Plasma Membrane and Cell Membrane - Cell: The Unit of Life | Class 11 Biology

Plasma Membrane

The plasma membrane, also called the cell membrane, surrounds cells. This lesson introduces its composition and arrangement, with a focus on the fluid mosaic model. Membrane transport functions are covered separately.

Main Ideas

• Studying the membrane: Electron microscopy helped scientists examine cell structures. Red blood cells (RBCs) were the first cells used in the chemical study of the membrane.
• Phospholipid bilayer: The membrane’s basic structure consists of two layers of phospholipids:
  • Each phospholipid has a polar, hydrophilic phosphate head that is attracted to water.
  • Its nonpolar, hydrophobic fatty-acid tails are repelled by water.
  • The heads face the watery environments outside the cell and inside the cytoplasm. The tails face one another, forming the middle of the bilayer.
• Membrane composition: The teacher gives an approximate composition of 60% lipids and 40% proteins, and also discusses cholesterol and carbohydrates.
• Membrane proteins:
  • Integral proteins are embedded in the bilayer. Some span the membrane and are called transmembrane proteins.
  • Some membrane proteins form channels.
  • Peripheral proteins sit on the membrane’s surface or edges.
• Carbohydrates: Carbohydrates attached to proteins form glycoproteins; those attached to lipids form glycolipids.
• Cholesterol: Cholesterol lies between phospholipids. It helps prevent the membrane from packing too tightly and becoming rigid when temperatures fall.
• Fluidity: The membrane is flexible rather than rigid. Lipids and some proteins can move within it.
  • Saturated fatty-acid tails pack closely, making the membrane less fluid.
  • Unsaturated tails pack less tightly, allowing greater movement and fluidity.
  • Temperature also affects membrane fluidity.

Fluid Mosaic Model

Singer and Nicolson proposed the fluid mosaic model in 1972. The teacher identifies it as the most widely accepted model discussed in the lesson.

• “Fluid” refers to the ability of membrane components—especially lipids—to move within the membrane.
• “Mosaic” refers to the varied proteins embedded in or associated with the lipid bilayer.

The membrane’s structure supports exchange between the cell and its surroundings. Active, passive, and facilitated transport are covered separately.

Speakers and Sources

• Teacher: Shipra Tiwari, presenting the lesson for Magnet Brains
• Scientific model attribution: Singer and Nicolson
• Textbook/source mentioned: NCERT