As you read these words, electricity is zinging through your brain, voracious killers are coursing through your veins and
corrosive chemicals bubble from your head to your toes. In
fact, your entire body is like an electrical company, chemical
factory, transportation grid, communications network,
detoxification facility, hospital and battlefield all rolled into
one. The workers that drive these activities are your cells.
Our bodies contain trillions of cells, organized into more
than 200 major types. At any given time, each cell is
doing thousands of routine jobs, like transforming and using
energy, manufacturing proteins and responding to
environmental cues. Different cell types also have special
duties, like building skin or bone, pumping out hormones or
making antibodies. hat accounts for about half the cell's volume (organelles make up the rest). In addition to the outer
membrane, which is made up of proteins and lipids (fats), the cells of humans and other higher organisms
have a pair of porous membranes that envelop the nucleus. Each organelle also has an outer membrane.
Endoplasmic Reticulum and Partners
Next to the nucleus are enormous, interconnected sacs called the endoplasmic reticulum or ER. From your
shrunken view, each sac is only a few inches across, but they can extend to lengths of 100 feet or more. The
sacs come in two types: a "rough" version covered with protein-making ribosomes and a "smooth" version
that makes lipids and breaks down toxic molecules.
The ER sends newly made proteins and lipids to the Golgi complex, a short and narrow structure inside the
cytosol. The Golgi complex processes them and sends the molecules to their final destinations inside or
outside the cell.
Mitochondria
Mitochondria are organelles that convert energy from our food into ATP, or adenosine triphosphate, to power
biochemical reactions. Credit: D. S. Friend, Brigham and Women's Hospital.
About the size of pickup trucks from where you're floating, the organelles called mitochondria convert
energy from your food into adenosine triphosphate, or ATP, to power biochemical reactions. A typical cell
burns through 1 billion molecules of ATP every 1 to 2 minutes.
Like all other organelles, mitochondria are enclosed in an outer membrane. But they also have an inner
membrane that's actually four or five times larger than the outer one. The inner membrane doubles over in
many places so it can fit, extending long, fingerlike folds into the center of the organelle. These folds vastly
increase the surface area for ATP production.
40,000-Foot View
Back in the human-sized world, many scientists funded by the National Institutes of Health are studying these
cellular structures—and many others not listed here—because knowledge about them underpins our
understanding of health and disease. For instance, recent research suggests why the nucleolinus (a cellular
compartment found in a range of species) is crucial for proper cell division, and how a special arrangement of
microtubules (cellular highways that transport raw materials) may help nerve cells rebuild after injury.

Structure and Purpose
Use the text and visuals to answer the following questions. Use complete sentences, and provide textual evidence to support your answers.
11. How do the authors structure (arrange) the article?
12. What is the purpose of the subheadings?
13. What is the purpose of the images and captions in the article?
14. What is the purpose of the article?
15. How does the final paragraph relate to the main idea of the article?