Red blood cells, for example, have a cell membrane protein with an internal channel that allows glucose to pass through it. Only glucose can pass through this channel, and it can move through in either direction. This cell membrane protein is said to facilitate, or help, the diffusion of glucose across the membrane.
The process, shown below, is known as facilitated diffusion fuh-SIL-uh-tayt-ud. Hundreds of different protein channels have been found that allow particular substances to cross different membranes. Although facilitated diffusion is fast and specific, it is still diffusion. Therefore, a net movement of molecules across a cell membrane will occur only if there is a higher concentration of the particular molecules on one side than on the other side.
This movement does not require the use of the cell's energy. Small molecules and ions are carried across membranes by proteins in the membrane that act like energy-requiring pumps. Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes.
Changes in protein shape, as shown in the figure at right, seem to play an important role in the pumping process. A considerable portion of the energy used by cells in their daily activities is devoted to providing the energy to keep this form of active transport working.
The use of energy in these systems enables cells to concentrate substances in a particular location, even when the forces of diffusion might tend to move these substances in the opposite direction.
Larger molecules and even solid clumps of material may be transported by movements of the cell membrane. One of these movements is called endocytosis en-doh-sy-TOH-sis.
Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The pocket that results breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm. Large molecules, clumps of food, and even whole cells can be taken up in this way. The cell then engulfs it.
Amoebas use this method of taking in food. Engulfing material in this way requires a considerable amount of energy and, therefore, is correctly considered a form of active transport. In a process similar to endocytosis, many cells take up liquid from the surrounding environment. Tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell.
This process is known as pinocytosis. Many cells also release large amounts of material from the cell, a process known as exocytosis ek-soh-sy-TOH-sis. During exocytosis , the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell. The removal of water by means of a contractile vacuole is one example of this kind of active transport. Key Concept Describe the functions of the cell membrane and cell wall.
Key Concept What happens during diffusion? Key Concept Describe how water moves during osmosis. What is the basic structure of a cell membrane?
What is the difference between phagocytosis and pinocytosis? Critical Thinking Comparing and Contrasting What is the main way that active transport differs from diffusion? Lesson in High School Biology. Measuring Concentration. How Osmosis Works. Osmotic Pressure. Facilitated Diffusion. Molecular Transport. Endocytosis and Exocytosis.
The authors of the present paper declare that none of them has any direct or indirect financial relations with any commercial identity mentioned in the paper that might lead to a conflict of interests for any of the authors. This study is financially supported by Project no. Minkov et al. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Minkov , 1,2 Emil D. Manev, 2 Svetla V. Sazdanova, 2 and Kiril H. Academic Editor: J. Received 12 Aug Accepted 07 Oct Published 26 Dec Abstract Osmosis is essential for the living organisms.
Introduction Osmosis plays a primary role in biological systems. Figure 1. Schematic of the two experimental osmotic regimes: a open cell variable volume ; b closed cell constant volume. Solvent influx [mol] as a function of osmotic pressure [bar] in the regime of variable solution volume. Table 1. Comparison of the osmotic pressure values at variable open and constant closed volume with the theoretical estimates. Figure 3. Comparison of equilibrium osmotic pressure values, as a function of solute concentration under regimes of constant and variable solution volume.
The dotted line indicates the theoretical dependence see 1. Table 2. Comparison of the kinetic characteristics of the osmotic process in aqueous sucrose solutions for the two experimental regimes of constant and variable solution volume. Active area of the semipermeable membrane. Figure 4. Osmotic pressure versus time dependence for three different initial sucrose concentrations at the two regimes: 1 0.
Figure 5. Solvent influx as a function of elapsed time dependences for the three studied solute concentrations: a constant volume regime: 1 0. Figure 6. Solvent rates of transfer dependences as a function of elapsed time for the three solute concentrations: a constant volume regime: 1 0. References J. View at: Google Scholar H.
Morse, J. Frazer, and F. View at: Google Scholar J. Frazer and R. View at: Google Scholar P. Lotz and J. View at: Google Scholar D. View at: Google Scholar V. Granik, B. Smith, S. Lee, and M. Grattoni, M. Merlo, and M. Krustev, H. Kolikov, D. Hristozov, and I. View at: Google Scholar A. Portella and B. Alleva, O. Chara, and G. View at: Google Scholar M. Paganelli and A. Longuet-Higgins and G. In living systems, diffusion is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.
Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Semi-permeable membranes are very thin layers of material which allow some things to pass through them, but prevent other things from passing through.
Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.
Example : If there was a semi-permeable membrane with more water molecules on one side as there were on the other, water molecules would flow from the side with a high concentration of water to the side with the lower concentration of water. This would continue until the concentration of water on both sides of the membrane were equal dynamic equilibrium is established. Osmotic Pressure Adding sugars to water will result in a decrease in the water concentration because the sugar molecules displace the water molecules.
If the two containers are connected, but separated by a semi-permeable membrane, water molecules would flow from the area of high water concentration the solution that does not contain any sugar to the area of lower water concentration the solution that contains sugar. This movement of water would continue until the water concentration on both sides of the membrane is equal, and will result in a change in volume of the two sides.
The side that contains sugar will end up with a larger volume. Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution.
Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and sugar would be characterized as a sugar solution.
The classic example used to demonstrate osmosis and osmotic pressure is to immerse red blood cells into sugar solutions of various concentrations.
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