Outside the Cell Membrane
The cell membrane marks the outer limit of cellular control, but not of cellular influence. Beyond their membranes cells secrete extra coverings to help protect them and keep them in shape. They can also make connections to other cells.
Almost all animal cells produce fibrous proteins which they transport to, and excrete from the cell surface. These proteins, which are mostly collagen, form a mesh or matrix outside the cell consisting of rope-like chains woven around and through one another. Mixed with other proteins, such as fibronectin this extracellular matrix has a profound effect on the life and function of the cells and the organism.
Free living, single and multiple cell organisms such as Volvox or Euglena also excrete an extracellular matrix. In Volvox, this matrix is very extensive and holds all the 'body' cells in the right spherical orientation.
Bacteria also secrete an external matrix, but one more complex in form and structure than that seen around animal cells. As well as proteins, this matrix material contains large amounts of modified carbohydrates and polysaccharides that link and cross link to form highly rigid structures. It is this outer bacterial cell wall that gives the cell its characteristic shape, which can be spherical, rod shaped or spiral.
Plant cells have what is perhaps the most complex outer coverings. Plant cell walls are made largely of cellulose which forms strong, highly rigid, almost indigestible coverings that protect the cell and gives it shape. After division, a new plant cell excretes a primary cell wall which is flexible and will stretch with the cell as it expands and grows. This wall is porous and allows the free passage of all kinds of molecules.
When the plant cell stops growing, matures and possibly specializes, it secretes an additional secondary cell wall inside the first. In many cases this wall is lignified (made woody) for extra strength and support.
In a multicellular organism, cells communicate and stick together. There are several types of connections that hold cells in place and help them transfer materials. Tight junctions are bands around a cell where it is 'spot welded' to an adjacent cell. No chemicals can be transferred at these points. Desmosomes come in a number of varieties from spot desmosomes to belt desmosomes. They hold cells together and anchor the internal intermediate filaments. Gap junctions are regions in the cell membrane when communication takes place. Cells on opposite sides of the junction place special protein in the cell membrane that can transport everything from sugars to electrical current between the connected cells. In plants plasmodesmata extend through the cell membrane and the cell wall to interconnect cells. These are delicate strands of cytoplasm.