Read This! When phospholipids are added to an aqueous environment (consisting mostly of water) the phospholipid molecules will spontaneously assemble into a phospholipid bilayer where the layers are held together by weak attractive forces between molecules. These structures are often seen in nature as cell and organelle membranes. 12. Consider animal cells, which are only bound by a cell membrane and plant cells which are bound by both a cell membrane and a cell wall. Are cell membranes flexible (fluid)? Provide specific examples to support your answer. 13. Explain why a phospholipid bilayer is flexible. terms of the strength of the forces that hold it together. 14. Refer to Model 1. a. What happens to the shape of the hydrophobic tail in a phospholipid when a double bond is present in the carbon chain? b. Explain why the flexibility (fluidity) of a membrane increases when more of the phospholipids in the layers contain double bonds. 15. The diagram below shows the chemical structure of cholesterol, which is a key component of membrane structure. H.C a. Is the cholesterol molecule mostly polar or mostly nonpolar? Explain. b. Circle the drawing below which illustrates the most likely placement of cholesterol in a phospholipid bilayer. +99 Mmm c. The cholesterol forms weak attractive forces with multiple phospholipids in the bilayer. Would this increase or decrease flexibility of the membrane? Explain your reasoning. 00000000 Extension Questions 16. Embedded proteins are often found spanning the membrane of a cell or organelle. These pro- teins serve as channels for specific molecules to travel through the membrane, either into or out of the cell 5 of 6 a. What sections of the embedded protein chain are most likely to contain amino acids hydrophobic R- > b. Explain why the flexibility (tluidity) of a membrane increases when more of the phospholipids in the layers contain double bonds. 15. The diagram below shows the chemical structure of cholesterol, which is a key component of membrane structure. HO HC a. Is the cholesterol molecule mostly polar or mostly nonpolar? Explain. b. Circle the drawing below which illustrates the most likely placement of cholesterol in a phospholipid bilayer. *80 c. The cholesterol forms weak attractive forces with multiple phospholipids in the bilayer. Would this increase or decrease flexibility of the membrane? Explain your reasoning. Extension Questions 16. Embedded proteins are often found spanning the membrane of a cell or organelle. These pro- teins serve as channels for specific molecules to travel through the membrane, either into or out of the cell a. What sections of the embedded protein chain are most likely to contain amino acids hydrophobic R-groups? Explain your reasoning b. What sections of the embedded protein chain are most likely to contain amino acids with hydrophilic R-groups? Explain your reasoning, 17. Some membranes have surface proteins on them. These proteins often serve a signaling func- tion between cells. Propose a mechanism by which these surface proteins are able to attach to the membrane.