Membrane Structure and Function Chapter 5, pt 1

Plasma Membrane - the Cell Boundary Controls what goes in and out of the cell Semipermeable - allows some things to cross more easily than others

Membrane Structure Phospholipids – main lipid in membranes Amphipathic molecule – has a hydrophilic region & a hydrophobic region

Membranes are fluid Held together by hydrophobic interactions Most lipids and some proteins can move laterally Mosaic-Model Mosaic-Model

Evidence for drifting of membrane proteins Mouse cell Membrane proteins Human cell Hybrid cell Mixed proteins after 1 hour RESULTS

Fluid Unsaturated hydrocarbon tails Viscous Saturated hydrocarbon tails (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane

Fluid Mosaic model of membrane structure

What’s part of the membrane Phopholipids Cholesterol Proteins: – Integral proteins – embedded in lipid bilayer – Peripheral proteins – not embedded Carbohydrate chains on proteins or lipids – found on outside face of membrane – Oligosaccharides (15 or fewer sugars)

Integral protein Hydrophobic region is in the membrane Hydrophilic regions are exposed on either side of membrane

Enzymes Signaling molecule Receptor Signal transduction Glyco- protein ATP (a) Transport (b) Enzymatic activity (c) Signal transduction (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Functions of Proteins inMembrane

Traffic across membranes What needs to go in and out of a cell?

Membrane structure results in selective permeability Small hydrophobic molecules – easily go through plasma membrane Ions, polar molecules do not – (water, glucose) Transport proteins aid in moving these molecules across membrane

Passive Transport -Energy is not required -Substance moves down concentration gradient

Osmosis Passive transport of water Hypertonic solution – more solute Hypotonic solution – less solute Isotonic solution – same amt of solute

Osmosis animation hill.com/sites/ /student_view0/ch apter38/animation_-_osmosis.html hill.com/sites/ /student_view0/ch apter38/animation_-_osmosis.html - What is plasmolysis? - What is turgor pressure?

Facilitated Diffusion Transport proteins help move polar molecules or ions across membrane Channels – corridor for specific molecules or ions Gated channels – open with a stimulus - physical, electrical, chemical Many change shape (conformation) in process

Facilitated diffusion hill.com/sites/ /student_view0/ch apter2/animation__how_facilitated_diffusion _works.html hill.com/sites/ /student_view0/ch apter2/animation__how_facilitated_diffusion _works.html

Aquaporins Water channel proteins (2003 Nobel prize – co winner Pete Agre for work on aquaporins) Move massive amounts of water molecules across the membrane – 3 billion per second Water molecules move through in a single file

Active Transport Requires energy (in form of ATP) Moves substances against the concentration gradient From low concentration to high concentration

Sodium Potassium Pump hill.com/sites/ /student_view0/ch apter38/sodium- potassium_exchange_pump.html hill.com/sites/ /student_view0/ch apter38/sodium- potassium_exchange_pump.html

Voltage across membranes Some ion pumps generate voltage across membranes – difference in charge across membrane An ion diffuses down electrochemical gradient

Cotransport

Endocytosis, Exocytosis Transport of large molecules Animation: Zc Zc Amoeba eating paramecium: 9gk 9gk

Types of Endocytosis

Receptor mediated endocytosis Coated pits have receptors for specific molecule Allow cells to get large amts of specific materials that may be in lower concentrations in the environment Humans – cholesterol travels in LDLs – attach to receptors in pits LDLs act as ligands – bind to LDL receptors in cell membrane If LDL receptors are defective, leads to hypercholesterolemia (build up of LDL and cholesterol in the blood)

Receptor mediated endocytosis – LDL animation or-Mediated-Endocytosis-of-LDL-Cholesterol or-Mediated-Endocytosis-of-LDL-Cholesterol

Practice problems – use the terms hypertonic, hypotonic, isotonic as appropriate If you soak your hands in dishwater, your hands absorb the water and swell into wrinkles. This is because your skin cells are to the dishwater. Why does lettuce wilt in a vinaigrette salad dressing? Why is seawater dangerous to drink? Why will a marine fish die in freshwater?