Can every substance cross the cell membrane why or why not?

Answer 1

The membrane is differentially permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. To do so, the membrane employs a number of transport mechanisms:

1. Diffusion : Some substances (small molecules, ions) such as carbon dioxide (CO2), oxygen (O2), and water, can move across the plasma membrane by diffusion, which is a passive transport process.

2. Osmosis : Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration difference across a semipermeable membrane can set up a osmotic flow for the solvent, in this case water. Water can thus be transported across the membrane by osmosis.

3. Mediated Transport : Nutrients such as sugars and materials of growth such as amino acid must enter the cell, and the waste of metabolism must leave. Such molecules are moved across the membrane by special proteins called transport proteins or permeases. Permeases form a small passageway through the membrane, enabling the solute molecule to cross the phospholipid bilayer. Permeases are usually quite specific, recognizing and transporting only a limited group of chemical substances, often even only a single substance.

4. Endocytosis : Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles (cell eating or phagocytosis), small molecules and ions (cell drinking or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.

5. Exocytosis : Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.

Answer 2

No, not every substance can cross the cell membrane. The cell membrane is selectively permeable, meaning it allows some substances to pass through while blocking others based on size, charge, and solubility. Substances like water, gases, and small nonpolar molecules can easily cross the cell membrane through diffusion, but larger or charged molecules may require specific transport mechanisms.