Final answer:
The 'primary cell wall' is the flexible structure that allows for plant cell growth, composed of cellulose, hemicellulose, and pectins, providing rigidity yet permitting expansion. After growth, plants develop a secondary cell wall reinforced by lignin for added strength and rigidity.
Step-by-step explanation:
The "primary cell wall" is flexible and allows cells to increase in size. In plant cells, the primary cell wall is composed of cellulose microfibrils embedded in a matrix of hemicellulose and pectins. This structure supports the plant cell during its growth phase as it maintains the cell's shape and rigidity while still allowing the cell to expand. Unlike the primary cell wall, plant cells may also develop a secondary cell wall after they have ceased to grow. This wall is much more rigid due to the presence of lignin, which provides additional structural support and is important for the plant's overall strength and stability. The secondary wall is not flexible, which is why mature plant parts such as wood are so tough. Also, unlike the expanding balloon analogy, plant cells do not diminish in wall thickness as they expand; this is due to the composition and arrangement of cellulose and other components in the cell wall matrix.