Lec.2

• cell membrane : All cells are contained by a cell membrane which is a biological membrane that separates the interior of all cells from the outside environment , The cell membrane is not a solid structure, it is made of millions smaller molecules that create a flexible and porous container
• Phospholipid molecules are shaped with a head and a tail region. The head section of the molecule

  likes water (hydrophilic) while the tail does not (hydrophobic) they tend to stick to each other . The two surfaces of molecules create the lipid bilayer
• Integral proteins: are embedded within the lipid bilayer. They cannot easily be removed from the cell membrane, are the only class of proteins that can perform functions both inside and outside of the cell
• Proteins in Plasma Membrane:

  1. integral protiens
  2. Peripheral protiens
  3. libid-bound protiens
  4. transpot protiens
• Peripheral Proteins: Peripheral proteins are attached to the exterior of the lipid bilayer.

  They are easily separable from the lipid bilayer, able to be removed without harming the

  bilayer in any way
• Lipid-Bound Proteins: Lipid-bound proteins are located entirely within the boundaries of the

  lipid bilayer
• Transport proteins (proteins channel
• Tight junction connects adjacent cells in a sheet in a

  way that prevents small molecules from leaking between the cells. In this junction, plasma

  membrane of the two adjacent cells are fused completely like a zipper and leave no space or gap bettwen them
• Function of Plasma Membrane

  1. It separates the contents of the cell from its outside environment and it regulates what enters an exits the cell
  2. protecting the integrity of the interior of the cell
  3. base of attachment for the cytoskeleton in some organisms the cell wall others. Thus the cell membrane supports the cell and helps in maintaining the shape of the cell
  4. Lipids help to give membranes their flexibility and proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane
• The main functions of the tight junction

  1. They holds cells toghter
  2. blocks the lateral movement of integral protiens
  3. controls the movements of ions and protiens by allowing only limited quantity to enter the cell membrane
• Gap Junctions: The gap junction or nexus is probably the most complex modification of adjacent

  plasma membranes. Gap junction are the connection tunnels that exist between adjacent cells,

  they allow molecules and ions up to a certain size to pass through from one cell to the next.

  Gap junction found in muscle tissue such as cardiac cell
• Maine function of gap junction

  1. Provides cytoplasmic channel from one cell to an adjacent cell

  b. Allows for communication through exchange of material and transmission of electrical impulses
• 3- Desmosomes: Desmosomes are localized patches that hold two cells tightly together. They are

  common in epithelia . Desmosomes do not prevent the

  movement of ions or molecules around the cell but but they are usually found in combination with tight

  junction ,while hemidesmosomes anchor epithelial cells to a basement membrane
• The main function of desmosomes

  1. Fasten cells together into strong sheets

  2. Attach muscle cells to each other
• microvillus) are microscopic cellular membrane protrusions that increase the

  surface area of cells and minimize any increase in volume, and are involved in a wide variety of

  functions, including absorption ,secretion , cellular adhesion.
• Examples for microvilli location:

  1. apical surface of some epithelial cells, such as the small intestines .
  2. the plasma surface of eggs, aiding in the anchoring of sperm cells
  3. on the cell surface of white blood cells