Membrane Proteins

Membrane Proteins

Plasma Membrane

  • Definition: The plasma membrane is the outer boundary of the cell.

    • Functions:

    • Controls the movement of substances in and out of the cell.

    • Separates the extracellular fluid from the cytoplasm (the material inside the cell).

Selective Permeability

  • The plasma membrane is selectively permeable.

  • Influence of Size and Charge:

    • The rate of diffusion across a membrane is affected by the size and charge of molecules.

Permeability Scale (cm/sec)

  • Phospholipid Bilayer: The primary component of all cell membranes.

    • High Permeability: Hydrophobic molecules (e.g., O extsubscript{2}, CO extsubscript{2}, N extsubscript{2}) have high permeability.

    • Small, Uncharged Polar Molecules (e.g., H extsubscript{2}O, indole, glycerol) have moderate permeability.

    • Large, Uncharged Polar Molecules (e.g., glucose, sucrose) have low permeability.

    • Ions (e.g., Cl extsuperscript{-}, K extsuplus{+}, Na extsuplus{+}) have very low permeability.

Fluid Mosaic Model of the Plasma Membrane

  • Components of the Plasma Membrane:

    • Phospholipids are the primary component.

    • Proteins, cholesterol, and carbohydrates contribute to its fluid mosaic characteristics.

    • Proteins interspersed among lipid molecules resemble tiles in a mosaic (referred to as the Fluid Mosaic Model).

    • Many proteins can move sideways through the bilayer, contributing to the membrane's fluid quality.

Cell Membranes and Extracellular Matrix

  • Adhesion to Extracellular Matrix:

    • Transmembrane proteins, known as integrins, attach cells to the extracellular matrix consisting of fibrous proteins embedded in a gel made of proteoglycans.

    • Example: Dr. Doris Taylor's lab created a beating heart.

Factors Affecting Membrane Fluidity

  • Temperature: Higher temperatures increase membrane fluidity.

  • Phospholipids: The presence of unsaturated phospholipids increases membrane fluidity.

  • Cholesterol: Integration of cholesterol decreases cell permeability.

Types of Membrane Proteins

  • Integral Membrane Proteins: Embedded within the phospholipid bilayer; those spanning the entire bilayer are termed transmembrane proteins.

  • Peripheral Membrane Proteins: Not embedded in the bilayer; associated with the outer or inner surfaces of the membrane.

Carbohydrate Component of Membranes

  • Carbohydrates on the outer surface of membranes serve as recognition sites for other cells and molecules, commonly found as glycolipids and glycoproteins.

Cell Adhesion and Recognition Structures

  • Specialized structures for cell adhesion called cell junctions are formed from proteins. Types of cell junctions include:

    • Tight Junctions:

    • Function: Help provide a tight seal between cells, preventing the movement of substances.

    • Example: Observed in the Blood-Brain Barrier and kidneys.

    • Desmosomes:

    • Function: Act like "spot welds" allowing some movement between cells.

    • Example: Found in cardiac muscle and bladder tissue.

    • Gap Junctions:

    • Function: Allow communication between cells through passage of chemicals or ions.

    • Example: Found in neurons, these junctions are channels between adjacent cells formed by proteins called connexons, which come together to form a channel roughly 1.5 nm wide—too small for proteins but suitable for signaling molecules.

Movement Across the Plasma Membrane

  • Molecules typically move down their concentration gradient from high to low concentration.

  • Energy Requirement: Moving a molecule against its concentration gradient requires energy.

    • Cellular Energy Currency: ATP (Adenosine Triphosphate) is used as the energy currency of the cell and is produced in cellular respiration.

Types of Transport Mechanisms

  • Active Transport Proteins:

    • Uniports: Move one item through the membrane.

    • Symports: Move two items in the same direction.

    • Antiports: Move two items in opposite directions.

  • Types of Active Transport:

    • Primary Active Transport: Involves direct participation of ATP.

    • Secondary Active Transport: Derived from the ion concentration gradient established by primary active transport.

Sodium-Potassium Pump

  • A specific example of primary active transport in animal cells.

    • Mechanism: Pumps Na extsuperscript{+} out of the cell and K extsuperscript{+} into the cell against their gradients.

    • Function:

    1. Cytoplasmic Na extsuperscript{+} binds to the sodium-potassium pump.

    2. Na extsuperscript{+} binding induces phosphorylation by ATP.

    3. Extracellular K extsuperscript{+} binds, triggering the release of the phosphate group.

    4. Loss of the phosphate restores the protein's original conformation.

    5. K extsuperscript{+} is released, and the cycle can repeat.

Energy Transformation in Membrane Proteins

  • Membrane proteins can facilitate energy transformation, as seen in photosynthesis.

    • Energy Source: Energy-rich pigments interact with reactions within the cell, contributing to mechanisms like ATP production through photosynthetic pathways.

Organization of Chemical Reactions

  • Membrane proteins can organize chemical reactions by providing favorable conditions for substrates to interact and react.

Membrane Proteins in Information Processing

  • Hormones: Bind to membrane proteins to initiate or inhibit specific chemical processes within the cell, illustrating a key role in cellular signaling and information processing.

Membrane Functions Related to Disease

  • Cholera Toxin:

    • Mechanism: One subunit binds to a cell surface receptor and prompts structural changes allowing another subunit entry into the cell. This subunit modifies a peripheral protein, leading to the opening of chloride channels in the membrane.

    • Outcome: Accumulation of Cl extsuperscript{-} and Na extsuperscript{+} in the intestines, resulting in osmotic loss of water and subsequent dehydration.

Cystic Fibrosis and CFTR

  • Cystic Fibrosis: A genetic disorder primarily caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which affects ion transport across epithelium and leads to thick, viscous secretions.

    • Resources: https://www.youtube.com/watch?v=_j99-xgOIaw

Techniques for Studying Membrane Proteins

  • Detergent Treatment: Membrane proteins can be extracted by applying detergents that incorporate into the membrane and disrupt the bilayer.

    • Mechanism: The hydrophobic tails of detergents interact with phospholipids and hydrophobic protein regions to form complexes.

  • Other Techniques Include:

    • Breaking cells open for examination.

    • Studying nucleic acids, vitamin absorption, lipid solubility, and absorption of fats facilitated by bile salts from the liver.