1 00:00:06,708 --> 00:00:09,585 You're in line at the grocery store when, uh oh, 2 00:00:09,585 --> 00:00:11,275 someone sneezes on you. 3 00:00:11,275 --> 00:00:13,736 The cold virus is sucked inside your lungs 4 00:00:13,736 --> 00:00:16,487 and lands on a cell on your airway lining. 5 00:00:16,487 --> 00:00:19,040 Every living thing on Earth is made of cells, 6 00:00:19,040 --> 00:00:21,073 from the smallest one-celled bacteria 7 00:00:21,073 --> 00:00:23,852 to the giant blue whale to you. 8 00:00:23,852 --> 00:00:27,190 Each cell in your body is surrounded by a cell membrane, 9 00:00:27,190 --> 00:00:30,278 a thick flexible layer made of fats and proteins, 10 00:00:30,278 --> 00:00:33,475 that surrounds and protects the inner components. 11 00:00:33,475 --> 00:00:34,493 It's semipermeable, 12 00:00:34,493 --> 00:00:37,048 meaning that it lets some thing pass in and out 13 00:00:37,048 --> 00:00:38,402 but blocks others. 14 00:00:38,402 --> 00:00:41,495 The cell membrane is covered with tiny projections. 15 00:00:41,495 --> 00:00:42,752 They all have functions, 16 00:00:42,752 --> 00:00:44,728 like helping cells adhere to their neighbors 17 00:00:44,728 --> 00:00:47,785 or binding to nutrients the cell will need. 18 00:00:47,785 --> 00:00:50,062 Animal and plant cells have cell membranes. 19 00:00:50,062 --> 00:00:52,052 Only plant cells have a cell wall, 20 00:00:52,052 --> 00:00:56,048 which is made of rigid cellulose that gives the plant structure. 21 00:00:56,048 --> 00:00:59,414 The virus cell that was sneezed into your lungs is sneaky. 22 00:00:59,414 --> 00:01:00,584 Pretending to be a friend, 23 00:01:00,584 --> 00:01:03,548 it attaches to a projection on the cell membrane, 24 00:01:03,548 --> 00:01:06,954 and the cell brings it through the cell membrane and inside. 25 00:01:06,954 --> 00:01:08,330 When the virus gets through, 26 00:01:08,330 --> 00:01:10,220 the cell recognizes its mistake. 27 00:01:10,220 --> 00:01:12,172 An enemy is inside! 28 00:01:12,172 --> 00:01:14,010 Special enzymes arrive at the scene 29 00:01:14,010 --> 00:01:16,224 and chop the virus to pieces. 30 00:01:16,224 --> 00:01:18,043 They then send one of the pieces back 31 00:01:18,043 --> 00:01:19,379 through the cell membrane, 32 00:01:19,379 --> 00:01:22,063 where the cell displays it to warn neighboring cells 33 00:01:22,063 --> 00:01:23,717 about the invader. 34 00:01:23,717 --> 00:01:25,332 A nearby cell sees the warning 35 00:01:25,332 --> 00:01:27,810 and immediately goes into action. 36 00:01:27,810 --> 00:01:29,419 It needs to make antibodies, 37 00:01:29,419 --> 00:01:31,189 proteins that will attack and kill 38 00:01:31,189 --> 00:01:33,068 the invading virus. 39 00:01:33,068 --> 00:01:35,656 This process starts in the nucleus. 40 00:01:35,656 --> 00:01:37,667 The nucleus contains our DNA, 41 00:01:37,667 --> 00:01:40,390 the blueprint that tells our cells how to make everything 42 00:01:40,390 --> 00:01:43,290 our bodies need to function. 43 00:01:43,290 --> 00:01:46,128 A certain section of our DNA contains instructions 44 00:01:46,128 --> 00:01:49,279 that tell our cells how to make antibodies. 45 00:01:49,279 --> 00:01:52,329 Enzymes in the nucleus find the right section of DNA, 46 00:01:52,329 --> 00:01:54,138 then create a copy of these instructions, 47 00:01:54,138 --> 00:01:56,427 called messenger RNA. 48 00:01:56,427 --> 00:02:00,423 The messenger RNA leaves the nucleus to carry out its orders. 49 00:02:00,423 --> 00:02:03,467 The messenger RNA travels to a ribosome. 50 00:02:03,467 --> 00:02:05,907 There can be as many as 10 million ribosomes 51 00:02:05,907 --> 00:02:07,302 in a human cell, 52 00:02:07,302 --> 00:02:09,140 all studded along a ribbon-like structure 53 00:02:09,140 --> 00:02:11,673 called the endoplasmic reticulum. 54 00:02:11,673 --> 00:02:14,914 This ribosome reads the instructions from the nucleus. 55 00:02:14,914 --> 00:02:18,091 It takes amino acids and links them together one by one 56 00:02:18,091 --> 00:02:22,003 creating an antibody protein that will go fight the virus. 57 00:02:22,003 --> 00:02:23,237 But before it can do that, 58 00:02:23,237 --> 00:02:25,776 the antibody needs to leave the cell. 59 00:02:25,776 --> 00:02:28,649 The antibody heads to the golgi apparatus. 60 00:02:28,649 --> 00:02:31,717 Here, it's packed up for delivery outside the cell. 61 00:02:31,717 --> 00:02:35,118 Enclosed in a bubble made of the same material as the cell membrane, 62 00:02:35,118 --> 00:02:38,798 the golgi apparatus also gives the antibody directions, 63 00:02:38,798 --> 00:02:41,355 telling it how to get to the edge of the cell. 64 00:02:41,355 --> 00:02:42,184 When it gets there, 65 00:02:42,184 --> 00:02:45,975 the bubble surrounding the antibody fuses to the cell membrane. 66 00:02:45,975 --> 00:02:47,954 The cell ejects the antibody, 67 00:02:47,954 --> 00:02:50,584 and it heads out to track down the virus. 68 00:02:50,584 --> 00:02:52,390 The leftover bubble will be broken down 69 00:02:52,390 --> 00:02:53,980 by the cell's lysosomes 70 00:02:53,980 --> 00:02:56,916 and its pieces recycled over and over again. 71 00:02:56,916 --> 00:03:00,079 Where did the cell get the energy to do all this? 72 00:03:00,079 --> 00:03:02,401 That's the roll of the mitochondria. 73 00:03:02,401 --> 00:03:04,719 To make energy, the mitochondria takes oxygen, 74 00:03:04,719 --> 00:03:07,366 this is the only reason we breathe it, 75 00:03:07,366 --> 00:03:09,386 and adds electrons from the food we eat 76 00:03:09,386 --> 00:03:11,457 to make water molecules. 77 00:03:11,457 --> 00:03:13,882 That process also creates a high energy molecule, 78 00:03:13,882 --> 00:03:18,244 called ATP which the cell uses to power all of its parts. 79 00:03:18,244 --> 00:03:20,964 Plant cells make energy a different way. 80 00:03:20,964 --> 00:03:22,103 They have chloroplasts 81 00:03:22,103 --> 00:03:24,205 that combine carbon dioxide and water 82 00:03:24,205 --> 00:03:26,048 with light energy from the sun 83 00:03:26,048 --> 00:03:27,769 to create oxygen and sugar, 84 00:03:27,769 --> 00:03:30,345 a form of chemical energy. 85 00:03:30,345 --> 00:03:32,294 All the parts of a cell have to work together 86 00:03:32,294 --> 00:03:34,532 to keep things running smoothly, 87 00:03:34,532 --> 00:03:36,646 and all the cells of your body have to work together 88 00:03:36,646 --> 00:03:38,949 to keep you running smoothly. 89 00:03:38,949 --> 00:03:40,520 That's a whole lot of cells. 90 00:03:40,520 --> 00:03:44,232 Scientists think there are about 37 trillion of them.