Cell walls may be complex in structure

The cytokinesis of a plant cell is completed when the two daughter cells are separated by a cell plate (see Figure 9.10b). The daughter cells then deposit a gluelike substance within the cell plate; this substance constitutes the middle lamella. Next, each daughter cell secretes cellulose and other polysaccharides to form a primary wall. This deposition and secretion continue as the cell expands to its final size (Figure 35.6).

Once cell expansion stops, a plant cell may deposit one or more additional cellulosic layers to form a secondary wall internal to the primary wall (Figure 35.6). Secondary walls

(a) Primary cell wall

Plasma membrane

The cell plate is the first barrier to form.

Each daughter cell deposits a primary wall.

The cells expand.

(a) Primary cell wall

Plasma membrane

The cell plate is the first barrier to form.

Each daughter cell deposits a primary wall.

The cells expand.

Primary Cell Wall Secondary

The primary cell wall thins and fractures.

After the cells stops expanding, they may deposit more layers, forming secondary walls.

The primary cell wall thins and fractures.

35.6 Cell Wall Formation cell division.

After the cells stops expanding, they may deposit more layers, forming secondary walls.

Plant cell walls form as the final step in

Plasmodesmata

(b) Endoplasmic reticulum Cell walls

Plasma membranes

35.7 Plasmodesmata (a)An electron micrograph shows that cell walls are traversed by strandlike structures called plasmodesmata (dark stain). The green objects are cytoskeletal microtubules (see Chapter 4). (b) Plasmodesmata contain desmotubules formed from endoplasmic reticulum.

are often impregnated with unique substances that give them special properties. Those impregnated with the polymer lignin become strong, as in wood cells. Walls to which the complex lipid suberin are added become waterproof.

Although it lies outside the plasma membrane, the cell wall is not a chemically inactive region. In addition to cellulose and other polysaccharides, the cell wall contains proteins, some of which are enzymes. Chemical reactions in the wall play important roles in cell expansion and in defense against invading organisms. Cell walls may thicken or be sculpted or perforated as cells differentiate into specialized cell types. Except where the secondary wall is waterproofed, the cell wall is permeable to water, small molecules, and mineral ions.

Localized modifications in the walls of adjacent cells allow water and dissolved materials to move easily from cell to cell. The primary wall usually has regions where it becomes quite thin. In these regions, strands of cytoplasm called plasmodesmata (singular, plasmodesma) pass through the primary wall, allowing direct communication between plant cells. A plasmodesma is a plasma-membrane lined canal traversed by a strand of endoplasmic reticulum called the desmotubule (Figure 35.7). Under certain circumstances, a plasmodesma can enlarge dramatically, allowing even macromolecules and viruses to pass directly between cells (see Chapter 15). Substances can move from cell to cell through plasmodesmata without having to cross a plasma membrane.

Even in cells with a waterproofed secondary wall, water and dissolved materials can pass from cell to cell by way of structures called pits. Pits are interruptions in the secondary wall that leave the thin regions of the primary wall, and

(b) Endoplasmic reticulum Cell walls

Plasma membranes

Primary Secondary Cell Wall

thus any plasmodesmata that are present, unobstructed (Figure 35.8).

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