3.4 The Cell Membrane

Cell membranes are like the skin of our cells, keeping the good stuff in and the bad stuff out. They're made of a special mix of fats and proteins that work together to control what goes in and out of the cell.

These membranes aren't just barriers, though. They're packed with important molecules that help cells talk to each other and respond to their environment. Understanding how cell membranes work is key to grasping how our bodies function at the smallest level.

Cell Membrane Structure and Function

Fluid mosaic model of membranes

Top images from around the web for Fluid mosaic model of membranes

• 

  Cell biology/Membrane Transport: Permeases and Channels - Wikiversity View original

  Is this image relevant?

• 

  Cell Membranes and the Fluid Mosaic Model | Boundless Anatomy and Physiology View original

  Is this image relevant?

• 

  Structure of the Membrane | Biology for Majors I View original

  Is this image relevant?

• 

  Cell biology/Membrane Transport: Permeases and Channels - Wikiversity View original

  Is this image relevant?

• 

  Cell Membranes and the Fluid Mosaic Model | Boundless Anatomy and Physiology View original

  Is this image relevant?

1 of 3

Top images from around the web for Fluid mosaic model of membranes

• 

  Cell biology/Membrane Transport: Permeases and Channels - Wikiversity View original

  Is this image relevant?

• 

  Cell Membranes and the Fluid Mosaic Model | Boundless Anatomy and Physiology View original

  Is this image relevant?

• 

  Structure of the Membrane | Biology for Majors I View original

  Is this image relevant?

• 

  Cell biology/Membrane Transport: Permeases and Channels - Wikiversity View original

  Is this image relevant?

• 

  Cell Membranes and the Fluid Mosaic Model | Boundless Anatomy and Physiology View original

  Is this image relevant?

1 of 3

• Describes the structure and organization of the cell membrane as a fluid, dynamic structure composed of a phospholipid bilayer with embedded proteins and other molecules
• Phospholipid bilayer forms the basic structure of the membrane
  • Phospholipids have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails that arrange themselves with heads facing the aqueous environment on both sides and tails facing each other in the interior
  • This arrangement makes phospholipids amphipathic molecules, contributing to the membrane's unique properties
• Proteins and other molecules are embedded in or attached to the phospholipid bilayer and can move laterally within the membrane, contributing to its fluid nature
• Arrangement of phospholipids, proteins, and other molecules in the membrane resembles a mosaic pattern (e.g., integral proteins, peripheral proteins, glycoproteins, glycolipids)

Components of membrane structure

• Phospholipids form the basic structure of the cell membrane (phospholipid bilayer) and act as a selectively permeable barrier, allowing some substances to pass through while restricting others, maintaining the integrity and fluidity of the membrane
• Proteins play crucial roles in cell communication, transport, and recognition
  • Integral proteins are embedded within the phospholipid bilayer and can function as channels, transporters (e.g., ion channels, glucose transporters), receptors, or enzymes
  • Peripheral proteins are attached to the surface of the membrane or to integral proteins and can function as enzymes, structural components, or in cell signaling pathways (e.g., G proteins, cytoskeletal proteins)
• Carbohydrates are found attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular side of the membrane, forming the glycocalyx, a carbohydrate-rich layer that plays a role in cell recognition, cell-cell adhesion, and protection against mechanical and chemical stresses

Roles of cholesterol and receptors

• Cholesterol is found in the phospholipid bilayer of animal cell membranes and helps to regulate membrane fluidity and stability
  1. At higher temperatures, cholesterol reduces fluidity by interacting with phospholipid tails, preventing them from moving freely
  2. At lower temperatures, cholesterol prevents the membrane from becoming too rigid by interfering with the close packing of phospholipids
  3. Contributes to the formation of lipid rafts, which are specialized membrane microdomains enriched in cholesterol and specific proteins (e.g., caveolae, signaling complexes)
• Receptors are membrane proteins that bind to specific ligands (e.g., hormones, neurotransmitters, growth factors) and trigger a cellular response, such as a change in the cell's metabolism or gene expression
  • Play a crucial role in cell signaling and communication by allowing cells to detect and respond to changes in their environment and enabling cells to communicate with each other and coordinate their activities within tissues and organs (e.g., insulin receptors, acetylcholine receptors, epidermal growth factor receptors)

Membrane Transport Mechanisms

• Passive transport: Movement of molecules across the membrane without energy expenditure
  • Diffusion: Random movement of molecules from an area of high concentration to an area of low concentration
  • Osmosis: Diffusion of water molecules across a selectively permeable membrane
• Active transport: Movement of molecules against their concentration gradient, requiring energy (usually in the form of ATP)
• Bulk transport:
  • Endocytosis: Process by which cells internalize materials from the external environment by engulfing them in membrane-bound vesicles
  • Exocytosis: Process by which cells release materials to the external environment through fusion of vesicles with the cell membrane

Membrane Potential

• The difference in electrical charge across the cell membrane, created by the unequal distribution of ions
• Plays a crucial role in various cellular processes, including nerve impulse transmission and muscle contraction