week 4: Cell membranes

Structure of Biological Membranes

Overview

• Biological membranes surround cells and organelles.

• Generalized structure consists of three major components:

  • Lipids

  • Proteins

  • Carbohydrates

Lipids

• Phospholipid Bilayer:

  • Core structure of biological membranes.

  • Embedded with proteins and other lipids.

  • Serves as a selectively permeable barrier.

• Cholesterol and Steroids:

  • Found within the lipid portion, helping maintain membrane stability.

Proteins

• Located within or attached to the lipid bilayer.

• Functions of membrane proteins include:

  • Transport: Move molecules and particles across the membrane.

  • Enzymatic Activity: Catalyze reactions.

  • Adhesion and Recognition: Essential for cell tissue formation and maintenance.

Carbohydrates

• Typically attached to lipids or proteins on the membrane's outer surface.

• Major function is cell identification and signaling.

Fluid Mosaic Model

• Most widely accepted structural model of biological membranes.

• Phospholipid bilayer acts as a sea of lipids where proteins float (like icebergs).

• This structure allows for fluidity and shape changes in the cell membrane.

Membrane Permeability

• Membranes are selectively permeable:

  • Some solutes cross freely, others require assistance, and some cannot cross.

• Factors affecting membrane permeability:

  • Temperature: Higher temperatures increase permeability due to increased kinetic energy.

  • Type of Lipids:

    • Saturated phospholipids create tighter arrangements, decreasing permeability and fluidity.

    • Unsaturated phospholipids (with kinks) increase fluidity and permeability.

  • Cholesterol:

    • Higher cholesterol levels decrease permeability and fluidity by restricting movement of phospholipid tails.

Cell Adhesion and Recognition

• Cells form tissues through recognition and adhesion processes.

• Example: Intestinal cells have finger-like extensions for increased surface area.

• Types of Cell Junctions in Animal Cells:

  • Tight Junctions: Seal cells to limit passage of molecules, blocking protein migration.

  • Desmosomes: Localized patches for strong adhesion, common in skin tissue for structural integrity.

  • Gap Junctions: Allow intercellular communication by enabling the passage of ions and small molecules between connected cells (e.g., cardiac muscle cells allow synchronized contraction).