WJEC Biology Practice Exam

The plasma membrane is the correct structure responsible for controlling what enters and exits a cell. It is a selectively permeable barrier formed primarily by a phospholipid bilayer, which has hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. This arrangement allows the plasma membrane to regulate the movement of substances in and out of the cell, maintaining homeostasis. Through various mechanisms, such as passive transport (diffusion and osmosis) and active transport (requiring energy), the plasma membrane can control the exchange of ions, nutrients, and waste products. Embedded proteins play critical roles in this process, acting as channels, carriers, or receptors that help facilitate specific transport functions. Other structures mentioned, such as the cell wall, cytoplasm, and nucleus, serve different purposes. The cell wall provides structural support and protection in plant cells and some bacteria but is not involved in selective transport. The cytoplasm is the gel-like fluid inside the cell where organelles reside and biochemical reactions occur, and it does not control transport across the membrane. The nucleus contains the cell's genetic material and controls cellular activities but does not directly regulate substance movement across the plasma membrane.