FSc Biology Part 1 Chapter 3 (Lecture 5) Cell Membrane

Cell membrane

All prokaryotic and eukaryotic cells have a plasma membrane that encloses their contents and serves as a semi-porous barrier to the outside environment. Structure of plasma membrane The fluid mosaic model is a widely accepted concept that describes the dynamic nature of plasma membrane. It was proposed by two American biologists S.J. Singer and Garth Nicolson in 1972. According to the fluid mosaic model, the basic foundation of plasma membrane is a lipid bilayer. This bilayer is made of phospholipids. A collection of proteins float within the lipid bilayer.

The phospholipids have a phosphate group at one end of each molecule. The interior of lipid bilayer sheet is hydrophobic. It repels water-soluble molecules that attempt to pass through it. If a cell was fully encased in pure lipid bilayer, it would be completely impermeable to water-soluble molecules e.g., sugars, polar amino acids etc. That is why, in addition to phospholipids molecules, the membranes also contain proteins that provide passageways across the membrane. Phospholipids are characteristically hydrophilic (\"water loving\") at their phosphate ends and hydrophobic (\"water-fearing\") along their tail regions containing C-H chains. In the lipid bilayer of plasma membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate groups are aligned outwards, either toward the cytoplasm of the cell or the extracellular environment. In eukaryotes, plasma membranes have cholesterol molecules, wedged into the phospholipid bilayer. They keep the fluidity of membrane at low temperatures. Many proteins float within the phospholipid bilayer of plasma membrane. Some other proteins simply adhere to the surfaces of the bilayer. The positioning of proteins is related to the organization of cytoskeleton. Plasma membrane proteins function in several different ways. Many proteins play role in the selective transport of certain substances across the phospholipid bilayer, either acting as channels or active transport molecules. Some proteins help in attachment of plasma membrane to cytoskeleton and external fibres. The ability to distinguish among different cells is crucial to life. In an embryo, it allows cells to sort themselves into tissues and organs. It also helps cells of the immune system to recognize and reject foreign cells, e.g., infectious bacteria. Some proteins, on the exterior surface, attach with sugars and make identification marks. Other proteins function as receptors, which bind messenger molecules (e.g., hormones) and transmit signals to the interior of the cell. Some proteins also exhibit enzymatic activity, catalysing various reactions related to the plasma membrane.

The surface outside of the plasma membrane has chains of sugars bonded to proteins and lipids. A protein with attached sugar is called a glycoprotein, whereas a lipid with attached sugar is called a glycolipid. The glycoproteins and glycolipids vary from species to species, from individual to individual in the same species, and even from one cell type to another in the same individual. The glycolipids and glycoproteins (collectively called glycocalyx) function as cell identification marks that are recognized by other cells.

Functions of plasma membrane:
Plasma membranes serve as semi-porous barriers to the outside environment. The membrane acts as a boundary, holding the cell constituents together. The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated. Eukaryotic cells also have membranes around some of their interior organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials into and out of organelles.

Techniques to study the structure of plasma membrane:

1. Transmission Electron Microscopy can reveal detailed structures of the lipid bilayer and associated proteins.

2. Scanning Electron Microscopy is useful for examining the surface topology of cells and membranes.

3. Confocal Microscopy uses laser scanning and fluorescence to create sharp, detailed images of the cell membrane.

4. Total Internal Reflection Fluorescence Microscopy is used for high-resolution images of the membrane and its interactions with the cytoskeleton and other cellular components.

5. Atomic Force Microscopy provides topographical images of the cell membrane at high resolution.

6. X-ray Crystallography is used to determine the atomic structure of membrane proteins.

7. Lipidomics involves the comprehensive analysis of lipids in the cell membrane using techniques like mass spectrometry.

8. Fluorescence Recovery After Photobleaching is used to study the mobility and dynamics of membrane proteins and lipids. It involves bleaching a fluorescently labeled region of the membrane with a laser and observing the recovery of fluorescence as unbleached molecules move into the area.