Here's why:
* Cytoskeleton: This internal network of protein filaments provides structural support and helps maintain the cell's shape. It also plays a role in cell movement, division, and transport of materials.
* Cell Membrane: While the cell membrane provides a boundary and regulates what enters and exits the cell, it is relatively flexible and doesn't provide significant structural support.
* Internal Pressure: In some cases, internal pressure from the cytoplasm can contribute to the cell's shape, but this is usually secondary to the cytoskeleton.
* External Factors: Environmental factors like surrounding fluid pressure or neighboring cells can influence a cell's shape to some degree, but the cytoskeleton is the primary determinant.
Examples:
* Animal cells: These cells rely heavily on their cytoskeleton for their diverse shapes, from the spherical red blood cells to the elongated muscle cells.
* Amoebas: These single-celled organisms use their cytoskeleton to extend pseudopodia (temporary projections) for movement and engulfing food.
In contrast, cells with cell walls, like plant cells, rely heavily on the rigid cell wall for their shape. The cytoskeleton still plays a role in intracellular organization and movement, but the wall provides the main structural support.
