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Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP g ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.
The Na+K+ Pump. This transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps.
The Na+K+ pump is a complex pump, simultaneously pumping three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP split. This means that, apart from moving ions around, it also generates a potential difference across the cell membrane. This is called the membrane potential, and all animal cells have it. It varies from 20 to 200 mV, but and is always negative inside the cell. In most cells the Na+K+ pump runs continuously and uses 30% of all the cell's energy (70% in nerve cells).
The rate of diffusion of a substance across a membrane increases as its concentration gradient increases, but whereas lipid diffusion shows a linear relationship, facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate being limited by the number of transport proteins. The rate of active transport also increases with concentration gradient, but most importantly it has a high rate even when there is no concentration difference across the membrane. Active transport stops if cellular respiration stops, since there is no energy.
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