Plasma Membrane: Structure, Function, and Transport Mechanisms

Plasma Membrane: Structure and Function

The plasma membrane is a semipermeable, fluid, flexible, and selective membranous structure that surrounds the entire cell. It consists of a phospholipid bilayer (semipermeable), proteins (transport and structural), carbohydrates (cell communication), and cholesterol (structural). Its functions are to:

• Protect the cell
• Contain the cytoplasm and its organelles
• Maintain cell shape
• Establish communication between the cell and its environment
• Limit the cell
• Manage transport within the cell

Types of Transport

Passive Transport

Passive transport is a type of transport that does not require energy expenditure. However, it does require certain conditions.

a) Diffusion

Diffusion is the transport of substances through a semipermeable membrane from an area of higher concentration to an area of lower concentration.

• Simple Diffusion: This type of diffusion requires that the molecule is small (e.g., oxygen, nitrogen, water, carbon dioxide) and lipid-soluble.
• Facilitated Diffusion: This type of diffusion is for molecules that are too large to pass directly through the lipid bilayer and need to pass through protein channels.

b) Dialysis

Dialysis is a transport process that occurs in the nephrons. In this case, low molecular weight molecules (solutes) can cross the membrane, passing from the more concentrated solution to the more dilute solution. It is the foundation of hemodialysis, which attempts to replace impaired renal filtration.

c) Osmosis

Osmosis is the diffusion of water through a cell membrane, i.e., the movement of water molecules from an area of higher water concentration (more purity) to an area of lower water concentration (lower purity).

• If the external solution is less concentrated than the cell, the cell is hypotonic, so water enters it, increasing its volume. If too much water enters, it can cause the rupture of the membrane, called cell lysis. When this occurs in red blood cells, it is called hemolysis.
• If the external solution is as concentrated as the cell, it is isotonic; therefore, there will not be any net flow of water, and the cell retains its volume.
• If the solution is more concentrated outside the cell, it is hypertonic. The cell's water will tend to flow out, so its volume will decrease. In cells like red blood cells, this process is called crenation because the cell's shape becomes stellate and irregular.

* In plant cells, the situation is different since they have a large central vacuole that allows them to increase and decrease in volume, compensating for volume changes. The cell wall also gives plant cells greater resistance to changes in intracellular volume.

Active Transport

Active transport is the process by which the cell concentrates substances against a concentration gradient using energy. This energy comes from cellular respiration. It has two modes:

• Through Protein Pumps: Membrane proteins pump ions against a gradient, i.e., from low to high concentration, using energy. An example is the Na+/K+ ATPase pump.
• Through Vesicles: Some particles are too large to cross the cell membrane, so they enter the cell by forming vesicles. This entry is called endocytosis.

Endocytosis

a) Phagocytosis: The capacity of certain specialized cells to engulf large solid particles. When the particle contacts the plasma membrane, the membrane invaginates (folds inward), initiating the cellular digestion process.

b) Pinocytosis: Similar to phagocytosis, but the substances encompassed by a vesicle are dissolved or liquid.

c) Receptor-mediated Endocytosis: A special type of endocytosis in which invagination occurs by stimulation of specific receptors present in the membrane.

Exocytosis

Exocytosis is the process that occurs when large substances exit the cell through membrane outgrowths. Examples include the release of enzymes such as salivary amylase for starch digestion, insulin for the regulation of blood glucose levels, and neurotransmitters that enable the transfer of nerve impulses between neurons.