plasma membrane

Examples of plasma membrane in the following topics:

• Components of Plasma Membranes

  • The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.
  • The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment.
  • The primary function of the plasma membrane is to protect the cell from its surroundings.
  • Just as a hole in the wall can be a disaster for the castle, a rupture in the plasma membrane causes the cell to lyse and die.
  • Among the most sophisticated functions of the plasma membrane is its ability to transmit signals via complex proteins.

• Selective Permeability

  • Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane.
  • Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.
  • This adds considerably to the selective nature of plasma membranes.
  • Recall that plasma membranes are amphiphilic; that is, they have hydrophilic and hydrophobic regions.
  • The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.

• Fluid Mosaic Model

  • Nicolson in 1972 to explain the structure of the plasma membrane.
  • Plasma membranes range from 5 to 10 nm in thickness.
  • Proteins make up the second major component of plasma membranes.
  • Carbohydrates are the third major component of plasma membranes.
  • The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins.

• Plasma Membrane Hormone Receptors

  • Hormones that cannot diffuse through the plasma membrane instead bind to receptors on the cell surface, triggering intracellular events.
  • Amino acid-derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble or fat-soluble); therefore, they cannot diffuse through the plasma membrane of cells.
  • Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.
  • The amino acid-derived hormones epinephrine and norepinephrine bind to beta-adrenergic receptors on the plasma membrane of cells.
  • Describe the events that occur when a hormone binds to a plasma hormone receptor

• The Plasma Membrane and the Cytoplasm

  • The plasma membrane is made up of a phospholipid bilayer that regulates the concentration of substances that can permeate a cell.
  • Despite differences in structure and function, all living cells in multicellular organisms have a surrounding plasma membrane (also known as the cell membrane).
  • In the case of the plasma membrane, only relatively small, non-polar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar).
  • The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
  • Explain the structure and purpose of the plasma membrane of a cell

• Facilitated transport

  • Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
  • Another type of protein embedded in the plasma membrane is a carrier protein.
  • This adds to the overall selectivity of the plasma membrane.
  • They may cross the plasma membrane with the aid of channel proteins.
  • Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins.

• Exocytosis

  • In exocytosis, waste material is enveloped in a membrane and fuses with the interior of the plasma membrane.
  • Next, the vesicle's membrane and the cell membrane connect and are held together in the vesicle docking step.
  • The final stage, vesicle fusion, involves the merging of the vesicle membrane with the target membrane.
  • Some examples of cells using exocytosis include: the secretion of proteins like enzymes, peptide hormones and antibodies from different cells, the flipping of the plasma membrane, the placement of integral membrane proteins(IMPs) or proteins that are attached biologically to the cell, and the recycling of plasma membrane bound receptors(molecules on the cell membrane that intercept signals).
  • In exocytosis, vesicles containing substances fuse with the plasma membrane.

• Endocytosis

  • The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.
  • In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane.
  • The endosomal membrane again becomes part of the plasma membrane.
  • The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin.
  • In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane.

• Basic Structures of Prokaryotic Cells

  • Most prokaryotes have a cell wall outside the plasma membrane.
  • The plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside.
  • Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids.
  • The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes.
  • Porins, proteins in this cell membrane, allow substances to pass through the outer membrane of gram-negative bacteria.

• Membrane Fluidity

  • The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
  • First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid.
  • However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst; the membrane will flow and self-seal when the needle is extracted.
  • It lies alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane.
  • The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins.