Cell+Transport



There are two types of transport across the cell membrane: **Passive Transport** and Active **Transport**


 * Passive Transport** occurs in the direction of the concentration gradient and does not require energy. This means that molecules move from an area of high concentration to an area of low concentration.

There are several types of passive transport:


 * Diffusion** is the spontaneous movement of molecules from an area of high concentration to an area of low concentration.
 * Osmosis** is diffusion of water through a membrane. The direction that the water will move will depend on the solution that the cells are in.

The three types of solutions are:
 * **Hypertonic**: The relative concentration of solutes is higher than inside the cell. The relative concentration of water is lower than inside the cell. Thus, the net movement of water will be out of the cell.


 * **Isotonic**: The relative concentration of water and salt is equivalent to the cell. There will be no net movement of water.


 * **Hypotonic**: The relative concentration of salts is lower than inside the cell. The relative concentration of water is higher than inside the cell. Thus, the net movement of water will be into the cell.

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 * Facilitated Diffusion** occurs when proteins help molecules move across the cell membrane. They can either be **carrier proteins** or **protein channels.**
 * **Carrier proteins** attach to the molecule on one side of the cell membrane and shuttle it across to the other side of the cell membrane.
 * **Protein channels** are tunnels through which molecules can get from one side of the membrane. They are usually quite specific with respect to the types of ions they allow through and often have 'gates' which are controlled by hormones, neurotransmitters or voltage.

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 * Active Transport** moves particles against the concentration gradient by using energy. This means that particles move from an area of low concentration to an area of high concentration.


 * Protein pumps** in the cell membrane use energy to move ions against the concentration gradient.

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 * Endocytosis** occurs when the cell membrane encloses materials and brings them into the cell.


 * **Phagocytosis** engulfs large particles by extending the cell membrane. White blood cells use this method to remove bacteria from tissue.


 * **Pinocytosis** encloses materials by forming pits in the cell membrane which pinch off.

Both phagocytosis and pinocytosis result in vesicles which can then merge with lysosomes in order to digest their contents or which merge with the cell membrane again to expel the contents.


 * Exocytosis** occurs when vesicles merge with the cell membrane to expel their content.

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The nerve cell is a fantastic example of cell transport in action:

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