Chapter 7 - Chapter 7 - Like all biological membranes, the...

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Chapter 7 - Like all biological membranes, the plasma membrane exhibits selective permeability ; that is, it allows some substances to cross it more easily than others. 7.1 – Cellular membranes are fluid mosaics of lipids and proteins - Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. - A phospholipid is an amphipathic molecule, meaning it has both hydrophilic region and a hydrophobic region. - In the fluid mosaic model , the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) or phospholipids. - Membranes with different functions differ in structure and chemical composition - Unlike proteins dissolved in the cytosol, membrane proteins are not very soluble in water because they are amphipathic. - There are 2 major populations of membrane proteins: integral proteins and peripheral proteins. - Integral proteins penetrate the hydrophobic interior of the lipid bilayer. - Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins. - 6 major junctions performed by proteins of the plasma membrane: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, & attachment to the cytoskeleton and extracellular matrix (ECM) - Glycolipid – a lipid with one or more covalently attached carbohydrates - Glycoprotein – a protein with one or more covalently attached carbohydrates 7.2 – Membrane structure results in selective permeability - Cell membranes are permeable to specific ions and a variety of polar molecules. - These hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. - Some transport proteins, called channel proteins , function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane.
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- The passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins . - Other transport proteins, called carrier proteins , hold onto their passengers and change shape in a way that shuttles them across the membrane. 7.3 – Passive transport is diffusion of a substance across a membrane with no energy investment - Diffusion – the movement of molecules of any substance so that they spread out evenly into available space. - In the absence of other forces, a substance will diffuse from where it is
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Chapter 7 - Chapter 7 - Like all biological membranes, the...

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