WEBVTT 00:00:06.708 --> 00:00:09.585 You're in line at the grocery store, when, uh oh! 00:00:09.585 --> 00:00:11.275 Somone sneezes on you. 00:00:11.275 --> 00:00:13.736 The cold virus is sucked inside your lungs 00:00:13.736 --> 00:00:16.487 and lands on a cell on your airway lining. 00:00:16.487 --> 00:00:19.040 Every living thing on Earth is made of cells, 00:00:19.040 --> 00:00:21.073 from the smallest one-celled bacteria, 00:00:21.073 --> 00:00:23.852 to the giant blue whale, to you. 00:00:23.852 --> 00:00:27.190 Each cell in your body is surrounded by a cell membrane, 00:00:27.190 --> 00:00:30.278 a thick flexible layer made of fats and proteins, 00:00:30.278 --> 00:00:33.475 that surrounds and protects the inner components. 00:00:33.475 --> 00:00:34.493 It's semipermiable, 00:00:34.493 --> 00:00:37.048 meaning that it lets some thing pass in and out 00:00:37.048 --> 00:00:38.402 but blocks others. 00:00:38.402 --> 00:00:41.495 The cell membrane is covered with tiny projections. 00:00:41.495 --> 00:00:42.752 They all have functions, 00:00:42.752 --> 00:00:44.728 like helping cells adhere to their neighbors, 00:00:44.728 --> 00:00:47.785 or binding to nutrients the cell will need. 00:00:47.785 --> 00:00:50.062 Animal and plant cells have cell membranes. 00:00:50.062 --> 00:00:52.052 Only plant cells have a cell wall, 00:00:52.052 --> 00:00:56.048 which is made of rigid cellulose that gives the plant structure. 00:00:56.048 --> 00:00:59.414 The virus cell that was sneezed in to you lungs is sneaky. 00:00:59.414 --> 00:01:00.584 Pretending to be a friend, 00:01:00.584 --> 00:01:03.548 it attaches to a projection on the cell membrane, 00:01:03.548 --> 00:01:06.954 and the cell brings it through the cell membrane and inside. 00:01:06.954 --> 00:01:08.330 When the virus gets through, 00:01:08.330 --> 00:01:10.220 the cell recognizes its mistake. 00:01:10.220 --> 00:01:12.172 An enemy is inside! 00:01:12.172 --> 00:01:14.010 Speical enzymes arrives at the scene 00:01:14.010 --> 00:01:16.224 and chop the virus to pieces. 00:01:16.224 --> 00:01:18.043 They then send one of the pieces back 00:01:18.043 --> 00:01:19.379 through the cell membrane, 00:01:19.379 --> 00:01:22.063 where the cell displays it to warn neighboring cells 00:01:22.063 --> 00:01:23.717 about the invader. 00:01:23.717 --> 00:01:25.332 A near by cell sees the warning 00:01:25.332 --> 00:01:27.810 and immediately goes in to action. 00:01:27.810 --> 00:01:29.419 It needs to make antibodies, 00:01:29.419 --> 00:01:31.189 proteins that will attack and kill 00:01:31.189 --> 00:01:33.068 the invading virus. 00:01:33.068 --> 00:01:35.656 This process starts in the nucleus. 00:01:35.656 --> 00:01:37.667 The nucleus contains our DNA, 00:01:37.667 --> 00:01:40.390 the blueprint that tells our cells how to make everything 00:01:40.390 --> 00:01:43.290 our bodies need to function. 00:01:43.290 --> 00:01:46.128 A certain section of our DNA contains instructions 00:01:46.128 --> 00:01:49.279 that tell our cells how to make antibodies. 00:01:49.279 --> 00:01:52.329 Enzymes in the nucelus find the right section of DNA, 00:01:52.329 --> 00:01:54.138 then create a copy of these instructions, 00:01:54.138 --> 00:01:56.427 called messenger RNA. 00:01:56.427 --> 00:02:00.423 The messenger RNA leaves the nucleus to carry out its orders. 00:02:00.423 --> 00:02:03.467 The messenger RNA travels to a ribosome. 00:02:03.467 --> 00:02:05.907 There can be as many as 10 million ribosomes 00:02:05.907 --> 00:02:07.302 in a human cell, 00:02:07.302 --> 00:02:09.140 all studded along a ribbon-like structure 00:02:09.140 --> 00:02:11.673 called the endoplasmic riticulumn. 00:02:11.673 --> 00:02:14.914 This ribosome reads the instructions from the nucleus. 00:02:14.914 --> 00:02:18.091 It takes amino acids and links them together one by one 00:02:18.091 --> 00:02:22.003 creating an antibody protein that will go fight the virus. 00:02:22.003 --> 00:02:23.237 But before it can do that, 00:02:23.237 --> 00:02:25.776 the antibody needs to leave the cell. 00:02:25.776 --> 00:02:28.649 The antibody heads to the golgi appartatus. 00:02:28.649 --> 00:02:31.717 Here it's packed up for delivery outside the cell. 00:02:31.717 --> 00:02:35.118 Enclosed in a bubble made of the same material as the cell membrane, 00:02:35.118 --> 00:02:38.798 the golgi apparatus also gives the antibody directions, 00:02:38.798 --> 00:02:41.355 telling it how to get to the edge of the cell. 00:02:41.355 --> 00:02:42.184 When it gets there, 00:02:42.184 --> 00:02:45.975 the bubble surrounding the antibody fuses to the cell membrane. 00:02:45.975 --> 00:02:47.954 The cell ejects the antibody, 00:02:47.954 --> 00:02:50.584 and it heads out to track down the virus. 00:02:50.584 --> 00:02:52.390 The leftover bubble will be broken down 00:02:52.390 --> 00:02:53.980 by the cell's lysosomes 00:02:53.980 --> 00:02:56.916 and its pieces recycled over and over again. 00:02:56.916 --> 00:03:00.079 Where did the cell get the energy to do all this. 00:03:00.079 --> 00:03:02.401 That's the roll of the mitochondria. 00:03:02.401 --> 00:03:04.719 To make energy, the mitochondria takes oxygen, 00:03:04.719 --> 00:03:07.366 this is the only reason we breathe it, 00:03:07.366 --> 00:03:09.386 and adds electrons from the food we eat 00:03:09.386 --> 00:03:11.457 to make water molecules. 00:03:11.457 --> 00:03:13.882 That process also creates a high energy molecule, 00:03:13.882 --> 00:03:18.244 called ATP, which the cell uses to power all of its parts. 00:03:18.244 --> 00:03:20.964 Plant cells make energy a different way. 00:03:20.964 --> 00:03:22.103 They have chloroplasts 00:03:22.103 --> 00:03:24.205 that combine carbon dioxide and water 00:03:24.205 --> 00:03:26.048 with light energy from the sun 00:03:26.048 --> 00:03:27.769 to create oxygen and sugar, 00:03:27.769 --> 00:03:30.345 a form of chemical energy. 00:03:30.345 --> 00:03:32.294 All the parts of the cell have to work together 00:03:32.294 --> 00:03:34.532 to keep things running smoothly, 00:03:34.532 --> 00:03:36.646 and all the cells of your body have to work together 00:03:36.646 --> 00:03:38.949 to keep you running smoothly. 00:03:38.949 --> 00:03:40.520 That's a whole lot of cells. 00:03:40.520 --> 00:03:44.232 Scientists think there are about 37 trillion of them.