Which part of the cell membrane is responsible for transportation?

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c67julie eNotes educator| Certified Educator

The cell membrane is made of a phospholipid bilayer that acts as a barrier to what molecules can or cannot enter the cell. There are different structures within the cell membrane to move molecules too large to enter between the phospholipids and those that are polar. Diffusion and osmosis are natural processes that do not require energy from the cell. Since no energy is required by the cell, diffusion and osmosis are types of passive transport. Diffusion allows non-polar molecules to flow from an area of higher concentration to an area of lower concentration. Osmosis is a specific type of diffusion involving only water molecules. Some molecules will need the aid of a channel protein to pass through the membrane. When the channel protein is open, the molecules will pass though by diffusion. This is still a form of passive transport because it requires no energy. If a molecule is too large to pass through the membrane by diffusion, transport proteins are needed. Carrier proteins mediate active transport. Active transport requires energy expenditure by the cell to get a molecule into or out of the cell. This is because the molecule needs to work against the gradient, meaning it must go from a lower concentration to a higher concentration.

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