Universal structure: fluid-mosaic model
All biological membranes, including the plasmalemma and organelle membranes (nucleus, mitochondria, endoplasmic reticulum), share a unified structural principle. Under an electron microscope, they appear as a trilaminar structure: two dark bands (hydrophilic regions) and a light intermediate layer (hydrophobic zone).
The membrane is a dynamic, or "fluid-mosaic", system. Its components are in constant motion:
- Lateral mobility — movement of molecules within the plane of their own layer.
- Rotational mobility (rotation) — rotation of integral proteins (e.g., carriers) for substance transport. Meanwhile, glycoproteins cannot rotate due to the high hydrophilicity of carbohydrate chains.
Chemical organization of the membrane
Lipid component
The basis of the membrane is a bilayer of amphiphilic lipids (phospholipids, sphingolipids, glycolipids). Lipid molecules spontaneously orient themselves in an aqueous environment:
- Hydrophobic "tails" (fatty acid residues) hide inside the bilayer.
- Hydrophilic "heads" face outward toward the aqueous environment.
Hydrophobic steroid molecules (mainly cholesterol) are tucked between the tails.
Protein component
Proteins determine the functional diversity of the membrane. Based on their location, they are divided into:
- Integral — span the lipid bilayer entirely.
- Peripheral — attached to one of the surfaces.
Carbohydrate component
Carbohydrates are represented by branched oligosaccharide chains. They do not exist freely, but are linked to lipids (glycolipids) or proteins (glycoproteins). An important feature is asymmetry: carbohydrates are always located on the outer surface of the membrane, forming a supramembranous layer (glycocalyx).
Functions of the plasmalemma
The plasmalemma performs a number of vital tasks:
- Barrier and transport. The lipid bilayer is impermeable to hydrophilic substances and ions. Transport of low-molecular-weight substances occurs via protein systems, and macromolecules via endocytosis and exocytosis.
- Supportive. The cytoskeleton attaches to the membrane from the inside, while on the outside it interacts with the extracellular matrix via adhesion proteins.
- Receptor. Specific proteins bind ligands (hormones, neurotransmitters). Ionotropic receptors thereby open/close ion channels, while metabotropic receptors trigger cascades of intracellular reactions.
- Intercellular interaction (adhesion). Membrane receptors ensure cell recognition and adhesion (involving integrins, selectins, cadherins). The set of these molecules can change (e.g., during inflammation, the endothelium "captures" leukocytes).
- Electrophysiological. The operation of the sodium-potassium pump (extrudes 3 $Na^+$ and imports 2 $K^+$) and potassium channels creates a transmembrane resting potential — the outer surface is positively charged. Upon excitation, $Na^+$ channels open, and the potential changes.
Briefly: answers to topical questions
How are membrane proteins classified according to their mode of attachment to the lipid bilayer?
Membrane proteins in relation to the lipid bilayer are classified into:
- Integral proteins — deeply embedded in the structure and span the lipid bilayer entirely.
- Peripheral proteins — associated with only one of the membrane surfaces and do not penetrate into the bilayer.
Surface proteins differ in their mode of attachment: they can be bound to integral proteins, attached to polar "heads" of the lipid layer; "anchored" proteins are held by a short hydrophobic terminal domain or a covalently linked acyl residue.
What mechanisms of substance transport across the plasmalemma exist?
Transport of substances across the plasmalemma is carried out via diffusion, protein systems, and cytoosis mechanisms. The following mechanisms are distinguished:
- Simple diffusion.
- Facilitated diffusion — carrier-mediated transport; for glucose, passive transport along a concentration gradient via the GLUT2 carrier protein is described.
- Primary active transport — transport requiring energy expenditure against a concentration gradient.
- Secondary active transport — e.g., symport with Na⁺ ions, where the transported substance is moved against a gradient using the energy of the Na⁺ flow.
- Endocytosis and exocytosis — mechanisms of macromolecule transport.
What types of cell receptors are distinguished by their signal transduction mechanism?
Based on the signal transduction mechanism, three main types of membrane receptors are distinguished. These include:
- Ionotropic receptors — contain a primary messenger-binding subunit and an ion channel; ligand binding alters the state of the channel.
- Catalytic receptors — receptors capable of exhibiting intrinsic enzymatic activity of the intracellular domain (e.g., receptors with tyrosine kinase or guanylyl cyclase activity).
- Metabotropic receptors — coupled with G proteins and activate the formation of secondary messengers to transmit signals to intracellular structures.
Which specific cytoskeleton proteins are attached to the plasmalemma from the inside?
Elements of the cytoskeleton attach to the plasmalemma from the inside: microtubules, microfilaments, and intermediate filaments.
A specific protein attachment scheme is described for the erythrocyte:
- Spectrin — the main cytoskeleton protein, forming a network on the inner surface of the membrane.
- Ankyrin — the protein through which the spectrin network attaches to the membrane.
- Band 3 protein — an integral transmembrane protein to which ankyrin is linked.
Class and exam questions
What structures form the resting potential?
The generation of the resting potential involves the sodium-potassium pump (actively exchanging $Na^+$ for $K^+$) and potassium channels, which provide partial efflux of $K^+$ down its gradient.
How does the plasmalemma differ from organelle membranes?
It is thicker (8–11 nm due to a higher protein content) and has pronounced asymmetry due to the carbohydrate glycocalyx on the outer surface.
What else to study on this topic
- Mechanisms of endocytosis and exocytosis
- Operation of the sodium-potassium pump in detail
- Types of cell junctions
- Cell adhesion molecules (integrins, selectins, cadherins)
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