Cell Membrane Function and Structure

The Working Cell: Membrane Structure and Function

Lecture Objectives

  • Understand the structure of the cell membrane

  • Examine membrane functions

  • Explore membrane movement mechanisms

Membrane Structure

  • Phospholipids:

    • Primary component of cell membranes.

    • Structure consists of:

      • Phosphate head: Hydrophilic (water-attracting)

      • Fatty acid tails: Hydrophobic (water-repelling)

    • Analogy: Phospholipid bilayer can be thought of as a sandwich, with heads as the bread and tails as the filling.

Fluid Mosaic Model

  • Describes the arrangement and movement within the cell membrane:

    • Cholesterol:

      • Present within the lipid bilayer, helps to maintain membrane fluidity; essential for functionality.

Membrane Components

  • Integral Membrane Proteins: Span the entire membrane bilayer, involved in various functions such as transport and signaling.

  • Peripheral Membrane Proteins: Located on the inner or outer surface of the membrane, associated with the cytoskeleton or extracellular matrix (ECM).

  • Glycoproteins and Glycolipids:

    • Play roles in cell recognition and signaling.

Functions of the Cell Membrane

  • Semi-permeable Barrier: Allows specific molecules to pass while blocking others.

  • Transport Mechanisms: Enables molecules to enter and exit the cell (includes passive and active transport).

  • Enzymatic Activity:

    • Catalyzes reactions affecting molecule structure and configuration.

  • Cell Adhesion:

    • Proteins can connect to cytoskeleton and ECM fibers, enabling structural integrity and communication between cells.

  • Cell Recognition:

    • Tolerance mechanisms allow cells to identify self from non-self using glycoproteins.

Cell Signaling and Membrane Attachment

  • Membrane plays a crucial role in cellular communication through signaling pathways initiated by receptor binding.

  • Attachment to the cytoskeleton supports shape and stability while allowing intracellular transport and movement.

Passive Transport Mechanisms

  • Cellular Diffusion:

    • Movement of molecules from high to low concentration without energy input (e.g., O2, CO2).

  • Facilitated Diffusion:

    • Similar to diffusion but requires protein channels or carriers for transport without energy use.

  • Osmosis:

    • Specific to water movement across a semipermeable membrane, influenced by solute concentrations.

Types of Solutions in Osmosis

  • Hypotonic Solution:

    • Lower solute concentration relative to the cell; can cause cell swelling or lysis.

  • Isotonic Solution:

    • Equal solute concentration; no net movement of water; cells remain normal.

  • Hypertonic Solution:

    • Higher solute concentration outside the cell; can lead to cell shrinkage (crenation).

Summary of Solution Effects

  • Animal Cell:

    • Normal (Isotonic)

    • Flaccid (Hypotonic)

    • Shriveled (Hypertonic)

  • Plant Cell:

    • Turgid (Normal in Hypotonic)

    • Flaccid (Isotonic)

    • Shriveled (Hypertonic)

Active Transport

  • Definition:

    • Movement of molecules from low to high concentration against the concentration gradient.

    • Energy Requirement: Uses ATP or other energy sources to function.