Cell membranes are composed of two phospholipid layers.

The cell membrane , or the plasma membrane, forms a boundary between a cell and the outside environment and controls the passage of materials into and out of a cell. The cell membrane consists of a double layer of phospholipids interspersed with a variety of other molecules. A phospholipid is a molecule composed of three basic parts:

• a charged phosphate group

• glycerol

• two fatty acid chains

Together, the glycerol and the phosphate group form the “head” of a phospholipid; the fatty acids form the “tail.” Because the head bears a charge, it is polar. Recall that water molecules are also polar. Therefore, the polar head of the phospholipid forms hydrogen bonds with water molecules. in contrast, the fatty acid tails are non-polar and cannot form hydrogen bonds with water. As a result, the non-polar tails are attracted to each other and repelled by water.

Because the membrane touches the cytoplasm inside the cell and the watery fluid outside the cell, the properties of polar heads and non-polar tails cause the phospholipids to arrange themselves in layers, like a sandwich.

The polar heads are like the bread. They form the outer surfaces of the membrane, where they interact with the watery environment both outside and inside a cell. The non-polar tails are like the filling. They are sandwiched between the layers of polar heads, where they are protected from the watery environment.

Molecules are embedded within the phospholipid layers. They give the membrane properties and characteristics it would not otherwise have. These molecules serve diverse functions. Here are a few examples:

• Cholesterol molecules strengthen the cell membrane.

• Some proteins extend through one or both phospholipid layers and help materials cross the membrane. Other proteins are key components of the cytoskeleton. Different cell types have different membrane proteins.

• Carbohydrates attached to membrane proteins serve as identification tags, enabling cells to distinguish one type of cell from another.

Fluid Mosaic Model

Scientists have developed the fluid mosaic model , which describes the arrangement of the molecules that make up a cell membrane. This model of cell membrane structure takes its name from two characteristics. First, the cell membrane is flexible, not rigid. The phospholipids in each layer can move from side to side and slide past each other. As a result, the membrane behaves like a fluid, similar to a film of oil on the surface of water. However, proteins embedded in the membrane do not flip vertically. If one part of a protein is outside the membrane, it will stay outside the membrane. Second, the variety of molecules studding the membrane is similar to the arrangement of colorful tiles with different textures and patterns that make up a mosaic.

Selective Permeability

The cell membrane has the property of selective permeability , which means it allows some, but not all, materials to cross. The terms semipermeable and selectively permeable also refer to this property. As an example, outdoor clothing is often made of semipermeable fabric. The material is waterproof yet breathable. Molecules of water vapor from sweat are small enough to exit the fabric, but water droplets are too large to enter.

Selective permeability enables a cell to maintain homeostasis in spite of unpredictable, changing conditions outside the cell. Because a cell needs to maintain certain conditions to carry out its functions, it must control the import and export of certain molecules and ions. Thus, even if ion concentrations change drastically outside a cell, these ions won’t necessarily interfere with vital chemical reactions inside a cell.

Molecules cross the membrane in several ways. Some of these methods require the cell to expend energy; others do not. How a particular molecule crosses the membrane depends on the molecule’s size, polarity, and concentration inside versus outside the cell. In general, small non-polar molecules easily pass through the cell membrane, small polar molecules are transported via proteins, and large molecules are moved in vesicles.

Chemical signals are transmitted across the cell membrane.

Recall that cell membranes may secrete molecules and may contain identifying molecules, such as carbohydrates. All these molecules can act as signals to communicate with other cells. How are these signals recognized?

A receptor is a protein that detects a signal molecule and performs an action in response. It recognizes and binds to only certain molecules, ensuring that the right cell gets the right signal at the right time. The molecule a receptor binds to is called a ligand. When a receptor and a ligand bind, they change shape. This change is critical because it affects how a receptor interacts with other molecules. Two major types of receptors are present in your cells.

Intracellular Receptor

A molecule may cross the cell membrane and bind to an intracellular receptor. The word intracellular means “within, or inside, a cell.” Molecules that cross the membrane are generally non-polar and may be relatively small. Many hormones fit within this category. For example, aldosterone can cross most cell membranes. However, it produces an effect only in cells that have the right type of receptor, such as kidney cells. When aldosterone enters a kidney cell, it binds to an intracellular receptor. The receptor-ligand complex enters the nucleus, interacts with the DNA, and turns on certain genes. As a result, specific proteins are made that help the kidneys absorb sodium ions and retain water, both of which are important for maintaining normal blood pressure.

Membrane Receptor

A molecule that cannot cross the membrane may bind to a receptor in the cell membrane.The receptor then sends the message to the cell interior. Although the receptor binds to a signal molecule outside the cell, the entire receptor changes shape—even the part inside the cell. As a result, it causes molecules inside the cell to respond. These molecules, in turn, start a complicated chain of events inside the cell that tells the cell what to do. For instance, band 3 protein is a membrane receptor in red blood cells. When activated, it triggers processes that carry carbon dioxide from body tissues to the lungs.

	Created by Madhu R about 8 years ago

9th grade Biology (Plasma Membrane and Cell Transport) Note on Plasma Membrane and Cell Transport, created by Madhu R on 01/10/2016.