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Published byLester West Modified over 9 years ago
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Cells & Their Environment
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Cell Membrane **Every cell is surrounded by a cell membrane! - protects cell - helps move substances in & out of the cell
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I. Homeostasis A.Definition – maintenance of stable internal conditions in a changing environment B.Cell Membrane & Homeostasis 1.Cell Membrane controls the movement of substances in & out of the cell 2.Provides structural support 3.Recognizes foreign material 4.Communicates with other cells
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II. Lipid Bilayer A.Phospholipid – cell membrane is a “sea” of phospholipids in which proteins “float” 1.Phosphate head is polar, hydrophilic, water- loving 2.Two fatty acid tails are nonpolar, hydrophobic, water-fearing
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B.Membrane Structure 1.Lipid Bilayer – hydrophobic tails are inside, protected by hydrophilic heads on outside 2.Barrier – phospholipids form a barrier through which only small, nonpolar substances can pass
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C.Cell Membranes are Fluid 1.Phospholipids are the most abundant lipid in the plasma membrane 2.The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
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Figure 7.3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein
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Figure 7.4 Knife Plasma membrane Cytoplasmic layer Proteins Extracellular layer Inside of extracellular layer Inside of cytoplasmic layer TECHNIQUE RESULTS
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3.Phospholipids in the plasma membrane can move within the bilayer 4.Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane
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Figure 7.6 Lateral movement occurs 10 7 times per second. Flip-flopping across the membrane is rare ( once per month).
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III. Membrane Proteins A.Proteins in Lipids 1.Integral Proteins – proteins stretch across the bilayer & face both inside and outside 2.Peripheral Proteins – proteins that are only on one side (face inside or outside) of the membrane
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B.Types of Proteins 1.Receptor Proteins – sense the cell’s surroundings by binding to substances outside the cell 2.Enzymes – proteins that help biochemical reactions in a cell 3.Transport Proteins – aid the movement of certain substances in & out of the cell
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Types of Proteins in the Membrane
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