Membrane Transport Flashcards

Composition and Structure of the Plasma Membrane

• The Lipid Bilayer: The plasma membrane is primarily composed of a lipid bilayer, which serves as a foundation for cell structure and function. It consists of two layers: an outward-facing layer of phospholipids and an inward-facing layer of phospholipids.

• Phospholipids (75%): These represent the majority of the membrane lipids. They are amphipathic molecules consisting of:

  • Hydrophilic Heads: These polar heads are water-loving and face the aqueous environments (extracellular fluid and cytoplasm).

  • Hydrophobic Tails: These nonpolar tails are water-fearing and point toward the center of the bilayer, away from water.

• Carbohydrates (5%): Linked to proteins and lipids, these components are essential for cell identity and interaction.

  • Glycolipids: Lipids with attached carbohydrates.

  • Glycoproteins: Proteins with attached carbohydrates.

  • Glycocalyx: A sugary coating on the cell surface formed by glycolipids and glycoproteins, functioning as a molecular signature for cell identity.

• Cholesterol (10%): Interspersed within the phospholipid bilayer to provide membrane stability and maintain appropriate fluidity.

• Cellular Environments:

  • Extracellular Fluid: The watery environment located outside of the cell.

  • Cytoplasm: The watery environment located inside the cell.

  • Cytoskeleton: Filaments of the cytoskeleton are located internally to provide structural support, often interacting with peripheral proteins.

Membrane Proteins and Their Functions

• Classification of Membrane Proteins:

  • Integral Proteins: Proteins deeply embedded in the lipid bilayer, often spanning the entire width of the membrane.

  • Peripheral Proteins: Proteins that are not embedded in the lipid bilayer but are typically attached to integral proteins or the cytoskeleton.

• Specialized Membrane Functions: Proteins are responsible for most of the specific functional tasks of the membrane, categorized into six primary roles:

  • Transport: Moving substances across the membrane; some transport proteins use $ATP$ as an energy source.

  • Enzymatic Activity: Proteins acting as enzymes to catalyze specific chemical reactions at the membrane surface.

  • Receptors for Signal Transduction: Membrane proteins that possess binding sites for specific chemical messengers (signals). Once bound, the protein may change shape to initiate a chain of chemical reactions in the cell.

  • Cell-Cell Recognition: Some glycoproteins serve as identification tags that are specifically recognized by other cells.

  • Intercellular Joining: Membrane proteins of adjacent cells may hook together in various kinds of junctions (e.g., using cell adhesion molecules or CAMs).

  • Attachment to the Cytoskeleton and Extracellular Matrix (ECM): Elements of the cytoskeleton and the ECM may anchor to membrane proteins, helping maintain cell shape and fix the location of certain membrane proteins.

Fundamentals of Membrane Transport

• Selective Permeability: The plasma membrane is selectively permeable, meaning it allows some substances to pass while excluding others. Permeability depends on:

  • Molecule Size: Smaller molecules generally pass more easily.

  • Lipid Solubility: Nonpolar, lipid-soluble molecules pass more readily through the lipid core.

  • "Size and Charge": These are the two primary determinants of whether a substance can cross the membrane.

• The Concentration Gradient: Defined as the difference in solute concentration between two areas separated by a semi-permeable barrier.

• Methods of Moving Substances:

  • Passive Transport Mechanisms: Process that moves substances across the membrane without the consumption of cellular energy ($ATP$).

  • Active Transport Mechanisms: Process that requires the cell to expend energy in the form of $ATP$ to move substances.

Passive Transport: Filtration and Simple Diffusion

• Filtration: The movement of water and solutes through a membrane or vessel wall due to hydrostatic pressure.

  • Mechanism: Blood pressure in capillaries forces water and small solutes (such as salts) through narrow clefts between capillary cells.

  • Constraints: Clefts hold back larger particles, such as red blood cells and large proteins.

  • Examples: Filtration of nutrients from blood capillary walls into tissue fluids; filtration of wastes from the blood in the kidneys.

• Simple Diffusion: The unassisted diffusion of solutes through the plasma membrane.

  • Direction: Substances move "Down or With" the gradient, from an area of high concentration to an area of low concentration.

  • Physical Requirements: Substrates must be small and nonpolar (lipid-soluble/hydrophobic).

  • Examples: Gases (like Oxygen and Carbon Dioxide), Steroid Hormones, and Fatty Acids.

Passive Transport: Facilitated Diffusion and Osmosis

• Facilitated Diffusion: A process for substances that cannot pass through the lipid bilayer by simple diffusion.

• Carrier-Mediated Facilitated Diffusion:

  • Used for large, polar molecules that are lipid-insoluble (hydrophilic).

  • Examples: Sugars such as Glucose, and Amino Acids.

  • Mechanism: The solute binds to a specific carrier protein, which then undergoes a shape change to release the solute on the other side of the membrane.

• Channel-Mediated Facilitated Diffusion:

  • Used for small, lipid-insoluble solutes, primarily specific polar molecules and ions.

  • Ions Involved: $Na^+$, $K^+$, and $Cl^-$.

  • Types of Channels:

    • Leakage Channels: These are always open, allowing for continuous flux.

    • Gated Channels: These open or close in response to specific stimuli, including Chemical, Electrical, or Mechanical signals.

• Osmosis:

  • The movement of a solvent (specifically Water) across the membrane.

  • Water is a polar molecule and lipid-insoluble, yet it crosses the membrane via two methods:

    • Simple diffusion through the lipid bilayer (at a slow rate).

    • Movement through Aquaporins, which are transmembrane proteins specialized for water transport.

Active Transport Mechanisms

• Core Principle: Movement occurs "Up or Against" the concentration gradient, traveling from an area of low concentration to high concentration.

• Primary Active Transport:

  • Energy is supplied directly by the hydrolysis of $ATP$.

  • The Sodium-Potassium Pump ($Na^+ - K^+$ Pump): A critical example of primary active transport. It moves ions against their electrochemical gradients to maintain cellular homeostasis.

  • Step-by-Step Mechanism of the $Na^+ - K^+$ Pump:

    1. Three cytoplasmic $Na^+$ ions bind to the pump protein.

    2. $Na^+$ binding promotes the hydrolysis of $ATP$. The energy released phosphorylates the pump (attaches a phosphate group).

    3. Phosphorylation causes the pump to change its conformation (shape), expelling the three $Na^+$ ions to the extracellular fluid.

    4. Two extracellular $K^+$ ions bind to the pump.

    5. $K^+$ binding triggers the release of the phosphate group. This dephosphorylation causes the pump to resume its original shape.

    6. The pump protein releases the two $K^+$ ions into the cytoplasm. The $Na^+$ binding sites are once again ready, and the cycle repeats.

• Secondary Active Transport: Uses the kinetic energy of a concentration gradient created by Primary Active transport to move other substances.

Vesicular Transport

• Definition: The transport of large particles, macromolecules, and fluids across the plasma membrane inside bubble-like sacs called vesicles.

• Endocytosis: The process of bringing substances into the cell.

  • Phagocytosis: The cell engulfs large particles (e.g., bacteria or cell debris) into a vesicle called a phagosome.

  • Pinocytosis: The cell "gulps" drops of extracellular fluid containing solutes into tiny vesicles; it is a non-specific process.

  • Receptor-mediated Endocytosis: A highly selective process where specific extracellular substances bind to receptor proteins on the membrane, triggering vesicle formation.

• Exocytosis: The process of releasing substances from the cell interior to the extracellular fluid.

  • Step-by-Step Process of Exocytosis:

    1. A membrane-bound vesicle (secretory vesicle) migrates to the plasma membrane.

    2. Proteins on the vesicle surface, known as v-SNAREs (vesicle SNAREs), bind with t-SNAREs (target plasma membrane proteins).

    3. The vesicle and plasma membrane fuse together, and a fusion pore opens up.

    4. The contents of the vesicle are released to the cell exterior.