Membrane Structure and Function Chapter 7 Structure of

Membrane Structure and Function Chapter 7

Structure of membrane • The membrane is constructed of a bi- layer of phospholipids , permeated with transport proteins • Exhibits selective permeability. • Arranged in a “fluid mosaic” pattern.

Phospholipids • The ability of a phospholipid to form a membrane is inherent in the molecular structure. • Phospholipids are amphipathic, having both a hydrophobic and hydrophilic portion on the same molecule. • Arranged in a double layer with the hydrophilic portion facing the cytoplasm/ICF and the hydrophobic portion facing the interior.

• Cholesterol (steroid) is wedged in between the phospholipids serves 2 important functions: ü Restrains movement of phospholipids at relatively warm temperatures. ü Hinder close packing of phospholipids lowering temperature required for the membrane to solidify. TEMPERATURE BUFFER

Proteins • Proteins can be integral (transmembrane) OR peripheral • Membrane proteins are also amphipathic in nature

Protein function There are 6 major functions of membrane proteins: 1. Transport 2. Enzymatic activity 3. Signal Transduction 4. Cell-cell recognition 5. Intercellular joining 6. Attachment to the cytoskeleton and ECM

The Role of Carbohydrates in cell Recognition • Glycoproteins and glycoplipids serve as identification tags for the cells • Sorting of cells into tissue and organs • Basis for the rejection of foreign cells by the immune system • Vary from species to species, among individuals and cell type.

Transport through Membrane The highly ordered membrane allows a steady traffic of movement both into and out at different rates; • Small, uncharged polar and small nonpolar pass freely. • Hydrophilic, large polar and ions move through embedded channel and transport proteins. • Water moves through channel proteins and aquaporins.

Transport Proteins • Carrier proteins hold the molecules and change shape in a way that shuttles the molecules across the membrane. • Channel proteins contain a hydrophilic channel that allow easy passage for these types of molecules. ie. Aquaporins Link (you tube transport)

What Determine Direction of Traffic? The mechanisms that drive molecules across a membrane are determined mainly by concentration gradients. • Diffusion • Osmosis • Facilitated Diffusion

Osmosis • Passive transport of water molecules. • Continues until the concentration of solute equal on both sides of the membrane. • Tonicity – the ability of a solution to cause a cell to gain or lose water based on the solute concentration and the permeability of the membrane.

Water Potential Water potential = solute + pressure • The direction of movemnt of water molecuels is based on the amount of “free” ater avaiable.

Tonicity • Isotonic: equal concentrations of solutes on each side of the membrane. • Hypertonic: high concentration of solutes. • Hypotonic: low concentration of solutes.

Active Transport • Uses ATP energy to transport molecules against a concentration gradient. ie. Sodium-Potassium pump

Cotransport • Cotransport occurs when a single ATP powered pump that transports a specific solute indirectly drives the active transport of several other substances. ie. H+ proton pump in ETC

Bulk Transport • Endocytosis : When the cell takes in macromolecules and particulate matter by forming vesicles. 3 types: 1. Phagocytosis 2. Pinocytosis 3. Receptor mediated endocytosis • Exocytosis: The cell secrets by fusion of vesicles with the plasma membrane