Extracellular Matrix, Cell Junctions, and Cellular Integration
Extracellular Matrix (ECM) of an Animal Cell
Composition and Structure:
Collagen fibers are embedded in a web of proteoglycan complexes.
Fibronectin is a key ECM protein that connects the ECM to the cell.
A proteoglycan molecule consists of a small core protein with many covalently attached carbohydrate chains, making up to carbohydrate.
Large proteoglycan complexes form when hundreds of proteoglycan molecules are noncovalently attached to a single long polysaccharide molecule.
Connection to the Cell:
ECM proteins, such as fibronectin, bind to cell-surface receptor proteins called integrins.
Integrins:
Are membrane proteins composed of two subunits.
Span the plasma membrane, binding to the ECM on the outside of the cell and to associated proteins attached to microfilaments of the cytoskeleton on the inside (cytoplasmic side).
The name "integrin" reflects their role in integrating changes occurring outside and inside the cell by transmitting signals between the ECM and the cytoskeleton.
Influential Role of the ECM:
The ECM plays a significant role in regulating cell behavior by communicating with the cell through integrins.
Examples of ECM influence:
Cell Migration: Cells in a developing embryo migrate along specific pathways by aligning the orientation of their microfilaments with the "grain" of fibers in the ECM.
Gene Activity: The ECM surrounding a cell can influence the activity of genes within the nucleus.
Signaling Pathways: Information from the ECM likely reaches the nucleus via a combination of mechanical and chemical signaling pathways.
Mechanical Signaling: Involves fibronectin, integrins, and microfilaments of the cytoskeleton.
Changes in the cytoskeleton can trigger intracellular signaling pathways, leading to alterations in the proteins produced by the cell and subsequently, changes in cell function.
The ECM of a particular tissue helps to coordinate the behavior of all cells within that tissue. Direct cell-to-cell connections also contribute to this coordination.
Cell Junctions
Cells in animals and plants are organized into tissues, organs, and organ systems, with neighboring cells often adhering, interacting, and communicating through direct physical contact sites.
Plasmodesmata in Plant Cells
Plant cell walls, though nonliving, are perforated with plasmodesmata (singular: plasmodesma), which are channels connecting adjacent cells.
Structure and Function:
The plasma membranes of adjacent cells line the channel of each plasmodesma, making them continuous.
These channels are filled with cytosol, ensuring that connected cells share the same internal chemical environment.
Plasmodesmata effectively unify most of the plant into a single living continuum.
Passage Through Plasmodesmata:
Water and small solutes can pass freely from cell to cell.
Experiments have shown that certain proteins and RNA molecules can also pass under specific circumstances (further discussed in Concept ).
Macromolecules are transported to neighboring cells by moving along fibers of the cytoskeleton to reach the plasmodesmata.
Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells
In animals, there are three primary types of cell junctions: tight junctions, desmosomes, and gap junctions.
These junctions are particularly common in epithelial tissue, which forms linings on the external and internal surfaces of the body.
Gap junctions are functionally similar to plasmodesmata in plants, though gap junction pores consist of proteins extending from each cell's membrane, rather than being lined with membrane.
Tight Junctions
Structure: Plasma membranes of neighboring cells are very tightly pressed against each other, bound by specific proteins.
Function: They form continuous seals around cells, creating a barrier that prevents the leakage of extracellular fluid across a layer of epithelial cells (e.g., as indicated by the red dashed arrow in Figure ).
Example: Tight junctions between skin cells provide a watertight barrier (e.g., making human skin waterproof).
Molecular Detail: The polypeptide chain of a tight junction protein weaves back and forth through the membrane four times, featuring two extracellular loops, one cytoplasmic loop, and short C-terminal and N-terminal tails in the cytoplasm. This structure suggests the presence of both hydrophobic and hydrophilic amino acid sequences consistent with membrane-spanning proteins.
Visual aid: TEM image shows tight junctions at µm scale.
Desmosomes (Anchoring Junctions)
Function: Act like rivets, fastening cells together into strong, cohesive sheets.
Anchoring: Desmosomes are anchored in the cytoplasm by intermediate filaments, which are composed of sturdy keratin proteins.
Example: They are crucial for attaching muscle cells to each other within a muscle.
Clinical Relevance: Some "muscle tears" involve the rupture of desmosomes, highlighting their importance in tissue integrity.
Visual aid: TEM image shows desmosomes at µm scale.
Gap Junctions (Communicating Junctions)
Function: Provide cytoplasmic channels from one cell to an adjacent cell, facilitating communication.
Structure: Consist of membrane proteins that extend from the membranes of the two cells, forming pores.
Passage Through Pores: Ions, sugars, amino acids, and other small molecules can pass through these pores.
Necessity for Communication: Gap junctions are essential for intercellular communication in various tissues, such as heart muscle, and play a vital role in animal embryos.
Visual aid: TEM image shows gap junction channels at µm scale.
A Cell Is Greater Than the Sum of Its Parts
Core Concept: The study of cellular organization consistently demonstrates a strong correlation between cellular structure and its function. No cellular component works in isolation; rather, all parts are integrated.
**Example: Macrophage Function (Figure )
Macrophages ( µm in diameter) are immune cells that defend the mammalian body by ingesting bacteria (smaller cells) into phagocytic vesicles.
Movement: A macrophage crawls along surfaces and extends thin pseudopodia (specifically filopodia) to reach bacteria.
These movements involve the interaction of actin filaments with other elements of the cytoskeleton.
Digestion: After engulfing bacteria, they are destroyed by lysosomes, which are produced by the elaborate endomembrane system and contain digestive enzymes.
Synthesis: Both the digestive enzymes of the lysosomes and the proteins of the cytoskeleton are synthesized by ribosomes.
Regulation: The synthesis of these proteins is programmed by genetic messages dispatched from the DNA housed in the nucleus.
Energy: All these complex cellular processes require energy, which is supplied in the form of ATP by mitochondria.
Conclusion: Cellular functions emerge from cellular order; a cell is a living unit whose capabilities exceed the simple sum of its individual components. The integration of cellular processes is fundamental to its operation.
Visualizing the Scale of Molecular Machinery in a Cell (Figure )
This figure provides a scaled visualization of various structures and molecules within a plant cell, illustrating their relative sizes and organization in the context of cellular structures and organelles.
Key Molecules and Structures Illustrated:
(a) Membrane Proteins (Chapter ): Proteins embedded in cellular membranes facilitate substance transport, signal conduction across membranes, among other crucial functions. Many can move within the membrane.
Examples include Proton pumps, Calcium channels, Aquaporins, and Receptors.
(b) Cellular Respiration (Chapter ): A multi-step process generating ATP from food molecules.
The initial two stages occur via enzymes in the cytoplasm and mitochondrial matrix (e.g., Phosphofructokinase, Hexokinase, Isocitrate dehydrogenase).
The final stage is carried out by proteins forming an electron transport chain within the inner mitochondrial membrane (e.g., Complex I, Complex II, Complex III, Complex IV, Cyt c).
(c) Photosynthesis (Chapter ): The process of producing sugars using light energy.
Initiated by large complexes of proteins and chlorophyll (shown in green) embedded in the thylakoid membranes (e.g., Photosystem II, Photosystem I, Cytochrome complex, Pq, Pc, Fd, ATP synthase, NADP+ reductase).
Light energy trapped by these complexes is used by Rubisco and other proteins in the stroma to synthesize sugars.
(d) Transcription (Chapter ): In the nucleus, the genetic information from a DNA sequence is transferred to messenger RNA (mRNA) by the enzyme RNA polymerase.
After synthesis, mRNA molecules exit the nucleus through nuclear pores.
A nucleosome consists of DNA wrapped around eight histone proteins. For RNA polymerase to transcribe this DNA, it must first be unwrapped from the histones.
(e) Nuclear Pore (Concept ): A complex that regulates the molecular traffic entering and exiting the nucleus, which is bounded by a double membrane.
Among the largest structures that pass through the pore are the ribosomal subunits, which are assembled in the nucleus.
(f) Translation (Chapter ): In the cytoplasm, the information carried by mRNA is used to assemble a polypeptide with a specific sequence of amino acids.
This process involves transfer RNA (tRNA) molecules and ribosomes.
Eukaryotic ribosomes are colossal complexes composed of four large ribosomal RNA (rRNA) molecules and more than proteins, split into large and small subunits.
Through transcription and translation, the nucleotide sequence of DNA in a gene ultimately determines the amino acid sequence of a polypeptide, with mRNA acting as an intermediary.
(g) Cytoskeleton (Concept ): Composed of polymers of protein subunits.
Microtubules: Hollow structural rods formed from tubulin protein subunits (specifically, dimers).
Microfilaments: Cables made of two chains of actin proteins wound around each other.
(h) Motor Proteins (Concept ): Proteins like myosin are responsible for the transport of vesicles and the movement of organelles within the cell.
A myosin motor protein can be observed "walking" on a microfilament, potentially moving an organelle like a vesicle.
Scale References:
The figure also shows a scale ruler, e.g., nm for cytoskeleton elements and enlarged structures. Other scales indicated earlier: macrophages at µm, tight junctions at µm, desmosomes at µm, and gap junctions at µm.
Relative Size Comparison Exercise: To list structures from largest to smallest, based on the provided figure: Nuclear pore, ribosome, proton pump, Cyt c (cytochrome c).