Lecture 1 part 2Cell Membrane Structure and Dynamics

The plasma membrane is a selective barrier that defines the cell's boundary, separating its internal components from the external environment. It is constructed based on the fluid-mosaic model, where proteins are embedded within a fluid lipid bilayer.

Membrane Structure: The Fluid-Mosaic Model

The fluid-mosaic model, proposed by S.J. Singer and Garth Nicolson, describes the plasma membrane's structure.

• Fluid: The phospholipid bilayer is viscous, allowing individual lipids and proteins to move laterally.

• Mosaic: A collage of different proteins is embedded or attached to the lipid bilayer.

Under a Transmission Electron Microscope (TEM), the membrane has a trilaminar appearance: two dark (electron-dense) layers separated by a light (electron-lucent) layer. The total thickness is approximately 8-10 nm.

Membrane Components

Membranes are primarily composed of lipids, proteins, and carbohydrates. The proportions vary by cell and organelle type.

Most plasma membranes are 40-60% lipids and 40-50% proteins by weight. The inner mitochondrial membrane is an exception, with ~76% protein, reflecting its high metabolic activity.

I. Membrane Lipids

Membrane lipids are amphipathic, meaning they have both a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This property drives the spontaneous formation of a lipid bilayer in aqueous environments.

Phospholipids

The most abundant lipids in the membrane.

• Structure: Consist of a hydrophilic phosphate head group and two hydrophobic fatty acid tails.

• Arrangement: They spontaneously form a bilayer where tails face inward (away from water) and heads face outward. This structure is energetically favorable and forms sealed compartments.

• Movement:

  • Lateral Diffusion & Rotation: Phospholipids move freely within their own monolayer.

  • Flip-flop: Movement from one monolayer to the other is rare and requires enzymes (flippases, floppases, scramblases).

Cholesterol

A crucial lipid found in animal cell membranes.

• Function: Acts as a fluidity buffer.

  • At high temperatures, it reduces fluidity and stiffens the membrane.

  • At low temperatures, it increases fluidity by preventing phospholipids from packing too tightly.

• Note: Cholesterol is absent in prokaryotic membranes and the inner mitochondrial membrane.

Membrane Fluidity

The fluidity of the membrane is critical for its function and depends on:

1. Temperature: Higher temperatures increase fluidity.

2. Fatty Acid Composition:

   • Saturated fatty acids have straight tails, allowing tight packing and reducing fluidity.

   • Unsaturated fatty acids have "kinks" in their tails due to double bonds, which prevent tight packing and increase fluidity.

3. Cholesterol: Modulates fluidity as described above.

Membrane Asymmetry & Lipid Rafts

The two layers of the plasma membrane have different lipid compositions, which reflects their different functions.

• Glycolipids are found exclusively in the outer (non-cytosolic) monolayer.

• Phosphatidylserine is normally found in the inner (cytosolic) monolayer. Its appearance on the outer surface is a signal for phagocytes to remove a dying cell (apoptosis).

• Lipid Rafts: Small (20-200 nm), specialized microdomains within the membrane. They are thicker, less fluid, and rich in cholesterol, sphingolipids, and specific proteins, playing a key role in cell signaling.

II. Membrane Proteins

Proteins carry out most of the membrane's specific functions. They can be classified based on their association with the bilayer.

Protein Structure and Mobility

• Structure: The most common structure for a polypeptide chain to cross a membrane is the -helix. Multiple -helices can bundle together to form a hydrophilic pore or channel (e.g., aquaporins).

• Mobility: Like lipids, proteins can rotate and diffuse laterally. Evidence comes from:

  • Hybrid Cell Experiment: Mouse and human proteins mixed over time on a fused cell surface.

  • FRAP (Fluorescence Recovery After Photobleaching): Measures the rate of protein diffusion into a bleached area.

• Restricted Mobility: Protein movement can be limited by attachment to the cytoskeleton, the extracellular matrix, or tight junctions in epithelial cells.

Functions of Membrane Proteins

1. Pumps & Channels: Transport ions and small molecules.

2. Receptors: Mediate signal transduction.

3. Enzymes: Catalyze reactions at the membrane surface.

4. Linkers: Anchor the cell to the extracellular matrix or cytoskeleton.

5. Structural Proteins: Provide cell shape (e.g., spectrin in RBCs).

6. Cell-cell recognition & joining.

III. Membrane Carbohydrates (Glycocalyx)

Carbohydrates are found exclusively on the outer surface of the plasma membrane, attached to proteins (glycoproteins) and lipids (glycolipids).

• This carbohydrate layer is called the glycocalyx or "sugar coat".

• Functions:

  • Protection: Shields the cell from mechanical and chemical damage.

  • Cell-Cell Recognition: Acts as a unique cell identifier. For example, lectins on endothelial cells recognize carbohydrates on neutrophils, allowing them to exit the bloodstream at sites of infection.

  • Adhesion: Helps cells stick together.

Key Takeaways

• The plasma membrane is a fluid mosaic of lipids and proteins that acts as a selective barrier.

• Lipids (phospholipids, cholesterol) provide the basic fluid structure and act as a permeability barrier.

• Proteins are responsible for most of the membrane's functions, including transport, signaling, and structural support.

• Carbohydrates on the outer surface form the glycocalyx, which is crucial for protection and cell-cell recognition.

• The membrane is asymmetric, with distinct lipid and carbohydrate compositions on its inner and outer faces.