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Leaf: How does a cell separate itself from its surroundings?

What is the purpose of the cell membrane?

The cell membrane (also called the plasma membrane) is a semi-permeable barrier made mostly of lipids. It is the outer boundary for a cell, allowing some molecules to cross while fencing water and the majority of organically produced chemicals inside the cell.

What is the cell membrane made of?

Most of a cell membrane is made of phospholipids and glycolipids. Phospholipids are molecules with two hydrophobic fatty acid "tail" and a polar "head group" made of a phosphate group (PO4-). Glycolipids are similarly shaped molecules, but have a slightly different chemical structure because they have a sugar instead of a phosphate group in their head group.

Diagram of the phospholipid bilayer.
A diagram of the phospholipid bilayer.


Based on high-magnification electron microscopic photos of cell membranes, and experiments with purified phospholipids, we know that the cell membrane is a lipid bilayer. Phospholipids and glycolipids form two layers, with the polar heads of each layer facing out, and the hydrophobic tails pointing in towards each other.

The head groups on one side of the membrane face the aqueous environment of the cytoplasm, while head groups on the other side of the membrane face the extracellular world. The hydrophobic layer between them forms a barrier that keeps hydrophilic molecules from moving from one side of the bilayer to the other.

Diagram of the phospholipid membrane.
A diagram of the phospholipid membrane.


By themselves, phospholipids and glycolipids form a reasonably water-impermeable barrier. Cells can make the barrier even more hydrophobic by adding cholesterol or similar small steroid molecules to the tail region.

The cell membrane is not just a barrier surrounding the cell. In addition to the lipid bilayer, cells use proteins to modify and extend the function of the hydrophobic barrier. Membranes have integral proteins that are embedded in the lipid bilayer. These proteins can relay information across the membrane, and group together to form channels.

Integral proteins also control what substances enter and leave the cell. Smaller substances enter the cell through channels that operate via a passive transport mechanism (no chemical energy is required for the substances to pass through the protein). Larger molecules enter through membrane proteins that contain “gates” that open and close to facilitate the active transport, which requires the addition of chemical energy.

The following video explains the importance of cell membranes and cell walls.


Peripheral proteins are proteins that attach to either the inner or outer side of the bilayer and attach more loosely then integral proteins. Some of these proteins are held in place by the attraction of opposite charges between the protein and a lipid's head group. Others attach to integral proteins. Still others have an attached fatty acid tail that inserts in the bilayer to anchor the protein in place.

Some functions of integral and peripheral proteins include:

  • The outer surface of the cell membrane has more glycolipids. Many of the integral proteins will have sugars attached to them too. These sugars form a chemical"fingerprint" that cells use to identify each other.
  • Some of the integral proteins that face out from the membrane are receptors. They bind molecules outside the cell membrane, and send a signal into the cell telling it to respond.
  • Other integral proteins are linkers that connect one cell to others.
  • Many integral proteins are carriers, or channel proteins. These help move molecules across the bilayer.
  • In the middle of the bilayer are the hydrophobic tails of phospholipids, glycolipids, and steroid molecules.
  • The majority of peripheral proteins are on the inner side of the cell membrane. They include cytoskeleton proteins that help give the cell its shape, proteins that pass along signals from receptors, and scaffold proteins that hold macromolecules together in functional groups.


How are all these parts arranged in space?

The 3D arrangement of lipids, steroids, and proteins in a typical membrane, is called the fluid mosaic model. A lipid bilayer makes up most of the membrane. Some proteins are embedded in the bilayer, and others float above or below it. The membrane is not homogeneous as we move from outside to inside a cell either. The composition changes significantly.

This video shows the fluid mosaic model


What are the differences between a cell membrane and a cell wall?

Many people confuse cell membranes with cell walls. One difference between them is that all cells have a lipid-rich cell membrane around them. However, not all cells have cell walls which are a second layer located outside the cell membrane.


References:

  1. http://www.emc.maricopa.edu/faculty/farabee/biobk/biobooktoc.html


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Page last modified on Sunday 02 of February, 2014 13:20:58 EST

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