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Renin Angiotensin Mechanism.

Transport of Macromolecules across the Plasma Membrane

 Cellular level of organization:  Transport of Macromolecules across the Plasma Membrane

Introduction:

  • Cell is defined as a basic structural and functional unit of the body.

  • Cytology is a branch of science that deals with the study of cells.

  • The animal cell is divided into two major parts as,

  1. Plasma Membrane.

  2. Subcellular organelles.

    1. Nucleus.

    2. Nucleolus

    3. Mitochondria.

    4. Golgi complex.

    5. Ribosomes.

    6. Endoplasmic reticulum.

    7. Lysosomes.

    8. Centrioles.

Plasma Membrane:

  • Also called as a Cell membrane.

  • It is a thin barrier made of lipids and proteins that separate internal environment of the cell and external environment.

  • The basic function of cell membrane is to contain the contents of the cell and its protection.

  • The cell membrane is “Semipermeable” in nature means it allows passage of certain materials while restricting others.

Figure 1.1 Structure of Cell Membrane

Transport of Macromolecules across the Plasma Membrane.

  • Plasma membrane is semipermeable in nature i.e. it allows passage of certain molecules while restricting others.

  • The transport of the micro molecules across the cell membrane takes place in two ways,

    • Endocytosis: Entry of molecules inside the cell.

    • Exocytosis: Exit of molecules from the cell.

Endocytosis:

  • The process by which cell allows the large molecules to enter inside the cells is called Endocytosis.

  • It is of two types,

    • Pinocytosis.

    • Phagocytosis.

Pinocytosis:

  • It is cellular uptake of fluids. ( Drinking of cell).

  • Also called fluid endocytosis.

  • It is a process by which most micromolecules like proteins, polynucleotides etc. can enter the cell.

  • The receptors for attachments of these micromolecules are present on the cell surface.

  • When these micromolecules get attached with the receptors the part of the cell membrane having this receptor-macromolecule complex invaginates and finally pinches off along with small amounts of extracellular fluid.

  • This results in formation of endocytic vesicles.

  • These vesicles apparently get dissolved to release the micromolecules.

Figure 1.2 Process of Pinocytosis

Phagocytosis

  • It involves ingestion of the bigger particles like viruses, bacterias and dead cells.

  • It occurs in specialized cells like White Blood Cells.

  • When foreign particles get attached to the cell receptors present on the cell surface, the complex gets invaginated and finally pinches off forming vesicles containing foreign substances.

  • The vesicle thus formed is called a “Phagosome”.

  • Lysosomes in the cytoplasm attach to these phagosomes and release their contents inside them hence digesting the foreign material.

Figure 1.3 Process of Phagocytosis

Exocytosis

  • Undigested substances called residual body are excreted through the cell membrane by a process called “Exocytosis”.

  • The residual body formed in the cytoplasm is enclosed in a vesicle called the Exocytic Vesicle”.

  • These exocytic vesicles migrate towards the cell membrane and after fusing with it release their contents into the outer environment.

  • The membrane of the vesicles left behind is reused.

  • This process is also used for releasing contents formed inside the cell like proteins and chemicals to the outer environment.

Figure 1.4 Process of Exocytosis

Commonly Asked Questions

  1. What do you mean by,

    1. Pinocytosis.

    2. Phagocytosis.

    3. Exocytosis.

  2. Write in short about transport of micromolecules across the cell membrane.
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