Cell Membrane: Definition, Function, Structure and Signaling

Cell Membrane Definition

The cell membrane, also known as the plasma membrane, is a thin, double layer of lipids and proteins that surrounds every living cell. It separates the interior of the cell from the extracellular environment and protects it.

The cell membrane is selectively permeable, meaning it allows some molecules to pass through while preventing others from doing so. It regulates the transport of materials entering and exiting the cell.

The function of the Cell Membrane

The cell membrane, also known as the plasma membrane, is a thin layer that surrounds every living cell and separates the interior of the cell from the outside environment. It consists of a lipid bilayer that is semipermeable, meaning it regulates the transport of materials entering and exiting the cell.

The cell membrane has two primary functions: to be a barrier keeping the constituents of the cell in and unwanted substances out and to be a gatekeeper regulating what enters and exits the cell.

The chemical structure of the cell membrane makes it remarkably flexible, allowing it to be an ideal boundary for rapidly growing and dividing cells. Yet, it is also a formidable barrier that allows some dissolved substances or solutes to pass while blocking others.

Lipid-soluble molecules and some small particles can diffuse spontaneously across the membrane. However, particles too large to diffuse or pump are often swallowed or disgorged whole by an opening and closing of the membrane.

The plasma membrane has several different functions. One function is to transport nutrients into the cell while transporting toxic substances out of it. Another function is that proteins on its surface interact with other cells through glycoproteins or lipid proteins.

The amount of cholesterol in the plasma membrane affects its fluidity at physiological temperatures. Cell membranes are fluid at normal temperatures but become gel-like at cooler temperatures.

The collection of transmembrane proteins can move laterally in the lipid bilayer, forming a fluid mosaic structure. Finally, endocytosis and exocytosis are processes where molecules enter or leave cells through vesicles formed by the invagination or evagination of plasma membranes.

Crossing the Membrane

The cell membrane is a semipermeable barrier that regulates what enters and exits the cell or organelle. The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane.

Facilitated diffusion is another way molecules can cross the membrane. It is diffusion that is helped along (facilitated by) a membrane transport channel.

These channels are glycoproteins (proteins with attached carbohydrates) that span the lipid bilayer of the plasma membrane. Membrane proteins such as channel proteins and carrier proteins facilitate transport across membranes.

Active transport requires energy to function. In active transport, an ion or molecule crosses the membrane and moves against its concentration gradient – from low to high concentration – using energy from ATP hydrolysis or an electrochemical gradient. Secondary active transporters use sodium to propel other molecules against their gradients.

Pinocytosis is a process by which extracellular fluid is engulfed by invaginating cell membranes, forming a vesicle that then separates from the membrane.

This vesicle may move through the cell cytoplasm and release its contents on the other side of the cell layer by means of exocytosis. Transcellular transport across a layer of cells requires carrier or channel molecules on the membranes of both sides of each cell layer.

Signaling at the Cell Membrane

Cell signaling is the process by which cells communicate with each other and their environment. Membrane signaling involves proteins shaped into receptors embedded in the cell’s membrane that biophysically connect the triggers in the external environment to the ongoing dynamic chemistry inside a cell.

Membrane receptors interact with both extracellular signals and molecules within the cell, allowing signaling molecules to affect cell function. Signals may come from the environment or other cells, and they must be transmitted across the cell membrane to trigger a response.

Activation of receptors can trigger the synthesis of small molecules called second messengers, which initiate and coordinate intracellular signaling pathways. For example, cyclic AMP (cAMP) is a common second messenger involved in signal transduction cascades.

cAMP is synthesized from ATP by adenylyl cyclase, which resides in the cell membrane. Membrane lipids also contribute to cell signaling and homeostasis. They can interact directly by associating with receptors or modulate receptor activity indirectly through changes in membrane properties such as curvature, thickness, and tension.

Bidirectional signaling across the plasma membrane is achieved by striking a delicate balance between the restriction and propagation of signals.

A phosphorylation gradient emanating from the plasma membrane was observed for microtubule regulator stathmin/OP18, locally switching off its system-induced local “cytoplasmic state”.

The duality of containing and propagating signals from the plasma membrane also becomes apparent in the gradient of mitogen-activated protein kinase (MAPK) activity emanating from shmoo: a mating projection that occurs in response to pheromone in budding yeast cells.

Structure of the Cell Membrane

The cell membrane, also known as the plasma membrane, is a thin layer that surrounds every living cell and separates the interior of the cell from its external environment.

The cell membrane consists of a lipid bilayer that is semipermeable, meaning it regulates the transport of materials entering and exiting the cell. The lipid bilayer is composed of two layers of phospholipids with their hydrophobic, fatty acid tails in contact with each other.

The phospholipids have an amphiphilic property, meaning they contain both a lipid-soluble and a water-soluble region.

The chemical structure of the cell membrane makes it remarkably flexible, which is ideal for rapidly growing and dividing cells. Membrane proteins are also present in the cell membrane and are embedded within the phospholipid bilayer.

There are two general types of membrane proteins: peripheral and integral membrane proteins. Peripheral membrane proteins dissociate from the membrane following treatments with polar reagents such as solutions while integral membrane proteins span across both layers of the lipid bilayer.

The fluid mosaic model describes membranes as two-dimensional fluids in which proteins are inserted into lipid bilayers. In this model, membranes are viewed as having two classes of membrane-associated proteins: peripheral and integral.

Transmembrane proteins are apparent on the internal faces of membranes after they split into their two leaflets. These transmembrane proteins usually consist of α helices that are inserted into the endoplasmic reticulum during the synthesis of polypeptide chains.

Phospholipid Bilayer

The phospholipid bilayer is a fundamental component of the plasma membrane that surrounds all cells. It consists of two layers of phospholipids, with a hydrophobic interior and a hydrophilic exterior. Each phospholipid molecule has a head and two tails – the head is hydrophilic, while the tails are hydrophobic. The hydrophilic heads face outward towards the aqueous environment, while the hydrophobic tails face inward, away from the aqueous environment.

Phospholipids are amphipathic molecules because they contain both hydrophilic and lipophilic regions. This property allows them to spontaneously arrange into a bilayer in water. The bilayer is held together by weak hydrophobic interactions between the tails.

The properties of the phospholipid bilayer restrict the passage of many substances, but individual phospholipids can move within the bilayer, allowing for some flexibility in membrane structure.

In addition to phospholipids, plasma membranes also contain proteins and carbohydrate groups attached to some of the lipids and proteins. Cholesterol and glycolipids are also present in animal cell membranes.

The asymmetric distribution of lipids between the inner and outer leaflets of the plasma membrane creates an electrochemical gradient across it that is important for cellular processes such as signal transduction.

Membrane-Associated Factors

Membrane-associated factors are proteins that are anchored to the plasma membrane or other cellular membranes. They play a crucial role in various cellular processes, including transcriptional regulation and viral restriction.

Plasma membrane-associated restriction factors, such as BST-2 and SERINC proteins, are involved in antiviral defense mechanisms. Membrane-bound transcription factors (MTFs) are transcription factors that are anchored in membranes in a dormant state.

Upon receiving a stimulatory signal, MTFs can be depalmitoylated, which releases them from the plasma membrane and allows them to translocate into the nucleus where they regulate gene expression.

The membrane-associated accessory protein (MAAP) is another example of a membrane-associated factor. It functions as an AAV egress factor by promoting AAV capsid formation in the nucleolus. Proteolytic activation of plant MB-TFs occurs when their cytosolic regions are degraded upon ubiquitination, leading to the release and nuclear translocation of the cytosolic MB-TF segment.

The conserved Npl4 protein complex mediates proteasome-dependent degradation of Mga2p and Spt23p, two membrane-bound transcription factors regulated in this manner.

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