Movement of Molecules Across Membranes
Plasma membrane is the outer boundary of any typical cell.One of the most important functions of the plasma membrane is to control the contents of the cell. This involves controlling the transport of materials across the membrane.There are two main types of movements,namely passive transport and active transport.
Passive Transport of Molecules Across Membranes
Movement of solutes (ions or molecules) across membranes without the use of energy is known as passive transport.
There are three types of passive transport ,diffusion and facilitated diffusion and osmosis.
Diffusion
Molecules and ions dissolved in water are in constant random motion, called Brownian movement. This random motion results in a net movement of these substances from regions of high concentration to regions of lower concentration, a process called diffusion.
Net movement of a substance down its concentration gradient,from higher concentration to Lower concentration, driven by diffusion, will continue until the concentration of that substance is the same in all regions,means when dynamic equilibrium is achieved.For Instance when you add a drop of colored ink to a bowl of water. Over time the ink becomes dispersed throughout the solution. This is due to diffusion of the ink molecules. In the special case of cells, we are usually concerned with differences in concentration of molecules across the plasma membrane,means outside the cell and inside it.
Many, but not all, substances can move into and out of cells because the plasma membrane is selectively permeable.The major barrier to cross a cell’s plasma membrane is the hydrophobic interior of the bilayer that repels polar molecules but not nonpolar molecules. For a nonpolar molecule, if a concentration difference exists on the two sides of a membrane, the nonpolar molecule will move freely across the membrane until its concentration is equal on both sides. At this point, movement in both directions still occurs, but there is no net change in either direction. This free passage includes molecules such as O2 and nonpolar organic molecules such as steroid
hormones.
The plasma membrane has only limited permeability for small polar molecules, and very limited permeability for larger polar molecules and ions. The movement of water, one of the most important polar molecules, is discussed later in this post.
Facilitated Diffusion
The movement of molecules or ions from their higher concentration to lower concentration through specific protein channels or carrier proteins is called facilitated diffusion.
Many important molecules required by cells are polar molecules andso cannot easily cross the plasma membrane. How do thesemolecules enter the cell? They gain entry by diffusing throughspecific protein channels or carrier proteins embedded withinthe plasma membrane. Their passage requires no energy,provided there is a higher concentration of the molecule outsidethe cell than inside. We call this process of diffusion mediatedby a membrane protein facilitated diffusion.
Ion channels have a hydrophilic interior that provides an aqueous channel through which ions can pass when the channel is open.
Carrier proteins, in contrast to channels, bind specifically to the molecule they assist, much as an enzyme binds to itssubstrate. These channels and carriers are usually selective forone type of molecule, and thus the cell membrane is said to beselectively permeable.
Facilitated Diffusion of Ions Through Channels
Because of their charge, ions are repelled by nonpolar moleculessuch as those that make up the interior of the plasma membrane’slipid bilayer. Therefore, ions cannot move between the cytoplasmof a cell and the extracellular fluid without the assistance of membrane transport proteins.
Ion channels possess a hydrated interior that spans the membrane. Ions can diffuse through the channel in either direction,depending on their relative concentration across the membrane.
Some channel proteins can be opened or closed in response to a stimulus. These channels are called gated channels; depending onthe nature of the channel, the stimulus can be either chemical orelectrical.
Three conditions determine the direction of net movementof the ions: (1) their relative concentrations on either side of themembrane, (2) the voltage difference across the membrane and for the gated channels, and (3) the state of the gate (open or closed).
A voltage difference is an electric potential difference across the membrane called a membrane potential.Changes in membranepotential form the basis for transmission of signals in the nervous system and some other tissues. Each type of channel is specific for a particular ion, such as calcium (Ca2+), sodium (Na+), potassium (K+), or chloride(Cl–), or in some cases, for more than one cation or anion. Ion channels play an essential role in signaling by the nervous
system.
Facilitated Diffusion By Carrier Proteins
Channels are not the only way into cells. Carrier proteins can also transport both ions and other solutes, such as some sugars and amino acids, across the plasma membrane.
Transport by a carrier protein is still a form of diffusion and therefore requires a concentration difference across the membrane. However, diffusion across a membrane using a carrier protein differs from simple diffusion in one key respect: As a concentration gradient increases, transport by simple diffusion shows a linear increase in rate of transport. For transported molecules bound to carrier proteins, on the other hand, as the concentration gradient increases, a point is reached where all carriers are occupied and the rate of transport can increase no further, having reached saturation.
This situation is somewhat like that of a stadium (the cell) where a crowd must pass through turnstiles (the carrier protein)
to enter. When ticket holders (transported molecules) are passing through all gates at maximum speed, the rate at which they enter cannot increase, no matter how many are waiting outside.
Several examples of facilitated diffusion can be found in the plasma membrane of vertebrate red blood cells (RBCs). One RBC carrier protein, for example, transports a different molecule in each direction: chloride ion (Cl–) in one direction and bicarbonate ion (HCO3–) in the opposite direction. This carrier is important in the uptake and release of carbon dioxide.
The glucose transporter is a second vital facilitated diffusion carrier in RBCs. Red blood cells keep their internal concentration of glucose low through a chemical trick: They immediately add a phosphate group to any entering glucose molecule, converting it to a highly charged glucose phosphate molecule that can no longer bind to the glucose transporter and therefore cannot pass back across the membrane. This maintains a steep concentration gradient for unphosphorylated glucose, favoring its entry into the cell.
The glucose transporter that assists the entry of glucose into the cell does not form a channel across the membrane. Instead,this transmembrane protein binds to a glucose molecule and then flips its shape, dragging the glucose through the bilayer and releasing it on the inside of the plasma membrane. After it releases the glucose, the transporter reverts to its original shape, and it is then available to bind the next glucose molecule that comes along outside the cell.
Next posts
1.Osmosis
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