Transport

views updated

Transport

Transport is the controlled movement of substances from one part of a cell to another, or from one side of a cell membrane to the other. Because each cell must maintain an internal environment different from the external environment, it must regulate the movement of ions, proteins, toxins, and other molecules both across the cell membrane and within its cytoplasm . This control over its molecular environment may be accomplished through a variety of measures, one of which is the establishment of a barrier membrane between the cell and the external world.

One such barrier, the bilipid membrane of a cell, is composed of two hydrophilic, or water-soluble, sheets of molecules separated by an intervening hydrophobic, also called oily or fatty, region. This property results from the structure of the phospholipid molecule composing the membrane: a polar, hydrophilic head region, and a nonpolar, hydrophobic tail region. The two layers of membrane are oriented so that the hydrophilic heads face the internal and external cell, and the fatty tails are positioned between the two head layers.

The structure of this membrane assures that any polar, water-soluble, molecules in the hydrophilic extracellular space will be unable to pass through the nonpolar, fatty, region within the membrane. If this barrier were completely impermeable, however, the cell would never be able to absorb nutrients or rid itself of wastes, let alone communicate with other cells. Thus the membrane is compromised by proteins that extend through both sides of the bilipid membrane. These proteins provide a watery pore region that connects the extracellular with the intracellular space, thereby allowing hydrophilic molecules to pass through.

There is a wide variety of membrane proteins, some of which are located within the outer cellular membrane, and some of which are imbedded in intracellular organelles . In addition to transport across membranes, substances must be transferred from one part of the cell to another, especially in very large cells and in single-celled organisms.

Osmosis

The simplest kind of cellular transport is osmosis . Osmosis is the passage of water molecules through a semipermeable membrane from a wet environment (a region of high water concentration), to a dry environment (a region of low water concentration). The defining characteristic of a wet environment is a low concentration of dissolved solute in the water. A dry environment has a high concentration of solute. Cells store a high concentration of proteins and other molecules within the membrane. If a cell were removed from biological conditions and placed in distilled water (water containing no dissolved substances), the small water molecules would rush through the bilipid membrane into the relatively dry interior of the cell. The membrane would expand with the increased water intake until the cell exploded.

This scenario does not normally occur in nature for two reasons. First, organisms balance their internal osmolarity. Internal osmolarity is the ratio of dissolved substance concentration between the inside and the outside of the cell. This balance is accomplished by maintaining a concentration of many small ions and molecules in the extracellular space, the areas between cells within an organism. Second, cells store many of their proteins within vesicles, within membranes, and inside organelles; this decreases the apparent concentration of free-floating soluble molecules.

Diffusion

The opposite of osmosis is diffusion, which means the passage of molecules from a region in which they are highly to a region in which they have a low concentration. This only occurs when the molecule has a concentration gradient, meaning that it exists in larger numbers per unit area in one location and in smaller numbers per unit area in an adjacent location. If a high concentration gradient is established, it means that the difference in concentration between two adjacent locations is great. In the case of a cellular membrane, this means that a certain substance is at very high concentrations on one side of the membrane and very low concentrations on the other side. The larger the concentration gradient, the stronger the driving force that powers the diffusion of molecules down the gradient. Whereas osmosis explains the movement of water molecules, diffusion explains the movement of other molecules within a liquid.

Passive Transport

For passive transport, no additional energy is needed to transfer molecules across the membrane. Instead, the concentration gradient of that molecule provides a driving force from high to low concentration, pushing the molecule across the membrane. This process is also known as facilitated diffusion because the membrane protein facilitates the natural diffusion tendency of the molecule by providing safe passage across the hydrophobic region of the lipid bilayer. Many sugars and amino acids are transported from the gut into the cells lining the gut through this mechanism.

When the protein forming the pore is constantly open to diffusable molecules, it is called a channel. The size of the pore, and any hydrophilic regions within the pore, can selectively allow certain molecules to pass while barring others. A uniport protein must first bind a molecule and then undergo a conformational change (a change in the shape of the protein) before it releases the molecule on the opposite side of the membrane.

One example of a passive transport mechanism is the gap junction. This is a molecular conduit between two cells formed by a hexagonal array of rigid proteins that leaves a permanent pore open between two cells' cytoplasms. This opening allows inorganic ions and very small hydrophilic molecules to pass directly between cells. Gap junctions are particularly useful in heart muscle cells because they allow ions carrying electrical charge to flow directly from cell to cell, thereby inducing the smooth muscle fibers to contract in a coordinated motion. This simple method of communication makes it unnecessary for neuronal connections at each and every heart muscle cell to induce contraction at exactly the right time for a smooth muscle heartbeat. If gap junctions were blocked, heart muscle cells would contract at the incorrect time, or not at all, resulting in irregular or weak heartbeats, or even heart seizure, and death.

Active Transport

Active transport requires that energy be expended to bring a molecule across the membrane. Most often active transport is used when the molecule is to be transferred against its concentration gradient. However, occasionally the target molecule is carried along its concentration gradient, when the gradient is not strong enough to ensure a sufficiently quick flow of molecules across the membrane. For uncoupled active transport, ATP (adenosine triphosphate ) in the cell's cytoplasm binds to the carrier protein at the same time as the targeted molecule binds. ATP is a cellular molecule that contains a great deal of energy in its molecular bonds. The carrier protein breaks a phosphate off the ATP and uses the energy released from the broken bond to undergo a conformational change that carries the targeted molecule to the other side of the membrane.

One example of active transport is the sodium/potassium ATPase pump, by which sodium is transferred out of a neuron while potassium is transferred into a neuron, both counter to their individual concentration gradients. This is essential for maintaining electrically active nerve cells because it helps to establish a concentration gradient and electrical gradient across the cell membrane. When the cell is active, sodium and potassium are allowed to flow down their electrochemical gradients, whereas during periods of inactivity the sodium/potassium pump restores the resting state polarization of the cell.

There are two kinds of coupled active transport. Symport uses the concentration gradient of one molecule to help transport another molecule. Symport membrane proteins have two binding sites on the same side of the membrane. The targeted molecule binds at one site, and a coactivating molecule with a high concentration gradient favoring movement across the membrane binds at the other site. The driving force of the coactivating molecule causes a conformational change in the membrane protein, which transports both molecules across the membrane in the same direction. The other type of coupled transport is antiport, by which the coactivating molecule and the targeted molecule bind at sites on different sides of the membrane bilayer. The concentration gradient of the coactivating molecule still provides the energy for the conformational change, but in this case, the molecules are transported simultaneously in opposing directions across the membrane. Symport and antiport systems are also called carrier-assisted transport, because energy from the coactivator helps to carry the target molecule in an energetically unfavorable direction.

The most common method for cells to transfer very large molecules across the membrane is through the intermediary of a vesicle. Vesicles are small spheres of membrane that contain large molecules, toxins, nutrients, or signaling molecules. Proteins that are created in the endoplasmic reticulum can be packaged in vesicles in the golgi apparatus. Similarly, vesicles may contain transporter proteins similar to those located in the external cellular membrane, so that they can bring in certain cytoplasmic proteins. Vesicles called lysosomes process and sequester harmful metabolic side-products so that they do not do damage to organelles in the cytoplasm.

In each of these cases, vesicles are transported to the outer cell membrane, a process known as exocytosis. The vesicles travel along a system of structural proteins called microtubules with the help of an associated motor protein called kinesin. Kinesin molecules "grab" vesicles with their globular head region and then take turns binding to the microtubules, so that the net result is that of a cartwheeling vesicle moving in one direction. Some proteins can be transported by binding directly to microtubules and slowly "riding" them as they slide toward the membrane. When it arrives, the vesicle lipid bilayer fuses with that of the outer cell membrane, so that the internal contents of the vesicle are released into the extracellular space. In endocytosis, the mechanism is reversed. Pockets of cell membrane that have bound a particular molecule dimple inwards toward the cell cytoplasm forming a deep pit, and then pinch off so that a vesicle forms on the intracellular side. Phagocytosis refers to the consumption of small cells or cell-pieces by larger cells by surrounding and engulfing them.

Rebecca M. Steinberg