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Transport across cell membranes may be either passive transport or active transport. Passive transport does not require energy. Active transport does require energy. Passive transport takes place when a substance moves from an area where it is more concentrated to an area where it is less concentrated. This movement is called diffusion. Passive transport requires no energy because a substance naturally moves from an area of higher to lower concentration. This is known as moving down the concentration gradient. It's like rolling a ball downhill. Types of passive transport are simple diffusion and facilitated diffusion. - Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. - Facilitated diffusion occurs when a substance diffuses through a cell membrane with the help of transport proteins. Active transport occurs when a substance moves from an area where it is less concentrated to an area where it is more concentrated. The substance is moving up, instead of down, the concentration gradient, and this takes energy. The energy comes from the molecule ATP. Types of active transport include pumps and vesicle transport. - Pumps are transport proteins that move certain substances to areas of higher concentration. An example is the sodium-potassium pump. - Vesicles are membrane-enclosed “containers.” They move substances into or out of the cell by fusing with the cell membrane.
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