Final answer:
The outer boundary of all cells that control the movement of substances in and out of the cell is known as the cell membrane or plasma membrane. It is selectively permeable and consists of a phospholipid bilayer with embedded proteins.
Step-by-step explanation:
The outer boundary of all cells, responsible for moving things in and out of the cell, is known as the cell membrane or plasma membrane. This structure is vital for maintaining the cell's environment by allowing selective transport of substances. It achieves this by being selectively permeable, meaning it lets certain molecules in and out based on size, charge, or the need for energy in the transport process. Additionally, the membrane's flexibility allows cells like red and white blood cells to change shape when necessary, for instance, when traveling through capillaries. The cell membrane is made up of a phospholipid bilayer, which consists of hydrophobic lipid 'tails' and hydrophilic phosphate 'heads', along with various proteins that assist in the transport of materials and communication between cells. The phospholipid bilayer provides a barrier that protects the internal components of the cell, such as the cytoplasm, organelles like mitochondria and ribosomes, and the genetic material within the nucleus. Transport through the cell membrane is a critical function, allowing for the uptake of nutrients and removal of waste products, ensuring the cell remains in homeostasis.