Membrane Composition and Structure

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms the fundamental structure of all biological membranes. Phospholipids are amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Structure: Each phospholipid consists of two fatty acid tails attached to a carbon backbone with a phosphate group as the head.

• Self-assembly: In aqueous environments, phospholipids spontaneously arrange into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward toward the cytoplasm or extracellular fluid.

• Fluid Mosaic Model: The membrane is a dynamic structure with proteins and steroids embedded within the bilayer, allowing for lateral movement and flexibility.

Membrane Fluidity and Components

• Lipid Composition and Temperature:

  • Membranes must remain fluid to function properly.

  • Unsaturated fatty acids increase fluidity; saturated fatty acids decrease fluidity.

  • At lower temperatures, membranes with more unsaturated fatty acids remain more fluid, while those with saturated fatty acids become more solid.

• Cholesterol:

  • Cholesterol molecules, with four fused rings, are interspersed within the bilayer.

  • At warm temperatures, cholesterol restrains phospholipid movement, reducing fluidity.

  • At cool temperatures, cholesterol prevents tight packing of phospholipids, maintaining fluidity.

• Proteins:

  • Integral proteins span the membrane (e.g., integrins), while peripheral proteins are attached to the membrane surface.

  • Functions include transportation, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

• Carbohydrates:

  • Located on the extracellular side, bound to proteins (glycoproteins) or lipids (glycolipids).

  • Form the glycocalyx, a sugar coating important for cell recognition and protection.

  • Serve as ID tags for cellular identification.

Membrane Structure and Selective Permeability

Selective Permeability

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Hydrophobic molecules pass through more readily than hydrophilic molecules.

• The membrane's composition, protein orientation, and carbohydrate tags differ on each side, contributing to its selective nature.

Diffusion and Transport Mechanisms

• Diffusion: The movement of particles from an area of higher concentration to an area of lower concentration due to random motion.

  • Temperature and chemical gradients affect the rate of diffusion.

  • At equilibrium, particles move at equal rates in both directions.

• Facilitated Diffusion: Movement of molecules across the membrane via channel or carrier proteins, without energy input.

  • Channel proteins (e.g., ion channels) allow specific ions or molecules to pass through.

  • Carrier proteins can regulate the rate and amount of transport.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

  • Example: Sodium-potassium pump in animal cells.

  • Rate is slower than facilitated diffusion.

Osmosis and Tonicity

• Osmosis: The diffusion of water across a semipermeable membrane from low solute concentration to high solute concentration.

• Osmolarity: The total solute concentration of a solution.

• Tonicity: The ability of a surrounding solution to cause a cell to gain or lose water.

  • Hypertonic: Solution with higher solute concentration; cell loses water and shrivels.

  • Hypotonic: Solution with lower solute concentration; cell gains water and may burst.

• Osmoregulation: The control of water and solute balance, essential for survival in varying environments.

  • Example: Paramecium caudatum uses a contractile vacuole to expel excess water in a hypotonic environment.

Membrane Pumps and Electrochemical Gradients

Membrane Potential and Electrochemical Gradient

• Membrane Potential: The voltage difference across a membrane, resulting from the unequal distribution of ions.

• Electrochemical Gradient: The combined effect of a chemical gradient (concentration difference) and an electrical gradient (voltage difference) on ion movement.

Types of Membrane Pumps

• Electrogenic Pump: A transport protein that generates voltage across a membrane.

  • Proton Pump: Main electrogenic pump in plants, fungi, and bacteria; moves protons (H+) out of the cell.

  • Sodium-Potassium Pump: Major electrogenic pump in animal cells; moves 3 Na+ out and 2 K+ in, creating a net negative charge inside the cell.

Cotransport

• Uniporter: Transports one molecule or ion at a time.

• Symporter: Transports two different molecules or ions in the same direction.

• Antiporter: Transports two different molecules or ions in opposite directions.

Bulk Transport

Bulk transport moves large substances across the membrane via vesicles and requires energy.

• Endocytosis: Brings substances into the cell via vesicles.

  • Phagocytosis: "Cell eating"; engulfs large particles or entire cells, merges with lysosomes for digestion.

  • Pinocytosis: "Cell drinking"; engulfs extracellular fluid and dissolved solutes.

  • Receptor-mediated Endocytosis: Specific molecules bind to receptors, triggering vesicle formation when enough ligand is present.

• Exocytosis: Substances are expelled from the cell via vesicles that fuse with the plasma membrane.

Cell Communication and Signaling

Types of Cell Signaling

• Intercellular Signaling: Communication between different cells.

• Intracellular Signaling: Communication within a single cell.

• Local Signaling: Direct contact between adjacent cells via gap junctions (animals) or plasmodesmata (plants).

• Synaptic Signaling: Electrical signals trigger neurotransmitter release across synapses (nerve cells).

• Autocrine Signaling: Cell targets itself.

• Paracrine Signaling: Cell targets nearby cells.

• Long-distance Signaling: Hormones travel through the circulatory system to target distant cells (endocrine signaling).

Stages of Cell Signaling

1. Reception: Detection of a signaling molecule (ligand) by a receptor protein, causing a conformational change.

2. Transduction: Conversion of the signal to a cellular response, often involving a signal transduction pathway.

3. Response: The specific cellular activity triggered by the signal.

Signal Receptors

• G protein-coupled receptors (GPCRs): Work with cytoplasmic G proteins; very diverse and important in many signaling pathways.

• Ligand-gated ion channels: Open or close in response to ligand binding, allowing ions such as Na+ or Ca2+ to cross the membrane and trigger electrical signals.

• Intracellular receptors: Located in the cytoplasm or nucleus; bind small or hydrophobic ligands (e.g., steroid and thyroid hormones) that can cross the membrane.

Signaling Molecules

• Small hydrophobic ligands: Steroid and thyroid hormones (act inside the cell).

• Water-soluble ligands: Small molecules, peptides, and proteins (act outside the cell).

• Gaseous ligands: Nitric oxide (NO), ethylene gas (C2H4).

Phosphorylation and Second Messengers

• Phosphorylation: Addition of a phosphate group to a protein to activate or deactivate it.

  • Kinase: Enzyme that adds phosphate groups.

  • Phosphatase: Enzyme that removes phosphate groups.

• Second Messengers: Small, non-protein, water-soluble molecules or ions that spread signals within a cell by diffusion.

  • Examples: cyclic AMP (cAMP), Ca2+

Homeostatic Regulation

Homeostatic regulatory mechanisms influence hormone concentration and maintain internal balance.

Key Table: Types of Membrane Transport

Key Equations

• Osmotic Pressure: Where = osmotic pressure, = van 't Hoff factor, = molarity, = gas constant, = temperature (Kelvin)

• Membrane Potential (Nernst Equation): Where = equilibrium potential, = gas constant, = temperature, = ion charge, = Faraday's constant