Extracellular Components, Matrices, and Cellular Junctions
Extracellular Structures and Cell Surfaces
Extracellular Synthesis and Secretion:
Although the plasma membrane serves as the outer boundary of the living cell, most cells synthesize and secrete materials into the extracellular space (outside the cell).
Extracellular structures play essential roles in protecting cells, maintaining structural integrity, and mediating cell-to-cell communication and coordination.
Plant Cell Walls
General Features and Functions:
The cell wall is an extracellular structure characteristic of plant cells, distinguishing them from animal cells (prokaryotes, fungi, and some protists also possess cell walls).
Protects the plant cell from physical damage.
Maintains cell shape and structural rigidity.
Prevents excessive uptake of water.
At the organismal level, strong walls of specialized cells support the plant against gravity.
Dimensions and Chemical Composition:
Thickness ranges from to several micrometers, making it substantially thicker than the plasma membrane.
Chemical composition varies across plant species and between different cell types within the same plant.
Architectural Design:
Consists of microfibrils made of the polysaccharide cellulose.
Cellulose microfibrils are synthesized by the membrane-bound enzyme cellulose synthase and secreted into the extracellular space.
Microfibrils are embedded in a ground substance (matrix) composed of other polysaccharides and proteins.
This composite design—strong fibers embedded in a matrix—mirrors structural materials such as fiberglass and steel-reinforced concrete.
Layers of the Plant Cell Wall:
Primary Cell Wall:
Secreted first by a young, growing plant cell.
Relatively thin and flexible to allow for cell expansion.
Middle Lamella:
A thin layer located between the primary walls of adjacent cells.
Rich in sticky polysaccharides called pectins.
Functions as cellular glue, adhering adjacent plant cells together (pectin is also extracted for use as a thickening agent in jams and jellies).
Secondary Cell Wall:
Added as the cell matures and ceases growth to strengthen the wall structure.
Some cells strengthen their wall simply by secreting hardening substances into the primary wall.
Other cells deposit a secondary cell wall between the plasma membrane and the primary cell wall.
Deposited in several laminated layers, featuring a strong, durable matrix providing enhanced protection and mechanical support.
Wood consists primarily of secondary cell walls.
Extracellular Matrix (ECM) of Animal Cells
Composition of the ECM:
Animal cells lack cell walls but possess an elaborate extracellular matrix (ECM).
Composed mainly of glycoproteins and other carbohydrate-containing molecules secreted by the cells (glycoproteins are proteins with covalently bonded carbohydrates).
Collagen:
The most abundant glycoprotein in the ECM of most animal cells.
Forms strong fibers outside the cell.
Accounts for approximately of the total protein in the human body.
Proteoglycans:
Collagen fibers are embedded in a woven network of secreted proteoglycans.
A proteoglycan molecule consists of a small core protein with numerous carbohydrate chains covalently attached, making it up to carbohydrate by weight.
Proteoglycan Complexes: Formed when hundreds of individual proteoglycan molecules attach noncovalently to a single long polysaccharide molecule.
Cellular Attachment and Integrins:
Fibronectin:
An ECM glycoprotein that attaches cells to the ECM.
Binds to cell-surface receptor proteins embedded in the plasma membrane.
Integrins:
Transmembrane receptor proteins composed of two subunits that span the plasma membrane.
Bind to ECM glycoproteins (such as fibronectin) on the extracellular side.
Bind to associated proteins attached to microfilaments of the cytoskeleton on the cytoplasmic side.
Derived from the word integrate: integrins transmit signals between the ECM and the cytoskeleton, integrating changes occurring outside and inside the cell.
Functions and Signaling Roles of the ECM:
Molecular composition and structural organization vary among different cell types.
Regulates cell behavior through integrin-mediated signaling.
Cell Migration: Embryonic cells migrate along specific developmental pathways by aligning their cytoskeletal microfilaments with the physical orientation ("grain") of fibers in the ECM.
Gene Expression Control: The ECM influences nuclear gene activity via combined mechanical and chemical signaling pathways.
Mechanical signaling involves physical connections through fibronectin, integrins, and cytoskeletal microfilaments.
Cytoskeletal reorganizations trigger intracellular signaling cascades, altering protein synthesis and cell function.
Coordinates collective behavior among all cells within a given tissue.
Intercellular Junctions
Direct physical contact sites allow neighboring plant and animal cells to adhere, interact, and communicate effectively.
Plasmodesmata in Plant Cells
Structure:
Channels perforating plant cell walls (singular: plasmodesma, from the Greek desma, meaning bond).
Membrane-lined channels filled with cytosol.
The plasma membranes of adjacent cells line the channel of each plasmodesma, making the plasma membranes continuous between cells.
Unifies most of a plant into a single living continuum (symplast).
Transport Capabilities:
Water and small solutes pass freely from cell to cell.
Under specific conditions, certain proteins and RNA molecules pass through.
Macromolecules are transported to plasmodesmata by moving along fibers of the cytoskeleton.
Animal Cell Junctions
Animal tissues contain three primary types of cell junctions, particularly abundant in epithelial tissue lining internal and external body surfaces:
Tight Junctions:
Structure: Plasma membranes of neighboring cells are pressed tightly against each other and bound together by specific transmembrane proteins.
Function: Form continuous, fluid-tight seals around cells to establish a barrier that prevents leakage of extracellular fluid across epithelial cell layers.
Example: Tight junctions between skin cells render the outer layer of human skin watertight.
Desmosomes (Anchoring Junctions):
Structure: Function like rivets, fastening cells together into strong, resilient sheets.
Cytoskeletal Linkage: Intermediate filaments composed of sturdy keratin proteins anchor desmosomes into the cytoplasm.
Function: Attach adjacent cells together under mechanical stress.
Example: Attach muscle cells together in muscle tissue; rupture of desmosomes contributes to muscle tears.
Gap Junctions (Communicating Junctions):
Structure: Consist of specialized membrane proteins that surround a pore, creating an open cytoplasmic channel between adjacent cells (not lined with a continuous plasma membrane, unlike plant plasmodesmata).
Function: Provide direct cytoplasmic channels for communication between cells.
Permeability: Allow ions, sugars, amino acids, and other small molecules to pass directly from one cell cytoplasm to another.
Importance: Critical for rapid electrical and chemical signaling across tissue cells, such as in heart muscle contraction and cellular coordination within animal embryos.