2.3: Cell wall, Vacuoles, and Plasmodesmata
Among eukaryotic cells, plant cells are largest. Some of them (for example, cells from green pepper and grapefruit) are well visible with the naked eye. Plant cells do not have well-developed internal cytoskeleton, but cell wall provides an external one. There are two kinds (or, better, two stages of development) of cell walls, the primary and the secondary . The primary cell wall is typically flexible, frequently thin and is made of cellulose, different carbohydrates and proteins. The secondary cell wall contains also lignin and highly hydrophobic suberin . These chemicals completely block the exchange between the cell and the environment which means that the cell with secondary wall will soon die. Dead cells can still be useful to plants in many ways, for example as a defense against herbivores, support and water transport. In fact, more than 90% of wood is dead.
Since every plant cell is surrounded with a cell wall, they need a specific way of communication. This is done through plasmodestata —thin cytoplasmic bridges between neighbor cells. A symplast is the name of continuous cytoplasm inside of cells. An apoplast is cell walls and space outside the cell where communication and considerable metabolic activity take place. Both the symplast and apoplast are important to the transportation of nutrients needed by the cell (Figure \(\PageIndex{1}\)).
If cells are surrounded by a smaller concentration of salts than in the cytoplasm, the water will flow into the cell. This process is called osmosis . In plant cells, most of the water with diluted chemicals is concentrated in vacuole(s). Turgor pressure is the combined pressure of the cell and vacuoles wall that supports the shape of cell (Figure 3.1.4).
You may think of plant tissue as about staked cardboard boxes where every box is made from wet cardboard paper (cell wall) but has the inflated balloon (vacuole) inside, and when the pressure of vacuole decreases (water deficit), plant organs droop. Please see the video http://ashipunov.info/shipunov/school/biol_154/mov/balloon.mp4 to understand this better.
Comparing with animal cells, plant cells have chloroplasts, vacuoles, cell walls, and plasmodesmata but they hardly have any phagocytosis and true cytoskeleton (Figure \(\PageIndex{2}\)). They are easy to explain: animals do not photosynthesize (no chloroplasts), instead, they need to move quickly (no cell walls and plasmodesmata); animals will support the shape of cell from cytoskeleton (no need for vacuole turgor system) and use molecular pumps to counterpart the osmosis.