Membrane Structure and Function Quiz

Membrane Structure and Function

• Organs are specialized structures in the body that perform specific life processes.
• Organelles are specialized structures inside cells, often surrounded by membranes, that perform specific cell processes.
• The plasma membrane (cell membrane) is a dynamic barrier surrounding the cell's cytosol. All cells and organelles are enclosed by a flexible membrane.
• Membrane-bound organelles are critical for healthy life. Malfunction or damage can lead to disease.
• Cystic fibrosis (CF) is a disease caused by a single gene mutation affecting the CFTR protein.
• CFTR is a membrane transport protein that normally moves chloride ions (Cl⁻) out of cells lining the lungs/intestines, creating an electrical gradient. This gradient facilitates sodium ion (Na⁺) movement, drawing water into the mucus. This hydration is essential for proper lung and intestine function
• In CF, the Cl⁻ channel malfunctions, reducing hydration of the mucus, which becomes thick. This thick mucus blocks airways, making breathing difficult, clogs pancreatic ducts, and impairs enzyme delivery, leading to possible pancreatic damage and digestive enzyme deficiency. CF patients often require dietary supplements.
• The fluid mosaic model, proposed by Singer and Nicolson (1972), describes membranes as fluid and mosaic.
  • Fluid implies dynamic movement of membrane components.
  • Mosaic implies a diverse array of molecules within the membrane.
  • The model suggests a membrane of fluid phospholipid bilayer in which proteins are embedded and can move laterally.
• Membranes are not rigid; proteins and lipid molecules are generally free to move laterally within the two layers of the bilayer.

Membrane Composition

• The plasma membrane is a mosaic of various molecules:
  • Phospholipids
  • Membrane proteins (integral and peripheral)
  • Carbohydrates
  • Cholesterol

Phospholipid Bilayer

• Membranes are primarily made of phospholipids. These lipids form a bilayer, two layers of phospholipids.
• Phospholipids have both hydrophobic (water-fearing) tails and hydrophilic (water-loving) heads. This amphipathic nature results in the hydrophobic tails facing inward and forming the hydrophobic core of the membrane. Hydrophilic heads face outwards, interacting with the aqueous environment.
• Lipid Density and Packing: The density and packing of individual lipid molecules depends on the fatty acid composition and temperature.
  • Saturated fatty acids are straight, allowing tighter packing and greater membrane rigidity.
  • Unsaturated fatty acids have double bonds and bends, which result in looser packing and increased fluidity.
  • Temperature lower temperatures tend to reduce fluidity. This is stabilized by Cholesterol.
  • Cholesterol, found in animal membranes, and sterols in plant membranes, stabilize the membrane at a variety of temperatures. It blocks movement at high temperatures and occupies space between lipids at low temperatures.

Membrane Proteins

• Membrane proteins have various functions:
  • Transport: Proteins facilitate movement of substances across the membrane. These can be passive or active.
  • Enzymatic activity: Some proteins are enzymes involved in biochemical pathways.
  • Signal transduction: Proteins act as receptors, binding chemicals to trigger cellular responses.
  • Attachment and recognition: Proteins act as attachment sites to cytoskeletal elements, aiding cell-cell recognition and extracellular matrix interactions. They can also trigger immune responses.
• There are two main types of membrane proteins:
  • Integral membrane proteins: embedded within the lipid bilayer. Most of these are transmembrane proteins.
    • Binding sites: Some proteins have specific regions for binding molecules. Carbohydrate surface groups, known as glycoproteins, identify a cell to other cells (e.g., antigens)
    • Channel proteins: Allow passive movement of small molecules or charged ions across the membrane; some are ungated (always open), and others are gated (open and close).
    • Carrier proteins/pumps: Active transport systems which allow for movement of molecules with conformational changes that involve energy input.
  • Peripheral membrane proteins: located on the surface of the membrane and do not interact with the hydrophobic core; they're bound non-covalently to either membrane surface.
    • Extracellular peripheral proteins: Involved in communication.
    • Intracellular peripheral proteins: Involved in structural support.

Extracellular Matrix (ECM)

• ECM is a mixture of glycoproteins and glycolipids (carbohydrate-containing proteins and lipids) secreted by cells and located outside cells.
• ECM's role includes:
  • Cell structure support
  • Cell anchoring
  • Separating tissues
  • Cell signaling