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Cell Physiology I Overview

• Spring 2025 Course: Cell Physiology I focuses on the structure and function of biological membranes, membrane permeability, and transport mechanisms.

• Instructor: Patricio E. Mujica, Ph.D.

• Department: Pharmacology, Physiology and Neuroscience, Rutgers–NJMS MSB H655.

Learning Objectives

1. Functions of Plasma Membrane: Correlate functions with molecular components.

2. Fluid Mosaic Model: Describe the structure of biological membranes.

3. Transport Mechanisms: Compare passive, primary active, and secondary active transport.

4. Ion Channel Gating: Contrast different gating mechanisms.

5. Carrier Proteins: Explain properties and types of transport mechanisms.

Cell Structure

• Basic Unit of Life: Cells are smallest functional units capable of all life processes.

• Surrounded by Plasma Membrane: Separates the internal (intracellular) from the external (extracellular) environment.

  • Intracellular Environment: Contains cytoplasm (cytosol and organelles).

  • Extracellular Environment: Provides access to nutrients and gases.

Fluid Compartments

• Total Body Fluids: Generally divided into two compartments:

  • Intracellular Fluid (ICF): 2/3 of total body fluids.

  • Extracellular Fluid (ECF): 1/3 of total body fluids, comprising interstitial fluid and blood plasma.

• Homeostasis: Maintained through composition differences between these compartments, requiring energy.

Functions of the Cell Membrane

• Barrier: Separation of body fluid compartments.

• Transport: Regulation of exchange of water and solutes.

• Communication: Relay signals between cell and environment.

• Structural Support: Links cell to its environment via cytoskeletal proteins.

Structure of the Cell Membrane

• Components of the Cell Membrane:

  • Lipids: Phospholipids, sphingolipids, cholesterol forming the lipid bilayer.

  • Proteins: Integral and peripheral proteins, glycoproteins, glycolipids.

• Fluid Mosaic Model: Explains the organization and dynamics of membrane components.

  • Integral Proteins: Cross the bilayer, involved in transport.

  • Peripheral Proteins: Temporarily associated with the membrane, modulate functions.

Membrane Proteins

• Classes:

  • Integral Proteins: Permanently bound, involved in transport and signaling.

  • Peripheral Proteins: Provide mechanical support, can be removed without disrupting membrane integrity.

• Transport Proteins:

  • Carrier Proteins: Undergo conformational changes to facilitate movement.

  • Channel Proteins: Form pores for molecule passage by diffusion.

Membrane Transport Mechanisms

• Passive Transport: No energy required; solutes move following concentration gradients.

  • Simple Diffusion: Small, nonpolar molecules cross the membrane directly.

  • Facilitated Diffusion: Involves protein channels or carriers for polar molecules.

• Active Transport: Energy required to move substances against concentration gradients.

  • Primary Active Transport: Direct use of ATP (e.g., Na+/K+ pump).

  • Secondary Active Transport: Utilizes gradients created by primary active transport mechanisms.

Types of Transport Proteins

• Ion Channels: Permit specific ions to flow across membranes.

  • Mechanically-gated: Open due to physical force.

  • Voltage-gated: Open due to electrical potential changes.

  • Ligand-gated: Open upon ligand binding.

• Transporters:

  • Uniporters: Transport one molecule.

  • Symporters: Transport two molecules in the same direction.

  • Antiporters: Transport two molecules in opposite directions.

Osmosis and Water Transport

• Aquaporins: Specialized channels for water movement across membranes.

• Definitions:

  • Osmosis: Movement of water from low to high osmotic pressure.

  • Osmolarity: Concentration of osmotically active particles.

  • Osmotic Pressure: Pressure needed to stop water movement.

Osmolarity and Tonicity

• Tonicity: Describes how a solute affects cell volume.

  • Isotonic: No volume change in the cell.

  • Hypertonic: Cell volume decreases.

  • Hypotonic: Cell volume increases.

• Calculation of Osmolarity:

  • Total osmolarity = sum of osmolarity from all solutes.

Clinical Applications

• Common Intravenous Solutions: Characterized by their osmolarity and tonicity.

  • Isotonic Solutions: E.g., 0.9% saline (154 mM NaCl).

  • Hypotonic Solutions: Cause cells to swell; careful monitoring required.