Chapter 5 - Chapter 5-Cell Membrane Structure and Function...

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Chapter 5—Cell Membrane Structure and Function The cell membrane is usually the outermost barrier of the cell. o An exception to this is found in cells which have a cell wall: Plants—cellulose Fungi—chitin Bacteria—chitin plus proteins The cell wall is mainly for structural support and protection. o It usually doesn’t act as a barrier to molecules. Plasma Membrane o The plasma membrane contains the cell but still allows the cell to interact with the environment. The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell’s chemical environment. Membranes are made primarily of lipid. Phospholipids are molecules which can form membranes naturally. Functions : Isolate the cell environment. Regulate exchange between inside and outside. Communicate with other cells. Identify the cell type. o Cell Identification Membrane proteins provide a ‘fingerprint’ for the cell type. HIV binds to cell receptors that are present on cells inside the body, but not present on skin cells.—called CD receptors , cluster of differentiation. This is why it is unlikely that you will contract HIV through casual contact. o Cell Membranes Phospholipid bilayers. Evidence: The phospholipid content of red blood cells is just enough to cover the cell with two layers. The red blood cell has no internal membranes. Biological Membranes Contains proteins Evidence: o Membranes isolated from red blood cells contain proteins in addition to lipids. o Fluid Mosaic Model Singer and Nicholson Fluidity —in membranes; means there is lateral (horizontal) two-dimensional movement of the lipids and some proteins in the plane of the membrane. However, transvers (flip-flop/vertical) movement across the membrane is rare— once per month. Mosaic —aspect of membrane that means there are proteins interspersed within the lipid bilayer o Functions of membrane proteins Transport proteins, channel proteins (pores), and carrier proteins (selective revolving doors Receptors Cell-to-cell recognition
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Transport Across Membranes o Transport is movement. Movement of molecules requires 2 factors: Movement must be through a fluid—liquid or gas. There must be a concentration gradient
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Chapter 5 - Chapter 5-Cell Membrane Structure and Function...

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