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Lecture 1 - Cells, Membranes & Proteins

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University of Toronto St. George
Jane Mitchell

Lecture 1 – Cells, Membranes & Proteins Cells: Extracellular Matrix Animal Cells Provides structural support to cell, secreted by cells Lysosome Animal Cells Digests damaged organelles, or things taken from the environment Cell Wall Plant Cells Provides support Some single celled orgs Vacuole Plant Cells Storage, and provides rigidity (Filled with fluid), Some singled celled orgs enzymatic Chloroplasts Plant Cells Site of photosynthesis Membranes: Processes @ Membranes:  Separate things into compartments  Scaffold for chemical reactions  Barrier to prevent nutrients from escaping or entering (Selectively permeable)  Proteins that transport nutrients across membranes  Receptors in plasma membranes allow cells to respond to external signals  Interactions between cells @ plasma membranes – leads to tissue coordination Properties of Cell Membranes  Fluid structure  2 leaflets (layers)  Proteins are in the bi-layer, protein move within bi-layer  Fluid mosaic Model – the exact composition of the bi-layer at one place at 2 moments is never the same; things always change, so there’s no pattern. Composition of Membrane  Amphiphilic Lipids – One side is hydrophilic, one side is hydrophobic  Phospholipids – 2 hydrophobic tails, 1 polar hydrophilic phosphate head o Conic – forms micelle o Cylindrical – Forms bi-layers Utility of Artificial Lipid Bi-layers (Liposomes)  Study the properties of the lipids – eg. Permeability  Properties of membrane proteins  Allow molecules into cells – put molecule in the liposome; liposome fuse with bi-layer and molecule is now inside the cell Lipid components of the Bi-layer 1. Phospholipids  Phosphate hydrophilic head  Can be saturated or unsaturated or both  Unsaturation leads to kinks in phospholipid  Can rapidly diffuse in membrane in any individual leaflet  Can rotate in circles, and tails can flex  Can flip flop between leaflets WITH HELP FROM PHOSPHOLIPID TRANSLOCATORS  Phospholipid Translocator  Catalyzes the flip flop  *the phospholipids are built onto the cytosolic leaflet*  Translocator flips some of the newly built phospholipids into the exoplasmic leaflet 2. Steroids  Interacts with phospholipid tails  Makes plasma membrane less permeable to small water soluble molecules  Stabilizes membrane  Plant sterols in plants  Cholesterol in animals and plant
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