Final answer:
The natural semi-permeable membrane in plant and animal cells is the plasma membrane, which selectively controls the movement of substances to sustain the cell's life. It allows the passage of water and lipid-soluble molecules while blocking ions and polar molecules, which are transported with the assistance of membrane transport proteins.
Step-by-step explanation:
The natural semi-permeable membrane in both plant and animal cells is the plasma membrane. This membrane is critical for maintaining the cell's integrity and controlling the movement of substances into and out of the cell. It is composed of a thin layer of protein and fat, and its semi-permeability allows for the selective passage of materials. The plasma membrane is freely permeable to water and lipid-soluble molecules, but it blocks electrically charged ions like Na+, Cl¯, and polar molecules such as sugars. These impermeable substances can be transported with the help of membrane transport proteins.
Semipermeability is an essential feature of a functional plasma membrane, allowing the cell to acquire necessary substances from the extracellular fluids while preventing harmful materials from entering and essential materials from exiting, thus sustaining the cell's life. The plasma membrane also contains protein receptors that bind to substances in the extracellular matrix, initiating internal cellular reactions. There are various types of junctions such as plasmodesmata in plant cells and gap junctions in animal cells, which provide additional pathways for communication and transport between cells.
It is fascinating to note that the study of membrane function involves the fields of physiology, chemistry, and physics, indicating the complexity and importance of this cellular component. The semipermeable nature of the plasma membrane is a subject of ongoing research, aiming to further understand how it maintains selectivity and contributes to overall cellular function.