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Transcript
In or Out? • 1. What are some things that can pass through a window screen? • 2. What are some things that cannot pass through a window screen? Why is it important to keep these things from moving through the screen? • 3. The cell is surrounded by a cell membrane, which regulates what enters and leaves the cell. Why is it important to regulate what moves into and out of a cell? 8-1 Cell Membrane Cell Membrane • Every cell is surrounded by a cell membrane Functions • provides structural support and protects the cell • recognizes foreign material • gatekeeper – allows only certain substances in and out of the cell • communicates with other cells Homeostasis • Maintenance of stable internal conditions in a changing environment. • Achieved by controlling the movement of substances across the cell membrane Lipid Bilayer Phospholipid • is a specialized lipid made of a phosphate “head” and two fatty acid “tails.” • Main component of the cell membrane • Cells are suspended in a fluid environment. – phosphate head = polar = attracted to water. – fatty acid tails = nonpolar = repelled by water. Lipid Bilayer Lipid Bilayer • the phospholipids form a double layer as a result of water inside and outside the cell • The nonpolar tails, repelled by water, make up the interior of the lipid bilayer. • The polar heads are attracted to the water, so they point toward the surfaces of the lipid bilayer. – One layer of polar heads faces the cytoplasm, while the other layer is in contact with the cell’s immediate surroundings. Lipid Bilayer • Only certain substances can pass through the lipid bilayer. – only small, nonpolar substances can pass. • Ions and most polar molecules are repelled by the nonpolar interior of the lipid bilayer. Checkpoint • What are the functions of the cell membrane? • What is homeostasis? • What is a lipid bilayer? – What are the two parts of a phospholipid? Membrane Proteins Where are proteins found within the membrane • face inside the cell • face outside the cell • stretch across the lipid bilayer and face both inside and outside. • Proteins are made of amino acids. Some amino acids are polar, and others are nonpolar. • The attraction and repulsion of polar and nonpolar parts of the protein to water help hold the protein in the membrane. Membrane Proteins Types of proteins • Cell-surface markers – act like a name tag – unique chain of sugars acts as a marker to identify each type of cell. • Receptor proteins – enable a cell to sense its surroundings – binds to certain substances outside the cell resulting in changes inside the cell. Membrane Proteins Types of proteins • Enzymes – specialized proteins in the cell membrane – helps with important biochemical reactions inside the cell. • Transport proteins – aid the movement of substances that the cell needs but cannot pass through of the lipid bilayer. Key Ideas • How does the cell membrane help a cell maintain homeostasis? • How does the cell membrane restrict the exchange of substances? • What are some functions of membrane proteins? Membrane Proteins Summary • One way that a cell maintains homeostasis is by controlling the movement of substances across the cell membrane. • The lipid bilayer is selectively permeable to small, nonpolar substances. • Proteins in the cell membrane include cellsurface markers, receptor proteins, enzymes, and transport proteins.