Cell membrane

Phospholipids Overview

  • Phospholipids are fundamental components of cell membranes.

    • Contain two parts:

    • Hydrophilic Phosphate Head

      • Attracts water (water-loving).

    • Hydrophobic Fatty Acid Tails

      • Repels water (water-fearing).

Hydrophobic and Hydrophilic Properties

  • Hydrophobic

    • Definition: Does not mix well with water.

  • Hydrophilic

    • Definition: Mixes well with water.

  • Amphipathic Nature of Phospholipids

    • Contain both hydrophobic and hydrophilic properties.

Structural Characteristics of Fatty Acid Tails

  • Fatty acid tails can exhibit bends or kinks due to:

    • Unsaturation: Presence of double bonds between carbon atoms in the fatty acid chain creates a kink.

Membrane Permeability

  • The cell membrane is referred to as partially permeable or selectively permeable.

  • Importance of Selective Permeability

    • Allows cells to control the movement of substances in and out.

    • Maintains homeostasis by ensuring necessary conditions for cellular reactions.

    • Varies depending on time of year or daily cycles, adapting to the cell’s needs.

Molecule Passage Through Membranes

  • Types of Molecules that Can Pass Through:

    • Small Nonpolar Molecules

    • Pass through fatty acid tails easily.

    • Small Polar Molecules

    • Pass through hydrophilic phosphate heads but must be sufficiently small.

  • Key Rule:

    • Permeability:

    • Greater permeability allows more substances to pass through the membrane.

    • Smaller molecules (like water) pass easily; larger molecules (like polysaccharides or proteins) do not.

Fluid Mosaic Model

  • Membrane Structure and Movement

    • Describes membranes as fluid and flexible due to molecular movement.

    • Fluidity: Increased temperature leads to the movement of phospholipids away from each other.

    • Mosaic Nature: The membrane is not a homogenous structure; it contains various embedded proteins.

  • Proteins Within Membranes

    • Membrane Proteins

    • Integral Proteins

      • Pass through the phospholipid bilayer (transmembrane proteins).

    • Peripheral Proteins

      • Located on the outer or inner surfaces of the membrane.

  • Integration of these proteins can indicate cellular activity and metabolic requirements.

Integral vs. Peripheral Proteins

  • Integral Proteins

    • Span the entire lipid bilayer and are also known as transmembrane proteins.

    • Example: Channel proteins; change configuration to facilitate transport.

  • Peripheral Proteins

    • Attach loosely to the membranes and do not penetrate the lipid bilayer.

    • Generally perform functions such as signaling or maintaining the cytoskeletal framework.

Functional Implications of Membrane Composition

  • The composition and type of membrane proteins indicate the cell's activity level.

  • Higher Concentration of Proteins

    • Indicates a more metabolically active cell (e.g., mitochondria and chloroplasts).

Glycoproteins and Glycolipids

  • Glycoproteins

    • Proteins with carbohydrate chains attached, located on the surface of the cell membrane.

    • Functions in cell recognition and signaling.

  • Glycolipids

    • Lipids with carbohydrate chains, also found on the cell surface but without a protein component.

    • Involved in cell-cell interactions and signaling.

  • Both structures are essential for cell adhesion and communication.

  • Orientation

    • Glycoproteins and glycolipids are oriented towards the exterior of the cell, allowing interaction with the external environment.

Drawing the Fluid Mosaic Model

  • Task: Create a two-dimensional representation of the fluid mosaic model.

  • Include:

    • Examples of integral and peripheral proteins.

    • Glycolipid and glycoprotein structures.

    • Indicate hydrophobic and hydrophilic regions.

    • Note that cholesterol is present in animal cells but absent in plant cells.

    • Importance of flexibility in cell shape and function, such as white blood cells engulfing pathogens.

Summary of Learning Goals

  • Understand the structural components of cellular membranes.

  • Differentiate between types of membrane proteins and their functions.

  • Appreciate the importance of structural features in various cellular processes including transport and communication.