Passive transport refers to the type of cellular transport wherein the net movement of substances is down the concentration gradient. In contrast, cellular transport wherein substances have to be moved to an area that is already saturated or high in concentration is called active transport.
Because the movement of substances in passive transport does not go against the concentration gradient it, therefore, does not require chemical energy e. ATP to proceed. Rather, it is driven by kinetic and natural energy.
Other examples of passive transport are filtration and osmosis. Diffusion is of two major types. The first one does not require assistance when moving down the concentration gradient. This type is called simple diffusion. In contrast, facilitated diffusion , as the name implies, is one in which assistance is required.
The assistance comes, for instance, from the proteins embedded in a biological membrane. Basically, these two types of diffusion differ in the mechanism by which substances move — one that occurs without assistance and the other one that occurs with the help of transport proteins. Thus, in facilitated diffusion, the transport only occurs when the molecule is able to bind to the membrane protein transporter.
Both of them result in the net downhill movement of substances and do not require chemical energy to proceed. While diffusion is the movement of particles down their concentration gradient, active transport is the movement of particles against the concentration gradient.
Since the movement is characteristically uphill this type of transport requires energy often in the form of adenosine triphosphate ATP. Diffusion and osmosis are both types of passive transport. Thus, both of them occur in a downhill manner and without energy expenditure. The difference is the diffusing molecules or particles. In diffusion, the diffusing particles are the solutes of a solution.
In osmosis, the diffusing particles are the solvent of the solution, i. In osmosis, water molecules diffuse from an area of high water concentration to an area of low water concentration across a biological membrane.
Water that is drawn towards a concentrated solution but does not involve a biological membrane is not osmosis. The cell regulates the entry and exit of substances through its plasma membrane. Not all molecules can readily pass across this selective membrane due to its structure. The lipid bilayer feature of the plasma membrane prevents the passage of polar molecules.
Nevertheless, small nonpolar molecules and ions can pass through the lipid bilayer. We can use simple equations and graphs to examine how particular molecules and their concentration affect the rate of diffusion.
We can also compare simple and facilitated diffusion. Question: How do rates of simple and facilitated diffusion differ in response to a concentration gradient? Method: The rate of simple diffusion can be expressed by a modification of Fick's Law for small, nonpolar molecules. We can describe the rate of diffusion as directly proportional to the concentration gradient by the following equation:.
P is a constant relating the ease of entry of a molecule into the cell depending on the molecule's size and lipid solubility. If we graph the rate of diffusion as a function of the concentration gradient, we get a simple linear function. Interpretation: Notice the rate of diffusion increases as the concentration gradient increases.
If the concentration of molecules outside the cell is very high relative to the internal cell concentration, the rate of diffusion will also be high. If the internal and external concentrations are similar low concentration gradient the rate of diffusion will be low. Method: Unlike simple diffusion, facilitated diffusion involves a limited number of carrier proteins.
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. Transport proteins within the membrane allow these molecules to pass through the membrane, and into or out of the cell. This way, polar molecules avoid contact with the nonpolar interior of the membrane, and large molecules are moved through large pores.
Every cell is contained within a membrane punctuated with transport proteins that act as channels or pumps to let in or force out certain molecules. The purpose of the transport proteins is to protect the cell's internal environment and to keep its balance of salts, nutrients, and proteins within a range that keeps the cell and the organism alive. There are three main ways that molecules can pass through a phospholipid membrane. The first way requires no energy input by the cell and is called passive transport.
The second way requires that the cell uses energy to pull in or pump out certain molecules and ions and is called active transport. The third way is through vesicle transport, in which large molecules are moved across the membrane in bubble-like sacks that are made from pieces of the membrane.
Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell.
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