Membrane Transport and Cell Signaling Lecture Notes
Introduction
Focus primarily on membrane structure and function.
Not delving into cellular signaling in this lecture.
Plasma Membrane: "Life at the Edge"
Every cell has a plasma membrane separating inside from outside.
Selectively permeable: Chooses what enters and exits to maintain homeostasis.
Composed of a phospholipid bilayer: Hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
Contains proteins (integral and peripheral), cholesterol, glycolipids, glycoproteins, and other molecules.
Phospholipid Bilayer
Structure: Two layers of phospholipids (fatty acid tails + phosphate group).
Properties: Amphipathic nature (both hydrophilic and hydrophobic regions).
Fluid Mosaic Model: Describes the membrane as a flexible and dynamic structure.
Proteins and lipids move laterally within the bilayer.
Proteins in the Membrane
Integral Proteins: Span the membrane, amphipathic, often alpha-helical.
Peripheral Proteins: Loosely bound to the surface, often involved in cellular processes.
Functions: Transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, attachment to the cytoskeleton, and extracellular matrix.
Carbohydrates and Cell Recognition
Carbohydrates on the exterior assist in cell recognition.
Glycolipids and glycoproteins facilitate cell-to-cell communication.
Cells have specific carbohydrate markers that vary between species and individuals.
Membrane Transport
Passive Transport: Movement of substances across the membrane without energy input.
Diffusion: Movement from high to low concentration.
Osmosis: Specifically the passive transport of water.
Facilitated Diffusion: Help from transport proteins (e.g., aquaporins).
Active Transport: Requires energy (ATP) to move substances against their concentration gradient.
Sodium-Potassium Pump: Moves Na+ out, K+ into the cell.
Electrochemical Gradients and Pumps
Electrogenic Pumps: Generate voltage across membranes (e.g., sodium-potassium pump in animals).
Proton Pump: Main electrogenic pump in plants, fungi, and bacteria.
Bulk Transport
Exocytosis: Vesicles fuse with the plasma membrane to release contents outside.
Endocytosis: Cell takes in materials via vesicle formation.
Types include phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.
Additional Concepts
Membrane Fluidity and Composition: Cholesterol plays a role in membrane fluidity, especially at varying temperatures.
Tonicity and Cell Environment: Isotonic, hypertonic, and hypotonic solutions affect cells differently.
Animal cells prefer isotonic solutions.
Plant cells prefer hypotonic environments for turgor pressure.
Conclusion
Focus on understanding structures, functions, and mechanisms of membrane components.
Recommended to read textbook Chapter 5 up to section 5.4 for exam preparation.