We'll start building our cell using a ready-made plasma membrane. The sizes of cells and organelles are usually given in micrometers, or 10-6 meters. Drag the ruler over the cell to measure its diameter to the nearest micrometer, and enter the value in the box. Click Calculate to find the volume of this cell.

feedback for correct answer:

Yes, that’s correct.

If we assume that the cell is a sphere, then its volume is four-thirds pi r-cubed, where r is the radius, or half the diameter, in units of cubic micrometers. Jot this value down so you can use it later! When you're ready, click Enter to go inside the membrane.

feedback for incorrect answer:

Sorry, your measurement isn't right. Take another look at the ruler and try again.

The plasma membrane is a phospholipid bilayer where the polar phosphate groups are on the outside and the fatty acid chains are on the inside. The membrane is about 5 nanometers thick. A nanometer is 10-3 micrometers, so the membrane thickness is about 4000 times smaller than the diameter of the cell. So what's in a cell? First there's the cytoplasm, which includes all of the contents except the nucleus. The cytoplasm includes the other organelles as well as the cytosol, an aqueous fluid containing ions, small molecules, and soluble macromolecules. Let's begin filling our cell!

The first thing we'll add is cytosol, which occupies about 65% of the cell's volume. How much cytosol should we add? Enter the value in units of cubic micrometers. If you're not sure, click Hint.

feedback for correct answer:

That's right! We take 65% of the total cell volume to estimate the volume of cytosol to add. Click Continue to return to constructing our cell.

feedback for incorrect answer:

Sorry, that's not right. We take 65% of the total cell volume to estimate the volume of cytosol to add. Click Reset to try again, or click Jump Ahead to skip this step.

audio for hint:

Earlier we calculated the total volume of the cell. If we take 65% of this value, we can estimate the volume of the cytosol. Click Return when you're ready to try again.

Let's pour the cytosol into the cell. We didn't quite fill the cell, because we need to save room for the nucleus and the organelles. We're ready to begin adding organelles! Most eukaryotic cells have a nucleus containing most of the cell's DNA. The nucleus is about 5 micrometers in diameter, making it the largest organelle in eukaryotes. The nucleus is surrounded by an inner and an outer membrane which together make up the nuclear envelope. Pores in the nuclear envelope allow RNA and small water-soluble molecules to enter or leave the nucleus. The cell's genetic material is in a form called chromatin, which is suspended in an aqueous solution called nucleoplasm. The nucleus also contains one or more nucleoli, which make the components of the protein-synthesizing organelles called ribosomes. Use your pointer to drag the nucleus into the cell.

Let's explore the ribosome in more detail. A ribosome is made of a small subunit and a large subunit, each containing RNA and proteins. The subunits are transported separately through the nuclear pores into the cytosol, where they're assembled into a functional ribosome. A ribosome is much smaller than the nucleus, only about 0.02 micrometer in diameter. Cells that synthesize a lot of protein, like the liver cell we're building, contain between ten thousand and twenty thousand ribosomes. It'd be hard to see that many, so we'll let one of these symbols represent a thousand ribosomes. How many 1000-ribosome symbols should we add to our model cell? Enter a number in the box, then click Submit.

feedback for correct answer:

Yes, that’s it.

feedback for incorrect answer:

Sorry, that's not right. We divide the total number of ribosomes by 1000 to figure out how many symbols to include in our model. Click Reset to try again or Jump Ahead to skip this step.

If each symbol represents 1000 ribosomes, then we need between 10 and 20 symbols. For simplicity, let's assume our liver cell contains 15,000 ribosomes, or 15 ribosome symbols. Ribosomes that float freely in the cytosol synthesize proteins that end up in the cytosol. However, many ribosomes are bound to a network of membranes called the endoplasmic reticulum, or ER, which is continuous with the nuclear envelope. The ER contains structures and molecules that synthesize and break down proteins, lipids, and carbohydrates. Rough ER, named for its bumpy appearance, contains ribosomes that synthesize proteins destined to be packaged inside organelles or released outside the cell. Smooth ER, which lacks ribosomes, is mainly responsible for the synthesis and breakdown of small molecules.

Many proteins synthesized in rough ER are eventually released outside the cell. But first, they need to be modified, packaged, and routed by the Golgi apparatus, conveniently located adjacent to the ER. Small membrane-enclosed transport vesicles containing newly synthesized proteins are pinched off from the vast ER membrane network and travel to the side of the Golgi apparatus closest to the ER. Click on one of the transport vesicles to see what happens to it and its contents as it moves through the Golgi apparatus.

We've just considered how molecules get from inside the cell to the outside. But what happens to food molecules that are taken into the cell? They are digested. This task is performed by lysosomes, which contain digestive enzymes to break down biological molecules. These enzymes work best around pH 5 and are mostly inactive at the pH of the cytosol (7.2). Lysosomes can be anywhere from 0.05 to 0.5 micrometer in diameter, depending on how much food they've taken in. This lysosome looks like it's about 0.25 micrometer in diameter. Each of these symbols represents 10 lysosomes. A typical liver cell has between 200 and 400 lysosomes. How many symbols should we include in our cell? Enter a number in the box, then click Submit.

feedback for correct answer:

Good job.

feedback for incorrect answer:

Sorry, that's not right. We divide the total number of lysosomes by 10 to figure out how many symbols to include in our model. Click Reset to try again or Jump Ahead to skip this step.

If each symbol represents 10 lysosomes, then we need between 20 and 40 symbols. For simplicity, let's assume our liver cell contains 300 lysosomes, or 30 lysosome symbols.

Lysosomes break down biological molecules, but similarly sized and shaped organelles called peroxisomes break down toxic substances. They do this using enzymes that transfer hydrogen atoms onto oxygen to form hydrogen peroxide. That's where their name comes from! But hydrogen peroxide is also toxic, so peroxisomes contain enzymes that convert the hydrogen peroxide to water. Liver cells contain, on average, 300 to 500 peroxisomes, since lots of toxic substances accumulate in the liver. Each of these symbols represents 10 peroxisomes. How many symbols should we include in our cell? Enter a number in the box, then click Submit.

feedback for correct answer:

Yes, that’s right.

feedback for incorrect answer:

Sorry, that's not right. Select a number of peroxisomes between 300 and 500, then divide by the number of peroxisomes per symbol to determine how many to include. Click Reset to try again, or click Jump Ahead to skip this step.

Since each symbol represents 10 peroxisomes, we need between 30 and 50 symbols in our cell. Let's add 40 symbols, equivalent to 400 peroxisomes.

Now let's think about where the cell gets most of the energy for the processes we've looked at so far. The mitochondrion is the cell's main "power plant". Mitochondria convert the chemical energy in food to a more usable form: adenosine triphosphate, or ATP. We'll learn about how that's done when we study metabolism. Mitochondria are shaped like an ellipsoid, or elongated sphere. They're about 0.5 micrometer across and between 1 and 5 micrometers long.

Mitochondria have two membranes. The outer mitochondrial membrane surrounds the organelle. The intermembrane space lies between the outer and inner mitochondrial membranes, while the inner mitochondrial membrane contains numerous folds, called cristae, that increase the surface area where the production of ATP takes place. The region inside the inner mitochondrial membrane is the mitochondrial matrix. It contains several of the key enzymes needed to convert food energy to ATP.

Liver cells contain between 1500 and 2000 mitochondria, which together occupy more than 20% of the total cell volume. Compare that to the nucleus, which occupies roughly 5% of the cell volume. Now that we've added a source of energy, we're done building our cell!

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education