Cell Membrane Transport: Mechanisms, Diffusion, and Active Transport

 

CELL MEMBRANE TRANSPORT PROTEINS, THE CELL MEMBRANE IS A LIPID BILAYER

The composition of the membrane that envelops each and every bodily cell. This membrane is made up nearly entirely of a lipid bilayer that contains several protein molecules, many of which are able to pass through the membrane completely. Neither the intracellular nor external fluids are miscible with the lipid bilayer. As a result, it acts as a barrier to prevent water molecules and other things soluble in water from moving between the fluid compartments inside cells and outside of them. Lipid-soluble compounds, however, can diffuse straight through the lipid material as indicated by the leftmost arrow.

Transport Mechanisms in Cell Membranes

The lipid bilayer is broken up by the membrane protein molecules, creating a different passageway through the cell membrane. Numerous of these penetrating proteins have transport capabilities. Certain proteins, referred to as channel proteins, have watery gaps that run the entire length of the molecule, allowing water and specific ions or molecules to flow freely. The substances to be transported bind to other proteins, known as carrier proteins, and are then moved through the protein molecules' interstices to the other side of the membrane by conformational changes in the protein molecules. The kinds of molecules or ions that are permitted to pass through the membrane are typically selected for by channel proteins and carrier proteins.

Diffusion vs. Active Transport Mechanisms

"Diffusion" against "Active Transport." Transport via one of two primary mechanisms diffusion or active transport occurs through the cell membrane, either via the proteins or the lipid bilayer directly. Diffusion is the random molecular movement of substances molecule by molecule, either across intermolecular spaces in the membrane or in conjunction with a carrier protein, however there are many variations of these fundamental principles. The energy of matter's regular kinetic motion is what leads to diffusion. On the other hand, active transport refers to the passage of ions or other materials across the membrane in conjunction with a carrier protein in a way that pushes the material against an energy gradient, like going from a state of low concentration to one of high concentration. In addition to kinetic energy, another source of energy is needed for this movement. 

Diffusion

Every molecule and ion in the bodily fluids, including dissolved materials and molecules of water, is constantly moving, each particle going in a different direction. These particles move in a way that scientists refer to as "heat"; the more they move, the hotter it gets. This motion never stops, with the exception of extremely cold temperatures. A moving molecule, A, repels a stationary molecule, B, with the help of its electrostatic and other nuclear interactions, giving molecule B some of the motion energy of molecule A. As a result, molecule B increases its kinetic energy whereas molecule A decreases its kinetic energy and slows down. A solitary molecule in a solution randomly bounces hundreds of times per second among the other molecules, first in one direction, then another, and so on. Diffusion is the term for the ongoing movement of molecules within liquids or gasses. Similar to entire molecules, lons disperse. Similar diffusion occurs between suspended colloid particles, with the exception that due to their huge size, colloids diffuse significantly more slowly than molecules.

Spreading Via The Cell Membrane

There are two varieties of diffusion through the cell membrane: simple diffusion and assisted diffusion. Simple diffusion is the process by which molecules or ions migrate kinetically through an opening in a membrane or through gaps between molecules without interacting with the membrane's carrier proteins. The amount of substance present, the kinetic motion velocity, and the quantity and size of membrane openings that allow molecules or ions to pass through all influence the rate of diffusion. A carrier protein's interaction is necessary for facilitated diffusion. By chemically attaching to molecules or ions and transporting them through the membrane in this manner, the carrier protein facilitates the passage of these substances through the membrane.

Two avenues exist for simple diffusion through the cell membrane:

(1) If the substance diffusing is lipid-soluble, through the lipid bilayer's interstices

(2) Through watery channels that reach all the way through some of the major transport proteins, as illustrated on the left.

Diffusion via the Lipid Bilayer of Lipid-Soluble Substances

A material's ability to dissolve quickly through the lipid bilayer is largely dependent on its lipid solubility. For instance, alcohols, carbon dioxide, nitrogen, and oxygen have high lipid solubilities and can all dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same way that water solutes diffuse in a watery solution. Each of these substances has a rate of diffusion through the membrane that is directly correlated with how soluble in lipids it is. This method is particularly effective in moving huge volumes of oxygen, making it possible to provide oxygen to the interior of the cell almost without the need for a cell membrane.

Diffusion via Protein Channels of Water and Other Lipid-Insoluble Molecules

Water easily flows via channels in protein molecules that penetrate all the way through the membrane, despite the fact that it is very insoluble in the lipids that make up the membrane. Aquaporins are protein "pores" found in many of the body's cell membranes that specifically allow water to pass through them quickly. There are at least 13 distinct varieties of highly specialized aquaporins found in different mammalian cells. It is remarkable how quickly water molecules may permeate through the majority of cell membranes. For instance, the volume of a red blood cell is only ten times larger than the total volume of water that diffuses through the membrane in all directions in a single second.

Size and Solubility in Membrane Transport

If they are tiny enough and water-soluble, other lipid-insoluble molecules can enter the protein pore channels in the same manner as water molecules. But as they get bigger, their penetration quickly decreases. For instance, urea has a width that is only 20% larger than water's, but it penetrates cell membrane pores at a rate that is roughly 1000 times slower than water's. Nevertheless, this level of urea penetration permits quick passage of urea across the membrane in a matter of minutes, despite the startling rate of water penetration.