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Renin Angiotensin Mechanism.

Movement of Molecules Across Plasma Membrane

 

Introduction:

  • In order for the cell cytoplasm to communicate with the external environment, materials must be able to move through the plasma membrane. 

  • This movement occurs through several mechanisms as follows,

    • Movement of Small Molecules.

      • Diffusion

        • Simple Diffusion.

        • Facilitate Diffusion.

        • Osmosis.

      • Active Transport.

    • Movement of Large Molecules.

      • Endocytosis.

      • Exocytosis.

Diffusion:

  • Also called as “Passive Transport” as it doesn't require energy utilization.

  • Simple Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. 

  • This movement occurs because the molecules are constantly colliding with one another. 

  • The net movement of the molecules is away from the region of high concentration to the region of low concentration.

  • Diffusion is a random movement of molecules down the pathway called the concentration gradient. 

  • Molecules are said to move down the concentration gradient because they move from a region of higher concentration to a region of lower concentration. 

  • A drop of dye placed in a beaker of water illustrates diffusion as the dye molecules spread out and color the water.

  • Mechanisms Involved:

    1. Molecules remain in an aqueous phase and enter the cell through the channels.

    2. May dissolve in the lipid bilayer of the plasma membrane and enter the cell.

    3. May combine with carrier molecules and enter the cell.

Osmosis:

  • It involves movement of molecules from higher concentration to the lower concentration through a semipermeable membrane.

  • It occurs when the semipermeable membrane is permeable to the water and to certain solutes.

Facilitated diffusion:

  • A third mechanism for movement across the plasma membrane is facilitated diffusion. 

  • Certain proteins in the membrane assist facilitated diffusion by permitting only certain molecules to pass across the membrane. 

  • The proteins encourage movement in the direction that diffusion would normally take place, from a region with a higher concentration of molecules to a region of lower concentration.

Active transport:

  • When active transport is taking place, a protein moves a certain material across the membrane from a region of lower concentration to a region of higher concentration. 

  • Because this movement is happening against the concentration gradient, the cell must spend energy that is usually derived from a substance called adenosine triphosphate, or ATP. 

  • An example of active transport occurs in human nerve cells. Here, sodium ions are constantly transported out of the cell into the external fluid bathing the cell, a region of high concentration of sodium. (This transport of sodium sets up the nerve cell for the impulse that will occur within it later.)

Endocytosis and exocytosis:

  • The final mechanism for movement across the plasma membrane into the cell is endocytosis, a process in which a small patch of plasma membrane encloses particles or tiny volumes of fluid that are at or near the cell surface. 

  • The membrane enclosure then sinks into the cytoplasm and pinches off from the membrane, forming a vesicle that moves into the cytoplasm. 

  • When the vesicle contains solid particulate matter, the process is called phagocytosis. When the vesicle contains droplets of fluid, the process is called pinocytosis. 

  • Exocytosis is the reverse of endocytosis, where internally produced substances are enclosed in vesicles and fuse with the cell membrane, releasing the contents to the exterior of the cell.

Commonly Asked Questions.

  1. Write a note on transport of materials across the plasma membrane.

  2. Write a note on,

    1. Passive Transport.

    2. Facilitate Diffusion.

    3. Active Transport.

    4. Osmosis.

    5. Endocytosis.

  3. Draw a well labelled diagram of the human cell. Explain transport of materials across the plasma membrane.
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