|Cells and Diffusion|
The process of active transport requires the direct input of energy tomove particles across the cell membrane.Diffusion and facilitated diffusion can occur without the direct expenditure of cellular energy.
If one were to drop a sugar cube into glass of water and immediately use a straw to sip a little water from the top of the glass, the water would not have a sweet taste. However, after a few hours, a sip of water from the top would taste sweet.
The reason for the change in the taste of the water is diffusion, the net movement of particles down a concentration gradient (that is, from an area of higher concentration to an area of lower concentration). Concentration is the number of particles or amount of substance per unit volume, and a gradient occurs when some factor such as concentration changes from one volume of space to another.
Hence, the sugar molecules move more frequently from around the cube where they were highly concentrated to other parts of the glass where they were less concentrated. There is always some movement in both directions, but the net movement is down the concentration gradient.
Diffusion is possible because molecules in a liquid or gaseous phase are not static; they are in constant motion as a result of kinetic energy, which exists at temperatures above absolute zero (–273.16 degrees Celsius, or –459.69 degrees Fahrenheit).
As the concentration of a substance increases, its free energy also increases. When molecules move, they collide with one another and exchange kinetic energy, and there is a random but progressive movement from regions of high free energy (high concentration) to regions of low free energy (low concentration).
Diffusion can occur quite rapidly over short distances but can be extremely slow over long distances. For example, a molecule of glucose can diffuse across a typical 50-micrometer diameter cell in 2.5 seconds, but it takes thirty-two years for it to diffuse a distance of 1 meter.
Role in Plants
Diffusion is an important process in the lives of plants.Water is an important component of all cells, and water moves into plant cells by the process of osmosis. Osmosis is the diffusion of water across a semipermeable membrane. Many plant nutrients reach the root surface via diffusion through the soil solution.
Some nutrient molecules diffuse across root cell membranes into the cytosol (cell sap or cytoplasm) or from the cytosol of the endodermal cells into the xylemtissue. Carbon dioxide diffuses from the atmosphere through the stomata and into the air spaces of leaves. Water vapor evaporates from the surface of a leaf by diffusion through the open stomata.
Diffusion also plays a role in the movement of photosynthetic products such as sugars into the phloem for transport throughout the plant. Because cellular membranes are composed of a lipid bilayer, lipid-soluble materials use simple diffusion to cross the membrane surface.
Substances with low lipid solubility can move across membranes via facilitated diffusion. In this process, the substance binds to a transporter molecule, generally called an ionophore, which transports the substance across the membrane and down its concentration gradient.
As previously mentioned, diffusion occurs rapidly over short distances. In order to move substances such as water over long distances—for example from the roots to the leaves, plants use a process referred to as bulk flow. Bulk flow is the concerted en masse movement of groups of molecules, usually in response to a pressure gradient.
A moving stream, water flowing through a garden hose, and wind currents are examples of bulk flow. Pressure-driven bulk flow is the major mechanism behind the movement of water over long distances through the xylem. This process is different from diffusion because it is independent of solute concentration.
The most common source of energy for active transport comes from adenosine triphosphate (ATP).When this high-energy phosphate is hydrolyzed, the stored energy is released to drive cellular reactions such as active transport. More specifically, the substance is moved across the membrane by a carrier protein embedded in the membrane.
The carrier protein uses energy from the hydrolysis of ATP (that is, the removal of one phosphate group). Although active transport is primarily for movement against a concentration gradient, it can also be used to move substance down their concentration gradient.
There are two important modifications of the active transport process: cotransport and counter transport, both of which involve the movement of one substance down its concentration gradient while simultaneously transporting another substance against its specialized membrane proteins. Although these proteins do not require an energy source to operate, ATP is still indirectly consumed.
The substance being moved down its concentration gradient would eventually be at equal concentrations on each side of the membrane. To counteract this, active transport, with hydrolysis of ATP as the energy source, is used to pump the substance across the membrane to maintain the gradient.