What differentiates plant cells from animal cells is that plant cells have a cell wall outside their plasma membrane. The cell wall protects plant cells and imparts stiffness to the plant, allowing it to grow tall. The composition of plant cell walls is mostly cellulose, a polymer of the sugar glucose.
All plant and animal cells have a plasma membrane, consisting of water-insoluble phospholipids and proteins. Inside these cells is a watery gel called cytoplasm and also various organelles. (The organelles include the nucleus which has the cell’s genetic information in DNA, and ribosomes which make proteins.) The space outside the cells is also watery, so the phospholipids successfully separate the inside of the cell from the outside. Plant cells have the addition of the cell wall which further protects the cell and keeps the outside of the cell rigid.
The plant cell wall is composed of three layers: the middle lamella, the primary wall, and the secondary wall. When the plant cell divides into two new daughter cells, a thin cell plate is formed which develops into the middle lamella of the mature cell. It becomes the layer that is in between the two adjacent cells.
The next layer is the primary wall. The primary wall is still rather soft, allowing the cell to grow. To keep the cell rigid, there is a large central vacuole that contains water under pressure. This turgor pressure maintains the rigidity of the cell during this phase of cell growth. The middle lamella and the primary wall both consist of cellulose fibers arranged in a random manner.
The third layer is the thickest layer, called the secondary wall. The secondary wall has a more uniform pattern of cellulose fibers, and is the layer that imparts the most rigidity.
Adjacent plant cells can exchange materials through pores in the cell wall, called plasmodesma, allowing plant cells to communicate. This is important because all adjacent cells need to communicate in multicellular organisms to coordinate responses to outside stimuli.
Plants also have starch, which is another polymer of glucose. Starch can be broken down by most organisms to glucose, but because the bonds between the glucose monomers in cellulose are different, cellulose cannot be digested by humans and most other animals, providing us with fiber. The guts of ruminants and termites have microorganisms that can digest cellulose. Since it cannot be easily broken down, cellulose is the most abundant biomolecule on earth.
Reference: Campbell et al, Biology, 8th Edition, 2008, United States
