Plasma Membrane
The ability of organisms to form fluid structures of some rigidity in an aqueous environment is central to the existence of life on this planet. ~ American physicist Peter Collings
The contents of every cell are held within a plasma membrane, which is a flexible container of controlled permeability. The cell membrane supports its cell and helps maintain its shape. Cell membranes are living liquid crystals: able to maintain a necessary, delicate balance between fluidity and rigidity. The structure of the plasma membrane is similar in prokaryotic and eukaryotic cells.
Organelles within cells are also enclosed within a membrane. As an evolutionary artifact of endosymbiotic incorporation, the nucleus, mitochondria, and chloroplasts in plants have 2 membranes.
Cell membranes are a mix of proteins and lipids, with respective percentages varying by cell type. While lipids structure the cell membrane, proteins manage traffic through the membrane and keep the membrane in good working order. Cell membranes are organized into distinct domains, with different proteins performing a range of necessary functions.
The lipid composition of membranes can have profound effects on the behavior and activity of its resident macromolecules. ~ Eric Schon
3 lipids are employed in cell membranes: phospholipids, glycolipids, and sterols. While the amount of each varies by cell type, phospholipids are consistently the most abundant.
The cell membrane scaffold is a double layer of phospholipids. A phospholipid has a hydrophilic (water-loving) phosphate head and 2 hydrophobic (water-fearing) fatty-acid tails. Phospholipids spontaneously arrange themselves into a double-layered structure, with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. Most biological membranes have this energetically favorable 2-layer structure, called a phospholipid bilayer.
A glycolipid is a lipid with sugar on top. In cell membranes, glycolipids self-assemble into highly organized domains. Glycolipids situated on a cell membrane surface act as signal markers, facilitating recognition by other cells. Their specific arrangement regulates numerous cellular processes which can cause disease if not properly done.
Lipids are hydrophobic. Their natural aversion to water causes them to clump together. That only partly explains the intricate integration of glycolipids on a cell membrane.
The sugar molecules in neighboring glycolipids interact to form an ordered crystalline structure, connected by hydrogen bonds. Thus, both lipid and carbohydrate molecules complement each other in forming well-organized glycolipid clusters.
All sterols share a common property: the ability to regulate dynamics in order to maintain membranes in a microfluid state where they can convey important biological processes. ~ French biochemist Erick Dufourc
Sterols are essential in eukaryotic cells. In animal cells, cholesterol disperses between membrane phospholipids, keeping cell membranes from becoming too stiff, by preventing phospholipids from being too tightly packed together. Plant cell membranes lack cholesterol, opting instead for a sophisticated mix.
In contrast to animal and fungal cells, which contain only one major sterol, plant cells synthesize a complex array of sterol mixtures. Sterols regulate membrane fluidity and permeability in a similar manner to cholesterol in mammalian cell membranes. Plant sterols can also modulate the activity of membrane-bound enzymes. ~ French molecular biologist Marie-Andrée Hartmann
A cell membrane protects the integrity of the cell, policing the molecules that go in or out. From without and within the cell, a cell membrane must process a wide variety of signals to initiate apposite responses to changing conditions.
Membrane dynamics is essential for cellular life. ~ Erick Dufourc
Membranes are constantly changing, allowing migration of proteins and other products. This fluidity is essential for engulfing food, discharging waste, and secreting cellular products.
Membrane proteins act as border guards, membrane maintainers, cellular communicators (signalers and receivers), nutrient jitneys, and enzymatic actors playing various roles. Generally, there are 2 types of proteins associated with a membrane. Integral membrane proteins are inserted into a membrane and may pass through the membrane. Portions of these transmember proteins may be exposed on both sides of the membrane. Peripheral membrane proteins are on the exterior and connected to the membrane via interactions with other proteins.
Every cell is mostly water. Maintaining cell pressure, an essential function, means managing the flow of water.
Animal cells move about by managing water flow. Cell motility requires tightly regulated membrane dynamics and snappy cell shape change in the cytoskeleton. A moving cell is a symphonic exercise, conducted by certain proteins. Morphogens are signaling molecules that tell a cell where to go.
Aquaporins are proteins that act as channels for the flow of water across biological membranes in response to osmotic pressure changes. They provide the plumbing system for cells. Each aquaporin acts as a meticulous membrane pore.
All sorts of molecules pass through cell membranes, albeit selectively. Aquaporins permit the flow of small polar molecules, such as glycerol, while blocking others. As keeping electrical homeostasis is crucial, protons and certain ions are precluded passage.
