Biological Membranes: Structure, Function and Transport

Tema 7: Biological Membranes

Membranes are structures that permit the selective passage of substances and act as barriers that keep physicochemical conditions of each cellular compartment distinct.

Plasma Membrane

The plasma membrane defines the limits of the cell and maintains differences between the cytosol and the extracellular environment. It is formed by a lipid bilayer composed of glycerophospholipids, sphingolipids and amphipathic cholesterol. It serves as a barrier permeable to polar molecules.

Properties of the Lipid Bilayer

• Self-assembly and self-sealing
• Fluidity: lipids move by lateral diffusion and exchange positions, but do not flip-flop between leaflets, maintaining lipid asymmetry.
• Cells can change membrane composition by increasing unsaturated fatty acids, which favor fluidity and lower viscosity.
• Cholesterol: provides stiffness and strength.
• Waterproof: the bilayer does not dissolve in water and interacts with membrane proteins.

Membrane Proteins

Membrane proteins are responsible for specific membrane functions:

• Integral (inclusive) proteins: embedded together with membrane lipids, in single-pass, multipass or covalently linked forms.
• Peripheral proteins: attached to either side of the membrane, bound weakly to polar heads of lipids or to integral proteins.

Glycocalyx

The glycocalyx protects the cell surface and acts as a physicochemical filter against damaging substances. It is the external arrangement of carbohydrates that enables interaction with extracellular molecules and plays an important role in cell recognition and adhesion.

Membrane Domains and Functions

Domains are specialized membrane regions that carry out specific functions. For example, in the gut epithelium, the apical domain captures nutrients, while the basal and lateral domains release them inside the body.

Functions: the membrane is selectively permeable, allowing passage of hydrophobic substances and small uncharged molecules, while large or ionized hydrophilic substances require transport systems.

Transport Across Membranes

Large molecules and particles are transported by vesicular systems:

• Endocytosis: the membrane invaginates to form vesicles that enter the cell. Phagocytosis involves very large particles, where the membrane extends pseudopods that surround them and form a phagosome, which fuses with a lysosome. Pinocytosis involves fluid uptake through clathrin-coated vesicles.
• Exocytosis: secretion of materials. Substances needed to renew the plasma membrane and extracellular matrix components are synthesized in the endoplasmic reticulum and Golgi apparatus.

Small molecules are transported by:

• Passive transport: molecules move spontaneously down their concentration gradient. Simple diffusion occurs through the lipid bilayer for gases, hormones and water. Facilitated diffusion uses transporter proteins that bind specific molecules, or aqueous channel proteins that allow ions to pass. Channels may be gated by ligands or voltage, or open in response to a signal.
• Active transport: molecules move against their concentration gradient, using energy, through carrier proteins.

Cell Junctions

Specialized regions of the plasma membrane in contact with other cells, containing specific proteins, are especially important in epithelia.

Types of Junctions

Occluding junctions: form a closed seal between adjacent cells, preventing molecules from passing between them.

Anchoring junctions: connect the cytoskeletal filaments of one cell to those of neighboring cells or to the extracellular matrix, giving tissues strength against mechanical tension. They include adherens junctions, desmosomes and hemidesmosomes.

Communicating junctions (gap junctions): allow direct communication between the cytoplasm of adjacent cells, through which ions and small molecules pass.

Cell Communication

Cell survival requires the ability to respond to external stimuli. This capability is called irritability. Stimuli are transmitted by signaling molecules produced by one cell, recognized by a signaling receptor present in the target cell, which converts the extracellular signal into an intracellular one through signal transduction systems.

Signaling Molecules

• Hormones (endocrine)
• Mediators (local chemical, paracrine)
• Neurotransmitters (neuronal)

Receptors

Receptors are distributed in the plasma membrane of target cells. Transmembrane proteins have an extracellular region (which recognizes a given signal molecule) and a cytosolic region (responsible for signal transduction). The active junction region starts the cytosolic signaling cascade that produces the cell response.

Signal Transduction Systems

These systems are responsible for transforming extracellular signals into intracellular ones. A receptor on the target cell acts as a first messenger receptor, and a second messenger in the cytoplasm spreads the signal throughout the cell. Enzymes act on other molecules in a cascade of events, interacting with target proteins to produce the cellular response.

Internal Membrane Systems

A characteristic and important feature of eukaryotic cells is the subdivision of the cytoplasm into internal compartments surrounded by membranes with specific lipid and protein compositions, which give them distinct structural and functional properties.

Endoplasmic Reticulum

The endoplasmic reticulum is a system of membranes forming a network of flattened sacs and tubules, interconnected and delimiting a single internal space. It is continuous with the outer membrane of the nuclear envelope.

• Rough ER: formed by flattened sacs studded with ribosomes. Functions include protein synthesis, glycosylation, and folding of proteins into their final structure.
• Smooth ER: formed by tubules without ribosomes. Functions include synthesis of glycerophospholipids and cholesterol, calcium ion accumulation, and detoxification of toxic liposoluble substances.

Golgi Apparatus

The Golgi apparatus is made up of several dictyosomes, formed by stacks of flattened sacs, located near the nucleus. Each dictyosome has three regions: the cis face (entry), the medial region, and the trans face (exit). Proteins move from cis to medial to trans through transport vesicles. In the trans face, they are sorted to their final destinations: lysosomes, secretory vesicles, or the plasma membrane.

• Constitutive secretion: continuous release of materials needed to renew the plasma membrane and extracellular matrix.
• Regulated secretion: release of products such as hormones and neurotransmitters, triggered only by specific stimuli.

Functions include glycosylation and protein processing started in the rough ER, sorting molecules for cellular traffic, and synthesis of glucosphingolipids and cell wall components in plants, as well as formation of the acrosome in sperm cells.

Lysosomes

Lysosomes are membrane-bound vesicles containing hydrolytic enzymes used for intracellular digestion of organic materials of varied nature. They participate in digestive processes:

• Primary lysosomes (inactive, containing enzymes) enter digestive processes when they fuse with vesicles.
• Secondary lysosomes (containing material being digested) finish the process, leaving residual bodies.

Heterophagy: digestion of nutrients taken in by endocytosis (pinocytosis or phagocytosis). The phagosome fuses with a primary lysosome to form a phagolysosome.

Autophagy: digestion of the cell's own materials or old organelles that must be destroyed. A membrane forms an autophagosome that fuses with a primary lysosome.

Peroxisomes

Membrane-bound compartments that carry out oxidative reactions and detoxification.

Vacuoles

In plant cells, vacuoles can occupy up to 90% of the cell volume. Their functions include storing nutrients and waste products, serving as compartments of degradation, and functioning as homeostatic devices.