Structure of a Cell - Plasma Membrane




  • Endocytosis: Endocytosis is a cellular process in which substances are brought into the cell.
  • Exocytosis: Exocytosis is the process by which a large amount of molecules are released out of the cell.

Plasma Membrane Or Cell Membrane:

  • This is the outermost covering of the cell that separates the contents of the cell from its external environment.
  • It is selectively permeable, i.e., allowing the entry and exit of selective molecules.

  • It is flexible (fluid mosaic model).

  • It helps in endocytosis in organisms like amoeba. i.e. engulfing food from the external environment.

  • Endocytosis: Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalised is surrounded by an area of the cell membrane, which then buds inside the cell to form a vesicle containing the ingested material. Endocytosis includes pinocytosis (cell drinking) and phagocytosis (cell eating). It is a form of active transport.
  • Exocytosis: An exocytosis is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell (exo- + cytosis) by secreting them through an energy-dependent process. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means.


How do substances like CO2 and water move in and out of the cell? Discuss.

The cell membrane is selectively permeable and regulates the movement of substances in and out of the cell.

Movement of CO2:-

CO2 is produced during cellular respiration. Therefore, it is present in high concentrations inside the cell. This CO2 must be excreted out of the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. Therefore, according to the principle of diffusion, CO2 moves from a region of higher concentration (inside the cell) towards a region of lower concentration (outside the cell). Similarly, Oenters the cell by the process of diffusion when the concentration of O2 inside the cell is low as compared to its surroundings.

Movement of water:-

Water moves from a region of high concentration to a region of low concentration through the plasma membrane. The plasma membrane acts as a semi-permeable membrane, and this movement of water is known as osmosis. However, the movement of water across the plasma membrane of the cell is affected by the amount of substance dissolved in water.


Why is the plasma membrane called a selectively permeable membrane?

The cell membrane or plasma membrane is a biological membrane that separates the interior of the cell from the outside environment. The plasma membrane is called a selectively permeable membrane because it regulates the movement of substances in and out of the cell. It means that the plasma membrane allows some material to pass through it while at the same time it blocks other material from entering through it.

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