Passive Transport and Osmosis
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.
Passive transport is the movement of substances into or out of the cell without the need of any energy expenditure. The cell does not have to act as a whole to make it possible, but instead, the structure of the plasma membrane and the laws of nature work to transport the substances. There are four main ways in which passive transport works: diffusion (passive), facilitated diffusion, filtration, and osmosis.
Diffusion is the net movement of material in the direction needed to achieve equilibrium, which is an equalized, balanced state. When two substances of different temperatures come into contact with each other, heat is always transferred from the substance with a higher temperature to the substance with the lower temperature, as per the Zeroth Law of Thermodynamics. If a closed structure contains a fluid (liquid or gas) that is under varying amounts of pressure from different locations of the structure, it will always flow from the area of higher pressure to the area of lower pressure, just because of the nature of pressure (total force applied over a surface area or cross section). Similarly, fundamental forces and natural laws and principles also cause the movement from a higher chemical potential to a lower chemical potential (like in batteries) or from a higher concentration of a substance to a lower concentration (also dubbed “down the concentration gradient”). These natural forces make substances work towards achieving equilibrium (equal temperatures, pressure, concentration, etc.) Due to the chemical properties of the phospholipid molecules and other fatty acids and proteins that form the plasma membrane, small, neutrally-charged, lipid-soluble particles or molecules (like oxygen and carbon dioxide) can easily pass through the small pores of the membrane. They do this until they achieve equilibrium, unless of course an outside force or barrier were to stop such movement.
Some molecules, like glucose, are too large to fit through the small pores of the membrane. Water soluble molecules can't pass through the membrane because of the hydrophobic nature of the fatty acid tails of the phospholipid molecules that make up the cell envelope. Electrically charged particles (like ions) can't pass through passively either. For these molecules, the plasma membrane hosts two types of integral, transmembrane proteins that facilitate their transport down the concentration gradient through the membrane (again, without the cell having to expend energy).
Ion channel receptors bind with special substances called ligands on the extracellular or intracellular side. This triggers the opening of their gates and allow specific types of ions or small polar molecules through (depending upon the channel, the ligand it binds with, and other factors). Polar molecules are neutrally charged but their positive and negative charges are on opposite ends of the molecule, making them polar with respect to electric charge. Because the binding with the ligand triggers the flow of the specific type of ions down the concentration gradient, the cell does not have to spend any energy in this transport.
Large molecules bind with a type of transmembrane carrier proteins called permeases within their conduit pores, and induce changes in the protein’s shape that force the molecule through the protein like a throat swallowing food. In permeases, the transported molecule is also the ligand itself.
Water soluble molecules are transported through carrier proteins that bind with a substance called retinol. Since the ligand binding induces this behavior in the carrier protein, this is a form of facilitated diffusion, and therefore passive transport.
In this type of passive transport, fluid (liquid or gas) moves down the pressure gradient and through the cell membrane through its pores. The pressure is generated by a force outside the cell, like the pumping of blood through blood vessels by the heart. In the human body, blood transports nutrients to all parts of the body, including oxygen that it receives from the lungs. In receiving and supplying nutrients, blood cells must repeatedly exchange materials through their plasma membranes. The blood cells themselves don’t need to expend energy to perform this transport, as the pressure from the pumping action of the human heart forces it through the vessels down the pressure gradient. Other types of cells engage in filtration, with membrane pore sizes suitable for the types of materials they need to exchange in order to function properly.
A cell is made up of mostly water (since the cytosol is mostly water and accounts for most of the volume of a cell), as is the extracellular matrix the cell is surrounded by. Regular exchange of water is essential for the maintenance of cells. Although some water does leak through the phospholipid bilayer cell membrane, the hydrophobic fatty acids repel most water and keep the plasma membrane near impermeable to water. Cells have special transmembrane proteins called aquaporins that are designed solely for the passage of water molecules. Water molecules enter and exit cells mainly by a type of passive transport known as osmosis.
Osmosis is the movement of a solvent – a liquid in which a substance dubbed the solute dissolves – through a semi-permeable or selectively permeable membrane to the area of higher solute concentration, until equilibrium is achieved with respect to the solute concentration. Since the solute cannot travel through the semi-permeable membrane, the solvent travels to achieve solute equilibrium. As mentioned before when discussing passive diffusion, the solute concentration being out of equilibrium drives the passive transport of the water.
The most common solvent for osmosis (and in general) is water. Osmosis is like facilitated diffusion, except that its focus is on the flow of water molecules through the cell membrane. If the concentration of solute is greater outside the cell, then this will drive water out of the cell, resulting in a hypertonic solution (when the solute concentration is high). If there is a lower amount of solute outside the cell, then water will flow into the cell, resulting in a hypotonic solution. If the solute concentrations are in equilibrium, then the solution is called an isotonic solution.
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.