Apr 10, 2025
Lecture Aims: After these lectures, you should be able to:
Name the major membrane systems of eukaryotic cells and the principal lipids of eukaryotic membranes
Draw a diagram showing a phospholipid bilayer
Draw a diagram showing the principal features of a membrane
List and illustrate the major functions of cell membranes
Describe the major groups of membrane ion transport proteins (pumps, coupled transporters, ion channels) and give examples of each
Identify the principal ion pumps of animals and plants and their respective coupling-ion circuits
I. Major Membrane Systems of Eukaryotic Cells and Principal Lipids:
Eukaryotic cells possess a complex network of internal membranes, each with specialized functions. The principal lipids in eukaryotic membranes are:
Phospholipids: These are the most abundant lipids, forming the bilayer. The most common are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. They are amphipathic, possessing both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
Sphingolipids: These lipids are structurally similar to phospholipids but have a sphingosine backbone instead of glycerol. Examples include sphingomyelin and glycosphingolipids (e.g., cerebrosides, gangliosides). They are also amphipathic.
Cholesterol: A steroid lipid found intercalated within the phospholipid bilayer. It modulates membrane fluidity.
Structure of the Cell Membrane
Cholesterol: Integrated within the membrane. Important for stability and fluidity.
Membrane Proteins Integral (Transmembrane) Proteins: Span entire membrane, involved in transport (e.g., ion channels, carrier proteins). Peripheral Proteins: Weakly attached to membrane surface, involved in enzyme activity and signaling.
Glycocalyx Composed of glycoproteins and glycolipids. Located on the extracellular surface of the membrane. Functions include water retention and antigenic recognition (immune responses).
Functions of the Cell Membrane
Glycocalyx Water Regulation: Prevents cell dehydration by regulating water movement. Antigenic Function: Helps the immune system recognize host versus foreign cells (e.g., blood typing).
Membrane Lipids Fluidity: Influenced by temperature, cholesterol, and types of fatty acids. High temperatures and low cholesterol increase fluidity, while low temperatures and high cholesterol decrease it. Saturated fatty acids decrease fluidity; unsaturated increase it.
Transport: Simple Diffusion: Movement of small, nonpolar, lipid-soluble molecules (e.g., O2, CO2). Lateral and Transverse Diffusion: Phospholipids move laterally or flip between the inner and outer membrane.
Major Membrane Systems:
Plasma membrane: The outer boundary of the cell, regulating transport and cell signaling. Endoplasmic reticulum (ER): A network of interconnected membranes involved in protein synthesis, folding, and lipid metabolism (Rough ER and Smooth ER). Golgi apparatus (Golgi body): Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. Mitochondria: The powerhouse of the cell, possessing its own inner and outer membranes, crucial for energy production. Lysosomes: Membrane-bound organelles containing hydrolytic enzymes for cellular digestion and waste recycling. Peroxisomes: Membrane-bound organelles involved in lipid metabolism and detoxification. Nuclear envelope: A double membrane system enclosing the nucleus, regulating the movement of molecules in and out of the nucleus. Vacuoles: Fluid-filled organelles; in plant cells, they maintain turgor pressure. II. Diagrams:
(A) Phospholipid Bilayer:
Hydrophilic Head | |
---------+--------- <- Phospholipid Bilayer | | Hydrophilic Head
Hydrophobic Tail Hydrophobic Tail
Draw two parallel lines to represent the bilayer. On the outside of each line, draw a circle to represent the hydrophilic head (polar). On the inside, draw two wiggly lines extending from each circle to represent the hydrophobic tails (nonpolar). Repeat this pattern along the lines.
(B) Principal Features of a Membrane:
Extracellular Space +---------+ | | Glycoprotein/Glycolipid | | Protein | Phospholipid Bilayer | | Cholesterol | Protein| | | +---------+ Cytoplasm
Integral Protein
Peripheral Protein
Cytoskeleton Filament
Draw a rectangle representing the membrane. Within the rectangle, draw two parallel lines for the phospholipid bilayer. Show cholesterol molecules interspersed between the phospholipids. Draw integral membrane proteins spanning the bilayer, and peripheral proteins attached to the surface. Show glycoproteins and glycolipids on the outer surface. Add a cytoskeleton filament (e.g., actin) on the cytosolic (inner) side.
III. Major Functions of Cell Membranes:
Compartmentalization: Membranes create distinct compartments within the cell, allowing for specialized metabolic processes. Selective Permeability: Membranes regulate the passage of substances in and out of the cell or organelles. This is crucial for maintaining homeostasis. Cell Signaling: Receptors on the cell membrane bind to signaling molecules, initiating intracellular responses. Cell Adhesion: Membranes facilitate cell-to-cell communication and adhesion through proteins such as cadherins and integrins. Energy Transduction: Membranes are the site of many energy-producing processes, such as photosynthesis (chloroplast) and cellular respiration (mitochondria). IV. Membrane Ion Transport Proteins:
(A) Pumps: Use energy (ATP hydrolysis) to transport ions against their concentration gradient.
Example: Na+/K+ ATPase (sodium-potassium pump) moves 3 Na+ out and 2 K+ into the cell. This maintains the electrochemical gradient across the membrane. Ca2+ ATPase pumps Ca2+ out of the cell. H+/K+ ATPase pumps H+ into the stomach lumen. (B) Coupled Transporters: Transport of one ion down its concentration gradient drives the transport of another ion against its concentration gradient. This transport occurs via symporters (same direction) or antiporters (opposite direction).
Example: Symporter: Sodium-glucose co-transporter (SGLT1) moves glucose and sodium into intestinal cells. Example: Antiporter: Sodium-calcium exchanger (NCX) moves Na+ into the cell and Ca2+ out of the cell. (C) Ion Channels: Form hydrophilic pores allowing ions to passively diffuse across the membrane down their electrochemical gradient. Gating mechanisms control opening and closing of these channels.
Example: Voltage-gated sodium channels (Nav) open in response to changes in membrane potential, critical for action potentials. Ligand-gated ion channels (e.g., nicotinic acetylcholine receptor) open upon binding of a specific ligand. V. Principal Ion Pumps in Animals and Plants:
Animals:
Na+/K+ ATPase: The primary ion pump, establishing the electrochemical gradients for Na+ and K+. Its coupling-ion circuit involves Na+ and K+. Ca2+ ATPase: Removes Ca2+ from the cytosol, regulating intracellular Ca2+ levels. Plants:
H+/ATPase: The primary ion pump, creates a proton gradient across the plasma membrane. This gradient is used to drive secondary active transport of other ions and nutrients. Its coupling ion circuit is H+ Remember that these are simplified representations. Membrane structure and function are incredibly complex, involving many different types of proteins and lipids interacting dynamically. Use these descriptions to create your diagrams, adding more detail as needed