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Simple/Passive diffusion, facilitated diffusion (through carrier/channel proteins), and active transport (requires ATP).
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The processes used by substances to move across the cell membrane are Osmosis, Active transport and Passive transport.
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Strictly speaking there are more than three ways of transporting molecules across a cellular membrane. However, the three passive processes ( do not require
Movement of substances across the cell membrane occurs in three main ways. We will look at each in turn
This is the most simple method of transport. Simple diffusion is the movement of a substance from a higher concentration to an area of lower concentration across the membrane. It does not require the use of membrane proteins. Molecules that use simple diffusion include oxygen, water and carbon dioxide. For this to happen, the molecules most travel down a concentration gradient. When molecules reach equilibrium, simple diffusion stops.
Some molecules cannot get across the membrane so easily. If a molecule is hydrophilic, it cant get across the hydrophobic plasma membrane. Molecules using FD are still moving down a concentration gradient from high to low, but they need some help to cross. This comes in the form of proteins which live on the plasma membrane. These might be channel proteins or carrier proteins. Channel proteins span the membrane and make hydrophilic tunnels across, which allow specific molecules to pass through by diffusion, like a bridge. A channel protein you might have heard of is an Aquaporin, which allow water to cross the membrane. You find them in plant cells, and in our own red blood cells and parts of the kidney. The other transmembrane proteins that we find in facilitated transport are carrier proteins. These work by changing shape to move a target molecule across the membrane.
Finally, active transport is used when the movement across the membrane needs energy! This may be because the molecule is NOT travelling down a concentration gradient. With AT, molecules travel against the gradient. This movement is not easy, and requires a cell to expend energy. There are two types we need to know about:
Primary active transport
This uses a source of chemical energy such as ATP to transport move molecules. The Sodium-Potassium pump is a good example. It moves Na+ out of cells, and K+ into cells. Movement of the ions directly uses energy from ATP, so is primary transport.
Secondary active transport, also known as co-transport uses an electrochemical gradient. This gradient is generated by active transport. It does not directly use the energy from ATP like primary transport does. Essentially, movement of one substance, such as an ion down a gradient is paired to the transport of other substance against a gradient by a shared carrier protein. These two molecules may move across the membrane in the same or opposite direction. If in the same direction, the transport protein is called a aymporter. When opposite directions, its an antiporter.
Diffusion, osmosis, active transport
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