Homeostasis and transport across cell membrane

Introduction

• Transport mechanisms are vital for delivering essential substances (e.g., nutrients, water, electrolytes) and removing waste materials (e.g., CO2) from tissues.

Basic Mechanisms of Transport

• Two main types:

  1. Passive mechanism

  2. Active mechanism

Passive Transport

• Movement along concentration/electrical gradient without energy (diffusion).

• Types:

  1. Simple diffusion

  2. Facilitated diffusion

Simple Diffusion

• Types:

  1. Through lipid layer (e.g., gases like O2, CO2)

  2. Through protein channels (e.g., electrolytes via selective channels like Na+, K+).

• Protein Channels:

  • Ungated channels: Continuously open

  • Gated channels: Open under specific conditions:

    • Voltage-gated (changes in electrical potential)

    • Ligand-gated (presence of hormones)

    • Mechanically gated (physical forces)

Facilitated or Carrier Mediated Diffusion

• Uses carrier proteins for larger, water-soluble substances (e.g., glucose and amino acids). Faster than simple diffusion.

Factors Affecting Rate of Diffusion

• Directly proportional to:

  1. Permeability of membrane

  2. Body temperature

  3. Concentration/electrical gradient

  4. Solubility of substances

• Inversely proportional to:

  1. Thickness of membrane

  2. Charge of ions

  3. Size of molecules

Special Types of Passive Transport

1. Bulk flow: Movement of large quantities from high to low pressure (e.g., gas exchange in lungs).

2. Filtration: Movement due to hydrostatic pressure (e.g., capillary exchange).

3. Osmosis: Movement of water through a semi-permeable membrane. Types:

   • Endosmosis: Water enters cells

   • Exosmosis: Water exits cells

Osmotic Pressure

• Pressure exerted by solutes in a solution. Creates osmotic pressure when water moves from low to high concentration.

Colloidal Osmotic Pressure and Oncotic Pressure

• Colloidal osmotic pressure is exerted by colloids; oncotic pressure is specifically due to plasma proteins (~25 mm Hg).

Active Transport

• Movement against the gradient (uphill transport) requiring energy (ATP) and carrier proteins.

Mechanism of Active Transport

• Substance binds to carrier protein, forming a complex that transports the substance across the membrane.

Carrier Proteins

• Types:

  1. Uniport: Carries one substance.

  2. Symport: Two substances in the same direction.

  3. Antiport: Two substances in opposite directions.

Substances Transported by Active Transport

• Ionic forms: Na+, K+, Ca2+, H+, Cl-

• Nonionic forms: Glucose, amino acids, urea.

Types of Active Transport

1. Primary Active Transport: Energy from ATP breakdown (e.g., Na+-K+ pump).

2. Secondary Active Transport: Utilizes gradients from primary transport (e.g., co-transport with Na+).

Special Categories of Active Transport

1. Endocytosis: Transport of large molecules into the cell (three types: pinocytosis, phagocytosis, receptor-mediated).

2. Exocytosis: Expulsion of substances from the cell.

3. Transcytosis: Movement of macromolecules across the cell.

Homeostasis

• Maintenance of a constant internal environment necessary for cell survival. Involves recognizing deviations from normal limits and correcting them.

Components of Homeostatic System

1. Detectors/Sensors: Recognize deviations.

2. Control Center: Processes information.

3. Effectors: Act on commands to restore balance.

Systems Involved in Homeostasis

• pH, temperature, nutrient supply, gas exchange, hormone regulation and water/electrolyte balance regulated by various systems (e.g., respiratory, nervous).

• Importance of Blood: Critical to transport nutrients and maintain internal environment.

Mechanism of Action of Homeostatic System

• Feedback mechanisms operate to maintain balance:

  1. Negative Feedback: Stops or reverses changes (e.g., thyroid hormone regulation).

  2. Positive Feedback: Amplifies changes in emergency conditions (e.g., blood clotting, childbirth).