Cell Membrane Differentiations and Cell Wall Structures Explained

Cell Membrane Differentiations: Junctions

Some areas of the cell membrane may have specific adaptations, such as invaginations (e.g., microvilli) to increase surface area, or structures that form unions between cells. These junctions are essential for stabilizing tissues and providing mechanical adhesion. They are particularly prevalent in tissues under heavy strain, like epithelial tissues or heart muscle, where their primary function is to provide mechanical strength to the union. These junctions are classified into:

Adherens Junctions (Bands of Adhesion)

• These junctions are formed by transmembrane proteins that bind cells together.
• On the intracellular side, these proteins are associated with actin filaments, which are part of the cytoskeleton.

Desmosomes

• These junctions act like spot welds or buttons at various levels between the plasma membranes of contiguous cells.
• The binding occurs via transmembrane proteins anchored to an intracellular disc-shaped plate (desmosomal plaque).
• This plate also binds to intermediate filaments of the cytoskeleton.

Tight Junctions

• Tight junctions seal adjacent epithelial cells and act as a barrier to the passage of molecules or ions.
• They are formed by a network of transmembrane protein strands.

Gap Junctions

• These structures are formed by transmembrane proteins, which assemble in groups of six to create hexagonal formations called connexons.
• Each connexon forms a watery channel in its center. When connexons from adjacent cells align, they create open channels between the cytoplasms, allowing direct communication.
• Gap junctions are present in most animal tissues.

Plasmodesmata

• These are channels of communication found in plant tissues, whose function is equivalent to that of gap junctions, although their structure is different.
• In plasmodesmata, the plasma membrane of one cell is continuous with that of its neighbor.
• Their cell walls, joined by a cementing substance, are interrupted at these points, allowing cells to share ions and small molecules.

Cell Walls

It is important to note that animal cells do not possess a rigid cell wall; instead, they are typically embedded in an extracellular matrix. In contrast, a rigid and insoluble envelope surrounds the cells of plants, algae, fungi, and monera (prokaryotes).

Plant and Algal Cell Wall

The cell wall of plants and algae is fundamentally composed of cellulose and proteins. In all cells, it consists of two main layers: the middle lamella and the primary wall. In many cells, a third layer, the secondary wall, develops once the cell has completed its growth and can also be reinforced by lignin.

Middle Lamella

• This is the outermost layer, primarily composed of pectin, which acts as a cementing substance between adjacent cells.

Primary Wall Characteristics

• It is relatively thin and semi-rigid, allowing for cell growth.
• Formed by the secretion of substances outside the cell membrane.
• Composed fundamentally of cellulose fibers immersed in a matrix of polysaccharides (hemicellulose and pectin), monosaccharides, and some structural proteins.
• Its structure is a network of cellulose fibers arranged in layers and interconnected by molecules of hemicellulose and pectin. Structural proteins are associated with these structures.

Secondary Wall Characteristics

• It is thick and rigid, impeding further cell growth.
• Formed in certain cells once growth ceases, by thickening or depositing new layers between the plasma membrane and the primary wall.
• Its composition is similar to that of the primary wall, but it often contains a higher proportion of cellulose and can be reinforced by lignin.
• Its structure is laminated, with highly ordered cellulose fibers having different orientations in successive layers, forming a highly resistant structure. Often, this is enhanced by lignin.

Fungal Cell Wall (Yeast Example)

• A rigid covering secreted by the cell itself, fundamentally consisting of polysaccharides.
• In fungi, the structural polysaccharide chitin is particularly abundant.

Moneran Cell Wall (Prokaryotes)

• A porous and rigid covering that determines the shape of the prokaryotic cell.
• Its main component is peptidoglycan, though there are considerable differences among the cell walls of various moneran groups.