Cell Membrane: Structure, Fluidity, Transport, and Signaling
Cell Membrane and Biomembranes
Each cell has a limiting boundary: the cell membrane, plasma membrane, or plasmalemma. It is a living membrane, outermost in animal cells but next to the cell wall in plant cells. It is flexible and can fold in or fold out. The plasma membrane is made of proteins and lipids, and several models have been proposed regarding the arrangement of proteins and lipids. The fluid mosaic model is widely accepted.
Fluid Mosaic Model
According to the fluid mosaic model:
- The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded.
- Each phospholipid molecule has two ends: an outer hydrophilic head and an inner hydrophobic tail pointing centrally.
- The protein molecules are arranged in two different ways:
- Peripheral proteins (or extrinsic proteins): These proteins are present on the outer and inner surfaces of the lipid bilayer.
- Integral proteins (or intrinsic proteins): These proteins penetrate the lipid bilayer partially or wholly.
Formation of the Cell Membrane
The cell membrane is formed due to the aggregation of membrane lipids in aqueous solutions. Aggregation is caused by the hydrophobic effect, where hydrophobic ends come into contact with each other and are sequestered away from water. This arrangement maximizes hydrogen bonding between hydrophilic heads and water while minimizing unfavorable contact between hydrophobic tails and water. The increase in available hydrogen bonding increases the entropy of the system, creating a spontaneous process.
Fluidity of the Cell Membrane
Fluidity is crucial for cellular functions, allowing rapid diffusion of membrane proteins, lipid and protein movement within the cell, membrane fusion, and even distribution of membrane molecules during cell division. Below a transition temperature, the bilayer becomes a gel-like solid, influenced by factors like hydrocarbon chain length and fatty acid saturation. Temperature-dependent fluidity is important for bacteria and cold-blooded organisms, which adjust their membrane lipid composition accordingly. In animal cells, cholesterol stiffens the bilayer, reducing permeability. Without fluidity, cellular processes such as growth and reproduction would be compromised.
Functions of the Cell Membrane
1. Transport and Selective Permeability
Passive transport: Diffusion, Osmosis, and Facilitated diffusion. Active transport: involves the movement of molecules against their concentration gradient, requiring energy provided by ATP. This process can occur through protein pumps, such as the sodium-potassium pump, which actively transports ions and molecules against their concentration gradient with the help of ATP. This mechanism is essential for cellular functions that require molecules to move from regions of lower concentration to higher concentration.
2. Cell Junctions
Cells adhere to each other and to the extracellular matrix using adhesion transmembrane proteins, which can form large macromolecular adhesion complexes crucial for tissue integrity. Cell junctions are classified based on morphology, adhesion molecule types, structures they adhere to, and interactions with the cytoskeleton. There are five types:
- Tight junctions
- Adherens junctions
- Desmosomes
- Gap junctions
- Hemidesmosomes
3. Cell Signaling: Reception and Transmission of Signals
Cell signaling is a fundamental property of all cells in living organisms, enabling them to receive, process, and transmit signals from their environment and within themselves. These signals can be physical (pressure, temperature, light) or chemical (e.g., small molecules, peptides). Signal transduction involves converting the signal into a chemical form, which amplifies the signal. Each cell responds to specific extracellular signals, governing processes like development, tissue repair, immunity, and homeostasis. Errors in signaling can lead to diseases such as cancer, autoimmunity, and diabetes.