Lecture 6: Membranes and Transport

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Today's Lecture: Membranes and Transport

Today's lecture is divided into two parts:

• Part 1: Membrane Structure: Focuses on what membranes are made of, comparing and contrasting their components. Introduces a new outcome: HD (Hypothesize and Diagnose): The Fluid Mosaic Model.
  • HD Outcome Explanation: This outcome asks students to hypothesize the effect of a change to a system. For example, if a cell is treated with a compound that removes all sugars from its membrane, how will this affect membrane function? In future physiology courses, this will extend to diagnostic scenarios in a medical context.
• Part 2: Membrane Transport: Explores processes for getting substances in and out of cells and how these are regulated. Will compare and contrast different types of membrane transport and sequence a specific type of bulk transport.

Part 1: Membrane Structure

We will cover four aspects of membrane structure:

1. Components: Phospholipids, proteins, and carbohydrates.
2. Integrated Model: How these components come together in the fluid mosaic model.

Membrane Components

Phospholipids

• Primary Structural Components: Phospholipids are the main builders of cell membranes.
• Structure Review: Composed of a glycerol molecule (a $3$-carbon backbone), two long fatty acid (hydrocarbon) tails, and one phosphate group (with an unspecified attachment).
• Spontaneous Bilayer Formation: In an aqueous solution, phospholipids spontaneously form a bilayer structure due to their amphipathic nature.
  • Amphipathic: Possessing both hydrophobic (water-fearing) and hydrophilic (water-loving) regions.
  • Orientation: The hydrophilic phosphate heads face outwards, interacting with the aqueous environment via hydrogen bonds. The hydrophobic fatty acid tails face inwards, away from water.
  • Stability: The bilayer is held together strongly by millions of weak, short-lived hydrophobic (Van der Waals) interactions between the hydrocarbon tails. The cumulative effect of these interactions is a robust, stable structure.
• Role: Phospholipids provide the membrane's structural integrity; other components are embedded or associated with this phospholipid foundation.

Proteins

• Functional Role: Membrane proteins perform most of the membrane's functions.
• Location: Can be on the surface of the membrane or span the entire membrane.
  • Transmembrane Proteins: Proteins that extend all the way through the bilayer. Like phospholipids, they must be amphipathic, with hydrophobic regions in the membrane's core and hydrophilic regions exposed to the aqueous environments on either side. They are held in place by Van der Waals interactions within the hydrophobic core.
• Movement (Fluidity): Many membrane proteins are not fixed in place and can move laterally within the membrane's plane.
  • Evidence: Cell hybridization experiments (e.g., mouse and human cell fusion showing mixed proteins over time) and fluorescent tagging demonstrate this movement.
  • Limitations: Proteins generally do not