Lecture 2. Cell Morphology_Plant Cell Wall (Week 2) PDF

Module I Cell Morphology: Structure and Functions External Cellular Components Morphology and Function of Plant Cell Wall  Plant Cell Wall Structure  Structure of Plasmodesmata  Plant Cell Wall Functions 0...

Module I Cell Morphology: Structure and Functions External Cellular Components Morphology and Function of Plant Cell Wall  Plant Cell Wall Structure  Structure of Plasmodesmata  Plant Cell Wall Functions 06-09-2024 Cell Morphology: External Cellular Components 1 The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer of the cell wall, these layers are identified as the 1). Middle Lamella; 2). Primary Cell Wall, and 3). Secondary cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary cell wall. 06-09-2024 Cell Morphology: External Cellular Components 2  Plant Cell Wall Structure  Middle Lamella: This outer cell wall layer contains polysaccharides called pectins (Calcium pectate). Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.  Primary Cell Wall: This layer is formed between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The primary cell wall provides the strength and flexibility needed to allow for cell growth. [Composition: 25% cellulose + 25% hemicellulose + 35% pectin + 1-8% proteins approximately]  Secondary Cell Wall: This layer is formed between the primary cell wall and plasma membrane in some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells. 06-09-2024 Cell Morphology: External Cellular Components 3   The primary cell and middle lamella never occur in the form of a continuous layer, but many minute apertures through the cells of a tissue maintain cytoplasmic relation with each other. Such cytoplasmic junctions or bridges between the adjacent cells are known as plasmodesmata. Plasmodesmata  Plasmodesmata are mostly found in plant cells. In animal cells, similar structures are presently called gap junctions.  Plasmodesmata permit to pass a molecule directly from one cell to another and are important in cellular communication.  They are essential for plant life because they serve as a channel for conveying water, fluids and transport of metabolites during developmental and defense signaling.  They permit the passage of molecules weighing less than 800 Da.  Transport through the plasmodesmata is also found under complex regulation which may involve Ca2+ and protein phosphorylation.  The plasmodesmata (singular, plasmodesma) were first reported by Strasburger in 1901.  The word plasmodesma is derived from the Latin word ‘plasmo’ meaning fluid and the Greek ‘desma’ meaning bond 06-09-2024 Cell Morphology: External Cellular Components 4  Plasmodesmata  The number of plasmodesmata may vary from one cell to another. For example, the number of plasmodesmata may vary from 1- 15 or greater per square micrometer of the wall surface. A typical plant cell will have plasmodesmata of 1-10 per µm2.  Callose, a plant polysaccharide, appears to serve as structural and functional element of plasmodesmata.  Plasmodesmata looks like H-shaped, and twinned structures.  Usually, young tissue has simple plasmodesmata and complex Types of Plasmodesmata: plasmodesmata developing later, after cell expansion. Primary Plasmodesmta is formed by trapping a portion of the endoplasmic reticulum cross the  A plasmodesma measures about 50 to 60 nm in diameter and it is middle lamella during the formation of the a roughly cylindrical membrane-lined channel. primary cell wall in the newly divided plant cells. Secondary Plasmodesmata: The lining that develops de-novo. Enzyme cytokinin in plants helps in its development. 06-09-2024 Cell Morphology: External Cellular Components 5  They are assembled in three compartments: Plasmodesmata Structur 1. Plasma Membrane Lining  Plasmodesmata have their own plasma membrane lining called e plasmalemma, which is the extension from the membrane of the cell.  Its structure is similar to having the phospholipid bilayer. 2. The Cytoplasmic Sleeve  A fluid-filled space surrounded by the plasmalemma is called a cytoplasmic sleeve and is a continuous extension of the cytosol.  Proteinaceous spike-like projections that are regularly positioned within the cytoplasmic sleeve are thought to create nanochannels of varying size.  The cytoplasmic sleeve helps to transport the ions and molecules through plasmodesmata via diffusion. 3. Desmotubule  The desmotubule runs from cell to cell through the center of plasmodesmata in most cases.  The desmotubule is a dense rod or narrower cylindrical structure, that is connected to the smooth endoplasmic reticulum of adjacent cells. 06-09-2024 Cell Morphology: External Cellular Components 6  Plasmodesmata Structur e 3. Desmotubule  Desmotubules are derived from the smooth endoplasmic reticulum of the connected cells (appressed ER).  It was initially called axial component.  Diameter of desmotubule is about 15 nm.  The annulus of the cytosol is present between the outside-inside of the desmotubule and cylindrical plasma membrane respectively.  The space between desmotubules and plasma membrane contains 8-10 microchannels.  They are used to transport lipid molecules. 06-09-2024 Cell Morphology: External Cellular Components 7  Plant Cell Wall Functions A major role of the cell wall is to form a framework for the cell. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include:  Support: The cell wall provides mechanical strength and support. It also controls the direction of cell growth.  Withstand turgor pressure: Turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.  Regulate growth: The cell wall sends signals for the cell to enter the cell cycle in order to divide and grow.  Regulate diffusion: The cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out.  Communication: Cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells).  Protection: The cell wall provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss.  Storage: The cell wall stores carbohydrates for use in plant growth, especially in seeds. 06-09-2024 Cell Morphology: External Cellular Components 8