Active and Passive Transport
What is passive transport?
Passive transport is the movement of molecules from one location to another in a somewhat random manner. There is no energy that is needed or used to move the molecules. There are three types of passive transport that we will discuss in class. They are diffusion, facilitated diffusion, and osmosis.
Diffusion
Diffusion is the process by which particles move from areas of high concentration to areas of low concentration. Diffusion can be observed by watching sugar crystals dissolve in water. Molecules of sugar break off from the crystal and gradually move away from it in the water until all the sugar molecules are dissolved in the water. Cell membranes allow some molecules to diffuse freely through them. Because those molecules move freely and randomly across the cell membrane, the cell does not have to use energy to obtain those particles, thus making the process an example of passive transport.
Diffusion
Diffusion is the process by which particles move from areas of high concentration to areas of low concentration. Diffusion can be observed by watching sugar crystals dissolve in water. Molecules of sugar break off from the crystal and gradually move away from it in the water until all the sugar molecules are dissolved in the water. Cell membranes allow some molecules to diffuse freely through them. Because those molecules move freely and randomly across the cell membrane, the cell does not have to use energy to obtain those particles, thus making the process an example of passive transport.
Facilitated Diffusion
Not all particles are able to freely pass through the phospholipid bilayer of the cell membrane, but the cell still needs to obtain certain particles without have to spend energy to get them. To do this, the cell membrane has certain proteins, called channel proteins, that provide pathways for specific particles to pass in and out of the cell. This is still passive transport, because the particles are still randomly passing through the channel proteins in both directions. The cell membrane just helps the diffusion out by providing specific channels for the particles to use.
Not all particles are able to freely pass through the phospholipid bilayer of the cell membrane, but the cell still needs to obtain certain particles without have to spend energy to get them. To do this, the cell membrane has certain proteins, called channel proteins, that provide pathways for specific particles to pass in and out of the cell. This is still passive transport, because the particles are still randomly passing through the channel proteins in both directions. The cell membrane just helps the diffusion out by providing specific channels for the particles to use.
Osmosis
Osmosis is similar to diffusion except for the fact that water molecules pass through the membrane instead of other particles. Also, water moves from areas of low concentration to areas of high concentration. For example, two solutions of differing concentrations of sugar water are separated by a selectively permeable membrane that will let water pass through the membrane, but nothing else can get through. Water from the lower concentration solution will pass through the membrane until the concentration on each side of the solution is equal. This will result in a change in volume on both sides of the membrane. |
The Effects of Osmosis on Cells
When comparing the concentrations of solutions on two sides of a membrane, there are three different states the solutions can be in, isotonic, hypertonic, and hypotonic.
Hypotonic Solutions
When a solution is hypotonic, the concentration of dissolved particles in the solution is lower than concentration of dissolved particles on the opposite side of the membrane. When a cell is placed in a hypotonic solution, it means that the cell has more dissolved particles inside of it than the solution that surrounds this. Because osmosis causes water to move from areas of low concentration to areas of high concentration, water will flow into the cell in an attempt to reach an equilibrium inside and outside of the cell. This will cause the cell to balloon out and potentially burst (called lysis).
Isotonic Solutions
When a solution is isotonic, the concentration of dissolved particles in the solution is equal to the concentration of dissolved particles on the opposite side of the membrane. It is said that the two solutions are in equilibrium. When a cell is placed in a isotonic solution, water will flow equally into and out of the cell at the same time keeping the cell at a constant size.
Hypertonic Solutions
When a solution is hypertonic, the concentration of dissolved particles in the solution is higher than the concentration of dissolved particles on the opposite side of the membrane. When a cell is placed in a hypertonic solution, water will flow out of the cell in an attempt to make the concentrations on both sides of the membrane equal. This will cause the cell to shrivel up.
When a solution is hypotonic, the concentration of dissolved particles in the solution is lower than concentration of dissolved particles on the opposite side of the membrane. When a cell is placed in a hypotonic solution, it means that the cell has more dissolved particles inside of it than the solution that surrounds this. Because osmosis causes water to move from areas of low concentration to areas of high concentration, water will flow into the cell in an attempt to reach an equilibrium inside and outside of the cell. This will cause the cell to balloon out and potentially burst (called lysis).
Isotonic Solutions
When a solution is isotonic, the concentration of dissolved particles in the solution is equal to the concentration of dissolved particles on the opposite side of the membrane. It is said that the two solutions are in equilibrium. When a cell is placed in a isotonic solution, water will flow equally into and out of the cell at the same time keeping the cell at a constant size.
Hypertonic Solutions
When a solution is hypertonic, the concentration of dissolved particles in the solution is higher than the concentration of dissolved particles on the opposite side of the membrane. When a cell is placed in a hypertonic solution, water will flow out of the cell in an attempt to make the concentrations on both sides of the membrane equal. This will cause the cell to shrivel up.
What is active transport?
Active transport occurs when particles move against a concentration gradient. This can be viewed of the opposite of diffusion. Instead of particles moving from areas of high concentration to areas of low concentration, which is what occurs naturally and with no energy, particles will move from areas of low concentration to areas of high concentration. For this to happen, energy is required to be used by a cell. There are two types of active transport, molecular transport and bulk transport.
Molecular Transport
When a cell needs a higher concentration of a particular particle inside the cell than outside, it will use specific proteins that are embedded in the cell membrane to be a pump and actively bring more of those particles into the cell. To do so the cell has to supply the protein pump with energy in the form of ATP. Every time the protein pump receives an ATP molecule, it opens up and transports the particle into the cell.
Molecular Transport
When a cell needs a higher concentration of a particular particle inside the cell than outside, it will use specific proteins that are embedded in the cell membrane to be a pump and actively bring more of those particles into the cell. To do so the cell has to supply the protein pump with energy in the form of ATP. Every time the protein pump receives an ATP molecule, it opens up and transports the particle into the cell.
Bulk Transport
Bulk transport involves the processes of endocytosis and exocytosis. Endocytosis is the process of a cell bringing materials into the cell by engulfing the materials. Exocytosis is the process of a cell expelling materials from the cell by fusing vesicles containing the materials with the cell membrane and opening the vesicle up to the environment outside of the cell.
Bulk transport involves the processes of endocytosis and exocytosis. Endocytosis is the process of a cell bringing materials into the cell by engulfing the materials. Exocytosis is the process of a cell expelling materials from the cell by fusing vesicles containing the materials with the cell membrane and opening the vesicle up to the environment outside of the cell.