Irregularly dispersed throughout the phospholipid bi-layer are aggregates of globular proteins, which are free to move within the layer, giving the membrane a fluid-like appearance. The membrane also contains water-filled pores with diameters of about 0.
The cell membrane acts as a barrier to the free flow of these ions and maintains concentration differences of these ions. In addition, the cell membrane acts as a barrier to the flow of water. Molecules can be transported across the cell membrane by passive transport or active processes. An active process is one that requires the expenditure of energy, while a passive process results solely from the inherent, random movement of molecules.
Osmosis , i. Simple diffusion accounts for the passage of small molecules through pores and of lipid-soluble molecules through the lipid bilayer. Oxy gen and carbon dioxide diffuse through the membrane readily because they are soluble in lipids. Some other mechanism must account for the transport of larger sugar molecules such as galactose, glucose, and sucrose, as they are too large to pass through membrane pores. Unable to display preview. Download preview PDF.
Skip to main content. For other types of molecules that need to cross the plasma membrane, cells use transport systems. Pumps push molecules against a concentration gradient. Channels open a gate for molecules to flow with their concentration gradient. Transporters bond to specific types of molecules and carry them through the membrane. The plasma membrane is absolutely essential for maintaining these conditions. Simply put, the plasma membrane separates the inside of the cell from everything else.
Without it, a cell is nothing more than a popped balloon, spilling its contents out into space. Plasma membranes are made up of a phospholipid bilayer. Phospholipids are chains of fatty acids attached to a phosphate group.
When phospholipids get together, they naturally form a double layer, with their phosphate groups facing out and their fatty tails pointing toward one another. The fatty interior of this layer is called "hydrophobic" because it repels water. The surrounding phosphates are called "hydrophilic" because they come into contact with fluid inside and outside the cell. The plasma membrane separates these two sets of fluids and their contents. A completely isolated cell soon runs out of fuel and fluid and drowns in its own waste.
The plasma membrane also maintains homeostasis by making sure materials can move in or out as necessary. Homeostasis depends on maintaining correct fluid levels within the cell and on exchanging useable materials, such as oxygen, for waste products, such as carbon dioxide.
Passive diffusion is the process in which molecules travel through a semipermeable barrier along a concentration gradient - that is, from an area of greater concentration to one of lower concentration. Only a small number of materials can pass through the plasma membrane by passive diffusion; if it were open to everything, it would not be a barrier.
Yet cells need to control the movement of a variety of other molecules in and out of their membranes to maintain homeostasis. To do this, cells have devised a variety of transport systems that use proteins embedded in the lipid bilayer as gates for cells to open and close. There are three major types of transport systems in the plasma membrane: pumps, channels and transporters.
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