BIO feb 10

Fluid Mosaic Model of the Cell Membrane

• The fluid mosaic model describes the structure of the cell membrane.

  • The membrane is not a rigid structure; rather, it is flexible and can move, similar to a fluid.

  • The term "mosaic" refers to the arrangement of various proteins embedded in or associated with the phospholipid bilayer, making it resemble a mosaic.

Structure of the Cell Membrane

• Four major components of the cell membrane:

  • Phospholipid bilayer: Fundamental structure, consisting of a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails."

    • Heads face outward toward the water inside and outside the cell, while tails face inward, away from the water. This orientation forms a bilayer.

  • Proteins: Embedded in the bilayer, these can assist in transporting molecules across the membrane and can act as receptors for signaling.

  • Cholesterol: Helps stabilize the membrane's fluidity, preventing it from becoming too rigid or too fluid under varying temperatures.

  • Carbohydrates: Often attached to proteins or lipids, these molecules play key roles in cell recognition and communication.

The Movement of Substances Across the Cell Membrane

• Passive Transport: Movement of substances across the membrane without energy input.

  • Diffusion: Movement from an area of high concentration to low concentration until equilibrium is reached.

    • Example: If many students (molecules) are packed in one corner of a room (high concentration), they'll spread out evenly when given space (low concentration).

  • Osmosis: A specific type of diffusion focused on the movement of water across a semipermeable membrane.

    • In isotonic solutions, water moves in and out equally, maintaining cell shape.

    • In hypertonic solutions, water moves out, leading to cell shrinkage (e.g., an egg in vinegar).

    • In hypotonic solutions, water moves in, potentially causing swelling and bursting of the cell.

• Active Transport: Movement against the concentration gradient, requiring energy (ATP).

  • Sodium-Potassium Pump: An example of active transport, which pumps sodium ions out of the cell and potassium ions into the cell against their gradients.

  • Important for maintaining cellular homeostasis; utilizes energy to reset the system.

Summary of Transport Mechanisms

1. Simple Diffusion: Small, nonpolar molecules pass through the phospholipid bilayer freely without energy.

2. Facilitated Diffusion: Larger or polar molecules use protein channels (carrier proteins) for transport without energy.

3. Active Transport: Requires energy to move substances against their concentration gradient; involves specialized pumps or channels.

Practical Examples and Analogies

• Imagine students in a classroom; if the door opens, they spread out (diffusion).

• The concentration gradient affects how fast substances move: more people (molecules) result in quicker movement.

• Active transport can be compared to swimming upstream in a river – it requires energy and effort, unlike passive diffusion.