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2The Cellular Level of OrganizationModule Overview
3This presentation is based on and includes content derivedfrom the following OER resource:Anatomy and PhysiologyAn OpenStax book used for this course may be downloadedfor free at:-and-physiologyAcknowledgments
4Thecell membraneis an extremely pliable structure composedprimarily of two adjacent (back-to-back) phospholipids (a bilayer).Cholesterol is also present, which contributes to the fluidity of themembrane, and there are various proteins embedded within themembrane that have a variety of functions.In the membrane bilayer, the nonpolar (hydrophobic, or “water-fearing”) lipid tails of one layer face the lipid tails of the other layer,meeting at the interface of the two layers. The polar (hydrophilic, or“water-loving”) phospholipid heads face outward, one layer facingthe water environment of the cell’s interior and one layer facing thewater environment of the cell’s exterior.Structure and composition of thecell membrane
5Achannel proteinis an example of an integral protein thatselectively allows particular materials, such as certain ions, to passinto or out of the cell.Areceptorprotein is another type of integral protein that canselectively bind a specific molecule outside the cell, and this bindinginduces a chemical reaction within the cell.Some integral membrane proteins are glycoproteins. A glycoproteinis a protein that has carbohydrate molecules attached, which extendinto the extracellular matrix.Membrane proteins
6The cell membrane has selective permeability that allows onlyrelatively small, nonpolar materials to move through the lipid bilayer.Some examples of these are other lipids, oxygen and carbon dioxidegases, and alcohol.Polar, water-soluble, materials—like glucose,amino acids, and electrolytes— need some assistance to cross themembrane because they are repelled by the hydrophobic tails of thephospholipid bilayer.•Passive transportis the movement of substances across themembrane without the expenditure of cellular energy.•Active transportis the movement of substances across themembrane using energy from adenosine triphosphate (ATP).Transport across the cell membrane
7Molecules and ions will spread/diffuse from where they are moreconcentrated to where they are less concentrated until they are equallydistributed in that space. That is, they move down a concentrationgradient.Diffusionis the movement of particles from an area of higherconcentration to an area of lower concentration.•Simple diffusionis the process for uncharged substances such asoxygen and carbon dioxide, and hydrophobic molecules such as lipids,to pass through the cell membrane, down their concentrationgradient.•Facilitated diffusionis the diffusion process for substances thatcannot cross the lipid bilayer due to their size, charge, and/or polarity.Facilitated diffusion is a passive process; it does not require energy.Passive transport