0:00:06.708,0:00:09.585
You're in line at the grocery store, when, uh oh!

0:00:09.585,0:00:11.275
Somone sneezes on you.

0:00:11.275,0:00:13.736
The cold virus is sucked inside your lungs

0:00:13.736,0:00:16.487
and lands on a cell on your airway lining.

0:00:16.487,0:00:19.040
Every living thing on Earth is made of cells,

0:00:19.040,0:00:21.073
from the smallest one-celled bacteria,

0:00:21.073,0:00:23.852
to the giant blue whale, to you.

0:00:23.852,0:00:27.190
Each cell in your body is surrounded by a cell membrane,

0:00:27.190,0:00:30.278
a thick flexible layer made of fats and proteins,

0:00:30.278,0:00:33.475
that surrounds and protects the inner components.

0:00:33.475,0:00:34.493
It's semipermiable,

0:00:34.493,0:00:37.048
meaning that it lets some thing pass in and out

0:00:37.048,0:00:38.402
but blocks others.

0:00:38.402,0:00:41.495
The cell membrane is covered with tiny projections.

0:00:41.495,0:00:42.752
They all have functions,

0:00:42.752,0:00:44.728
like helping cells adhere to their neighbors,

0:00:44.728,0:00:47.785
or binding to nutrients the cell will need.

0:00:47.785,0:00:50.062
Animal and plant cells have cell membranes.

0:00:50.062,0:00:52.052
Only plant cells have a cell wall,

0:00:52.052,0:00:56.048
which is made of rigid cellulose that gives the plant structure.

0:00:56.048,0:00:59.414
The virus cell that was sneezed in to you lungs is sneaky.

0:00:59.414,0:01:00.584
Pretending to be a friend,

0:01:00.584,0:01:03.548
it attaches to a projection on the cell membrane,

0:01:03.548,0:01:06.954
and the cell brings it through the cell membrane and inside.

0:01:06.954,0:01:08.330
When the virus gets through,

0:01:08.330,0:01:10.220
the cell recognizes its mistake.

0:01:10.220,0:01:12.172
An enemy is inside!

0:01:12.172,0:01:14.010
Speical enzymes arrives at the scene

0:01:14.010,0:01:16.224
and chop the virus to pieces.

0:01:16.224,0:01:18.043
They then send one of the pieces back

0:01:18.043,0:01:19.379
through the cell membrane,

0:01:19.379,0:01:22.063
where the cell displays it to warn neighboring cells

0:01:22.063,0:01:23.717
about the invader.

0:01:23.717,0:01:25.332
A near by cell sees the warning

0:01:25.332,0:01:27.810
and immediately goes in to action.

0:01:27.810,0:01:29.419
It needs to make antibodies,

0:01:29.419,0:01:31.189
proteins that will attack and kill

0:01:31.189,0:01:33.068
the invading virus.

0:01:33.068,0:01:35.656
This process starts in the nucleus.

0:01:35.656,0:01:37.667
The nucleus contains our DNA,

0:01:37.667,0:01:40.390
the blueprint that tells our cells how to make everything

0:01:40.390,0:01:43.290
our bodies need to function.

0:01:43.290,0:01:46.128
A certain section of our DNA contains instructions

0:01:46.128,0:01:49.279
that tell our cells how to make antibodies.

0:01:49.279,0:01:52.329
Enzymes in the nucelus find the right section of DNA,

0:01:52.329,0:01:54.138
then create a copy of these instructions,

0:01:54.138,0:01:56.427
called messenger RNA.

0:01:56.427,0:02:00.423
The messenger RNA leaves the nucleus to carry out its orders.

0:02:00.423,0:02:03.467
The messenger RNA travels to a ribosome.

0:02:03.467,0:02:05.907
There can be as many as 10 million ribosomes

0:02:05.907,0:02:07.302
in a human cell,

0:02:07.302,0:02:09.140
all studded along a ribbon-like structure

0:02:09.140,0:02:11.673
called the endoplasmic riticulumn.

0:02:11.673,0:02:14.914
This ribosome reads the instructions from the nucleus.

0:02:14.914,0:02:18.091
It takes amino acids and links them together one by one

0:02:18.091,0:02:22.003
creating an antibody protein that will go fight the virus.

0:02:22.003,0:02:23.237
But before it can do that,

0:02:23.237,0:02:25.776
the antibody needs to leave the cell.

0:02:25.776,0:02:28.649
The antibody heads to the golgi appartatus.

0:02:28.649,0:02:31.717
Here it's packed up for delivery outside the cell.

0:02:31.717,0:02:35.118
Enclosed in a bubble made of the same material as the cell membrane,

0:02:35.118,0:02:38.798
the golgi apparatus also gives the antibody directions,

0:02:38.798,0:02:41.355
telling it how to get to the edge of the cell.

0:02:41.355,0:02:42.184
When it gets there,

0:02:42.184,0:02:45.975
the bubble surrounding the antibody fuses to the cell membrane.

0:02:45.975,0:02:47.954
The cell ejects the antibody,

0:02:47.954,0:02:50.584
and it heads out to track down the virus.

0:02:50.584,0:02:52.390
The leftover bubble will be broken down

0:02:52.390,0:02:53.980
by the cell's lysosomes

0:02:53.980,0:02:56.916
and its pieces recycled over and over again.

0:02:56.916,0:03:00.079
Where did the cell get the energy to do all this.

0:03:00.079,0:03:02.401
That's the roll of the mitochondria.

0:03:02.401,0:03:04.719
To make energy, the mitochondria takes oxygen,

0:03:04.719,0:03:07.366
this is the only reason we breathe it,

0:03:07.366,0:03:09.386
and adds electrons from the food we eat

0:03:09.386,0:03:11.457
to make water molecules.

0:03:11.457,0:03:13.882
That process also creates a high energy molecule,

0:03:13.882,0:03:18.244
called ATP, which the cell uses to power all of its parts.

0:03:18.244,0:03:20.964
Plant cells make energy a different way.

0:03:20.964,0:03:22.103
They have chloroplasts

0:03:22.103,0:03:24.205
that combine carbon dioxide and water

0:03:24.205,0:03:26.048
with light energy from the sun

0:03:26.048,0:03:27.769
to create oxygen and sugar,

0:03:27.769,0:03:30.345
a form of chemical energy.

0:03:30.345,0:03:32.294
All the parts of the cell have to work together

0:03:32.294,0:03:34.532
to keep things running smoothly,

0:03:34.532,0:03:36.646
and all the cells of your body have to work together

0:03:36.646,0:03:38.949
to keep you running smoothly.

0:03:38.949,0:03:40.520
That's a whole lot of cells.

0:03:40.520,0:03:44.232
Scientists think there are about 37 trillion of them.