Cell Membrane Structure and Function

Definition: The cell or plasma membrane is a selectively permeable membrane that regulates the steady traffic that enters and leaves the cell.
Fluid Mosaic Model:
- Proposed by S. J. Singer in 1972.
- Describes the structure of the plasma membrane as a fluid mosaic, consisting of a phospholipid bilayer with proteins dispersed throughout.
- Emphasizes the dynamic nature of the membrane.

Phospholipid Bilayer: Composed of phospholipids, which are amphipathic molecules having both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.
Proteins:
- Integral proteins have nonpolar regions that span the hydrophobic interior of the membrane.
- Peripheral proteins are loosely bound to the membrane's surface.
Cholesterol: Embedded in the bilayer to stabilize membrane structure and fluidity.
Composition: Average plasma membrane consists of approximately 40% lipid and 60% protein and has a consistency similar to olive oil.

Movement of Proteins and Lipids:
- Phospholipids move rapidly within the plane of the membrane.
- Some proteins are anchored via cytoskeleton attachments, while others can drift slowly.
Hydrophobic Interactions:
- Membranes are held together mainly by hydrophobic interactions, which are weaker than covalent bonds.
- Lateral movement of lipids and some proteins is common; transverse flip-flops across the membrane are rare.

Glycolipids and Glycoproteins:
- Extend from the extracellular surface of the plasma membrane.
- Glycolipids are carbohydrates covalently bonded to lipids, while glycoproteins are carbohydrates attached to proteins.
- Function as signaling molecules that help distinguish between different cell types.
Example: Glycoproteins on red blood cells are responsible for ABO and Rh blood types.

Transport:
- Channels, pumps, carriers, and electron transport chains facilitate the movement of molecules, electrons, and ions through the membrane.
Enzymatic Activity:
- Example: Adenylate cyclase synthesizes cyclic AMP (cAMP) from ATP.
Signal Transduction:
- Protein receptors bind chemical messengers (e.g., hormones), altering protein shape and relaying signals inside the cell.
Cell-to-Cell Recognition:
- Certain glycoproteins act as identification flags recognized by other cells.
Cell-to-Cell Attachments:
- Includes structures such as desmosomes, gap junctions, and tight junctions.
Attachment to Cytoskeleton and ECM:
- Helps maintain cell shape and stabilizes membrane protein locations.

Definition of Transport: The movement of substances in and out of a cell, categorized into active and passive transport.
Active Transport: Requires energy (ATP) to move substances against their concentration gradient.
Passive Transport: Does not require energy; substances move down their concentration gradient until equilibrium is reached.

Types of Passive Transport:
- Diffusion: Random movement from higher to lower concentration.
- Simple Diffusion: Does not involve protein channels.
  - Example: Solutes diffusing from blood into Bowman’s capsule in kidneys.
- Facilitated Diffusion: Requires hydrophilic protein channels to help specific substances across the membrane.
- Ion channels may be gated in response to stimuli (e.g., voltage-gated ion channels in axons).
Countercurrent Exchange: A special case of simple diffusion involving adjacent fluids moving in opposite directions to maximize diffusion rates, exemplified in fish gills.

Diffusion: Movement of molecules from high to low concentration.
Osmosis: Specific diffusion of water across a membrane.
Solvent: Substance that dissolves solutes.
Solute: Substance being dissolved.
Hypertonic: Higher solute concentration compared to another solution.
Hypotonic: Lower solute concentration compared to another solution.
Isotonic: Equal solute concentrations in two solutions.
Osmotic Potential: Tendency of water to move across a membrane into a solution.
Water Potential: Influenced by solute concentration and pressure; pure water has a water potential of 0, while solute addition lowers it to a negative value.

Symbol: Water potential denoted by ψ, influenced by solute concentration and pressure.
Water Movement Dynamics: Water moves from areas of higher water potential to lower water potential; movement can cause cells to swell or shrink based on surrounding solution tonicity.
Effects on Plant Cells:
- Cells become turgid when swollen; loss of water leads to wilting in plants due to turgor pressure.

Definition: Special water channel proteins facilitating rapid water movement across membranes.
Function: Facilitate the diffusion of up to 3 billion (3 × 10^9) water molecules per second; do not alter water potential gradients.
Specificity: Aquaporins are selective, allowing rapid passage of water but not of charged molecules like hydronium ions (H3O+).

Definition: Movement of substances against their concentration gradient requiring energy (usually ATP).
Examples:
- Sodium-Potassium Pump: Pumps Na+ and K+ ions across nerve cell membranes. Moves 3 Na+ ions out and 2 K+ ions into the cell, crucial for returning nerve cells to resting state.
- Proton Pumps: Actively transport protons (H+) across membranes; generate voltage for energy storage for cellular work, such as ATP production in mitochondria.
- Cotransport: A type of membrane pump system linking proton transport to the movement of a secondary substance (e.g., sucrose).
Types of Endocytosis:
- Exocytosis: Vesicles fuse with the plasma membrane to secrete substances; e.g., neurotransmitters from nerve cells.
- Endocytosis: Formation of vesicles to take in macromolecules; includes:
- Pinocytosis: Uptake of dissolved particles (cell drinking).
- Phagocytosis: Engulfing of large particles (cell eating), used by white blood cells.
- Receptor-Mediated Endocytosis: Specific uptake method where substances bind to receptors, cluster, and form coated vesicles for ingestion.

Homeostasis: Maintenance of cellular balance via continuous transport across membranes.
Bulk Flow: General term for one-directional fluid movement in an organism, such as blood circulation in humans due to heart action and sap flow in trees driven by active transport in phloem.