Cell Membrane, Transport, and Biomolecules: Key Concepts
Cell Membrane and Transport
The cell membrane, composed of a phospholipid bilayer, is selectively permeable, allowing certain molecules to pass while restricting others. Small non-polar molecules (O₂, CO₂) and small uncharged polar molecules (H₂O, glycerol) can pass freely, while ions (Na⁺, K⁺, Cl⁻) and large polar molecules (glucose, amino acids) require transport proteins. Membrane proteins serve various functions: structural support (desmosomes), enzymatic activity, signal transduction (receptors), and transport.
Transport across the membrane occurs through passive transport (no energy required), including simple diffusion (movement from high to low concentration), facilitated diffusion (movement through carrier or channel proteins), and osmosis (water moving toward higher solute concentration). In contrast, active transport (ATP required) moves molecules against their concentration gradient, such as the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in) or bulk transport via endocytosis and exocytosis.
Transport proteins are classified into channels (passive only, gated or ungated) and carriers, which can be uniporters (transporting one molecule), symporters (transporting two molecules in the same direction, e.g., SGLT: Na⁺ + glucose), or antiporters (transporting two molecules in opposite directions, e.g., Na⁺/K⁺ pump).
Osmosis and tonicity determine water movement, where hypotonic solutions (low solute, high water) cause cells to swell and burst (lysis), hypertonic solutions (high solute, low water) cause cells to shrink (crenation), and isotonic solutions result in no net movement. Understanding membrane transport, protein functions, and osmosis is essential for cellular function and medical applications, such as maintaining IV fluid balance and preventing osmotic stress on cells.
Phospholipid Bilayer and Permeability
The cell membrane is a phospholipid bilayer, self-sealing and selectively permeable, allowing small nonpolar molecules like O₂ and CO₂ to pass while requiring transport proteins for charged ions and large molecules. Transport occurs through passive mechanisms such as diffusion and osmosis, facilitated transport via channels and carrier proteins, and active transport like the sodium-potassium pump, which moves 3 Na⁺ out and 2 K⁺ in using ATP. Additionally, secondary active transport uses an existing ion gradient (e.g., Na⁺) to move another molecule against its gradient, such as glucose via SGLT transporters.
Membrane Proteins and Their Functions
Membrane proteins include structural support, enzymatic activity, receptor-mediated signaling, and transport. Transporters can function as channels, carriers (uniport, symport, antiport), or pumps. Gated channels include ligand-gated receptors such as the nicotinic acetylcholine receptor, voltage-gated channels involved in nerve signaling, and mechanically-gated channels used in sensory perception. These proteins play crucial roles in communication and molecule transport across the membrane.
Diffusion, Osmosis, and Water Movement
Diffusion is the natural movement of molecules from high to low concentration, while osmosis refers specifically to the diffusion of water. In a hypotonic solution, water enters a cell, causing it to swell and potentially burst. In a hypertonic solution, water leaves the cell, causing it to shrink. Isotonic solutions result in no net movement of water. Osmolarity, or the total solute concentration, determines the direction of water movement.
Carbohydrates and Energy
Monosaccharides such as glucose, fructose, and galactose serve as primary energy sources. Disaccharides include maltose (glucose + glucose), sucrose (glucose + fructose), and lactose (glucose + galactose). Polysaccharides like starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (structural component of plants, indigestible by humans) play vital roles in biological systems. Glucose transporters include GLUT (passive transport) and SGLT (sodium-glucose symport, secondary active transport).
Nucleic Acids and Genetic Information
DNA is a double-stranded molecule containing deoxyribose and thymine, while RNA is single-stranded with ribose and uracil. Nucleotides consist of a sugar, phosphate, and nitrogenous base (A, T, C, G, U). The central dogma of molecular biology describes the processes of replication (DNA to DNA), transcription (DNA to RNA), and translation (RNA to protein). Mutations, which are changes in the DNA sequence, can lead to altered protein function, potentially causing genetic disorders or variations in traits.
Lipids and Cell Membranes
Hydrophobic biomolecules such as fats, oils, phospholipids, and steroids. Phospholipids form bilayers, with hydrophilic heads facing outward and hydrophobic tails inward, creating the cell membrane. Cholesterol, a type of steroid, stabilizes membranes and serves as a precursor for steroid hormones. Triglycerides, composed of glycerol and three fatty acids, act as long-term energy storage molecules and are stored in adipose tissue.
Membrane Potential and Nerve Signaling
Membrane potential (Vm), the voltage difference between the inside and outside of a cell. A neuron’s resting potential is typically around -70 mV, maintained by the Na⁺/K⁺ pump. Depolarization occurs when Na⁺ channels open, allowing positive charge to enter, while hyperpolarization happens when K⁺ channels open, making the inside more negative. The equilibrium potential (Eion) represents the voltage at which an ion’s diffusion is balanced by electrical force. The sodium equilibrium potential (E_Na) is approximately +61 mV, while the chloride equilibrium potential (E_Cl) is approximately -61 mV.
Second messengers, such as cAMP and cGMP, are molecules created inside a cell in response to an external signal, amplifying cellular responses. G-protein coupled receptors activate second messenger cascades, leading to signal amplification. Neurotransmission occurs when action potentials propagate along neurons, causing the release of neurotransmitters at synapses. Ligand-gated ion channels, such as the acetylcholine receptor, open when bound to neurotransmitters, allowing ion flow and generating a response in the target cell.
This study summarizes key concepts essential for understanding cell function, biomolecules, and physiological processes. By mastering these principles, one gains insight into fundamental biological mechanisms and their role in human physiology.