Biology Video 1 - Cell Transport
(4)(B) investigate and explain cellular processes, including homeostasis and transport of molecules
Hey, y’all. It’s your boy, Chucky D, and I just got a text from Andre in Houston. He said, “Hey, D, you’re
supposed to be the science pro, so I’ve got a question for you. Why do our fingers get all wrinkled up in
the pool or shower?”
Well, H-Town, that’s a pretty awesome question, but the answer is easy - osmosis. Let’s back it up a bit.
Let’s start with diffusion, okay?
Diffusion is just a big word that means molecules will spread out from an area of high concentration to
an area of low concentration. Duh! Right? It’s just like when a guy walks into a party wearing waaaay
too much cologne. At first, the people by the door have to breathe in his odor, and eventually we’re all
suffering! Those odor molecules all start off concentrated at the beginning, and then as they start to
spread out, they’ll bounce off each other, and eventually the whole room stinks.
Well, H-Town, all molecules diffuse this way, alright? What we care about is water. When there’s a
higher concentration of water on the inside of a cell, it will diffuse outside, and when there’s more water
outside, it’ll move on in. And since our cells are surrounded by membranes, that H2O has to pass through
that membrane. That’s what osmosis is. Cool?
Let’s talk about our cell membranes real quick, alright? The membranes around our cells are selectively
permeable; they select which molecules can come and go. They’re like the security guard on the outside
of a mall. Big molecules can’t just come on in, but water can come and go as it pleases.
Whenever there’s a high concentration of water on one side, it’ll naturally diffuse to the other side. The
way I remember it, it’s like water on one side of a dam. And if I punch a hole in the dam, it will be
permeable to water, and the water will move. That’s important.
So what does that have to do with H-Town’s wrinkled up fingers? Here’s what: If there’s a high
concentration of solutes, like molecules and cell parts, inside the cell, that means there’s a low
concentration of water in the cell.
You following me, H-Town? Here, let’s look at the worst drawing of a cell ever.
Here’s the cell membrane, and here’s all the solutes and cell stuff inside. But remember, we care about
how water moves, so there’s always water inside the cell. Those blue dots, those are water molecules,
but in this case, there’s a way higher concentration of water outside the cell, right? So how’s the water
gonna move? Well, it’s all gonna diffuse into the cell, right? Just like our water behind the dam! Easy,
right?
And then, the opposite is gonna happen. If the concentration of water inside your cell is higher than
outside, this is like when people are in salt water for too long. Since they’re in a solution with a lot of
salt solutes, there’s a higher concentration of water in their cells. What’s gonna happen? Let’s go back
to our awesome cell drawing – same cell, different solution. See? There’s not any water molecules the
outside the cell. There’s more water on the inside of the cell, so all the water will naturally diffuse
outside. This is just common sense, right, H-Town? And think about this – if the concentration of water
is the same as the outside, what’s gonna happen? Well, every time two or three water molecules come
in, another two or three will go out. Everything’s gonna be equal.
So what’s the big deal? Well, if you ask me, biology is like a foreign language, and now we gotta speak
Greek, okay? Let’s talk about hypotonic, hypertonic, and isotonic. A hypotonic solution was our first
one where there was a higher concentration of water outside the cell. I remember, “H2O rhymes with
hypo.” A solution that’s mostly H2O is hypotonic. Whenever a solution is mostly H2O, the water is going
to move into the cell, and the cell membrane might even explode from all the water inside.
The opposite was when the solution had a lower water concentration. If you put a cell in sugar water,
there’s gonna be more water inside than outside, and the cell’s gonna shrink because all the water’s
leaving. I think, “Sugar makes you hyper - hypertonic.” Awesome, right? This happens in salty solutions,
too.
Well, when everything’s all equal, that’s isotonic. I think, “Isotonic, nice-otonic.” This is why hospitals
have to give you a specific I.V. solution and not just pure water. The I.V. has to have the equal
concentration of water that our blood cells do.
Here, just look at this picture. In a hypotonic solution, the water is moving in, and the cell swells up,
maybe even pops. In a hypertonic solution, the water is leaving. The cell shrinks. In an isotonic solution,
there is an equal ratio of water going in and coming out. The cell is happy.
Whew! Pretty easy to understand. Just think about which side of the cell has the higher concentration
of water, and it makes sense. Then, just try to remember those Greek words.
One last thing, H-Town. Osmosis is all good for water, but what about other molecules? A lot of
important molecules need to get into our cells, but they’re just too big to just pass on through. Well it’s
no big deal. The cell membrane is more than just a plastic baggy. Look here. This is a phospholipid
bilayer. It’s made of two layers of lipids. You know – fats, oils, waxes. The membrane can do active
transport. It moves specific molecules in and out. This uses up a bit of energy, though.
So, whenever we need sugar out of our blood, it can go through one of these protein channels. That’s
what insulin does. It’s the key that opens up the protein channel. Also, the protein can form these
bubbles around the cell’s waste like a garbage bag. The vesicle melds with the membrane and gets spit
out. Plus, there are other ways to get specific molecules in and out. Kinda cool.
Alright, H-Town, back to your wrinkled up fingers. Let’s think it out. You’re in a pool. Is that a hypotonic
solution or a hypertonic solution? Well, H2O – hypo. Pool water is, like, pure water practically, right?
There’s a higher concentration of water outside of your cells, so what’s gonna happen? Duh! The
water’s gonna diffuse inside, and the cells in your fingers are gonna swell up. So they’re not really
“wrinkled up,” they’re all “puffed-up!” Crazy, right?
So short answer, H-Town – osmosis. ‘Til then, it’s your boy Chucky D saying, “Catch ya later.”