Membrane Structure and Osmosis

Learning Objectives

• By the end of this lecture, students should be able to:

  • Describe the structure of the plasma membrane and its functions for the cell.

  • Explain the fluid mosaic model of the plasma membrane.

  • Identify components embedded in or attached to the membrane.

  • Describe how membrane fluidity is regulated.

  • Understand the impacts of temperature on membrane fluidity and structure.

  • Explain the roles and types of membrane proteins.

  • Define diffusion and osmosis, including tonicity, and the effects on cells in different environments.

Plasma Membrane Structure

• Phospholipid Bilayer:

  • Composed of phospholipids, which make the membrane selectively permeable (allows certain substances to pass).

  • Plasma membrane: main barrier surrounding the cell (lipid bilayer)

• Functions of Plasma Membrane:

  1. Selective barrier for substances (like oxygen, nutrients, waste).

  2. Separation of the cell from its environment.

  3. Interface for cellular communication with the environment.

Fluid Mosaic Model

• Describes the plasma membrane as a fluid combination of:

  • Phospholipids

  • Proteins

  • Other molecules

• Allows for mobility within the membrane, leading to dynamic interactions among its components.

Membrane Components

• Embedded in the Lipid Bilayer (inside of the membrane):

  • Integral proteins

  • Cholesterol

  • Phospholipids

• Attached to the Lipid Bilayer (outside of the membrane):

  • Peripheral proteins

  • Carbohydrates

Membrane Fluidity

• Influences:

  • Type of phospholipids

  • Amount of embedded cholesterol

  • Cold environments: More unsaturated fatty acids increase fluidity to avoid becoming too solid (no transport)

  • Hot environments: More saturated fatty acids be have less fluidity, preventing disintegration to avoid falling apart

Membrane Proteins

• Types of Membrane Proteins:

  • Integral Membrane Proteins: Majority are transmembrane proteins which span the entire bilayer

    • Characteristics: Amphipathic (hydrophobic & hydrophilic)

  • Peripheral Membrane Proteins: Loosely bound to the membrane surface

    • Characteristics: Hydrophilic

• Functions of Membrane Proteins:

  1. Transport: Move molecules across the membrane.

  2. Enzymatic Activity: Speed up chemical reactions

  3. Signal Transduction: Receive and internalize signals

  4. Cell-Cell Recognition: Identify cells to each other.

  5. Intercellular Joining: Link cells together.

Diffusion and Osmosis

• Diffusion (P): movement of molecules from high concentration to low concentration region

  • Passive movement from high to low concentration.

  • Does not require energy.

  • Concentration gradient: change in solute concentration across regions.

• Osmosis (P):

  • Passive movement of water [no good]

  • Movement of water across selective permeable membrane [better definition]

  • Water move towards side with more solute (higher solute concentration

Tonicity

• Definitions:

  • Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

  • Hypertonic: Higher solute concentration outside the cell; cell loses water and shrivels.

  • Hypotonic: Lower solute concentration outside the cell; cell gains water and may burst.

• Impact on Cells:

  • Isotonic: Cell size remains stable.

  • Hypertonic: Cell shrivels (crenation in animal cells, plasmolysis in plant cells).

  • Hypotonic: Cell swells (lysed in animal cells, turgid in plant cells).

Osmoregulation

• Cells controls solute concentration and water balance

• The process of maintaining solute concentrations and water balance in cells.

• Essential for survival in varying tonicity environments (hypertonic vs. hypotonic).

• Cells have adaptations for controlling osmosis and fluid balance.

Vocabulary

• Isotonic: Equal solute concentrations.

• Osmoregulation: Regulation of water balance.

• Selectively Permeable: Membrane allows specific substances to pass.

• Cholesterol: Maintains fluidity in the membrane.

• Transmembrane Proteins: Span the membrane.

• Integral Proteins: Permanently attached to the membrane.

• Peripheral Proteins: Temporarily attached to the membrane surface.

• Diffusion: Movement from high to low concentration.

• Concentration Gradient: Difference in solute concentration.

• Osmosis: Movement of water across a membrane.

• Tonicity: Effect of solution concentration on cell volume.

• Solvent/Solute: The substance that does the dissolving/the substance that is dissolved.

• Hypotonic/Hypertonic: Solutions with lower/higher solute concentrations than the cell.