Final answer:
Cells flatten out upon contacting a surface through the action of sending out projections that establish stable attachments, leading to increased adhesion and spreading of the cell. This is part of how cells interact with the extracellular matrix and is necessary for their anchoring to surfaces.
Step-by-step explanation:
Cells flatten out as they make contact with a surface due to a process involving the spreading and adhering to increase their stability on the surface. This phenomenon can be understood with reference to simple soft matter models, like a liquid droplet adhering to a surface governed by surface tension, or a solid elastic sphere which gains adhesion energy by forming a contact region. When a cell encounters an external surface covered with a specific ligand, it undergoes adhesion steps that lead to spreading, involving the actions of the plasma membrane and the actin cortex underneath it.
An important factor in this process is the formation of stable attachments through projections like filopodia or lamellipodia. These projections reach out and touch the surface, creating links thanks to cell adhesion molecules. Over time, the spread area of the cell increases as more projections find anchor points, which results in a flattening of the cell's overall shape.
Therefore, the correct answer to why cells flatten out as they make contact with a surface is that they send out projections that make increasingly stable attachments (3). This process is part of how animal cells communicate with their extracellular matrices and is crucial for their adhesion to substrates.