Chapter 11: What Is Inside Cells?
What are the parts of a cell? How are eukaryotic cells different than prokaryotic cells? Come find out in a visit to a special castle...and a tank!
Welcome to the first installment in the Kingdoms of Creation home education science program. We’re glad you’re here. Each chapter is split into two parts: Part 1 is for younger students (usually K-4); Parts 1 and 2 together are for older students (usually 5-8). The Kingdoms of Creation is a comprehensive biology program. See the full table of contents here. If you would like supplementary literature suggestions and activity guides (and to get in on all the fun!), consider subscribing.
Part 1: The Cell – A Castle in Disguise
A mighty fortress is our God,
A bulwark never failing
-A Mighty Fortress is Our God, Martin Luther
You have met the many types of cells, from long nerve cells to blobby fat cells, in all their wonderful variety. But, what if I told you that all of those types of cells are actually castles in disguise. That’s right! Those tiny building blocks of life, so small you can’t even see them without a microscope, are actually made of many parts that work together, just like the magnificent castles of old.
Imagine you are a knight or a damsel, riding a horse down a country lane. You turn a corner and before you lies a rocky hill topped with a castle. Green banners flutter from the tops of the bright white towers that peak over the imposing stone walls. Those walls seem awfully forbidding, and for good reason: they are there to protect the lord and his people.
A castle’s walls are like a cell’s membrane – a thin layer that encircles the cell. Just like there are gates in the castle walls to let people in and out, a cell’s membrane allows food, water, and wastes to enter and exit the cell. Plants cells are even more like castles; along with their cell membrane, they also have a cell wall. This stiff outer coating gives plants like oak trees the strength and stability they need to reach to the sky.
You guide your horse through the gates and into the courtyard into the bailey. This courtyard tucked safely behind the walls is abuzz with activity as workers move to and fro between the stables, the blacksmith, the carpenter. Ahead of you rises the castle proper, known as the keep, where the lord lives who directs his subjects about the crops to grow and the defenses to build.
Inside the cell membrane, we find an area much like the bailey – the cytoplasm [sy-toe-plaz-um]. This is a substance rather like jelly which gently holds all the organelles (the smaller parts that make up the cell, each with their own job). One of those organelles is the nucleus, like the keep of a castle it is the “brains” of the operation. It contains the information that makes a particular creature the way that it is in the form of DNA.1 This rather miraculous molecule is tightly coiled like a never-ending spiral staircase inside the nucleus and sends out the information to build the different parts of the creature.
Back in the castle, you are not here today to see the lord. No, today you are in search of a sword or perhaps some cheese or maybe a chair. You are looking for one of the many artisans that work here in the castle. They make all the things that the lord needs, from food and clothing to pots and pans.
Many of the organelles are involved in making things as well. Like the kitchens of a castle, the mitochondria
[my-toe-con-dree-uh] take the food that a creature eats and turns it into energy for the cell to use. This pill-shaped organelle (often pictured with a squiggle down the middle) is the powerhouse for animal cells. Plant cells have another source of energy – chloroplasts [klor-oh-plasts]; these little pill-shaped features can take the energy from the sun and turn it into sugar to power the cell.
The endoplasmic reticulum (ER) [en-doe-plaz-mic ruh-tick-you-lum] is like the artisans of the cell. Just like the cluster of shops might be tucked into one corner of the castle, the ER is maze-like structure that makes the fats and proteins for the creature.2 The smooth ER specializes in fat production, while the rough ER is covered in little ribosome balls which make the proteins.
Of course, no castle or cell would be complete without some storage space. After all, where else than the root cellar are we going to store our apples until we are ready to take them to the kitchens to make apple pie? In cells, extra food and water are stored in the vacuoles until the mitochondria is ready to use it. Animals have smaller, ball-shaped vacuoles while plants have larger ones that are often shaped like kidneys. These little storage spaces, in cells or castles, help to make the whole run smoothly.
Note: More parts of the cell will be introduced in Part 2, but there are even more that we won’t meet even there. These are the most important part of the cells, but there is always more to discover in this world made by an infinite Creator.
1 Or deoxyribonucleic [dee-ox-ee-rye-boe-new-clay-ick] acid.
2 Proteins, for those who don’t know, make muscles, send messages, and do many other jobs in your body.
Part 2: From a Castle to a Tank - Eukaryotic vs. Prokaryotic Cells
Our gates were strong, our walls were thick,
So smooth and high, no man could win
A foothold there, no clever trick
Could take us, have us dead or quick.
Only a bird could have got in.
-The Castle, Edwin Muir
Back in the castle’s bailey some of the serfs are hard at work loading up a cart to take to the market in the next village. Some put vegetables into baskets, some carefully wrap up some bread loaves, still others stack the cheese neatly in the corner. You have retrieved your shiny new sword (or perhaps candelabra) from the forge and area ready to head out with the market wagon, but first you have to stop at the privy (you know, ye olde water closet...without the water), which often fed into the moat to be later used as fertilizer for fields.1 Then you are on your way back to your own castle of artisans, serfs, and even privies.
Like the members of the castle, cells also need to package up the fats and proteins made in the endoplasmic reticulum into spheres2 called vesicles [vess-ick-uhls]. This job is done by the delightfully named Golgi [goal-gee] body (also known as the Golgi apparatus), which looks like a series of differently sized rubber bands. The vesicles can then leave the cell like the market wagon to take the fats and proteins where they need to go. Waste management in the cell is handled by the lysosomes [lie-zoe-zomes], more spherical structures that are a bit bigger than the vesicles. They protect the cell from viruses and other invaders and recycle parts of the cell that are worn out.
So far, we have been talking about eukaryotic cells: those with a nucleus, including plants, animals, fungi, and protists. But what about prokaryotic cells that lack a nucleus, such as bacteria and archaea? Do they have all the same organelles as plant and animal cells? To illustrate how these cells are similar and different, let’s leave a defensive war machine for an offensive one – a tank.
Tanks are mobile strongholds, significantly smaller and simpler than a castle but still highly effective. As you ride along on your horse, you come upon this strange contraption with its thick armor, treads, and top hatch. You are surprised to see such a thing – like a boat made of metal crawling across the land. Suddenly, the top hatch opens, and a young man’s head appears. He invites you inside, and you see some amazing wonders – in the small space inside, there are four people looking at glowing tablets. They are working together to tell the tank which way to move using something called a “computer.”
….Okay, perhaps that was rather silly, but it illustrates the point. Prokaryotic cells have cell membranes to protect them, just like the eukaryotic cells. Prokaryotic cells are usually significantly smaller than eukaryotic cells but more mobile. Many bacteria have cilia (like little hairs) or flagella (like tails) that allow them to move around, while most plant and animal cells stay in one place because they are a part of a larger creature. Like the bailey in the castle, the tank has an open area inside; this inside area is filled with cytoplasm in both. Bacteria cells do not have a nucleus just like a tank does not have a lord; instead it has a nucleoid [new-klee-oid], DNA distributed around the cell. Like a team of soldiers working together, the bacteria’s DNA still gives instructions to the cell but in a more diffuse way. The only other organelles that prokaryotes have are ribosomes. These protein-producing powerhouses are like the computer in a tank that takes the instructions from the soldiers and then actually makes the tank move.
You might be asking yourself, why are bacteria cells missing such random things – a packing center for proteins, storage spaces, a container for its DNA? Why these things? They don’t seem to have anything in common. The thing that connects all the missing structures is that they all have membranes – wrappers around them that separate them from the cytoplasm. While bacteria do have a cell membrane to separate themselves from the rest of the world, they do not have any membrane-bound organelles (as we call this category of cell parts). Although it may seem like this is a weakness, the Creator must have known what He was doing, after all bacteria and archaea are everywhere, doing amazing things in our world.
1This practice is still done today with the use of “biosolids,” which is just a fancy name for human waste that can be used to grow food.
2You might have noticed an abundance of spheres in cells. Thinking back to fat cells, can you figure out why that might be? Spheres are excellent for storage, which is often needed in cells of all types.