axis, forming multiple right-handed helical surfaces.

Where is the thylakoid membrane location?

The thylakoid membrane location in plant cells is the chloroplasts.

Thylakoid membrane photosynthesis

Photosynthesis occurs in chloroplasts, which contain chlorophyll. The chloroplasts in plant cells are surrounded by a double membrane and have a third inner membrane known as the thylakoid membrane, which forms long folds within the organelle. Thylakoid membranes appear like coins stacked together in electron micrographs, despite the fact that the compartments they form are connected like a maze of chambers. The green pigment chlorophyll which plants use to absorb light for photosynthesis is found within the thylakoid membrane.

Role of the thylakoid in photosynthesis

The thylakoid membrane in photosynthesis houses photosynthetic pigments (such as chlorophyll). These pigments are embedded in the thylakoid membrane. The thylakoid membrane chlorophyll is the site of the light-dependent reactions of photosynthesis. The shape of the grana’s stacked coil provides the chloroplast with a high surface area to volume ratio, which aids photosynthesis efficiency.

During photosynthesis, the thylakoid lumen is used for photophosphorylation. Protons are pumped into the lumen by light-dependent reactions in the membrane, lowering its pH to 4.

Thylakoid membrane system in algae and cyanobacteria

Cyanobacteria are photosynthetic prokaryotes with complex membrane systems. They have an internal thylakoid membrane system that houses the fully functional electron transfer chains of photosynthesis and respiration. The presence of multiple membrane systems gives these bacteria cells a distinct level of complexity.

Cyanobacteria must be able to reorganize membranes, synthesize new membrane lipids, and target proteins to the appropriate membrane system. The cyanobacterial cell’s outer membrane, plasma membrane, and thylakoid membrane all serve specific functions.

In contrast to the thylakoid network of algae and higher plants, which is divided into grana and stroma lamellae, cyanobacteria’s thylakoids are organized into multiple concentric shells that split and fuse to form a highly connected network. This produces a continuous network that encloses a single lumen (as seen in the chloroplasts of higher plants) and allows water-soluble and lipid-soluble molecules to diffuse through the entire membrane network.

Besides that, perforations are frequently seen within the parallel thylakoid sheets. These gaps in the membrane allow particles of various sizes, such as ribosomes, glycogen granules, and lipid bodies, to move throughout the cell. The relatively large distance between the thylakoids allows for the phycobilisomes, which are external light-harvesting antennae. This macrostructure, like that of higher plants, exhibits some flexibility in response to changes in the physicochemical environment.

Proteins in the thylakoid membrane

  • Integral membrane proteins
  • Lumen proteins

Many integral and peripheral membrane proteins, as well as lumenal proteins, are found in thylakoids. The thylakoid is made up of at least 335 different proteins. 89 of these are in the lumen, 116 are integral membrane proteins, 62 are stroma-side peripheral proteins, and 68 are lumen-side peripheral proteins.

Integral membrane proteins

Integral membrane proteins in thylakoid membranes play an important role in light-harvesting and photosynthesis’s light-dependent reactions. The thylakoid membrane contains four major protein complexes namely photosystems I and II, cytochrome b6f complex, and ATP synthase.

Photosystem II is mostly found in the grana thylakoids, while photosystem I and ATP synthase are mostly found in the stroma thylakoids and the grana’s outer layers. The cytochrome b6f complex is evenly distributed across thylakoid membranes.

Because the two photosystems in thylakoid membrane are located separately in the thylakoid membrane system, mobile electron carriers are required to shuttle electrons between them. Therefore, these mobile electron carriers are plastoquinone and plastocyanin. Plastoquinone transports electrons from the photosystem II complex to the cytochrome b6f complex, whereas plastocyanin transports electrons from the cytochrome b6f complex to the photosystem I complex.

Light energy is used by these proteins to drive electron transport chains, which generate a chemiosmotic potential across the thylakoid membrane and NADPH, a product of the terminal redox reaction. During photophosphorylation, the ATP synthase uses chemiosmotic potential to produce ATP.

Photosystems I and II

These photosystems are light-driven redox centers comprised of an antenna complex that harvests light at various wavelengths using chlorophylls and accessory photosynthetic pigments such as carotenoids and phycobiliproteins. Each antenna complex contains between 250 and 400 pigment molecules, and the energy they absorb is transferred to a specialized chlorophyll a at the reaction center of each photosystem via resonance energy transfer. When either of the 2 chlorophyll a molecules at the reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule.

Photosystem I contains a pair of chlorophyll a molecules, designated P700, at its reaction center that absorbs light at a maximum wavelength of 700 nm. Photosystem II contains P680 chlorophyll, which absorbs light best at 680 nm.

To fully understand the abbreviations, the letter P stands for pigment, and the number represents the specific absorption peak in nanometers for chlorophyll molecules in each reaction center.

Cytochrome b6f complex

The cytochrome b6f complex is a component of the thylakoid electron transport chain that links electron transfer to proton pumping into the thylakoid lumen. It is located between the two photosystems and transfers electrons from photosystem II-plastoquinone to plastocyanin-photosystem I.

ATP synthase

The CF1FO-ATP synthase is a thylakoid ATP synthase that is similar to the mitochondrial ATPase. The CF1-part sticks into the stroma and is integrated into the thylakoid membrane. Thus, ATP synthesis occurs on the stromal side of the thylakoids, where ATP is required for light-independent photosynthesis reactions.

Lumen proteins

Based on their targeting signals, luminal proteins can be predicted computationally. Out of the predicted lumenal proteins containing the Tat signal in Arabidopsis, the largest groups with known functions are 19% involved in protein processing (proteolysis and folding), 18% in photosynthesis, 11% in metabolism, and 7% in redox carriers and defense.

Protein expression

Chloroplasts have their own genome that encodes several thylakoid proteins. However, extensive gene transfer from the chloroplast genome to the cell nucleus occurred during plastid evolution from their cyanobacterial endosymbiotic ancestors. As a result, the four major thylakoid protein complexes are encoded partially by the chloroplast genome and partially by the nuclear genome.

To ensure proper stoichiometry and assembly of these protein complexes, plants have developed several mechanisms to co-regulate the expression of the different subunits encoded in the two different organelles.

For example, regulation of the transcription of nuclear genes encoding components of the photosynthetic apparatus is done by light. Whereas, phosphorylation via redox-sensitive kinases in thylakoid membranes regulates the biogenesis, stability, and turnover of thylakoid protein complexes.

The presence or absence of assembly partners influences the translation rate of chloroplast-encoded proteins (controlled by epistasy of synthesis). This mechanism involves negative feedback via excess protein binding to the 5′ untranslated region of chloroplast mRNA.

Chloroplasts must also balance the photosystem I and II ratios for the electron transfer chain. The redox state of the electron carrier plastoquinone in the thylakoid membrane has a direct effect on the transcription of chloroplast genes encoding proteins of the photosystems’ reaction centers, thus balancing the electron transfer chain.

FAQs on Thylakoid membrane

The region of the chloroplast between the inner membrane and the thylakoid membrane is called?

The region of the chloroplast between the inner membrane and the thylakoid membrane is called the lumen.

A light-absorbing molecule in thylakoid membrane is?

Chlorophyll a, chlorophyll b, and carotenoids are the light-harvesting molecules found in thylakoid membranes.

What causes the gradient across the thylakoid membrane?

The formation of a proton gradient across the thylakoid layer is caused by the accumulation of protons on the internal side of the lumen and the decrease of protons in the stroma.

Where is the thylakoid membrane?

The thylakoid membrane is located between the stroma and the aqueous lumen of the chloroplast.

What does the thylakoid membrane do?

It is the location where chlorophyll is found, which is used to absorb light, which they use in biochemical reactions.

Which of the following is a proton pump in thylakoid membrane?

Platoquinol—plastocyanin reductase proton pump is an enzyme related to Complex III that is found in the thylakoid membrane of chloroplasts of plants, cyanobacteria, and green algae. Electron transport drives this proton pump, which catalyzes the transfer of electrons from plastoquinol to plastocyanin.

What is the thylakoid membrane?

The thylakoid membrane is the site for oxygenic photosynthesis using photochemical and electron transport reactions. This also answers the question of what is a thylakoid membrane.

What is a series of proteins embedded in the thylakoid membrane?

Photosystem II (PSII), cytochrome b6/f complex, photosystem I (PSI), and ATP synthase are thylakoid membrane protein complexes or series.

What is thylakoid membrane definition in biology?

The thylakoid membrane is a saclike membrane in plant chloroplasts that are often arranged in stacks called grana and serves as the site of photosynthesis light reactions.

Which part of photosynthesis does not occur inside the thylakoid membrane?

The part of photosynthesis that does not occur in the thylakoid membrane is the light-independent photosynthesis otherwise known as the Calvin cycle.

Why is it important that nadp+ reductase is on the stromal side of the thylakoid membrane?

Because it helps in the transfer of electrons during photosynthesis from photosystem I to NADPH. 

What happens at a proton pump of the thylakoid membrane?

The transfer of electrons from one protein pump to another protein pump happens.

The three molecular complexes contained in the thylakoid membrane are?

The three molecular complexes contained in the thylakoid membrane are photosystem II (PSII), cytochrome b6f (cytb6f), and photosystem I (PSI),

What pigments are found in the thylakoid membrane?

The pigments in thylakoid membrane are chlorophyll and carotenoids.

The proton gradient across the thylakoid membrane is required to?

It is required to synthesize ATP.

Does the thylakoid membrane contain a protein called ATP synthase?

Yes, the thylakoid membrane contains a protein called ATP synthase.

Where is the thylakoid membrane located?

The thylakoid membrane is located in the chloroplasts.

Is ATP synthase found in the thylakoid membrane?

Yes, the ATP synthase is found in the thylakoid membrane.

What is the thylakoid membrane stacked into?

The thylakoid membrane is stacked into the grana.

What is inside the thylakoid membrane?

Embedded inside the thylakoid membrane are chlorophyll molecules.

What is the thylakoid membrane light reaction?

This is the reaction that occurs in the thylakoid membrane in the presence of sunlight.

What effect would a damaged thylakoid membrane have on photosynthesis?

The damage will affect the thylakoid membrane mitochondria electron flow and the splitting of the water molecule would be affected if the thylakoid was damaged.

The flow of what particle across the thylakoid membrane powers the production of ATP?

The flow of H+ ions across the thylakoid membrane through the ATP synthase produces ATP

Which part of the molecule goes through the thylakoid membrane?

The lumen proteins especially plastocyanin moves through the thylakoid membrane.

What causes proton gradient across the thylakoid membrane?

When the number of protons in the stroma decreases, protons accumulate, and the pH value in the lumen decreases, the proton gradient across the thylakoid membrane is created and it increases.

What is the relationship between chloroplast thylakoid membrane and inner mitochondrial membrane?

The relationship is such that the membrane is critical in chloroplasts, where it replaces the inner mitochondrial membrane in electron transport and chemiosmotic ATP generation.

What is the role of thylakoid membrane?

The thylakoid membrane is used to absorb light that is used in biochemical reactions. This is because the membrane is where chlorophyll is found.

What is in the thylakoid membrane?

Embedded in the thylakoid membrane are two photosystems, named photosystem I and photosystem II.

The b6-f complex in the thylakoid membrane receives electrons from?

PSII and energy from PSI pump protons from the stroma to the lumen.

What is the function of thylakoid membrane in chloroplast?

The thylakoid membrane function as the site for light-dependent photosynthesis reactions.

What is the thylakoid membrane made of?

The thylakoid is made up of membrane, lumen, granum and stroma lamellae.
This is also the thylakoid membrane structure and function in light dependent reactions.

What do mitochondrial and thylakoid membrane have in common?

Both organelles are involved in energy transformation and they both have multiple membranes that separate their interiors into compartments.

Where is the electron transport chain in the thylakoid membrane?

The electron transport chain components are embedded in the thylakoid membrane.

Chemiosmosis in the thylakoid membrane is directly responsible for?

Chemiosmosis in the thylakoid membrane is directly responsible for the formation of molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis.

Why are protons pumped across the thylakoid membrane important?

Protons pumped across the thylakoid membrane are essential for the synthesis of ATP.

What events occur in the thylakoid membrane during the light-dependent reactions?

In order to trace what events occur in the thylakoid membrane during the light-dependent reactions, one has to view the whole stages of photosynthesis. In the first stage, light dependent reactions happen in the thylakoid membrane where light energy is captured and stored as ATP.

Protons are pumped across the thylakoid membrane of the chloroplast during the?

Protons are pumped across the thylakoid membrane of the chloroplast during the electron-transport process.

What are the clusters in the thylakoid membrane of chlorophyll and other pigments?

The clusters are known as photosystems.

How is a proton gradient established across the thylakoid membrane?

A proton gradient across the thylakoid membrane is established, with a higher concentration of protons and lower pH in the thylakoid lumen.

What is the purpose of the thylakoid membrane?

The purpose of the thylakoid membrane is to absorb light for photosynthesis.

What are the light harvesting units of the thylakoid membrane are called?

They are called antenna complexes; LH or LHC

What structures are found on the thylakoid membrane?

The specialized structures of photosystem I (PSI) and photosystem II (PSII), are found in the thylakoid membrane systems of cyanobacteria, as well as plant and algal chloroplasts (PSII),

What are the groups of photosynthetic pigment molecules organized in the thylakoid membrane are called?

They are called light-harvesting molecules.

What holds chlorophyll in the thylakoid membrane?

In thylakoid membranes, chlorophyll pigments are found in packets called quantasomes.

What does thylakoid membrane do in photosynthesis?

The thylakoid membrane houses the pigments that are needed for the light reaction of photosynthesis.

If one punctures a hole through the thylakoid membrane what happens to ATP synthase?

The thylakoid lumen will not be intact in the punctured thylakoid, so the H+concentration ions will not be developed. As a result, ATP synthase will not function and no ATP will be synthesized.

What happens at the thylakoid membrane?

Light-dependent reactions occur in the chloroplast thylakoid membrane.

Can thylakoid membrane be found in animal cells?

The thylakoid membrane in an animal cell is not present.

What pigments are found in thylakoids?

A class of pigments that are present in the thylakoid membrane is chlorophylls.

What causes proton gradient across the thylakoid membrane?

The creation of proton gradient across the thylakoid membrane is caused by the accumulation of protons on the internal side of the lumen and the decrease of protons in the stroma.

What is the importance of protons thylakoid membrane?

Light-induced -proton gradients are required for ATP synthesis.

What is a series of carriers that pass electrons in thylakoid membrane?

A series of carriers that pass electrons in thylakoid membrane are called photosystems.

Is the isolation of thylakoid membrane proteins possible?

Yes, it is possible.

The pH inside the thylakoid membrane is _ than the stroma?

The pH inside the thylakoid membrane is less than the pH of the stroma.

The video briefly explains the thylakoid membrane.

Leave a Comment