1 00:00:06,370 --> 00:00:08,710 PROFESSOR: Hi, my name is Nicole and welcome to 2 00:00:08,710 --> 00:00:10,170 recitation. 3 00:00:10,170 --> 00:00:13,460 Today we are going to go over the practice problem for the 4 00:00:13,460 --> 00:00:16,790 nucleic acids, carbohydrates, and lipids section of the 5 00:00:16,790 --> 00:00:18,710 biochemistry module. 6 00:00:18,710 --> 00:00:20,940 First, if you haven't had a chance to look at this 7 00:00:20,940 --> 00:00:24,860 problem, please go back and look at it and then come back 8 00:00:24,860 --> 00:00:27,055 to recitation and we'll go over it together. 9 00:00:33,940 --> 00:00:35,040 Welcome back. 10 00:00:35,040 --> 00:00:38,430 To start this problem, we are going to go through parts A 11 00:00:38,430 --> 00:00:41,680 through D and then we'll take a break and come back and do 12 00:00:41,680 --> 00:00:43,760 parts E and F. 13 00:00:43,760 --> 00:00:49,400 So, this problem asks us about the molecule shown here. 14 00:00:49,400 --> 00:00:54,050 First, we are asked to name this molecule. 15 00:00:54,050 --> 00:00:57,590 By looking at this molecule, it's really easy to figure out 16 00:00:57,590 --> 00:00:58,610 what it is. 17 00:00:58,610 --> 00:01:04,150 All you have to do is notice the fatty acid tail and the 18 00:01:04,150 --> 00:01:06,550 phosphate head. 19 00:01:06,550 --> 00:01:09,955 If you remember from lecture, this should tell you that it 20 00:01:09,955 --> 00:01:11,205 is a phospholipid. 21 00:01:23,090 --> 00:01:27,380 Next, we are asked where we find these molecules in living 22 00:01:27,380 --> 00:01:32,140 cells and what the function of these molecules is. 23 00:01:32,140 --> 00:01:36,110 So if you remember from lecture, we are told that we 24 00:01:36,110 --> 00:01:38,685 find these molecules in cell membranes. 25 00:01:49,280 --> 00:01:51,970 We learned that they are an integral part of cell 26 00:01:51,970 --> 00:01:56,530 membranes and their function is to control the permeability 27 00:01:56,530 --> 00:02:01,120 of the cell, which is a very important function. 28 00:02:01,120 --> 00:02:07,260 Their structure allows them to keep out polar molecules and 29 00:02:07,260 --> 00:02:11,610 let in nonpolar molecules so they can allow the cell to 30 00:02:11,610 --> 00:02:16,320 have a high solute concentration on the inside as 31 00:02:16,320 --> 00:02:18,580 compared to the outside. 32 00:02:18,580 --> 00:02:23,840 So their critical function is to form a permeability barrier 33 00:02:23,840 --> 00:02:25,090 on the cell. 34 00:02:38,810 --> 00:02:41,230 So now that we understand what the function of these 35 00:02:41,230 --> 00:02:46,500 molecules is, this question asks us what would happen if 36 00:02:46,500 --> 00:02:51,770 we dropped these molecules into a glass of water. 37 00:02:51,770 --> 00:02:56,000 So sometimes these molecules are drawn like this, with a 38 00:02:56,000 --> 00:02:59,230 circle as the phosphate head and two squiggly 39 00:02:59,230 --> 00:03:01,140 lines as the tail. 40 00:03:01,140 --> 00:03:03,790 So if we dropped these in water, we can get three 41 00:03:03,790 --> 00:03:05,280 possibilities. 42 00:03:05,280 --> 00:03:08,930 Either they can form a micelle, which is drawn with 43 00:03:08,930 --> 00:03:12,920 all of the heads towards the outside, facing the water 44 00:03:12,920 --> 00:03:19,050 molecules and the tails all hidden on the inside. 45 00:03:19,050 --> 00:03:26,120 Or, it can form a liposome, which is two layers with a 46 00:03:26,120 --> 00:03:31,600 very tight layer of phospholipids on the inside 47 00:03:31,600 --> 00:03:35,240 and a larger layer of phospholipids on the outside 48 00:03:35,240 --> 00:03:42,390 with the tails facing inwards and the heads facing outwards 49 00:03:42,390 --> 00:03:43,640 towards the water. 50 00:03:48,010 --> 00:03:52,220 And there is also water in the middle as well 51 00:03:52,220 --> 00:03:53,990 facing these nonpolar-- 52 00:03:53,990 --> 00:03:56,440 I mean, these polar heads. 53 00:03:56,440 --> 00:04:00,530 The third structure they can form is a bilayer. 54 00:04:00,530 --> 00:04:04,050 This is a structure that is found that makes cell 55 00:04:04,050 --> 00:04:07,500 membranes, is this phospholipid bilayer. 56 00:04:07,500 --> 00:04:10,115 So this is actually a much larger structure-- 57 00:04:10,115 --> 00:04:13,140 I've only drawn part of it here-- 58 00:04:13,140 --> 00:04:17,490 and this is where you have water on both sides of it. 59 00:04:17,490 --> 00:04:28,180 So you have water molecules out here and on the inside of 60 00:04:28,180 --> 00:04:31,080 the bilayer. 61 00:04:31,080 --> 00:04:34,330 So those are the three molecules that-- 62 00:04:34,330 --> 00:04:37,460 three structures that this molecule will form when you 63 00:04:37,460 --> 00:04:39,910 drop it into water. 64 00:04:39,910 --> 00:04:44,770 Now the most important part of this problem is asking why do 65 00:04:44,770 --> 00:04:46,950 these molecules form these structures when 66 00:04:46,950 --> 00:04:49,150 dropped into water. 67 00:04:49,150 --> 00:04:52,350 And we can find the answer to this very easily in just 68 00:04:52,350 --> 00:04:55,370 looking at the properties of phospholipids and the 69 00:04:55,370 --> 00:04:57,270 properties of water. 70 00:04:57,270 --> 00:05:01,190 If you look at a phospholipid, you'll see that the head, the 71 00:05:01,190 --> 00:05:05,510 phosphate head, is negatively charged, so that means that 72 00:05:05,510 --> 00:05:15,620 it's polar, whereas the tail is a fatty acid, and we have 73 00:05:15,620 --> 00:05:18,630 learned that that means that it is nonpolar, 74 00:05:18,630 --> 00:05:20,080 because it has no charge. 75 00:05:23,540 --> 00:05:25,880 And if you look at water, water is a 76 00:05:25,880 --> 00:05:28,500 polar molecule as well. 77 00:05:28,500 --> 00:05:34,110 It has two polar bonds to hydrogen. 78 00:05:34,110 --> 00:05:37,890 And the reason that they form these structures is because 79 00:05:37,890 --> 00:05:41,960 the polar heads of the phospholipids want to interact 80 00:05:41,960 --> 00:05:45,530 with polar water and the nonpolar tails of the 81 00:05:45,530 --> 00:05:49,610 phospholipids do not want to interact with the polar water. 82 00:05:49,610 --> 00:05:54,040 So they have to find the confirmations in which the 83 00:05:54,040 --> 00:05:56,760 tails are hidden from water and the heads 84 00:05:56,760 --> 00:06:00,170 are exposed to water. 85 00:06:00,170 --> 00:06:03,140 So now that we understand why phospholipids form these 86 00:06:03,140 --> 00:06:05,660 structures, we're going to take a break and come back to 87 00:06:05,660 --> 00:06:06,950 the second part of the problem. 88 00:06:10,810 --> 00:06:11,630 Welcome back. 89 00:06:11,630 --> 00:06:15,400 Now we are going to solve parts E and F of this problem. 90 00:06:15,400 --> 00:06:18,720 Part E asks us to draw another molecule that contains 91 00:06:18,720 --> 00:06:23,180 phosphate that is essential to biological systems. 92 00:06:23,180 --> 00:06:27,400 Right off, you should be thinking about nucleic acids. 93 00:06:27,400 --> 00:06:32,570 I have written an example of a nucleic acid up here. 94 00:06:32,570 --> 00:06:39,550 This, as you can see by the A in the base position, is ATP. 95 00:06:39,550 --> 00:06:42,880 And you can see where phosphate is essential as the 96 00:06:42,880 --> 00:06:47,740 triphosphate portion of ATP. 97 00:06:47,740 --> 00:06:51,560 The next part of the question asks us what is the function 98 00:06:51,560 --> 00:06:53,800 of these type of molecules? 99 00:06:53,800 --> 00:06:57,640 So the molecule that I chose to draw, ATP, is very 100 00:06:57,640 --> 00:07:01,120 interesting because it has multiple functions. 101 00:07:01,120 --> 00:07:08,720 It serves as a building block of RNA through oligomerization 102 00:07:08,720 --> 00:07:12,120 through this bond by adding multiple 103 00:07:12,120 --> 00:07:13,505 nucleic acids together. 104 00:07:18,850 --> 00:07:24,010 It also serves as an energy source for the cell, the 105 00:07:24,010 --> 00:07:32,980 energy source for the cell, through breaking of this bond, 106 00:07:32,980 --> 00:07:36,560 which is an exergonic reaction and releases energy and allows 107 00:07:36,560 --> 00:07:40,160 a lot of the processes in the cell that wouldn't normally be 108 00:07:40,160 --> 00:07:42,530 able to occur to occur. 109 00:07:42,530 --> 00:07:47,010 So I'm going to put as a function of this molecule as 110 00:07:47,010 --> 00:07:57,240 both energy and RNA. 111 00:08:00,740 --> 00:08:04,690 So now that we understand the properties of phospholipids 112 00:08:04,690 --> 00:08:08,070 and we've been able to identify another very 113 00:08:08,070 --> 00:08:12,310 important molecule that contains phosphate, we can 114 00:08:12,310 --> 00:08:15,090 move on to the rest of the biochemistry section. 115 00:08:17,680 --> 00:08:20,560 In this problem, we have learned about the properties 116 00:08:20,560 --> 00:08:24,600 of phospholipids as well as the importance of phosphate in 117 00:08:24,600 --> 00:08:26,510 nucleic acids. 118 00:08:26,510 --> 00:08:30,050 I hope you've enjoyed watching this video and I hope to see 119 00:08:30,050 --> 00:08:31,300 you next time.