The granas are structures arising from grouping the thylakoid located within the chloroplasts of plant cells . These structures contain photosynthetic pigments (chlorophyll, carotenoids, xanthophyll) and various lipids. In addition to the proteins responsible for generating energy, such as ATP-synthetase.
In this regard, thylakoids constitute flattened vesicles located on the inner membrane of chloroplasts. In these structures, light capture is carried out for photosynthesis and photophosphorylation reactions . In turn, the stacked and granum thylakoids are embedded in the stroma of the chloroplasts.
In the stroma, the thylakoid stacks are connected by stromal laminae. These connections usually go from one granum through the stroma to the neighboring granum. In turn, the central aqueous zone called the thylakoid lumen is surrounded by the thylakoid membrane.
Two photosystems (photosystem I and II) are located on the upper silvers. Each system contains photosynthetic pigments and a series of proteins capable of transferring electrons. Photosystem II is located in the grana, responsible for capturing light energy during the first stages of non-cyclic electron transport.
For Neil A. Campbell, author of Biology: Concepts and Relationships (2012), grana are bundles of solar energy from chloroplast. They are the places where chlorophyll traps energy from the sun .
The grana – singular, granum – originate from the internal membranes of chloroplasts. These hollowed-out pile-shaped structures contain a series of closely packed, thin, circular compartments: the thylakoids.
To exert its function in photosystem II, the grana within the thylakoid membrane contains proteins and phospholipids. In addition to chlorophyll and other pigments that capture light during the photosynthetic process.
In fact, the thylakoids of a grana connect with other grana, forming within the chloroplast a network of highly developed membranes similar to that of the endoplasmic reticulum.
Grana is suspended in a liquid called the stroma, which has ribosomes and DNA , used to synthesize some proteins that make up the chloroplast.
The structure of the granum is a function of the grouping of thylakoids within the chloroplast. The grana is made up of a pile of disk-shaped membranous thylakoids, submerged in the stroma of the chloroplast.
Indeed, chloroplasts contain an internal membranous system, which in higher plants is designated as grana-thylakoids, which originates from the inner membrane of the envelope.
In each chloroplast there is usually a variable number of granum, between 10 and 100. The grains are linked to each other by stromal thylakoids, intergranal thylakoids or, more commonly lamella.
An examination of the granum with the transmission electron microscope (TEM) allows to detect granules called quantosomes. These grains are the morphological units of photosynthesis.
Likewise, the thylakoid membrane contains various proteins and enzymes, including photosynthetic pigments. These molecules have the ability to absorb the energy of photons and initiate the photochemical reactions that determine the synthesis of ATP .
Grana as a constituent structure of chloroplasts, promotes and interacts in the photosynthesis process. Thus, chloroplasts are energy converting organelles.
The main function of chloroplasts is the transformation of electromagnetic energy from sunlight into energy from chemical bonds . Chlorophyll, ATP synthetase and ribulose bisphosphate carboxylase / oxygenase (Rubisco) participate in this process.
Photosynthesis has two phases:
- A light phase, in the presence of sunlight, where the transformation of light energy into a proton gradient occurs, which will be used for ATP synthesis and for the production of NADPH.
- A dark phase, which does not require the presence of direct light, however, does require the products formed in the light phase. This phase promotes the fixation of CO2 in the form of phosphate sugars with three carbon atoms.
The reactions during photosynthesis are carried out by the molecule called Rubisco. The light phase occurs in the thylakoid membrane, and the dark phase in the stroma.
Phases of photosynthesis
The photosynthesis process fulfills the following steps:
1) Photosystem II breaks down two water molecules, giving rise to an O2 molecule and four protons. Four electrons are released to the chlorophylls located in this photosystem II. Separating other electrons previously excited by light and released from photosystem II.
2) The released electrons pass to a plastoquinone that gives them to cytochrome b6 / f. With the energy captured by the electrons, it introduces 4 protons inside the thylakoid.
3) The cytochrome b6 / f complex transfers the electrons to a plastocyanin, and this to the photosystem I complex. With the energy of light absorbed by the chlorophylls, it manages to raise the energy of the electrons again.
Related to this complex is ferredoxin-NADP + reductase, which modifies NADP + into NADPH, which remains in the stroma. Likewise, the protons attached to the thylakoid and the stroma create a gradient capable of producing ATP.
In this way, both NADPH and ATP participate in the Calvin cycle, which is established as a metabolic pathway where CO2 is fixed by RUBISCO. It culminates in the production of phosphoglycerate molecules from ribulose 1,5-bisphosphate and CO2.
On the other hand, chloroplasts perform multiple functions. Among others, the synthesis of amino acids, nucleotides and fatty acids. As well as the production of hormones, vitamins and other secondary metabolites, and participate in the assimilation of nitrogen and sulfur.
Nitrate is one of the main sources of available nitrogen in higher plants. Indeed, in chloroplasts the process of transformation from nitrite to ammonium occurs with the participation of nitrite-reductase.
Chloroplasts generate a series of metabolites that contribute as a means of natural prevention against various pathogens, promoting the adaptation of plants to adverse conditions such as stress, excess water or high temperatures. Likewise, the production of hormones influences extracellular communication.
Thus, chloroplasts interact with other cellular components, either through molecular emissions or by physical contact, as occurs between the granum in the stroma and the thylakoid membrane.
- Atlas of Plant and Animal Histology. The cell. Chloroplasts Dept. of Functional Biology and Health Sciences. Faculty of Biology. University of Vigo. Recovered at: mmegias.webs.uvigo.es
- León Patricia and Guevara-García Arturo (2007) The chloroplast: a key organelle in life and in the use of plants. Biotecnología V 14, CS 3, Indd 2. Retrieved from: ibt.unam.mx
- Jiménez García Luis Felipe and Merchant Larios Horacio (2003) Cellular and Molecular Biology. Pearson Education. Mexico ISBN: 970-26-0387-40.
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