Unit 3 – Membranes & Transport

Selectively Permeable There are barriers separating cells and organelles (cell parts) from world around it Plasma (Cell) Membrane Each cell is a protected compartment Boundary in/outside of cell YET Liquid, changing, dynamic Selectively Permeable Some things can/cannot pass across membrane Plasma Membranes are composed mainly of Phospholipids, Proteins

Chemical Structure of a Plasma Membrane PHOSPHOLIPID Primary structure component Modified Triglyceride Phosphate group (PO4-3) replaces one of the three fatty acids normally found. The addition gives: Polar “head” Two non-polar “tails”

Structure of the Lipid Bi-layer Cell membranes are composed of 2 layers of phospholipids and protein Lipids are main “fabric” of membrane Proteins are “floating” in membrane other amphipathic, organic compounds also floating Amphipathic = Having both hydrophobic & hydrophilic end

Structure of the Lipid Bi-layer

This is the Fluid Mosaic Model of Lipid Bi-layers Model of cell membrane structure; individually inserted proteins drifting laterally in fluid layer of phospholipids Mosaic - Structure made up of many different parts Lipid bi-layer forms because Hydrophobic ends of molecules face inward packed together away from water, Hydrophilic ends face outward toward the water

Helps maintain fluidity Two factors maintain fluidity Fatty Acid Tails If phospholipids were SATURATED, non-polar tails would be… Straight If phospholipids were UNSATURATED, tails would be … Kinked Kinks in tails keep molecules from packing close together Helps maintain fluidity

Fluidity Unsaturated/Polyunsaturated F.A. tails Cholesterol Non-polar Serves as a blocker in the fatty acid tails

What does AMPHIPATHIC mean What does AMPHIPATHIC mean? What are the two factors that affect fluidity? Describe the polarity of things that are Hydrophobic and Hydrophilic? What does it mean to be Selectively Permeable?

WEBPAGE

Fluidity means that Lipids can float around each Fluidity means that Lipids can float around each other Different factors affect this…

Lipids can drift about randomly in the plane of membrane Movement is vital to membrane function Lateral, Rotate Will NOT switch/flip-flop planes Separates cell from the external environment AND regulate flow of materials in and out of cell

Selectively Permeable Membrane has specific properties that allow things to pass through or not pass through - CAN vs. CANNOT Not all molecules are equally soluble in PM Size Polarity/Non-polarity Charged/Uncharged

Selectively Permeable Rules: Small and/or hydrophobic (NP) pass VERY easily Large and/or hydrophilic (polar) NOT easily Permeable to many lipids, lipid-like substances, and gasses Impermeable to such things as proteins and amino acids, nucleic acids, and carbohydrates

Permeability

Fluid Mosaic Model To complete the F.M.M. – need to include the role of PROTEINS– Lipid bi-layer is a great model, but not complete story Need proteins to move Large, Charged, Polar Substances across membrane

Passive Transport High Concentration to Low Concentration With/Down the Concentration Gradient Does not requires energy (ATP)

Types of Passive Transport Diffusion- movement of small, nonpolar, and /or uncharged molecules directly across the membrane. EX: O2, CO2 Osmosis- diffusion of water directly across the membrane Facilitated Diffusion- diffusion of larger, polar, and/or charged molecules across the membrane with the help of a transport protein. EX: Amino Acids, monosaccharides, ions

Passive Transport

Integral Intrinsic Trans-membrane Embedded in membrane COMPLETELY spans membrane Have Hydrophobic and Hydrophilic regions 70-80% of the proteins in membranes Not easily extracted from membrane Most often involved in transport / reception

Peripheral / Extrinsic Proteins not embedded in lipid bi-layer Appendages LOOSELY bound to surface of membrane 20-30% Various roles Enzyme activity, Signal transduction, intercellular joining, cell-cell recognition, ECM attachment

Roles / Functions Transport Channel Proteins – Non-selective Receptor Gated Channel - Selective (Specific Ion/Molecule) Carrier – Carries materials across Receptor Peripheral – Ex. : INSULIN Antigens Communication to immune system that cells belong Membership card If bacteria, virus, other; begin immune defense

Movement Proteins crucial portion of the membrane structure Assist in movement of material through the membrane Provide structure for the membrane

Active Transport Low Concentration to High Concentration Against/Up Concentration Gradient Requires Energy (ATP) Used to keep high concentrations high and/ or low concentrations low (wastes)

Types of Active Transport Active Transport with a transport Protein EX: Ion Pump Endocytosis/ Exocytosis EX: huge molecules like proteins

Active Transport