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  • You're in line at the grocery store when, uh oh, someone sneezes on you.

  • The cold virus is sucked inside your lungs and lands on a cell on your airway lining.

  • Every living thing on Earth is made of cells, from the smallest one-celled bacteria, to the giant blue whale, to you.

  • Each cell in your body is surrounded by a cell membrane, a thick flexible layer made of fats and proteins, that surrounds and protects the inner components.

  • It's semipermeable, meaning that it lets some things pass in and out but blocks others.

  • The cell membrane is covered with tiny projections.

  • They all have functions, like helping cells adhere to their neighbors or binding to nutrients the cell will need.

  • Animal and plant cells have cell membranes.

  • Only plant cells have a cell wall, which is made of rigid cellulose that gives the plant structure.

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

  • Pretending to be a friend, it attaches to a projection on the cell membrane, and the cell brings it through the cell membrane and inside.

  • When the virus gets through, the cell recognizes its mistake.

  • An enemy is inside!

  • Special enzymes arrive at the scene and chop the virus to pieces.

  • They then send one of the pieces back through the cell membrane, where the cell displays it to warn neighboring cells about the invader.

  • A nearby cell sees the warning and immediately goes into action.

  • It needs to make antibodies, proteins that will attack and kill the invading virus.

  • This process starts in the nucleus.

  • The nucleus contains our DNA, the blueprint that tells our cells how to make everything our bodies need to function.

  • A certain section of our DNA contains instructions that tell our cells how to make antibodies.

  • Enzymes in the nucleus find the right section of DNA, then create a copy of these instructions, called messenger RNA.

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

  • The messenger RNA travels to a ribosome.

  • There can be as many as 10 million ribosomes in a human cell, all studded along a ribbon-like structure called the endoplasmic reticulum.

  • This ribosome reads the instructions from the nucleus.

  • It takes amino acids and links them together one by one, creating an antibody protein that will go fight the virus.

  • But before it can do that, the antibody needs to leave the cell.

  • The antibody heads to the golgi apparatus.

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

  • Enclosed in a bubble made of the same material as the cell membrane, the golgi apparatus also gives the antibody directions, telling it how to get to the edge of the cell.

  • When it gets there, the bubble surrounding the antibody fuses to the cell membrane.

  • The cell ejects the antibody, and it heads out to track down the virus.

  • The leftover bubble will be broken down by the cell's lysosomes and its pieces recycled over and over again.

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

  • That's the roll of the mitochondria.

  • To make energy, the mitochondria takes oxygen - this is the only reason we breathe it - and adds electrons from the food we eat to make water molecules.

  • That process also creates a high energy molecule, called ATP, which the cell uses to power all of its parts.

  • Plant cells make energy a different way.

  • They have chloroplasts that combine carbon dioxide and water with light energy from the sun to create oxygen and sugar, a form of chemical energy.

  • All the parts of a cell have to work together to keep things running smoothly, and all the cells of your body have to work together to keep you running smoothly.

  • That's a whole lot of cells.

  • Scientists think there are about 37 trillion of them.

You're in line at the grocery store when, uh oh, someone sneezes on you.

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