Unit 2: Membrane and Endomembrane Systems

1. Various Models of Plasma Membrane Structure

The plasma membrane (or cell membrane) is a selectively permeable barrier that surrounds the cell. Several models have been proposed to describe its structure.

Historical Models

• Gorter and Grendel (1925): Deduced that the membrane was a lipid bilayer. They found that the lipid extracted from red blood cells covered an area twice the surface area of the cells.
• Davson and Danielli (1935): Proposed the "sandwich model" or "protein-lipid-protein" model. They suggested a phospholipid bilayer was sandwiched between two layers of globular proteins.

The Fluid Mosaic Model (Singer and Nicolson, 1972)

This is the currently accepted model. It describes the plasma membrane as a dynamic and flexible structure.

Fluid Mosaic Model: The membrane is a "mosaic" of components (phospholipids, cholesterol, proteins, carbohydrates) that are "fluid" and can move and flow relative to each other.

• Phospholipid Bilayer: The foundation of the membrane. Phospholipids are amphipathic, having a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. They arrange tail-to-tail, creating a barrier to water-soluble substances.
• Proteins:
  • Integral Proteins: Embedded within the bilayer, often spanning it completely (transmembrane proteins). Function as channels, pumps, or receptors.
  • Peripheral Proteins: Loosely bound to the surface of the membrane (inner or outer), often attached to integral proteins. Function as enzymes or in cell signaling.
• Cholesterol: (In animal cells) Tucked within the tails of the bilayer. It acts as a "fluidity buffer," preventing the membrane from becoming too fluid at high temperatures or too rigid at low temperatures.

2. Transport Across Membranes

The plasma membrane controls what enters and leaves the cell. Transport mechanisms are broadly divided into passive and active.

3. Cell Junctions

In multicellular organisms, cells are often connected by specialized structures called cell junctions. These are crucial for tissue structure and function.

Types of Cell Junctions (in Animals)

• Tight Junctions (Zonula Occludens):
  • Structure: Belts of proteins that fuse the plasma membranes of adjacent cells together, forming a quilt-like seal.
  • Function: Prevent the passage of molecules and fluids through the space *between* cells. They create a "watertight" barrier.
  • Example: Found in the epithelial lining of the bladder (to prevent urine leakage) and the intestine (to force nutrient absorption *through* the cells).
• Desmosomes (Macula Adherens):
  • Structure: "Spot welds" or "rivets" that anchor cells together. Cadherin proteins on the surface link, while intermediate filaments (like keratin) inside the cell provide mechanical support.
  • Function: Provide immense mechanical strength and resist tearing in tissues subjected to stress.
  • Example: Found in the epidermis (skin) and heart muscle tissue.
• Gap Junctions (Nexus):
  • Structure: Channels (made of proteins called connexins) that directly connect the cytoplasm of two adjacent cells.
  • Function: Allow for rapid chemical and electrical communication. Small molecules and ions can pass directly from one cell to the next.
  • Example: Found in heart muscle (for coordinated contraction) and developing embryos.

4. Structure and Functions of Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is a vast, continuous network of interconnected membranes (cisternae and tubules) that is contiguous with the nuclear envelope. It is a key part of the endomembrane system.

Rough Endoplasmic Reticulum (RER)

• Structure: Appears "rough" because its surface is studded with ribosomes.
• Functions:
  1. Protein Synthesis: Synthesizes proteins destined for secretion (export from the cell), insertion into membranes, or delivery to organelles (like lysosomes).
  2. Protein Modification: Proteins are folded into their 3D shape and modified (e.g., glycosylation - adding carbohydrates) inside the RER lumen.

Smooth Endoplasmic Reticulum (SER)

• Structure: Lacks ribosomes, giving it a "smooth" appearance. More tubular in shape.
• Functions:
  1. Lipid Synthesis: Synthesizes lipids, phospholipids, and steroids (e.g., in adrenal glands, testes, ovaries).
  2. Detoxification: Detoxifies drugs, poisons, and metabolic wastes (abundant in liver cells).
  3. Calcium Storage: Stores and releases Ca²⁺ ions, which is critical for muscle contraction and cell signaling.

5. Structure and Functions of Golgi Apparatus

The Golgi apparatus (or Golgi complex/body) is the "post office" of the cell. It receives, modifies, sorts, and packages molecules from the ER.

• Structure: A stack of flattened, membrane-bound sacs called cisternae. It has two distinct faces:
  • Cis face: The "receiving" side, oriented towards the ER. Transport vesicles from the ER fuse here.
  • Trans face: The "shipping" side, oriented towards the plasma membrane. Vesicles bud off from here to go to their final destinations.
• Functions:
  1. Modification: Further modifies proteins and lipids received from the ER (e.g., alters carbohydrate chains).
  2. Sorting and Packaging: Sorts materials into different transport vesicles based on their destination (e.g., for secretion, to the lysosome).
  3. Synthesis: Synthesizes some polysaccharides (e.g., in plant cell walls).
  4. Lysosome Formation: Forms primary lysosomes.

6. Structure and Functions of Lysosomes

Lysosomes are the "digestive system" or "stomach" of the cell.

• Structure: Small, spherical, single-membrane vesicles that bud off from the Golgi apparatus. They are filled with powerful hydrolytic enzymes (like proteases, nucleases, lipases) that can break down all major macromolecules.
• Internal Environment: The lysosomal membrane actively pumps H⁺ ions (protons) inside to maintain a very acidic pH (around 5.0), which is optimal for its enzymes.
• Functions:
  • Phagocytosis: Fusing with a food vacuole (or phagosome) to digest food particles or engulfed pathogens (like bacteria).
  • Autophagy: The process of "self-eating," where lysosomes break down and recycle the cell's own old, damaged organelles. This is essential for cellular renewal.
  • Autolysis: Programmed self-destruction of a cell by releasing its lysosomal enzymes (e.g., in the tail of a tadpole during metamorphosis).