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You're in line at the grocery store when, uh oh,

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someone sneezes on you.

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The cold virus is sucked inside your lungs

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and lands on a cell on your airway lining.

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Every living thing on Earth is made of cells,

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from the smallest one-celled bacteria

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to the giant blue whale to you.

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Each cell in your body is surrounded by a cell membrane,

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a thick flexible layer made of fats and proteins,

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that surrounds and protects the inner components.

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It's semipermeable,

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meaning that it lets some thing pass in and out

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but blocks others.

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The cell membrane is covered with tiny projections.

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They all have functions,

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like helping cells adhere to their neighbors

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or binding to nutrients the cell will need.

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Animal and plant cells have cell membranes.

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Only plant cells have a cell wall,

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which is made of rigid cellulose that gives the plant structure.

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The virus cell that was sneezed into your lungs is sneaky.

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Pretending to be a friend,

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it attaches to a projection on the cell membrane,

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and the cell brings it through the cell membrane and inside.

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When the virus gets through,

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the cell recognizes its mistake.

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An enemy is inside!

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Special enzymes arrive at the scene

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and chop the virus to pieces.

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They then send one of the pieces back

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through the cell membrane,

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where the cell displays it to warn neighboring cells

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about the invader.

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A nearby cell sees the warning

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and immediately goes into action.

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It needs to make antibodies,

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proteins that will attack and kill

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the invading virus.

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This process starts in the nucleus.

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The nucleus contains our DNA,

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the blueprint that tells our cells how to make everything

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our bodies need to function.

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A certain section of our DNA contains instructions

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that tell our cells how to make antibodies.

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Enzymes in the nucleus find the right section of DNA,

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then create a copy of these instructions,

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called messenger RNA.

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The messenger RNA leaves the nucleus to carry out its orders.

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The messenger RNA travels to a ribosome.

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There can be as many as 10 million ribosomes

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in a human cell,

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all studded along a ribbon-like structure

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called the endoplasmic reticulum.

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This ribosome reads the instructions from the nucleus.

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It takes amino acids and links them together one by one

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creating an antibody protein that will go fight the virus.

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But before it can do that,

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the antibody needs to leave the cell.

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The antibody heads to the golgi apparatus.

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Here, it's packed up for delivery outside the cell.

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Enclosed in a bubble made of the same material as the cell membrane,

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the golgi apparatus also gives the antibody directions,

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telling it how to get to the edge of the cell.

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When it gets there,

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the bubble surrounding the antibody fuses to the cell membrane.

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The cell ejects the antibody,

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and it heads out to track down the virus.

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The leftover bubble will be broken down

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by the cell's lysosomes

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and its pieces recycled over and over again.

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Where did the cell get the energy to do all this?

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That's the roll of the mitochondria.

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To make energy, the mitochondria takes oxygen,

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this is the only reason we breathe it,

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and adds electrons from the food we eat

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to make water molecules.

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That process also creates a high energy molecule,

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called ATP which the cell uses to power all of its parts.

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Plant cells make energy a different way.

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They have chloroplasts

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that combine carbon dioxide and water

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with light energy from the sun

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to create oxygen and sugar,

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a form of chemical energy.

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All the parts of a cell have to work together

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to keep things running smoothly,

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and all the cells of your body have to work together

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to keep you running smoothly.

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That's a whole lot of cells.

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Scientists think there are about 37 trillion of them.