Study Guide

Cells Introduction

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Cells Introduction

Seeing the word cell triggers so many memories. You may recall your last dropped call, solar panels, your current Fantasy Baseball spreadsheet, or your last school field trip to the local penitentiary. No? Anyway, onto real biology.

All living things (we like to call them organisms because no one wants to be called a "thing") have five characteristics in common. Cells are the basic units of life because they are the smallest entities on Earth to have all five of the attributes of Life.

The Five Basic Attributes of Life, Round Two

  1. Complexity and organization. Cells are complex and highly organized. If you don’t believe us, skip to the In Depth section right now. Go ahead. We dare you.
  2. The ability to acquire and use matter and energy. Cells eat and drink substances that provide them with energy.
  3. Homeostasis. Cells maintain a constant internal environment.
  4. The ability to reproduce. Cells reproduce all on their own.
  5. The abilities to adapt and evolve. Cells adapt to new environments and evolve through time.

(If you are totally confused right now, it probably means you need to go back to our handy dandy little intro unit. You might want to do that now, before reading on. Otherwise, on with regularly scheduled programming.)

Cells live and behave in a variety of ways. Some cells require the services of other cells to survive. The cells in your liver, and every other part of your body for that matter, require red blood cells to deliver oxygen. Without red blood cells, all the other cells in your body would die. Kaput. Lights out.

Other cells are able to grow and reproduce without the services of other cells. Many bacteria fall into this category. The entire bacterial "organism" is just a single cell. The human "organism," in contrast, is estimated to have between 10 and 100 trillion cells. You read that right: trillions of cells. Take that, US federal deficit. What’s more, some biologists estimate that there are 20 times more bacterial cells living inside the human body than there are actual human cells making up the body. We hope you aren't a germophobe, and if you are, this unit just might cure you.

There are two major types of cells:

  • Prokaryotic cells
  • Eukaryotic cells

While both types of cells have some attributes in common, we usually focus on the differences (glass-half-empty theory), like the fact that eukaryotic cells are generally larger and more complex than prokaryotic cells. The defining difference between the two cell types, however, is that eukaryotic cells have membrane-bound organelles (if you are thinking "mini organ," you are on the right track), including the nucleus. The name "eukaryote" means "true kernel," where "kernel" refers to the nucleus. "Prokaryote," on the other hand, means "before or without a kernel." In the end, it is the presence or absence of a membrane-bound nucleus that determines whether a cell is classified as prokaryotic or eukaryotic. Everything else is just details.

While you're here, we might as well give you the back story of the cell's discovery. A couple of geeks (a term of endearment around these parts), going by the names of Robert Hooke (in 1665) and Anton van Leeuwehoek (in 1673), were spending their days waiting around for the invention of DVR and YouTube. In an attempt to kill 300+ years of time, Bob used optical lenses to magnify the parts of a tree: the cork, the bark, the stems, the roots, and so on. He found these parts had little "boxes" in common that reminded him of the cells that the monks lived in. Hence, the name.

Also needing to occupy his hours, Anton ground the lenses further to produce more magnification (10x) so that he could observe the germs in pond water. Yep, you read that correctly. Someone wanted to study pond water. Biologists, what can you do…

These boxes were consistently observed in many living organisms over the next few years, and eventually, the cell theory was developed. With continued technological development and dedication to cell theory, scientists were able to decipher the inner workings of the cells. And so began a revolution that has fascinated scientists ever since. And…oh, you know…has saved hundreds of millions of lives every day through the practice of modern medicine.

Cells Resources


The Biology Project
Interactive biology quizzes! Now, we know how much you loooove quizzes, but these ones have super detailed answer explanations. Very helpful, we promise.

Organelle Cheat Sheet
A pretty sweet cheat sheet with hand-drawn representations of each of your favorite organelle friends.

Bacteria Talk
A Q&A session with a Princeton professor who is changing the way we think about bacteria.

Cells and Cellular Organization
An image-intense explanation of the parts of the cell. And when we say intense, we mean intense.

Molecular Expressions
We linked you to the home page on cells, but there are separate pages on animal cells, plant cells, bacteria, plasma membranes, cell images, and cells in motion.

BioVisions at Harvard
Killer animations brought to you by BioVisions at Harvard University. We've linked you to "Mitochondria," but there are a number of others. Obligatory uplifting/dramatic music included, of course.


Cells in Action
A no-less-than-awesome '80s video featuring cells doing their things. Enjoy the excessively dramatic '80s music and narrator.

Plasma Membranes
'80s video number two, with a featured clip on cell murder.

Voyage Inside the Cell
As we move into the '90s, another super dramatic video of how molecules get into cells.

Cells by They Might Be Giants
They Might Be Giants sing about cells. So many songs about science! We <3 TMBG.

Cellular State of Mind
If you can handle the off-key singing, the song is kind of catchy… In fact, we're humming it to ourselves right now.

MicroscopyU Live Cell, Swept Field, and Confocoal Microscopy Imaging Live Cell
Swept FieldConfocalWatch live cell motion and the behavior of organelles.


Bacteria Outnumber Cells in Human Body
An explanation of how we are basically a bunch of "hotels" that house a lot of bacterial houseguests. Gross.

Where Would We Be Without Bacteria?
NPR discusses infection and where we would be without bacteria.


How a Cell Sounds
NPR throws a party to demonstrate what the interior of a cell might look like.

Beautiful Cells
Some pretty mind-blowing scanning electron micrographs of different types of cells in your body.

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