Often, substances have to be moved from a low to a high concentration - against a concentration gradient. Active transport is a process that is required to move molecules against a concentration gradient. The process requires energy. If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.
To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane.
A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it.
In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure.
Osmosis proceeds constantly in living systems. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.
In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water. This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.
An important distinction that concerns living systems is that osmolarity measures the number of particles which may be molecules in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules than there are cells.
Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell.
In living systems, the point of reference is always the cytoplasm, so the prefix hypo — means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm. It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell. Because the cell has a relatively higher concentration of water, water will leave the cell.
In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out.
Blood cells and plant cells Figure 7 in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances. Figure 7.
Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. A doctor injects a patient with what the doctor thinks is an isotonic saline solution.
The patient dies, and an autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic? In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane.
There is no net water movement; therefore, there is no change in the size of the cell. In a hypertonic solution, water leaves a cell and the cell shrinks. Remember, the membrane resembles a mosaic, with discrete spaces between the molecules composing it.
If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell.
Various living things have ways of controlling the effects of osmosis—a mechanism called osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution.
The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available.
This inflow of water produces turgor pressure, which stiffens the cell walls of the plant Figure 7. In nonwoody plants, turgor pressure supports the plant.
Conversly, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt Figure 8.
Figure 8. Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it right.
Vicente Selvas. Figure 9. Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment Figure 9. Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live.
Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells.
Choose the Right Synonym for transport Verb banish , exile , deport , transport mean to remove by authority from a state or country. Examples of transport in a Sentence Verb A van at the hotel transports guests to and from the airport. The illness was first transported across the ocean by European explorers.
The movie transports us to a world of stunning beauty. While reading, I was transported back to the year He was transported for stealing. Noun the transport of manufactured goods I was left without transport when the car broke down. She relies on public transport. Recent Examples on the Web: Verb She's thought about using planes again to transport products from the warehouses to retailers' distribution centers. First Known Use of transport Verb 14th century, in the meaning defined at sense 1 Noun , in the meaning defined at sense 1.
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The passage of ions or molecules across a cell membrane against an electrochemical or concentration gradient, or against the normal direction of diffusion. Published by Houghton Mifflin Company. The movement of ions or molecules across a cell membrane in the direction opposite that of diffusion, that is, from an area of lower concentration to one of higher concentration. Active transport requires the assistance of a type of protein called a carrier protein, using energy supplied by ATP.
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