Intro to Membrane Movement

Functions of Cell Membrane (CM)

  • Regulation of Exchange: The cell membrane acts as a selective barrier, controlling what substances enter and exit the cell, ensuring the internal environment remains stable (homeostasis).

  • Communication: The cell membrane facilitates communication between the cell and its environment through receptors that detect signals (hormones, neurotransmitters) and initiate cellular responses.

  • Structure: The cell membrane provides structural support to the cell, maintaining its shape and integrity, and anchoring the cytoskeleton for stability and shape.

  • Protection: It serves as a physical barrier, protecting the internal components of the cell from harmful substances and pathogens.

What is Physiology?

Physiology is the scientific study of the functions and mechanisms in a living system, particularly how organisms, organs, and cells perform their vital processes.

Two Categories:

  1. Mechanical:

    • Focuses on the processes of how the body executes functions.

    • Example: The contraction of the heart muscle to pump blood throughout the body, driven by electrical impulses.

  2. Teleological:

    • Concentrates on the purpose behind each function and how it contributes to overall health and survival.

    • Example: The heart beats to distribute oxygen (O2) to bodily tissues.

Cell Membrane Composition

  • The cell membrane is constructed from a phospholipid bilayer, consisting of:

    • Hydrophobic tails: Made from fatty acids that repel water.

    • Hydrophilic heads: Composed of phosphate groups that attract water, allowing interaction with the aqueous environment.

Permeability of Molecules

Permeability of a molecule through the cell membrane is influenced by several factors:

  • Lipid Solubility: Hydrophobic (lipid-soluble) molecules pass more easily.

  • Size: Smaller molecules diffuse more readily than larger molecules.

  • Charge: Uncharged and nonpolar molecules are more likely to permeate the membrane than charged ions.

Molecules That are Permeable:

  • Small Hydrophobic/Lipophilic Molecules:

    • Examples: Oxygen (O2), Carbon Dioxide (CO2), Nitrogen (N2)

  • Small Uncharged Polar Molecules:

    • Example: Urea

  • Cholesterol-Based Molecules:

    • Example: Steroid hormones (e.g., cortisol).

Molecules That are NOT Permeable:

  • Large Uncharged Polar Molecules:

    • Examples: Amino acids, glucose, nucleotides

  • Ions:

    • Examples: H+ , Cl- , HCO3-

Transport Mechanisms

Movement of molecules across the cell membrane can be classified based on energy requirements:

Active Transport:

  • Definition: Moves substances against their concentration gradient, requiring energy.

    • Primary Active Transport: Directly uses ATP to transport molecules.

    • Secondary Active Transport: Utilizes the electrochemical gradient created by primary active transport for assistance.

  • Role of Membrane Proteins: Integral in mediating the transport of substances across the membrane.

Passive Transport:

  • Definition: Does not require energy and moves substances along their concentration gradient.

    • Simple Diffusion: Molecules pass directly through the lipid bilayer from high to low concentration.

    • Facilitated Diffusion: Involves membrane proteins that assist the movement of substances across the membrane without utilizing ATP.

Specialized Transport Mechanisms:

  • Endocytosis/Exocytosis/Pinocytosis:

    • Use membrane-bound vesicles to transport large particles or volumes of fluid into (endocytosis) or out of (exocytosis) the cell.

Diffusion

  • Energy Requirements: Yes, diffusion requires kinetic energy, but does not require ATP (energy from external sources).

  • Effect of Variables on Rate of Diffusion:

    • Temperature: Increase in temperature boosts diffusion rates.

    • Molecular Size and Weight: Larger molecules diffuse more slowly, while smaller molecules diffuse rapidly.

    • Distance from the Initial Site: As distance increases, diffusion rate decreases.

Stopping Point for Diffusion:

Diffusion continues until there is no concentration gradient (equilibrium is reached).

  • Additional Factors Influencing Diffusion Across Membranes:

    • Surface area and thickness of the membrane play a role as well.

Differences between Carrier and Channel Proteins

Channel Proteins:

  • Open pores that allow water and certain ions to flow through.

  • Composed of transmembrane proteins and can be gated (mechanically or ligand-gated) to regulate passage.

Carrier Proteins:

  • Not open to both the extracellular fluid (ECF) and intracellular fluid (ICF) simultaneously, preventing backflow.

  • Composed of transmembrane proteins and transport molecules more slowly than channel proteins.

    • Note: Carrier proteins are different from ATPase pumps, which actively move ions against their gradients.

GLUT4 Transporters

  • Function: GLUT4 transporters are sensitive to insulin and are predominantly located in muscle and adipose (fat) tissue.

  • Role in Glucose Uptake: They facilitate glucose uptake by allowing glucose to enter cells in response to insulin signaling, playing a crucial role in glucose homeostasis.