Transport through the Cell Membrane Help (page 2)
Introduction to Transport through the Cell Membrane
The cell organelles, whether normal or abnormal, require transport systems across the cell membrane to keep themselves functioning. Transport systems within the cell, like highway systems in a city, provide a nearly constant movement of particles or objects, some of them moving in, others moving out.
Transport systems are needed because the plasma membrane is a selectively permeable ( PER -me-ah-bl) membrane . By selectively permeable, it is meant that certain types of particles are able to permeate ( PER -me-ate) or “pass through” the cell membrane, while others are not able to pass.
There are two basic types of transport systems that move particles across the cell membrane: passive transport systems versus active transport systems . Passive transport systems are passive in the sense that they do not require any active input of ATP energy to function. Active transport systems, in direct opposite, are those systems in which free energy from splitting ATP energy is needed.
Passive Transport Systems
There are three common types of passive transport systems serving cells. These are simple diffusion , osmosis (ahs- MOH -sis), and facilitated (fah- SIH -lih- tay -ted) diffusion . Diffusion is literally a “process of scattering” ( diffus ). The scattering process of diffusion arises from the fact that all particles are constantly moving in random directions. During simple diffusion, particles move by chance from a region where their concentration is high, to a region where their concentration is low.
Oxygen (O 2 ) molecules, for instance, tend to have a much higher concentration (crowding together) in the fluid outside of most cells, compared to their concentration within the cell. Why does this make sense? The reason is that oxygen molecules are being constantly used within cells for their aerobic metabolism. Thus, the intracellular ( IN -trah- sell -yew-lar) fluid present “within” ( intra -) the cell, generally has a low O 2 concentration. But the extracellular ( EKS -trah- sell -yew-lar) fluid “outside” ( extra -) the cell typically has a high O 2 concentration. Hence, there is a net (overall) simple diffusion of oxygen molecules from the extracellular fluid, across the plasma membrane, and into the intracellular fluid. [ Study suggestion: Open a bottle of perfume. Place it onto a table in a quiet room. After half an hour or so, return to the room. Do you smell the perfume in the far corners of the room? Explain this observation.]
Closely related to simple diffusion is osmosis. Osmosis is a “condition of thrusting” ( osm -). Specifically, osmosis is the simple diffusion of water (H 2 O) molecules only, from a region where the water concentration is high, to a region where the water concentration is low. Think about a row of small green plants. In dry soil, their leaves and stems soon wither. But when the soil is freshly watered, the leaves and stems expand and stiffen. Osmosis of millions of H 2 O molecules occurs from the moistened soil (having a high water concentration) into the cells of the green plants (which have a lower water concentration). The “thrusting” origin of the word osmosis therefore reflects osmotic pressure, the pushing or thrusting force associated with the simple diffusion of large numbers of water molecules.
Finally, facilitated diffusion (as its name strongly suggests) is diffusion that is facilitated or helped by the use of protein carrier molecules. Facilitated diffusion is basically just a process of “scattering” (like simple diffusion). It is special in that certain large molecules, such as glucose, need extra help in crossing the cell membrane after they “scatter” randomly to contact it. A glucose carrier protein, located right within the plasma membrane, combines with the glucose molecule. The carrier protein then changes its shape, dropping the glucose molecule off into the intracellular fluid. Here, the glucose can serve as an important source of cell energy.
Active Transport Systems
Active transport systems actively split ATP, unleashing free energy to power the transportation process.
Active transport, itself, is the ATP-requiring active pumping of particles from an area where their concentration is low, to an area where their concentration is high. Like facilitated diffusion, active transport uses a protein carrier molecule within the plasma membrane. Unlike facilitated diffusion, however, active transport runs “uphill” (energetically speaking), in that particles are moved from an area of low concentration “up” to an area of high concentration.
For example, sodium (Na + ) ions are removed from the intracellular fluid of many human and animal cells by an active transport system or “ATP pump.” Sodium is at a much higher concentration in the extracellular fluid, compared to the intracellular fluid. Sodium ions rapidly pass through the plasma membrane and enter the intracellular fluid of nerve cells when a person is stimulated or excited. A sodium “ATP pump,” consisting of a special carrier protein in the membrane, combines with the Na + ions that leaked into the cell, then splits ATP. This ATP-splitting generates enough free energy to actively carry Na + particles from the intracellular fluid of the nerve cells, back out into the extracellular fluid. A resting or nonstimulated state of the nerve cell membrane is thus re-established.
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