Cell Membrane and Transport

Phospholipids: Structure and Properties

• Amphipathic Molecules: Phospholipids are described as amphipathic, meaning they possess two ends that behave differently.

  • The prefix "amphi-" (as in amphibian) implies "two different things" or "both," similar to how amphibians inhabit both land and water.

  • Polar Head: The spherical region (head) of a phospholipid is polar and hydrophilic (water-loving), meaning it readily interacts with water.

  • Nonpolar Tails: The two linear regions (tails) are nonpolar and hydrophobic (water-fearing), meaning they do not like water and prefer to associate with each other.

• Tail Characteristics: The tails can vary:

  • Kinked Tails: Result from carbon-carbon double bonds, leading to less tightly packed structures.

  • Straight Tails: Lack double bonds, resulting in a more tightly packed structure, similar to a saturated fat.

Cell Membrane Structure: The Fluid Mosaic Model

• Phospholipid Bilayer: Due to their amphipathic nature and the presence of water both outside (extracellular) and inside (intracellular) the cell, phospholipids spontaneously form a bilayer.

  • Formation: The polar heads face outward towards the aqueous environments (extracellular fluid and cytoplasm), while the nonpolar tails face inward, associating with each other to create a hydrophobic internal environment.

  • Spontaneity: This self-assembly is spontaneous, akin to how olive oil forms droplets in water, as polar regions interact with water and nonpolar regions interact with each other.

  • Fluidity: The cell membrane maintains fluidity, which is crucial for cellular health. Loss of fluidity can be associated with diseases.

• Components of the Plasma Membrane:

  • Transmembrane Proteins: Proteins that span across the entire membrane (trans means across).

  • Interior Protein Network: Includes peripheral proteins (loosely associated with the inner surface) and connections to the cytoskeleton.

  • Cell Surface Markers: Proteins or lipids (often with attached sugar groups) that sit in the membrane and facilitate communication with the outside of the cell, allowing other cells to recognize "self" vs. "foreign invaders".

Microdomains and Heterogeneity

• Definition: Microdomains are small, functional regions within the cell membrane.

  • Micro means small, and domain refers to a functional region.

  • The cell membrane is heterogeneous (hetero means different), not homogeneous (homo means same), meaning different areas have specific functions or compositions (e.g., more of a certain protein or lipid).

• Lipid Rafts: A specific type of microdomain enriched in cholesterol and modified lipids (sphingolipids). These rafts are important functional areas within the membrane.

Membrane Proteins: Functions and Domains

• Membrane proteins allow cells to interact with their environment and perform various functions.

• Six Major Classes of Membrane Protein Function:

  1. Transport Proteins: Allow ions or small, polar molecules to cross the membrane that otherwise cannot due to size or the hydrophobic environment of the tails (e.g., water requires an aquaporin).

  2. Enzymes: Catalysts embedded within the membrane; not all enzymes are membrane-bound, but it is a common location.

  3. Cell Surface Receptors: Transmembrane proteins that bind a ligand (signaling molecule) from outside the cell, inducing a change (signal) inside the cell. (Detailed in a later chapter).

  4. Cell Surface Identity Markers: Often heavily sugar-coated proteins (glycoproteins) that face the outside of the cell.

     • These sugar groups indicate to the immune system that the cell is "self" and should not be destroyed.

     • Autoimmunity: A loss or alteration of these markers (e.g., inappropriate sugars) can lead to the immune system attacking the body's own cells, causing diseases like lupus or certain types of diabetes.

  5. Cell-to-Cell Adhesion Proteins: Help cells stick to each other, crucial for tissue formation (though not extensively covered in this single-cell-focused class).

  6. Attachment to the Cytoskeleton: Proteins that link the cell membrane to the cytoskeleton, helping the cell maintain its shape. An example is the attachment to actin filaments (double helical cytoskeletal elements).

• Peripheral Proteins: Located on the inside surface of the plasma membrane, loosely associated, and do not span the membrane.

• Transmembrane Protein Domains: Distinct regions with specific functions:

  • Extracellular Domain: Interacts with the outside environment.

  • Transmembrane Domain: Anchors the protein within the membrane; may have other functions depending on the protein type.

  • Intracellular Domain: Impacts the inside (cytoplasmic) side of the cell.

Transport Across Membranes: Passive Transport

• Passive Transport: Movement of substances across the membrane without the use of cellular energy (ATP).

• Key Definitions:

  • Solution: A mixture of solute (dissolved substance) and solvent (substance doing the dissolving). Example: Salt water (salt is solute, water is solvent).

  • Solute: The substance that is dissolved (e.g., salt).

  • Solvent: The substance that does the dissolving (e.g., water, often called the universal solvent for polar substances).

  • Concentration: Described as the amount of solute per unit of solvent or total solution (e.g., moles per liter $( ext{mol/L})$, molarity, grams per liter $( ext{g/L})$, percentage).

Types of Passive Transport

1. Simple Diffusion:

   • Does not require a protein helper.

   • Substances move down the concentration gradient, meaning from an area of higher concentration to an area of lower concentration.

   • Example: A sodium ion moving from a high concentration extracellular environment $( ext{E})$ to a low concentration intracellular environment $( ext{I})$ to balance concentrations.

2. Facilitated Diffusion:

   • Requires a helper protein because the substance is either too large or too polar to cross the hydrophobic membrane core unaided.

   • Still moves down the concentration gradient and does not use energy.

   • Channel Proteins: Form an open pore (often a beta-barrel structure) through which ions or molecules can flow (e.g., ion channels).

   • Carrier Proteins: Bind the substance on one side of the membrane, undergo a conformational change, and release it on the other side.

3. Osmosis (Movement of Water):

   • The passive movement of water across a selectively permeable membrane.

   • Technically a type of facilitated diffusion in cells, as water is polar and requires aquaporin proteins to cross the membrane efficiently.

   • Aqueous Solution: A solution where water is the solvent.

   • Describing Solutions Relative to Each Other:

     • Hypertonic: The side with more concentrated solute.

     • Hypotonic: The side with less concentrated solute.

     • Isotonic: When solute concentrations are equal on both sides.

   • Water Movement Rule: Water moves to the hypertonic side (the side with the higher solute concentration) in an effort to dilute the solute and balance the overall concentration (part solute / part solvent).

   • Simultaneous Balancing Act: In living cells, both solutes and water move simultaneously to achieve equilibrium: solutes down their concentration gradient (high to low), and water to the more concentrated solute side.

Aquaporins and Nephrogenic Diabetes Insipidus (NDI)

• Aquaporins: Water channel proteins that facilitate the rapid movement of water across cell membranes. They are crucial for water balance.

• Nephrogenic Diabetes Insipidus (NDI):

  • A disease caused by non-functioning (mutated) aquaporins, particularly in the kidney.

  • Mechanism: The kidneys fail to reabsorb water, leading to excessive excretion of very dilute urine.

  • Symptoms: Dehydration and seemingly high blood sugar.

  • Explanation: The high blood sugar is not due to an insulin issue (like in Type 1 or Type 2 diabetes mellitus) but rather an imbalance of part solute (glucose) over part solvent (water). As water is lost, the concentration of glucose in the blood increases, leading to apparent hyperglycemia.

  • Differentiation: NDI is distinct from diabetes mellitus, which involves insulin dysfunction. Specific diagnostic tests (e.g., urine analysis for glucose/protein) are used to differentiate.

Water Balance and Cell Shape

• Importance: Maintaining proper water balance is critical for preserving cell shape and preventing cells from shriveling (crenation) or bursting (lysis).

• Pressures Involved:

  • Osmotic Pressure: Pushes water into the cell.

  • Hydrostatic Pressure: Pushes water back out of the cell.

  • Isotonic Environment: The ideal state where osmotic and hydrostatic pressures are equal, preventing significant net water movement.

• Effects on Red Blood Cells:

  • Dehydration: In a hypertonic environment (too much salt, too little water), red blood cells shrivel.

  • Excess Water: In a hypotonic environment (too much water too quickly), red blood cells swell and can burst (e.g., from excessive rapid water intake).

• Cells with Cell Walls (e.g., Plant Cells):

  • Protection: Cell walls provide structural support and prevent bursting in hypotonic conditions.

  • Turgid: In isotonic conditions, the membrane meets the cell wall in a placid (normal) state.

  • Turgid State (Hypotonic): When plant cells take in excess water, the cell wall maintains shape, and the large central vacuole can absorb the water, increasing turgor pressure but preventing lysis.

  • Wilting (Hypertonic): In a high-salt (hypertonic) environment, plant cells lose water, leading to plasmolysis (shrinking of the protoplast away from the cell wall), causing wilting of leaves.