4. Yr 11 Biology - Plasma Membrane Structure (RAN)

The Plasma Membrane

Year 11 Biology Unit 1 AOS1

Learning Intention

  • To understand the structure and function of the plasma membrane

Success Criteria

  • Explain why the plasma membrane is referred to as a fluid mosaic.

  • Describe the function of the plasma membrane.

  • Identify the parts of the plasma membrane and their function.

  • Explain the hydrophilic and hydrophobic components of the plasma membrane and how they affect movement across the membrane.

Overview of the Plasma Membrane

  • Cells exist in a watery environment of extracellular fluid (ECF).

  • The plasma membrane regulates the movement of substances between the ECF and intracellular fluid (ICF).

  • Intracellular Fluid: Fluid inside the cell.

  • Extracellular Fluid: Fluid outside the cell.

Extracellular Fluid in Unicellular Organisms

  • Unicellular organisms are largely influenced by their environment.

  • May die if environmental conditions change significantly.

    • Example 1: Yeast can become dormant until conditions return to normal.

    • Example 2: Unicellular algae can slowly move towards light.

    • Example 3: Some bacteria can move towards nutrients or away from toxic substances.

Extracellular Fluid in Multicellular Organisms

  • Multicellular organisms have more control over their internal environment.

  • They possess outer layers that act as protective barriers.

  • Can regulate the internal environment precisely for optimal cell function:

    • Temperature

    • Oxygen concentration

    • Carbon dioxide concentration

    • pH levels

    • Osmotic pressure (concentration of salts or ions)

    • Nitrogen waste concentration

    • Glucose concentration

Characteristics of the Plasma Membrane

  • All cells are encased in a plasma membrane.

  • Composed of lipids, creating a thin boundary that separates ICF and ECF.

    • Lipids: Organic compounds not soluble in water, e.g., fats and oils.

Structure of the Plasma Membrane

  • Fluid Mosaic Model:

    • The plasma membrane is a mosaic of diverse molecules and is referred to as fluid because the phospholipids and proteins are constantly moving.

  • Phospholipid Bilayer:

    • Consists of two layers of phospholipids that separate ICF from ECF.

  • Functions:

    • Controls the entry and exit of substances.

    • Communicates with other cells.

    • Cell recognition.

  • Selectively Permeable:

    • Only specific molecules can enter and exit the cell, allowing specialized internal environments.

Phospholipid Structure

  • Phospholipid Molecule:

    • Consists of:

      • Hydrophilic Phosphate Heads: Face the aqueous environments (ICF & ECF).

      • Hydrophobic Fatty Acid Tails: Face inward, away from water.

  • Amphipathic Nature:

    • Due to their dual properties, they stabilize the membrane.

Cholesterol in the Plasma Membrane

  • Situated between phospholipids in animal cell membranes.

  • Functions:

    • Reduces permeability to water-soluble molecules.

    • Regulates membrane fluidity:

      • At higher temperatures, cholesterol stabilizes phospholipids.

      • At lower temperatures, it prevents solidification.

Membrane Proteins

  • Integral Proteins / Transmembrane Proteins:

    • Embedded in the bilayer.

    • Includes:

      • Channel Proteins: Transport substances passively, highly selective.

      • Carrier Proteins: Transport substances actively/passively, slower due to shape change.

  • Peripheral Proteins:

    • Temporary part of the membrane, located outside the bilayer.

  • Aquaporins:

    • Specialized channel proteins facilitating rapid water transport.

Roles of Plasma Membrane

  • Structure: Defines cell boundaries and provides shape.

  • Protection: Acts as a barrier from surroundings.

  • Selective Permeability: Regulates entry/exit of substances.

  • Cell Signaling: Communicates with other cells.

  • Cell Identity: Glycoproteins act as markers to identify cells.

Summary of Membrane Structure

  • Double layer of phospholipids with hydrophobic tails inward and hydrophilic heads outward.

  • The membrane is fluid with proteins that can embed or surface and is stabilized by cholesterol.

  • Glycoproteins and glycolipids play crucial roles in cell recognition and attachment.

Phospholipids consist of:

  • Phosphate Head: Made of a glycerol and phosphate group, negatively charged, hydrophilic, and polar.

  • Fatty Acid Tails: Made of long chains of carbon and hydrogen, uncharged, hydrophobic, and nonpolar.

Due to their dual characteristics, phospholipids are amphipathic molecules. This nature contributes to the stability of the plasma membrane, as the hydrophobic fatty acids are repelled by water while the hydrophilic phosphate heads are attracted to it, leading to the formation of a bilayer.