What are plant cell walls and why are they important?

The walls of plant cells must have sufficient tensile strength to withstand internal osmotic pressures of several times atmospheric pressure that result from the difference in solute concentration between the cell interior and external water. Plant cell walls vary from 0.1 µm to 10 µm in thickness.

The structure of the primary cell wall of plants, the following three primary layers make up the cell wall:

1. The middle lamella, a layer rich in pectins. This outermost layer forms the interface between adjacent plant cells and glues them together.

2. The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing.

3. The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem, possess a secondary wall containing lignin, which strengthens and waterproofs the wall.

Typical structure and composition

In the primary (living and flexible boundary) plant cell wall, the major carbohydrates are cellulose, hemicellulose and pectin. The cellulose microfibrils are linked through hemicellulose tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronoarabinoxylan, another type of hemicellulose. Primary cell walls characteristically grow by a mechanism called acid growth, which involves turgor-driven movement of the strong cellulose microfibrils within the weaker hemicellulose and pectin matrix, catalysed by expansin proteins. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.

The secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major polymers that make up wood, largely secondary cell walls, include: cellulose (35-50%), xylan (20-35%) a type of hemicellulose, lignin (10-25%), which is a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.

Structural proteins (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins, arabinogalactan proteins (AGPs), glycine-rich proteins, and proline-rich proteins. Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Most are glycosylated, contain hydroxyproline and become cross-linked in the cell wall. These proteins are often concentrated in specialized cells and in cell corners. Cell walls of the epidermis and endodermis may also contain suberin or cutin, two polyester polymers that function as permeability barriers. The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age. Plant cells walls also contain numerous enzymes, such as: hydrolases, esterases, peroxidases, and transglycosylases, which cut, trim and cross-link wall polymers.

The walls of cork cells in the bark of trees are impregnated with suberin, and suberin also forms the permeability barrier in primary roots known as the Casparian strip. Secondary walls, particularly in grasses, may also contain microscopic silica crystals, which may strengthen the wall and protect it from herbivores.

Cell walls in some plant tissues also function as storage vacuoles for carbohydrates, which can be broken down and re-adsorbed to supply the metabolic and growth needs of the plant. The endosperm cell walls in the seeds of cereal grasses, nasturtium, and other species, are rich in glucans and other polysaccharides that are readily digested by enzymes during seed germination to form simple sugars that nourish the growing embryo. However, cellulose microfibrils are not readily digested by plants.

Formation

The middle lamella is laid down first, formed from the cell plate during cytokinesis, and the primary cell wall is then deposited inside the middle lamella. The actual structure of the cell wall is not clearly defined and several models exist; the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall can be defined as composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium salts of pectic acid. Cells interact though plasmodesmata, which are interconnecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.

In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plasma membrane and primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction and with each additional layer the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible through pits in the secondary cell wall that allow plasmodesmata to connect cells through the secondary cell walls.

Comparison of dicotyledons and grasses

The primary cell walls of flowering plants can be placed into two basic categories in regards to their biochemistry, Type I and Type II. Type I cell walls, found in dicotyledons and non-commelinoid monocotyledons contain equal levels of cellulose microfibrils and xyloglucan. This cellulose-xyloglucan network is embedded within a pectin matrix, which is rich in homogalacturonans, rhamnogalacturonans I and II. Heteromannan is also present within the matrix of Type I cell walls. Type II cell walls found in commelinoid monocotyledons, which includes the order of Poales and within that order the family of Poaceae or true grasses, contain less xyloglucan than cellulose. Glucuronoarabinoxylan and mixed-linkage glucans are the major glycans that cross-link the cellulose microfibrils instead of xyloglucan. Type II cell walls also contain less pectin and heteromannan, and higher levels of phenylopropanoids, and arabinoxylan networks.

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