Cell Membrane structure and function.

The cell membrane, also frequently referred to as the plasma membrane or biomembrane, is the selectively permeable barrier that defines the boundary of the cell, separating the intracellular fluid (ICF) from the extracellular fluid (ECF).

I. Structure and Composition

The structure of the cell membrane is fundamentally based on the fluid-mosaic model, which describes a dynamic arrangement of lipids and proteins.

Lipids (Compositional Framework)

The primary structural component is the lipid bilayer.

1. Phospholipids: Most lipids that form biomembranes are phospholipids.

    ◦ They are amphiphilic (or amphipathic), possessing both hydrophilic (polar, water-loving) and hydrophobic (nonpolar, water-fearing) regions.

    ◦ The molecule has a phosphorylated glycerol backbone (head), which is hydrophilic and faces the aqueous environment (ICF or ECF).

    ◦ It has two fatty acid tails, which are hydrophobic.

    ◦ The lipid bilayer forms because the hydrophobic tails face each other, sequestering themselves in the interior of the membrane, while the hydrophilic heads point outward toward the aqueous solutions.

2. Other Lipids: Biomembranes typically also contain other lipids, such as cholesterol and sphingolipids.

    ◦ Cholesterol, a nonphospholipid, alters the fluidity of the membrane and is present in significant amounts in animal cell membranes.

3. Asymmetry: Biomembranes are generally not uniform structures. The lipid compositions of the inner and outer leaflets of the bilayer often differ, a condition known as asymmetry.

Proteins

Proteins are the other principal constituents of the cell membrane.

1. Integral Proteins: These are embedded in, and anchored to, the lipid bilayer by hydrophobic interactions.

    ◦ Many are transmembrane proteins that span the entire membrane one or more times, contacting both the ECF and ICF.

2. Peripheral Proteins: These are loosely attached to the surfaces of the membrane (only partially embedded in one face) and can separate easily.

Carbohydrates

Carbohydrates: The plasma membrane contains carbohydrates, often attached to outer surface proteins and lipids, forming glycoproteins and glycolipids. These molecules play a part in cell communication, growth, and development.

II. Functions of the Cell Membrane

The specialized functions and selective transport properties of the cell membrane are primarily attributed to its protein content. The processes mediated by biological membranes are essential for physiological function and homeostasis.

1. Maintaining Homeostasis and Compartmentation

• The membrane provides the requisite barrier to maintain the chemical composition of the cell interior, which is different from its surroundings.

• It is selectively permeable to small molecules, but generally impermeable to large molecules such as proteins and nucleic acids, ensuring their retention within the cytosol.

• The hydrophobic core acts as an effective barrier to the transport of charged water-soluble substances, such as inorganic ions (Na+, K+, Cl-, Ca2+), which barely permeate pure lipid bilayers.

2. Transport of Solutes and Water

The membrane controls the flux of solutes and fluid between the lumen/ECF and blood/ICF.

• Passive Transport: Net movement of substances down an electrochemical gradient, not requiring metabolic energy.

    ◦ Simple Diffusion: Lipid-soluble substances (e.g., O₂, CO₂, steroid hormones) can dissolve directly through the hydrophobic lipid portions of the membrane. Water molecules can also diffuse through the membrane to a limited degree due to their small size and lack of net charge.

    ◦ Facilitated Diffusion: The transport of molecules, such as glucose, is mediated by specific carrier proteins down a concentration gradient.

   ◦ Ion Channels: These integral proteins form gated pores that dramatically increase the permeability of the membrane to specific ions (e.g., Na+, K+, Cl−, Ca²⁺), allowing rapid transport down their electrochemical gradient.

• Active Transport: Net movement of substances against an electrochemical gradient, requiring metabolic energy (ATP).

    ◦ Primary Active Transport (Pumps): Proteins like the Na+-K+ ATPase (Na+-K+ pump) hydrolyze ATP to transport Na+ out of the cell and K+ into the cell, establishing and maintaining the large concentration gradients necessary for nearly every physiological function.

    ◦ Secondary Active Transport (Cotransporters/Exchangers): Uses the energy stored in the concentration gradient of one solute (typically Na+) to drive the uphill transport of another solute.

• Osmosis: The net diffusion of water across the membrane, often aided by specific aquaporin (water) channels, driven by a difference in solute concentration (osmotic pressure) across the membrane.

3. Electrical Activity and Signaling

• Electrical Behavior: The membrane capacitance allows the lipid bilayer to act as an electrical insulator, enabling charges to accumulate at the surface. The asymmetric distribution of ions across the membrane creates an unequal distribution of charges, resulting in a membrane potential (Vm), where the inside of the cell is typically negative relative to the outside.

• Receptors: The membrane contains receptor proteins that bind to specific extracellular chemical regulators, such as hormones or neurotransmitters, transmitting signals from the outside to the cell interior via signal transduction processes.

• Enzymatic Activity: Integral membrane proteins can function as enzymes. For example, the apical membranes of enterocytes contain hydrolytic enzymes responsible for membrane digestion (or contact digestion).

4. Structural and Mechanical Roles

• Structural Connections: Membrane proteins connect to the cytoskeleton within the cell and to extracellular proteins outside the cell.

• Adhesion: Integral membrane proteins, such as integrins, act as adhesion molecules, binding to components of the extracellular matrix to help integrate the intracellular and extracellular compartments and confer structural integrity.