Membrane Transport 1

Cellular Biology & Homeostasis

• Presentation by: Clara Camargo, DVM, Cert AqV

• Theme: Membrane Transport in Cellular Biology

Learning Objectives

1. **Characteristics affecting membrane permeability:

   • Factors include polarity and size of molecules.**

2. Cell Membrane Potential:

   • Understanding what defines cell membrane potential and the influencing factors.

3. Membrane Proteins:

   • Recognizing the various types of membrane proteins and their arrangements (topology).

4. Transport Proteins:

   • Providing examples of common membrane transport proteins.

5. Channel vs Transport Proteins:

   • Identifying key differences between channel proteins and transport (carrier) proteins.

6. Functions of Channel Proteins:

   • Defining channel proteins and explaining their various functions, such as gated, leak, and hormone-dependent channels.

Cystinuria in Dogs

• Definition:

  • Presence of cystine crystals in urine; can lead to urinary obstruction, bladder and kidney stones.

• Genetic Mutations:

  • Three types related to genetic mutations affecting the transport proteins responsible for reabsorbing cystine, particularly in the kidneys and intestines.

• Crystallization Factors:

  • Cystine crystals form in acidic urine due to low solubility in acidic conditions.

Cellular Organization

• Membrane Architecture:

  • Structure includes a bilayer membrane with embedded membrane proteins that regulate the movement of substances in and out of the cell.

• Fluid Compartments:

  • The extracellular fluid surrounds the cell's outer layer and interacts with intracellular contents through membrane proteins.

Membrane Transport

Membrane Permeability and Transport

• Hydrophobic Nature of Lipid Bilayer:

  • Restricts passage of polar molecules due to hydrophobic interior.

• Cytosol and Cytoplasm:

  • Liquid matrix surrounding organelles and all materials inside a cell except the nucleus.

• Homeostasis:

  • Maintaining concentrations of solutes in the cytosol different from extracellular fluids.

Transport Mechanisms

• Transport Proteins:

  • Comprise 15-30% of all membrane proteins; facilitate selective molecule passage.

• Active vs Passive Transport:

  • Passive transport occurs via diffusion (high to low concentration); active transport moves substances against their concentration gradient and requires energy.

Ion Concentrations

Comparison of Ion Concentrations:

• Intracellular vs Extracellular Concentrations:

  • Sodium (Na+), Potassium (K+), Calcium (Ca2+), and Magnesium (Mg2+).

  • Example: Na+ (5-15 mM intracellular vs. 145 mM extracellular).

Membrane Potential

• Definition:

  • Electrical potential difference across a membrane due to ion distribution; influences ion movement in and out of cells.

• Resting Membrane Potential:

  • Maintained by selective permeability and sodium-potassium pump activity, typically ranging from -70 to -90 mV.

Electrochemical Gradient

• Importance in Transport:

  • Affects the direction and rate of ion or molecule transport across membranes crucial for processes like nutrient uptake and signal transmission.

Membrane Transport Proteins - Topology

Structural Features

• Types of topology include:

1. Single alpha helix

2. Multiple alpha helices

3. Rolled-up beta sheet (beta barrel)

4. One-layer attachment

5. Covalent lipid binding

6. Oligosaccharide attachment

7. Protein-protein attachments

Transporter Classes

• Common Features:

  • Typically consist of transmembrane alpha helices, substrate binding sites, and can exist in inward-open or outward-open conformations for molecule transfer.

Examples of Transport Proteins

• Sodium/Glucose Cotransporter (SGLT): Facilitates glucose absorption in the intestines.

• Na+/Ca2+ Exchanger (NCX):

  • Bidirectional transporter crucial for calcium removal, operates on electrochemical gradients.

• NCKX2:

  • Involved in neuronal function and synaptic plasticity.

Membrane Channels vs Transporters

• Transporters (Carriers):

  • Undergo conformational changes to transfer substrates across membranes, can be passive or active.

• Channels:

  • Create pores for solute transport (ions, water); typically facilitate passive transport.

Leak Channels

• Structure & Functionality:

  • Allow selective ion passage based on size and charge; crucial for maintaining membrane potential.

• Potassium (K+) Channels:

  • K+ flows down concentration gradient. Interaction with hydration shells essential for selective filtration.

Aquaporins

• Specific Water Channels:

  • Facilitate osmotic water flow; expressed in high concentrations in kidneys and other secretory tissues.

• Hormonal Response:

  • Hormone-responsive aquaporins play roles in water reabsorption processes, notably during water deficits.

Supplemental Resources

• Channel-Mediated Diffusion:

  • Educational videos available for concepts related to cell transport mechanisms.

Contact Information

• Ross University School of Veterinary Medicine

• Contact: ccamargo@rossvet.edu.kn