Cell Physiology Overview

Cell Physiology Overview

• Focus areas include passive and active solute transport, osmolarity, tonicity, and cell communication principles.
• Specialized functions cover electrophysiology and muscle physiology across various lectures.

Passive Solute Transport

• Energy expenditure is not required for passive transport.
• Solutes move along concentration gradients without energy.

Learning Objectives

• Understand system feedback: negative, positive, and feedforward.
• Memorize intracellular and extracellular concentrations for Na+, K+, Ca++, and Cl-.
• Identify membrane functions including selective substance passage, chemical messenger detection, immune recognition, tissue formation, and cell communication.

Membrane Structure and Function

• Membranes consist of phospholipids and cholesterol, which create permeability and fluidity.
• Integral proteins permanently attach to membranes, facilitating specific functions (e.g., nACh receptors).

Membrane Functions

• Regulate substance entry and exit.
• Detect and respond to chemical signals.
• Enable immune functions through cellular recognition.
• Form tissue through attachment to extracellular matrix.
• Facilitate communication and transport through membrane junctions.

Membrane Junctions

• Tight Junctions: Encircle cells, controlling solute movement and forming barriers.
• Gap Junctions: Allow direct communication between cells, important for synchronized muscle contractions.

Transport Mechanisms

• Solute transport across membranes can be passive or active.
• Passive transport includes simple diffusion, pores, channels, and facilitated diffusion.

Diffusion Equation

• J = PA (Co-Ci), where J is net flux, A is surface area, Co-Ci is concentration difference, and P is permeability coefficient.
• Hydrophilic solutes diffuse slowly, while hydrophobic solutes pass quickly through lipid bilayers.

Solute Permeability

• Lipid-soluble substances (e.g., O2, CO2, fatty acids) have high membrane permeability.
• Large polar (glucose) and charged molecules (amino acids) generally do not diffuse freely across membranes.
• Water's diffusion is limited yet occurs slowly through pores.

Transport for Essential Solutes

• Normal cell functioning depends on transport systems for large polar molecules, charged molecules, and ions.
• Various transport systems enable this across the plasma membrane.

Passive Transport via Membrane Proteins

• Pores: Always open, allowing solutes to pass down their concentration gradient.
• Ion Channels: Facilitate ion transport, typically closed but can open ("gating"), allowing for selective ion passage.

Types of Ion Channels

• Specificity is crucial; only solutes with the right shape and charge can be transported.
• Molecules that share structural similarities may compete for the same transporter.

Carrier Protein Characteristics

• Competition: Similar molecules compete for transport systems (e.g., glucose vs. galactose).
• Saturation: Transport systems can become fully occupied, limiting transport capacity.### Transport Mechanisms Overview
• Transport plateaus occur when all transporters are occupied, reaching maximal transport capacity (Tm).
• Simple diffusion occurs without saturation, whereas carrier-mediated transport saturates with increasing solute concentration.

Types of Transport

• Simple Diffusion:
  • Passive movement of molecules directly through the membrane, not reliant on transport proteins.
• Facilitated Diffusion:
  • Also called "mediated transport"; passive transport via carrier proteins (e.g., glucose, urea).
  • Occurs down concentration gradients, requires no energy, and allows bidirectional transport.
  • Involves specific proteins that transport solutes, distinct from simple diffusion.

Examples of Facilitated Diffusion

• Water molecules utilize aquaporins for facilitated diffusion.
• Ions use ion channels for transport according to electrochemical gradients.

Distinction of Transport Types

• Carrier Proteins:
  • Essential for facilitated diffusion, transporting larger molecules such as glucose.
  • Operate in cycles, which limit the maximum transport rate and allow for saturation.
• Pores and Channels:
  • Pores (e.g., aquaporins) are always open allowing continuous and fast transport.
  • Channels (e.g., K+ channels) open transiently, facilitating rapid but controlled passage of ions.

Transport Rates Comparison

• Pores:
  • Achieve maximum transport rates of approx. 2 x 10^9 particles/sec, the fastest among transport types.
• Channels:
  • Transport rates range between 10^6 and 10^8 particles/sec.
• Carriers:
  • Operate at slower rates of 200 to 50,000 particles/sec.

Key Takeaways

• Concept of Tm emphasizes the limitation of carrier-mediated transport as compared to simple diffusion.
• Saturation of transporters is a critical aspect of understanding facilitated diffusion.
• Understanding the differences in transport mechanisms informs how various substances are moved across cellular membranes.