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Derrick has taught biology and chemistry at both the high school and college level. He has a master's degree in science education.
All cells are enclosed with a cell membrane. A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport. Active transport processes require the cell to expend energy to move the materials, while passive transport can be done without using cellular energy. It is important to note that the membrane can also block the passage of materials depending on the conditions inside or outside of the cell.
The method used by a cell to pass molecules in or out is determined by the size of the molecule, as well as the concentration of the molecules on the inside and outside of the cell. Most cell membranes are covered with tiny protein channels that allow things to move in and out of the cell. These channels act as gateways.
If a molecule is tiny enough to fit through a special protein channel in the plasma membrane, it will use a form of active or passive transport to move through. If the molecule is too large to fit through a protein channel, it will have to enter or exit the cell by forming a vesicle. A vesicle can be thought of like a tiny pouch that surrounds the molecule and ushers it through an opening in the cell membrane.
Selectively permeable membranes can be found around a variety of cells and places. The most common example is the phospholipid bilayer cell membrane that surrounds every cell in our bodies. Another example of a selectively permeable membrane is the inner membranes of an egg.
All cells in our body are surrounded by a phospholipid bilayer. Phospholipids are molecules composed of a hydrophilic, or water-loving, head and a hydrophobic, or water-fearing, tail. In cell membranes the phospholipids are arranged with the heads facing the outer and inner surfaces of the cell with the tails aligned in the middle.
These cell membranes are also dotted with protein channels that can open or close to allow or prevent molecules from entering the cells. One example of this is the sodium-potassium pump. Cells need higher concentrations of potassium than sodium. These protein pumps move three sodium ions out for every two potassium ions they let into the cell. This keeps the proper concentration levels inside and outside of the cell.
If a cell needs to remove large waste particles it can form a vesicle (a large sac-like container) to move all of the particles out at one time. First, the vesicle surrounds the materials. Then, it connects with the cell membrane and creates an opening to the outside of the cell where the materials are released.
Have you ever seen a baby chick that just hatched from an egg? Two selectively permeable membranes allow the baby chick to grow and survive inside the egg. These membranes, found just inside the egg shell, are crucial because they keep the yolk and cellular material of the baby chick inside the egg but allow air to flow into and out of the egg. They also act as a barrier to keep out tiny bacteria and dust particles. This makes them selectively permeable.
Selectively permeable membranes are crucial to all cells. Every cell must have a means of taking in materials and releasing unneeded things. Cells accomplish this in a variety of ways and have specific tools depending on the material being moved.
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High School Biology: Homework Help Resource36 chapters | 435 lessons
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