Extra cellular components

Extra cellular components and connections Extra cellular components and connections between the cells help coordinated cellular activities. 1. Cell wall of plant cells 2. Extracellular matrix of animal cells 3. Intercellular Junctions

Extra cellular matrix components of animal cells

cEll wall Cell wall is an extracellular structure of plant cell. It protects the plant cell, maintain its shape, and prevents excessive uptake of water. On the level of whole plant the walls of specialized cells holds the plant up against gravity. Prokaryotes, fungi and some protists also have cell walls.

Plant cell wall

TEM image of cell wall

Layers of plant cell wall

Layers of plant cell wall

Plant cell wall Plant cell walls are thick structures ranging from 10 μm to several μm. The chemical composition of cell wall varies from species to species and cell type to cell type. The major constituent of cell wall is cellulose a polysaccharide molecule. form the microfibrils which are embedded in the matrix of other polysaccharides pectins and proteins.

There are three layers of the cell wall: 1. Primary cell wall: It is present in young cells, relatively thin and flexible. 2. Middle Lamella: It is present between the primary walls of two adjacent cells. It is a thin layer made up of sticky material pectins. The middle lamella glues the two adjacent cells together. 3. Secondary wall: When the cells matures they secrete hardening material in the cell wall. Some cell add this material in the primary cell wall while others secretes an other layer of cell wall called secondary cell wall.

Cellulose

Pectin molecule

Plasmodesmata cellulose microfibrils

pits The cell wall is interrupted by narrow pores carrying fine strands of cytoplasm,which interlink the contents of adjacent.They are called plasmodesmata. They form a protoplasmic continuum called symplast. Plasmodesmata consists of a canal, lined by plasmamembrane. It has a simple or branched tubule known as desmotubule which is an extension of endoplasmic reticulum.

Secondary wall is deposited between the plasma membrane and primary cell wall. Secondary wall has several laminated layers which are hard and protect and support the plant cell. Wood consists of secondary cell wall. Cell wall is perforated to make channels between the two cells. These perforations are celled plasmodesmata. The deposition of secondary cell wall material deposited in such a way that it makes a structure called pit.

Extra cellular matrix of animal cell Animal cell lack cell walls but they have elaborate extra cellular matrix (ECM). ECM consists of glycoproteins secreted by the cell. i.e. Collagen which form strong fiber out side the cells. Collagen accounts half of the total proteins in the cell. Collagen fibers are embedded in network of proteoglycans, a glycoprotein of another class. Prteoglycans consists of 95% carbohydrates and small amount of proteins. Large proteoglycan complex can form when hundreds of proteoglycans become non-covalently attached to a single long polysaccharide molecule.

ECM of animal cell

ECM of animal cell

ECM OF ANIMAL CELL

Some cells are attached to ECM by other glycoproteins, including fibronectin. Fibronectin and other ECM protein bind to cell surface receptor protein called integrins. Integrins are built inside the plasma membrane than span the membrane and bind on the cytoplasmic side to associated protein attached with the microfilaments of cytoskeleton. Integrins transmit changes between the ECM and cytoskeleton thus integrate changes occurring outside and inside the cell.

Fibronectin and integrins reveals an influential role of ECM in the lives of cells. With the help of these ECM regulates cell’s behavior. i.e. some cell in the developing embryo migrates along specific path ways by matching the orientation of their microfilaments to the grain of the fibers in ECM. ECM around a cell can influence the activity of genes in the nucleus by mechanical (fibrinectin, integirns and microfilaments of cytoskeleton) and chemical (changes in proteins thus cell functions) signalling pathways. ECM help coordinate the behavior of all the cells within that tissue.

Intercellular junctions Many cells of plants and animals are organized into tissues, organs and organ systems. Neighboring cells interacts and communicate through special patches of direct physical contacts. Plants cells have Plasmodesmata: Plant cell wall is perforated with channels called plasmodesmata. Cytosole passes through the plasmodesmata and connect the chemical environments of adjacent cells. This makes the plant a continuum. Water, small solutes, proteins and RNA molecules pass freely through plasmodesmata via channels made by fibers of cytoskeleton.

LM Images of plasmoadesmata

Plasmodesmata LM image confocal image

Plasmodesmata Tem image lm image

High mag. model of plasmodesmata

TEM images of plasmodesmata

Plasmodesmata cell junctions

Animals: Tight Junctions, Desmosomes, and Gap Junctions. These three types are common in epithelial tissue which line the internal surface of the body. Tight Junctions: membranes of the neighbouring cells are very tightly pressed against each other, bound together by specific proteins. Form continuous seal around the cell. They prevents the extracellular leakage across the epithelial cells.

Tight junctions Desmosome gap junctions

Animal cell junctions

Cell connection during growth

Tight junction

Desmosomes: Anchoring junctions, function like rivets, fastening cells together into strong sheets. Intermediate filament of cytoskeleton are made of keratin protein anchor desmosome into the cytoplasm. Gap Junctions: Communicating junctions, provide cytoplasmic channels from one cell to another cell. Gap junctions consist of special proteins that surround a pore through which ions, sugar molecules, Aas and other small molecules may pass. Gap junctions are important for communication between cells in many types of tissues, including heart muscles and animal embryos.

Desmosome