Biological membrane

A biological membrane, or biomembrane, is a selectively permeable structure. It acts as a boundary that separates the interior of a cell from its external environment. These membranes also create intracellular compartments. These are small, enclosed spaces within the cell. One example is the membrane around peroxisomes. This membrane shields the rest of the cell from peroxides, which are toxic chemicals. Most organelles are defined by such membranes and are called membrane-bound organelles.

The fundamental structure of these membranes is the phospholipid bilayer. This bilayer forms due to the aggregation of lipids in aqueous solutions. This happens because of the hydrophobic effect. Lipids are amphipathic, meaning they are both hydrophobic and hydrophilic. They have hydrophilic heads that interact with polar water. They also have hydrophobic tails that hide from water. In a bilayer, the tails meet in the middle. This arrangement maximizes hydrogen bonding between the heads and water. It also minimizes contact between the tails and water. This process increases the entropy of the system, making it spontaneous.

Membranes are characterized by a distinct asymmetry. This means the outer leaflet and inner leaflet are not identical. Certain proteins and lipids rest only on one surface. This organization is vital for functions like cell signaling. In eukaryotic cells, new phospholipids are made by enzymes in the endoplasmic reticulum. These enzymes deposit all new lipids into the cytosolic half of the bilayer. To ensure even growth, enzymes called flippases transfer half of these molecules to the opposite layer. In the plasma membrane, flippases selectively transfer specific phospholipids. This creates a concentrated distribution in each monolayer.

Proteins are embedded within this lipid matrix to perform many tasks. Integral proteins span the entire membrane. They have different domains on each side. These proteins have a strong association with the bilayer. They only detach if chemical treatments break the membrane. Peripheral proteins are different. They hold weak interactions with the bilayer surface. They stay on only one face of the membrane. This helps maintain membrane asymmetry. Additionally, some lipids and proteins are linked to oligosaccharides. These sugar-containing polymers form glycolipids or glycoproteins. Glycolipids are highly asymmetric and help with cell recognition. Glycoproteins are integral proteins that assist in immune responses.

A crucial feature of the biomembrane is selective permeability. This means the membrane acts as a gatekeeper. The size, charge, and chemical properties of a molecule determine if it can cross. Small hydrophobic molecules can pass through simple diffusion. However, particles required for life often cannot diffuse freely. These particles must enter through membrane transport proteins. Cells can also use endocytosis. In this process, the membrane allows a vacuole to join and push contents inside. Specialized membranes exist for many structures, such as the sarcolemma in muscle cells or the membranes of cilia.

The membrane is not a solid wall; it is a fluid environment. The hydrophobic core is in constant motion due to rotations in the lipid tails. This fluidity allows proteins to diffuse and interact for cell signaling. It also lets membranes fuse and distribute molecules during cell division. Fluidity is temperature-dependent. Below a transition temperature, the bilayer can become a gel-like solid. Bacteria and cold-blooded organisms manage this by changing their fatty acid composition. In animal cells, cholesterol regulates this fluidity. Cholesterol makes up approximately 20% of the lipids in the plasma membrane by weight. Its rigid structure fills gaps between phospholipids, making the membrane less permeable and more stable.

Red blood cells, or erythrocytes, provide a unique example of membrane composition. Their bilayer consists of cholesterol and phospholipids in equal proportions by weight. The erythrocyte membrane is also vital for blood clotting. In these cells, a lipid called phosphatidylserine is usually on the cytoplasmic side. During clotting, it is flipped to the outer membrane. This specific movement demonstrates how the control of membrane components can trigger important biological processes.