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Transport of materials across cellular membranes is essential to the functioning of plants and other living organisms. It is the movement of materials across these semipermeable barriers that provides the conditions necessary for life, not only for the plasma membrane separating a cell from its environment but also for membranes surrounding organelles within cells.
Unless cells are able to maintain a stable internal environment (homeostasis), growth, development, and metabolism are not possible.
Thus, understanding how substances move across membranes and how membranes select which substances to admit and exclude leads to a better understanding of homeostasis and its maintenance.
Some substances require metabolic energy to cross membranes in a process called active transport. All other movement across membranes, however, is through either simple or facilitated diffusion. Osmosis is a special case of simple diffusion involving just the movement of water.
Cell membranes, typically about eight nanometers thick, serve as barriers between the cell’s cytoplasm and the extracellular environment. The cell membrane’s lipid constituents (primarily phospholipids) make it fundamentally insoluble in water and therefore impermeable to all but the smallest uncharged polar molecules and a variety of lipid soluble molecules. It is necessary, though, for some of these substances to cross this barrier.
The currently accepted structure of the cell membrane was proposed by Jonathan Singer and Garth Nicolson in 1972. According to their fluid mosaic model, a membrane consists of a double layer of phospholipids, called a lipid bilayer.
Phospholipids are more or less linear molecules with a hydrophilic end (water-soluble or, literally, "water-loving"), often called the "head", and a hydrophobic end (water-insoluble or "water-fearing"), often called the "tail".
Within the bilayer, the lipids naturally orient themselves with their hydrophilic heads facing toward the aqueous fluid on each side—the cytoplasm on one side, the extracellular fluid on the other—and their hydrophobic tails touching one another within the bilayer.
Phospholipids on each side of the membrane are free to move around, a property called fluidity. Among the phospholipids float various proteins. They, too, are free to move unless constrained by anchors to cytoplasmic or extracellular structures or by limits on the fluidity of the lipid.
Lipids and proteins that are exposed to the outside of the cell may also have oligosaccharides (short chains of sugar molecules) attached to them, making them glycolipids and glycoproteins. The carbohydrate portion recognizes and is recognized by specific binding substances on other cells or in the environment.
Concentration and Electrochemical Gradients Substances crossing the membrane barrier by osmosis or diffusion will cross only from the side of the membrane with a higher concentration of the substance to the side with a lower concentration of the substance.
If the substance is an ion (all of which are electrically charged), the concentration gradient of the ion and the difference in electrical charge across the membrane (called the membrane potential) together determine the direction of diffusion. The concentration gradient and the membrane potential together are referred to as an electrochemical gradient. Ions will only go down their electrochemical gradient.
If the concentration gradient and the membrane potential are lined up in the same direction, it is easy to determine which way the ion involved will travel. For example, if chloride ions (Cl–), which have a negative charge, are in higher concentration outside the cell than inside the cell, they will be able tomove into the cell by facilitated diffusion.
This is because all cells have a negative membrane potential, which is to say, the electrical charge in the cytoplasmis more negative than the electrical charge on the outside.
Thus, for this Cl– example, both the concentration gradient and the membrane potential are aligned in the same direction. On the other hand, if the Cl– concentration is higher inside the cell than outside, Cl- could potentially travel in either direction.
If the concentration gradient is very large and the membrane potential is only slightly negative, Cl– would likely move out of the cell, whereas if the concentration gradient is small, and the membrane potential is very negative, Cl– will likely move into the cell.
Routes Across the Membrane
Regardless of the direction substances will potentially be able to travel across the membrane, they will be able to do so by only one of three different processes: osmosis, simple diffusion, or facilitated diffusion.
Water molecules, which are very small, can penetrate the lipid bilayer by passing between the phospholipids in a process called osmosis, although the exact mechanism is poorly understood.
A few other hydrophilic molecules, such as methanol and ethanol, can cross a lipid bilayer. Apparently they, too, can penetrate the otherwise inhospitable hydrophobic environment of the membrane because of their small size and lack of charge.
Hydrophobic substances, including dissolved gases such as oxygen, can also cross the membrane essentially unrestrained, and because they are lipid-soluble they simply "dissolve" into the membrane and through to the other side.
Hydrophilic substances, particularly charged molecules (ions), however, cannot cross a lipid bilayer by simple diffusion. They can cross membranes only with assistance from specialized transport proteins.
These proteins penetrate both sides of the bilayer, with one part exposed to the cytoplasm and the opposite part to the extracellular fluid. Many transport proteins appear to work by a “shuttle” type of mechanism, binding to recognized substances on one side of the membrane and releasing them on the other side.
Like many other processes involving proteins, the activity of transport proteins depends critically on the shape of the protein itself and on its ability to recognize a limited group of substances. Some membrane proteins form complexes of several proteins that form aqueous channels of the proper dimensions and charge distribution to serve as pores for specific molecules or ions.
Because living cells occupy an aqueous environment, water will always be present on both sides of amembrane. If one side has a higher concentration of solutes (dissolved material) in it than the other side, it will have a correspondingly lower concentration of water.
Driven by random molecular movement, more water molecules will bump into one another and move out of the region of lower solute concentration than move into it, until eventually the water is uniformly distributed on both sides and the solute concentrations are the same. This net movement driven by the difference in solute concentration is called osmosis.
Osmosis is of fundamental importance in a variety of living systems. Organisms originally evolved in the ocean, so the cytoplasm of most cells has a solute concentration similar to seawater.
Even terrestrial organisms have evolved mechanisms that maintain proper solute concentrations in tissue fluids. If the balance deviates from relatively narrow limits, there can be serious consequences.
For example, the freshwater green alga Spirogyra has a higher solute concentration than the surrounding water. Consequently, the net movement of water is from outside the algal cells to the cytoplasm.
If this movement of water were to continue indefinitely, it would eventually cause such a buildup of pressure inside the algal cells that they would burst, except that algal cells (as well as all plant cells) are surrounded by a rigid cell wall. As the pressure builds, the cytoplasm swells in volume and presses the cell membrane against the cell wall, and the cell is said to be turgid.
This same process occurs in terrestrial plants when roots absorb water from the soil, and this enables stem and leaf cells to maintain the turgidity required to keep the plant from wilting. In these examples, the cytoplasmis said to have a lower (more negative) water potential than the water outside.
A solution with a lower water potential than another solution across a semipermeable membrane is said to be hypertonic (or hyperosmotic), while the other solution is said to be hypotonic (or hypo-osmotic). If the solute concentrations are the same on both sides of a semipermeable membrane, they are said to be isotonic (or iso-osmotic).
If the same green algae is placed in concentrated saltwater, the cytoplasm will now be hypotonic (and the saltwater will be hypertonic), and the net flow of water will be out of the algal cells. When this occurs in plants, the cytoplasm shrinks in volume, and the cell membrane pulls away from the cell wall in a process called plasmolysis.
As a result, turgor pressure drops, and in the case of terrestrial plants in salty or dry soil, they begin to wilt. Some freshwater protozoa have specialized structures called contractile vacuoles that serve as pumps to maintain hypertonic conditions in the cytoplasm.
Both simple and facilitated diffusion involve the movement of a substance down a concentration or electrochemical gradient between the two compartments separated by a membrane.
The main difference between the two mechanisms is that substances moving by simple diffusion move through the lipid portion of the membrane, while substances using facilitated diffusion require specialized membrane proteins to allow their transport.
In simple diffusion, dissolved gases, such as oxygen and carbon dioxide, can cross membranes, as can lipid soluble substances. One type of important small molecule that crosses membranes by free diffusion are steroid hormones.
These extremely potent regulators of cell metabolism readily diffuse through cellular membranes because their hydrophobic nature makes them dissolve readily in lipids. Within cells, they bind to specific receptors, which passmessages on to other proteins that control various biochemical events.
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Such proteins allow larger, polar molecules such as sugars and amino acids to be taken up by cells. They control the response of cells to certain growth factors and hormones, whose binding to the cell membrane causes channels for facilitated diffusion to open or close selectively.
Extremely specific recognition by the transport protein enables material to be transported. For this reason, membranes may readily transport one type of molecule but be completely impermeable to another molecule, even a closely related one.
Membranes with selective permeability probably represent one of the most important innovations in living organisms. Without membranes, separate compartments could not be maintained to separate the various incompatible biochemical reactions of living systems.
Without the semipermeability of biological membranes, the environment within the various compartments could not be continuously replenished and would be unable to respond to a continuously changing environment.