Structure of the Cell - Plasma Membrane

It is a thin, fragile, and elastic covering that separates cell components from the outer environment. 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 amoebae, i.e., engulfing food from the external environment.

Structure of Plasma Membrane:

The plasma membrane is made of two layers of phospholipids with protein molecules embedded within them. It is selectively permeable, meaning it allows only certain substances, like water, salt, and oxygen, to enter the cell while blocking others and letting waste, like carbon dioxide, exit. The plasma membrane maintains a stable internal environment by responding to changes outside the cell, a process called homeostasis.

Structure of Plasma Membrane

1. Endocytosis: A process where the cell takes in substances by surrounding them with the cell membrane, forming a vesicle inside the cell. Includes pinocytosis (cell drinking) and phagocytosis (cell eating). It is an active transport method that requires energy.
2. Exocytosis: A process where the cell uses energy to release substances, like proteins or neurotransmitters, out of the cell (exo+cytosis). Used to expel waste or secrete important molecules. Both endocytosis and exocytosis are essential for transporting large molecules that cannot pass through the cell membrane on their own.

1. Diffusion: A process where small molecules like oxygen (O₂) and carbon dioxide (CO₂) move in and out of the cell. Movement occurs from an area of high concentration to an area of low concentration until balance is achieved. It does not require energy (a passive process).
2. Osmosis: A specific type of diffusion involving the movement of water. Water moves from a region with more water (high concentration) to a region with less water (low concentration) through a selectively permeable membrane. Helps maintain proper hydration levels in cells. Essential for processes like nutrient absorption and waste removal in cells. Does not require energy, as it is also a passive process. It is a physical process with 3 possibilities:

Osmosis

i. Isotonic Solution: The concentration of water is the same both inside and outside the cell. The cell remains in a stable state with no net movement of water. Water neither enters nor leaves the cell, maintaining balance.

For example, blood cells in an isotonic saline solution maintain their shape and size.

ii. Hypotonic Solution: The outside medium has more water compared to the cell’s interior. This process of water entering the cell is called endosmosis. Plant cells become firm and turgid when placed in a hypotonic solution due to the influx of water. Water enters the cell through osmosis, causing the cell to swell.

For example, raisins placed in water absorb water, swell, and become turgid.

iii. Hypertonic Solution: The outside medium has less water than the cell’s interior. This process of water leaving the cell is called exosmosis.

• In plant cells, loss of water causes the cytoplasm to contract away from the cell wall, a process called plasmolysis.
• In animal cells, excessive water loss can cause the cell to shrivel and become dehydrated, affecting cell function. Water exits the cell through osmosis, causing the cell to shrink. 

For example,

• Fruit pieces in a thick sugar solution lose water, shrink, and become dehydrated.
• A plant wilts when placed in a hypertonic solution because water leaves the cells.