Dr._Wilson_Set_4_1A.09

Dr._Wilson_Set_4_1A.09 - Functions of Cellular Membranes:...

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Unformatted text preview: Functions of Cellular Membranes: the plasma membrane that surrounds all cells, and the internal (endo) membrane system of eukaryotic cells Set 4 Important Features of Cell Membranes Semi-permeable phospholipid bilayers ~ 10 nm thick that contain and hold the contents of the cell prevent passage of some molecules from one side to the other, and permit passage of others Figure 5.2 A Phospholipid Bilayer Separates Two Aqueous Regions Lipid environment Consistency of olive oil Lipid bilayer Phospholipids spontaneously adopt a bi-layer organization in an aqueous environment Figure 5.1 The Fluid Mosaic Model Various proteins Cell membranes have proteins embedded within them -- the number and types varies from one cell to another Transport of materials across membranes -three basic mechanisms 1) Passive Diffusion- the simplest mechanism permits crossing of molecules that can dissolve in (thus, penetrate) the lipid portion of the membrane Membrane Transport Mechanisms 1) Passive Diffusion- the simplest mechanism permits crossing of molecules that can dissolve in (thus, penetrate into) the lipid portion of the membrane Important Rules the molecules equilibrate across the membrane, they do not become concentrated on either side the Fow is with the concentration gradient Other important features of passive diffusion No energy needed No carrier molecules What kinds of molecules can passively diffuse across cell membranes?? water (because of its small size- passes through imperfections in the lipid bilayer) lipid-soluble uncharged molecules (e.g., uncharged lipids, nucleosides) Molecules that do not diffuse passively across cell membranes : Most water-soluble molecules Molecules carrying positive or negative charges sugars, amino acids, proteins, peptides, nucleic acids When such molecules cross membranes, they must have help Carrier Mediated Diffusion Transported molecules are helped by receptor proteins or carriers in the membrane that can bind to the molecule. Still a type of diffusion- no energy required transport can occur in either direction with the concetration gradient Figure 5.12 A Carrier Protein Facilitates Diffusion (Part 1) Glucose binding site No ATP Required Transport is in both directions- with the concentration gradient glucose Figure 5.12 A Carrier Protein Facilitates Diffusion (Part 2) Active (ATP-dependent) Transport carriers use ATP to pump molecules against their concentration gradient transport only goes one way (is unidirectional) Three classes: 1. uniports 2. antiports 3. symports Uniports: transport one substance in one direction Symports: transport two different substances in the same direction Antiports: transport two different substances in opposite directions Figure 5.13 Three Types of Proteins for Active Transport uniports symports antiports Additional Important Concepts about Transport Systems Active transport Channels can be gated either by chemical substances or voltage across the membrane Figure 5.10 A Gated Channel Protein Opens in Response to a Stimulus The process by which complex molecules and large...
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Dr._Wilson_Set_4_1A.09 - Functions of Cellular Membranes:...

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