The chemical structure and integrated components of the cell membrane are what give it selective permeability. The plasma membrane is a bilayer, meaning two layered, phospholipid envelope. The membrane is perforated with very small pores and a rich mixture of integral, transmembrane proteins that enable the entry and exit of specific types of substances.. The main ingredient of the bilayer membrane is the phospholipid molecule, featuring a hydrophilic phosphate “head” and a hydrophobic lipid “tail”. Because of the chemical natures of the head and tail, these molecules form a bilayer membrane in which two lipid tails face each other and are joined by a glycerol molecule while the phosphate heads face away from each other, attracted to the water of the extracellular matrix (ECM) and the water of the intracellular cytosol of the cell. In addition, the plasma membrane features an assortment of lipids and proteins that add to the stability of the membrane, as well as its ability to regulate substance trafficking.
Active transport is the movement of substances into or out of the cell that requires the cell to expend its energy to produce the transport. In all cases, active transport is powered by energy-carrier molecules like ATP (whether directly or indirectly) and performed using transport vesicles. As discussed in this article, substances can enter or exit the cell by passive transport, and technically that is endocytosis and exocytosis, but, typically, the words “endocytosis” and “exocytosis” are used more in the context of active transport than passive transport.
Endocytosis occurs when a cell uses its stored energy to transport substances into the cell. There are three main types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated. Phagocytosis is a specific kind of endocytosis by which cells swallow food particles whole so they can be broken down by the cell once they enter. To accomplish this, the cell membrane first has to fold so that it acts as a mouth that can engulf the solid food. In many phagocytes, like many single-celled species among kingdom protozoa, cilia or pseudopods are used to guide the food particle into the cell. Pinocytosis, also called fluid endocytosis, works essentially the same way as phagocytosis, except the cell engulfs liquid or small particles instead of large, solid food; and as such, the energy and nutrients gained are small in comparison to what the cell gets from phagocytosis. There are many types of receptors that can perform receptor-mediated endocytosis; two main types are the G-protein-coupled receptor (GPCR) and receptor tyrosine kinase (RTK), where RTK is an enzyme-linked receptor. Essentially, these work like an ion channel receptor in that they bind with a ligand on the extracellular side, except they work with energy-carrier molecules like ATP on the intracellular side to scission off transport vesicles from the cell membrane (induce the budding off of vesicles).
Exocytosis is a mechanism by which cells are able to replenish and reinforce the cell membrane with membrane proteins, lipids, and other nutrients the rest of the cell could not use, and what even the plasma membrane cannot use is secreted into the extra cellular matrix. Exocytosis is not varied much in types. Constitutive exocytosis is not triggered by a calcium ion and is where the vesicle simply merges with the plasma membrane as the substances meant to be secreted are expelled out of the cell. In certain types of cells, like neurons, the process is the same except that an external signal that involves a calcium ion trigger must be made in order to carry out the exocytosis, and this type is called regulated exocytosis.
The complex structure of the plasma membrane is what both defines the border of the cell and makes it a feature-rich envelope, capable of selective permeability and intercellular collaboration. The cell membrane is a phospholipid bilayer envelope fitted throughout with integrated, transmembrane proteins that facilitate both the passive and active transport of materials into and out of the cell. The chemical properties of the phospholipid molecules that form the plasma membrane make it impermeable to most materials, keeping the cell protected from pathogens, but allowing various types of substances through its variety of channels and carrier proteins. The cell membrane is so flexible that it can fold up to ingest materials and use its own membrane material to bud off a vesicle that can transport important nutrients to the various parts of the cell. It is even fitted with many different types of receptors that can trigger endocytosis and many other important cellular functions. Once the cell has had its fill from the nutrients its membrane brought in, it both reinforces the plasma membrane and secretes the waste products out of the cell through its versatile membrane. The structure and flexibility of the cell membrane is what makes passive and active transport possible.