Eli5:What makes cells different?



Ok, so high school bio got me as far as the concept of animal cells and plants cells. Great. Putting that behind us, animal cells can then be broken down into smaller categories right? From what I can tell, they are at least broken down by species and function, but are there further categorical reductions to be made? Just as an example; red blood cells vs white blood cells (both human), how and why are they structurally different? Do their organelles differ in size, shape, number or composition? What about two red blood cells from the same person, are they structurally identical? Could all human red blood cell mitochondria be considered interchangeable? Identical? I know the cell nuclei can’t at least. I really want to stress structure here, not function. I know I just typed a lot of question marks, but I am not looking to have them all answered, just trying to convey the gist of what I am after. I think I am trying to understand the gap between the intracellular level and the atomic level.

PS. slightly unrelated; when ATP is hydrolyzed into ADP and phosphate; is the energy released by that reaction the same heat energy that our bodies radiate? Is the phosphate the same phosphate excreted in our urine? Seems like it to me but I cant find explicit confirmation.

In: Biology

Some good questions. A cell’s structure is going to be tied to what it does. Since there are a lot of cells with different functions in your body, there are a lot of differently structured cells of all different size.

For example, white blood cells are tasked with keeping your body safe from invaders. They have special components to them that a brain cell would not have, and brain cells have special components that white blood cells would not have.

So cells can have different organelles, be different sizes, have different structural components, and have different numbers of organelles/components inside of them. Some cells have multiple nuclei (certain muscle cells), some have one, and some have none (red blood cells). Also, you mentioned red blood cell mitochondria- RBCs don’t have mitochondria because they are tasked with carrying oxygen through the body, and mitochondria eat up oxygen to make energy.

To your other question: ATP hydrolysis does generate energy (or transfer energy, if we’re being precise in language). Simplistically, energy can be defined as either heat or work. Heat is what you think it is and work is when something does something. That thing could be pushing a box, forcing a sodium ion against it’s concentration gradient, or anything really. The energy used in ATP hydrolysis is coupled with a reaction to provide some extra energy. Note: nothing is 100% efficient, so when performing work, some energy will be lost as heat.

Please let me know if I’ve answered everything or if there’s anything you’d want me to clarify on

When cells differentiate in a multicellular organism the entire structure can be completely different. It’s kindof the point.

Taking Red blood cells, as an example RBCs have almost nothing in them compared to most other cells. No mitochondria, no nucleus. They’re basically sacs of hemoglobin.

Within a species though, barring mutations, and some protien markers, (things that lead to tissue rejection when you transplant organs,) any cell of one type will be virtually identical to the same cell type in the same species.

I’m gonna go ahead and open with a nitpick / fun fact: red blood cells actually don’t have *any organelles at all*. Not even a nucleus. They’re just a membrane with almost nothing inside except tons and tons of hemoglobin.

Now getting on to your actual question:

Cells of different types (say, a human liver cell vs a human skin cell) are genetically identical, because of the “human” part. All human cells within the same human will be (theoretically) genetically identical. So why are there so many types of cells if they’re all identical? Because a liver cell and a skin cell will express different *portions* of your genome. The liver cell will have the “liver-functions” part turned *on* and the “skin-functions” turned *off*. Vice versa for the skin cell. What does that actually mean? The liver, for example, is an important site of metabolism so the liver cell will have a variety of metabolic genes turned on. You’re not actively metabolizing your lunch in your skin, so the skin will have those genes turned off.

Structure is similar to function on the cellular level. Structure is generally defined by a cytoskeleton, and also by all the stuff going on inside the cell. Some types of cells, like neurons, have their cytoskeleton made of very long actin bundles: this forms the axon of the neuron. Other cells, like red blood cells, don’t have that specific function turned on so they are more round. The “default” shape of a cell is round, and any other cell shapes you see are either the result of a specifically-shaped cytoskeleton, or simply the result of packing a bunch of cells together. You can picture this like squeezing a bunch of balloons together: they won’t be quite as round anymore but the parts that touch will become a bit flatter.

As far as organelles go, cells absolutely differ in that way as well. Muscle cells (myocytes) are basically mitochondria factories because they need so much ATP. On the other hand, fat cells (lipocytes) are basically one big fat-storage vesicle. So the type and number of organelles absolutely changes from cell type to cell type.

So you can see that by selectively turning on/off different parts of the genome, you can create different types of cells that do different things. In my favorite analogy with the genome being a book, each type of cell is reading a different chapter and not reading any of the other chapters.

The “book” (genome) for different humans varies by, like, a couple of letters. 99.999% of what’s going on inside you vs your neighbor is exactly the same. The cells all look the same, unless you have something like sickle cell disease or any number of blood conditions (microcytic anemia will cause your red blood cells to be smaller, etc).

But barring any diseases or weird mutations, your liver cell looks exactly like any other human liver cell. Your neurons look exactly like other human neurons. The biggest differences between people genetically speaking will be in the *exact detail* of a protein’s structure, not whether you have the protein or not.

Addressing your PS:

The heat that warms your body does not come from ATP hydrolysis. It is true that ATP hydrolysis does release *some* heat but the whole point of ATP is to use that energy for catalysis rather than just letting it escape as heat.

On the other hand, you have brown adipose tissue (BAT) which lies near your abdominal organs and is the main thing warming your body. Instead of hydrolyzing ATP, BAT uses a protein called thermogenin to generate heat. When ATP is being formed normally, the enzyme making it uses a proton gradient as energy. In BAT, that proton gradient is repurposed so instead of producing ATP the gradient is allowed to dissipate through thermogenin, and this dissipation creates a *lot* of heat.

Addressing the other point, you’re usually not excreting phosphates from ATP hydrolysis simply because those phosphates are used to reform ATP (via the reverse, Pi + ADP -> ATP). If you were massively excreting phosphates you wouldn’t have enough to recycle the ADP. The phosphate in your urine most likely comes from free phosphates in your blood which are part of the phosphate buffering system (a minor buffer that helps keep your blood at the right pH; the major buffer here is bicarbonate).

I tried to answer most of your questions, let me know if you have any other questions!