21. Why does water need a protein channel to move across the cell membrane?
22. What is needed to fuel active transport? Where is this made in the cell?
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All cells acquire the molecules and ions they need from their surrounding extracellular fluid (ECF). There is an unceasing traffic of molecules and ions • in and out of the cell through its plasma membrane ◦Examples: glucose, Na+, Ca2+
•In eukaryotic cells, there is also transport in and out of membrane-bounded intracellular compartments such as the nucleus, endoplasmic reticulum, and mitochondria. ◦Examples: proteins, mRNA, Ca2+, ATP
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The concentration of most molecules inside a cell is different than the concentration of molecules in the surrounding environment. The plasma membrane separates the internal environment of the cell from the fluid bathing the cell and regulates the flow of molecules both into and out of the cell. The second law of thermodynamics states that molecules, whether in the gas or liquid state, will move spontaneously from an area of higher concentration to an area of lower concentration or down their concentration gradient.
A concentration gradient can be likened to water stored behind a dam. The water behind the dam will flow through the dam via any available channel to the other side. The energy from the water moving through the dam can be harnessed to make electricity. Water can also be pumped in the opposite direction from the river below the dam up to the reservoir behind the dam, with an expenditure of energy. Cellular membranes act somewhat like a dam. They block the movement of many types of molecules and have specific channels, transporters and pumps to provide pathways for the movement of certain molecules across the membrane.
When a molecule moves down its concentration gradient using one of these membrane channels or transporters, the process is called facilitated diffusion. In facilitated diffusion, no input of energy is needed to move the molecules. Instead, the potential energy of the concentration gradient powers the movement, just like water flowing out of a dam. For further diffusion, the channel or transporter does not determine in which direction the molecules will move, it only provides a pathway for the movement.
In cells, some molecules must be moved against their concentration gradient to increase their concentration inside or outside the cell. This process requires the input of energy and is known as active transport. As with facilitated diffusion, special transporters in the membrane are used to move the molecules across the membrane. The plasma membrane is not the only cellular membrane that requires active transport. All organelles surrounded by membranes must concentrate some molecules against their concentration gradients.