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.