Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas.
Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the blood to the body tissues. This spread of particles through random motion from an area of high concentration to an area of lower concentration is known as diffusion.
This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, dynamic equilibrium exists.
The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of diffusion. 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. Since this is an energetically unfavorable reaction, energy is needed for this movement. The source of energy is the breakdown of ATP. If the energy of ATP is directly used to pump molecules against their concentration gradient, the transport is called primary active transport.
Note that the substance indicated by the triangles is being transported from the side of the membrane with little of the substance to the side of the membrane with a lot of the substance through a membrane protein, and that ATP is being broken down to ADP. In some cases, the use of ATP may be indirect. On the left side of the picture below, a substance represented by an X is being transported from the inside of the cell to the outside even though there is more of that substance on the outside indicated by the letter X being larger on the outside of the cell.
This is primary active transport. In the picture on the right side, substance S, already at higher concentration in the cell, is brought into the cell with substance X. Since S is being transported without the direct use of ATP, the transport of S is an example of secondary active transport. For substance X primary active transport of X is occurring. The high concentration of X outside the cell is being used to bring in substance S against its concentration gradient.
It is possible for large molecules to enter a cell by a process called endocytosis, where a small piece of the cell membrane wraps around the particle and is brought into the cell. If the particle is solid, endocytosis is also called phagocytosis. If fluid droplets are taken in, the processes is called pinocytosis. Illustration of endocytosis.
Note that the particle entered the cell surrounded by a piece of cell membrane. The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete molecules too large to pass through the cell membrane by any other mechanism. Click on the button above to open a problem solver to help you practice your understanding of membrane transport with the following examples:. Potassium permanganate is placed into a beaker of water.
Particles diffuse from an area of high concentration to an area of low concentration. The contents of the beaker are now all the same concentration. Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion. Their concentration is higher in the small intestine than their concentration in the blood, so there is a concentration gradient from the intestine to the blood.
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