Study Guide: The Cell Although your cell drawings were intended to be a study guide of sorts, here are a few additional questions that address some things that may have been less well emphasized in the drawings, or expand on things that would not have been obvious to draw or explain. . Your best bet here is to try can answer these questions from memory. If you can, great! If not, study, and try again. Next best thing: use this as a work sheet while you study – filling in as you find the answers I would not recommend: getting the answers and just memorizing the questions/answers. They will not be the same on the exam! 1. Describe the structure of a lipid bilayer. Two layers of phospholipids, with the hydrophobic fatty acid chains (“tails” of each layer facing towards each other, and the polar (hydrophilic) “heads” on the surfaces. 2. Describe the structure of the plasma membrane. It is composed of a lipid bilayer (see above), but includes many proteins that are embedded in the bilayer. These may go through the bilayer, or just be anchored to one ore the other side. One can envision this as proteins floating around in a layer of lipids. 3. What is the function of the plasma membrane? To restrict and control the passage of molecules into and out of the cell. It helps the cell maintain an appropriate internal environment. 4. Compare the plasma membrane with the nuclear membrane (sometimes called the nuclear envelope) – how are they similar in structure, but different? As noted above, the plasma membrane is a single lipid belayed with proteins studded throughout. The nuclear envelope is composed of TWO lipid bilayers (i.e., 4 lipid layers). It is studded with very large pores (nuclear pores). 5. Why are there large pores in the nuclear envelope? Molecules of mRNA (which are not only very large but also charged) need to pass from the nucleus, where they have been copied from the DNA, into the cytoplasm where they can be transcribed. 6. Besides a nucleus, what do eukaryotic cells have that prokaryotic cells do not? Other membrane bound organelles, such as mitochondria and chloroplasts. 7. What structures do plant cells have, that animal cells lack? A central vacuole, a cell wall, and chloroplasts 8. How are mitochondria different in structure, and in function, from lysosomes? Structurally, lysosomes are simple vesicles (a membrane “bubble”), whereas the mitochondrion has one outer membrane, and an inner membrane – this inner membrane is quite large, but is folded up on itself. Functionally, lysosomes contain enzymes that help the cell break larger molecules into smaller, more useful bits. Mitochondria are used in aerobic respiration – breaking down sugar to obtain energy in the form of ATP. 9. What is the difference between the smooth and rough endoplasmic reticulum? The SER and RER are continuous – the RER, however, has ribosomes attached to it. Therefore the functions of the two are different. In the RER, synthesis of the secreted and membrane proteins is carried out. In the SER, synthesis of lipids for the elaborate membrane system of the cells takes place. 10. What is the difference between the functions of “free” ribosomes vs. those that are attached to the endoplasmic reticule? The ribosomes that are attached to the ER are synthesizing proteins that will be secreted or are membrane proteins. As they are synthesized they are allowed to either pass through the membrane into the lumen (inside) of the ER [secreted proteins] , or get stuck/anchored in the membrane, and become “membrane proteins”. Proteins are so large, and tend to be so hydrophilic on the outside (unless they are membrane proteins) that it is difficult for them to cross the bilayer “after the fact”. Synthesis in the ER puts them inside the lumen of the ER, which is effectively OUTSIDE the cell already. They get modified as they pass through the Golgi, and ultimately get dumped outside (and its similar fro membrane proteins except that they get turned inside out when vesicles that pinch of from the Golgi fuse with the plasma membrane Free ribosomes are actively translating (because remember that the subunits do not come together unless there is a message to translate), but are making cytoplasmic membranes. .