What provides structural support and rigidity to plants, allowing them to grow upright?

The cell wall is the correct answer because it plays a crucial role in providing structural support and rigidity to plants. Composed primarily of cellulose, the cell wall surrounds the plant cell membrane and maintains the shape of the cell. It protects the plant against physical stress and enables it to withstand various environmental factors such as wind and gravity. The rigidity of the cell wall is integral for allowing plants to grow upright and reach toward sunlight, which is essential for photosynthesis.

While the cell membrane is important for controlling the movement of substances in and out of the cell, it does not contribute to the structural support of the plant's overall structure. The cytoplasm, which contains organelles and the cytoskeleton, facilitates cellular functions but also does not provide the external structural support that the cell wall does. Chloroplasts are involved in photosynthesis but do not play a role in maintaining the plant's structural integrity.