The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that surrounds the cell and regulates the movement of substances in and out of the cell. There are several ways through which substances can move across the cell membrane:
Passive diffusion: Small, non-polar molecules, such as oxygen and carbon dioxide, can freely diffuse across the cell membrane without the need for energy input. This occurs due to the concentration gradient, where substances move from an area of high concentration to an area of low concentration until equilibrium is reached.
Facilitated diffusion: Larger or charged molecules, such as glucose and ions, require the assistance of membrane proteins called transporters or channels for their movement across the cell membrane. This process also occurs along the concentration gradient and does not require energy.
Active transport: Some substances, such as ions, amino acids, and larger molecules, may need to move against the concentration gradient from an area of low concentration to an area of high concentration. This requires the input of energy in the form of ATP (adenosine triphosphate), and it is carried out by specific membrane proteins called pumps.
Endocytosis: This is a process by which cells engulf substances from the extracellular environment by forming a vesicle around the substance and bringing it into the cell. There are different types of endocytosis, including phagocytosis (cellular eating) and pinocytosis (cellular drinking), which allow cells to take in larger particles or fluids.
Exocytosis: This is the reverse process of endocytosis, where substances that are synthesized or packaged within the cell are released outside the cell. The substance is enclosed in a vesicle that fuses with the cell membrane, allowing the contents to be expelled into the extracellular environment.