Question
Asked Mar 14, 2019

what is the plasma membrane-its composition, movement through it and the different proteins that make it up- integral and peripheral?

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Step 1

The plasma membrane is a microscopic biological membrane, made up of lipids and proteins, that separates the interior of all the cells from their outside environment. It protects the cell from its environment and is made up of a lipid bilayer with embedded proteins. The plasma membrane controls the movement of substances in and out of the cell organelles.

Step 2

Composition of Plasma Membrane:

The plasma membrane contains a variety of biological molecules. It is a fluid mosaic, which means that it is flexible and made up of many different types of molecules. The plasma membrane is a mosaic of lipids, proteins and carbohydrates.

1. Lipid bilayer: The main fabric of the membrane is composed of dual-loving lipid molecules. The lipids form the double layered surface of the cells, hence called lipid bilayer. They are amphiphilic and have their hydrophilic head soluble in water and the hydrophobic tail soluble in fat. This bilayer forms the barrier that marks the boundaries of the cell. Lipid bilayers are generally impermeable to ions and polar molecules. This allows the cell to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates. The three major classes of membrane lipids are phospholipids, glycolipids and cholesterol. 

a. Phospholipids: Phospholipids account for more than half of the lipid in most membranes. The different kinds of phospholipids are asymmetrically distributed between the two halves of the membrane bilayer. 

b. Glycolipids: Glycolipids are found exclusively in the outer leaflet of the plasma membrane. Carbohydrate groups are present on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids.

c. Cholesterol: Cholesterol is another lipid composed of four fused carbon rings, found alongside phospholipids in the core of the membrane. It does not form a membrane by itself, but inserts into the bilayer of phospholipids with its polar hydroxyl group close to the phospholipid head groups. Depending on the temperature, cholesterol has distinct effects on membrane fluidity.

2. Proteins: Proteins make up the second major component of the plasma membrane. They are embedded in the lipid bilayer and contain residues with hydrophobic side chains that help them to anchor to the membrane. There are two classes of membrane-associated proteins: peripheral proteins and integral membrane proteins.

a. Peripheral proteins: These proteins are found on the outer and inner surfaces of the membranes, attached either to integral proteins or to phospholipids. They are not inserted into the hydrophobic interior of the lipid bilayer. Thus, they are more loosely attached and tend to dissociate from the membrane to carry out its work in the cytoplasm. 

b. Integral membrane proteins: These proteins are integrated into the membrane and have at least one hydrophobic region that anchors them to the hydrophobic core of the phospholipid bilayer. Some of the integral proteins stick only partway into the membrane, while others stretch from one side of the membrane to the other and are exposed on either side. Thus, integral membrane proteins span the membrane and have a hydrophilic cytosolic domain, which interacts with the internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. Proteins that extend all the way across the membrane are called transmembrane proteins.

3. Carbohydrates: Carbohydrates are the third major component of the plasma membrane. They are always found on the exterior surface of the cells. The carbohydrates are covantly attached to the proteins, called glycoproteins, or to lipids, called glycolipids. These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allows cells to recognize each other. 

Step 3

Movement through the Plasma Membrane:

The movement of a substance across the selectively permeable plasma membrane can either be passive or active. The cell employs a number of transport mechanisms that are as follows:

A. Passive transport include diffusion, osmosis and facilitated diffusion. Small molecules without charges such as oxygen and carbon dioxide flow through a plasma membrane without assistance and without expending energy. 

B. Active transport requires the cell to use energy, usually in the form of ATP. Active transport creates a charge gradient in the cell membrane. They keep the unwanted ions or other molecules out of the cell. For example, in the mitochondrion, the hydrogen ion pumps pump hydrogen ions into the intermembrane space of the org...

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