BIO LAB CHPT 2 PDF

BIO LAB CHPT 2 PDF - Chapter 2 Properties of Biological...

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15 Chapter 2 Properties of Biological Membranes: Osmosis/Diffusion MEMBRANES All cells, both prokaryotic and eukaryotic, are surrounded by a cell membrane. This membrane is also known as a plasma membrane. The cell membrane defines a cell’s boundary and regulates the interaction of the cell with its environment. Biological membranes are composed of lipids, proteins and carbohydrates. A category of lipids known as phospholipids form a bilayer that provides a backbone for the membrane and a barrier, which prevents many substances from entering the cell. Proteins inserted into the phospholipid bilayer carry out many of the functions of membranes. The external surface of mammalian cell membranes is covered with a layer of carbohydrates that function in cell recognition and cell adhesion. In addition to a plasma membrane, eukaryotic cells also contain smaller compartments known as organelles that are surrounded by lipid bilayers. Examples of organelles are mitochondria, chloroplasts, endoplasmic reticulum, and Golgi. Membranes have the following functions. 1. Barrier. The cell membrane functions as a barrier between the cell and the envi- ronment. The lipid bilayer blocks the entry into the cell of most substances except water, glycerol and small lipid-soluble molecules. 2. Transport. A second function of cell membranes is that of transport. The cell must take up metabolites and eliminate waste products. As mentioned above, the structure of the phospholipid bilayer prohibits most substances from moving di- rectly across its span. Water, glycerol, and small, lipid-soluble molecules can move across the membrane directly, and do so readily. The net direction of movement is such that the molecules move from the side of the membrane where they are found in higher concentration to the side of the membrane where they are found in lower concentration. This diffusion of substances across a biological membrane does not require energy and thus is termed a passive process . Water movement across a biological membrane is an extremely important process to the health of a cell, as you will observe today. This diffusion of water across a selectively permeable membrane—like the plasma membranes that surround all cells—is known as os- mosis . (See the following pages for more information on diffusion and osmosis.)
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16 BIO 100 Properties of Biological Membranes: Osmosis/Diffusion What happens to the substances like ions and polar molecules that cannot move directly across the phospholipid bilayer? These substances must move into and out of cells with the help of membrane proteins inserted in the phospholipid bilayer. If substances move through a protein channel or carrier, down (or along ) their concentration gradients (from the side of the membrane where they are in higher concentration to the side of the membrane where they are in lower concentration), no energy is required. This is a type of diffusion known as facilitated diffusion (with the membrane proteins doing the facilitating). Facilitated diffusion is a type of
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This note was uploaded on 10/27/2010 for the course BIO 102 taught by Professor Alaie during the Fall '08 term at CUNY Hunter.

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BIO LAB CHPT 2 PDF - Chapter 2 Properties of Biological...

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