Cell Membrane and Osmosis: Diffusion and Water Movement

Diffusion allows molecules to move from high to low concentration, impacting cells and osmosis.

### Diffusion and Osmosis in Cells: Understanding Molecular Movement
* 🔍 Diffusion is the movement of molecules from an **area of high concentration to low concentration**, while osmosis is the **diffusion of water across a semi-permeable membrane**.
* 🔬 The plasma membrane, composed of phospholipids, is semi-permeable, allowing **small, uncharged molecules** to pass through directly.
* In animal cells, osmosis affects red blood cells placed in solutions of varying salt concentrations, leading to **crenation in hypertonic solutions** and **hemolysis in hypotonic solutions**.
* 🌱 Plant cells have an additional cell wall, which affects how they respond to **osmotic changes**.
* 🏥 In a hypertonic solution, plant cells undergo plasmolysis, causing wilting, as **water moves out** and the **central vacuole shrinks**.
* 💧 In an isotonic solution, there is **no net movement of water**, maintaining **equilibrium**.
* 💦 In a hypotonic solution, water moves into the plant cell, increasing **turgor pressure** and maintaining **cell rigidity**.
* 🌿 Turgor pressure is crucial for maintaining the structure of non-woody plants and can be observed in houseplants that **wilt and then recover after watering**.

so let's talk about diffusion. Diffusion is a process that does not require any energy input, and it is where molecules or solute molecules, all come move from an area where they have high concentration to an area of low concentration. So in other words, they're moving down their concentration gradient. And let's talk about how that specifically works with cells or plasma membranes. So remember, a plasma membrane is made up of individual phospholipids, and each phospholipid has a hydrophilic head, a polar head. It's water-loving, and it has hydrophobic tails. So this phospholipid molecule has both. It has a hydrophilic head and a hydrophobic tail. These come together. So you see here's one phospholipid, here's the second one, and we have essentially one layer. Here are the phospholipids. And then we call it a bilayer because we have a second layer. But the way that they orient themselves in the layers is you have a phospholipid from one layer and a

phospholipid from the other layer that are tailed to tail. So if you continue around and you imagine that this is the cell membrane of a cell, this would be inside the cell and this would be outside the cell. Then you see that the hydrophilic heads are both on the exterior most part of the plasma membrane and on the interior most pointing in the cell and pointing out. But in between those heads is this big layer, or actually it's two layers, of hydrophobic tails. And what that means is the plasma membrane, the cell membrane, is going to be semi-permeable. It allows some molecules to pass directly across the membrane and some it prohibits. So we can usually think that small and uncharged molecules can cross the plasma membrane, but if they're too large or they're too charged, then they will not be able to directly diffuse across. They'll need some kind of protein carrier or they'll have to be energy expended in order for them to cross the membrane. But in this case, what we see is

here our solute molecules represented by these blue hexagons, and they are permeable to the membrane. They can cross the plasma membrane. And you see at the beginning, we have a high concentration of these solute molecules outside the cell. And because they can cross the membrane, diffusion says they're going to move from where they're most concentrated to where they're least concentrated, so from outside in. And as we see over time, we end up with an equal concentration of solute molecules inside and outside the cell because they've reached equilibrium just based on diffusion alone. Now we're going to talk about another process called osmosis, which specifically deals with the diffusion of water. Let's talk about why we would need to have the diffusion of water. Well, we said that some solute molecules are too large or too charged to cross the plasma membrane. So when that's the case, water actually will move to try to make up for the difference in the solute concentration outside the cell versus inside the cell.

want to get this written down for you so water moves and what you have to here from high concentration of water to low concentration of water, and at the same time that also means it's moving from low solute concentration to high solute concentration. So let's pick a solute that we're going to talk about so that we can give an example of that. So let's say salt, sodium chloride is salt, and in water what happens is it breaks apart because it's an ionic bond into sodium ion and chloride ion, and because they're both charged, they are not able to diffuse across the plasma membrane. They can't get through that hydrophobic portion of the membrane. So if, as we see here with our red blood cells, so these are animal cells that we're looking at, red blood cells, they have a plasma membrane, and we're gonna put them in varying solutions of salt concentrations. So a normal red blood cell inside the cell has about 0.9 percent salt. So in this first solution

we're going to say we have about a 10 percent salt solution. Okay, isotonic. This is 0.9 percent salt. In other words, it's the same as what the normal red blood cell is. And over here in the hypotonic solution, we're gonna say this is just water. In other words, no salt in this solution. Well, let's talk about what we mean by that. So hypertonic. Hyper means more. So we're referring to this solute. In this case, our solute is salt. So that means the hypertonic solution has more salt in the solution than then the cell has inside of the cell. A hypotonic solution is the opposite. It has hypo means less. It has less than inside the cell. Now, if one is hypo, the other is always hyper. So for example, on the left, if the solution is hyper, that means the cell is hypo. Over here, if the solution is hypo, that means the cell must be hyper. In the middle with our isotonic solution, because they are exactly the same, they're identical, we have no net movement of water. So

water is moving into the cell, but water is also moving out of the cell. We don't have any net movement because our solute concentration is exactly the same. Now let's look at our hypertonic solution here because there is more solute concentration outside of the cell. Osmosis says that water is going to move from where there is a low solute concentration to where there is a high solute concentration. So water moves from the cell out into the solution. So that's depicted here. It shows you there's net movement of water from the cell out, and that's why the cells are shaped that way. It causes the cells to become crenate, or the process is called cremation. Now on the right side in a hypotonic solution, the opposite is occurring. So water is always going to move to where there is the higher concentration of solute, and in this case, there's more salt or more solute inside the cell. So water is actually moving into the cell from outside. And you can see this here. The

net movement of water. And that may actually, if it happens to a great enough degree, may cause the red blood cells to burst. And this is called hemolysis when this happens. So a little trick to remember this, remember that a hyperactive child has too much energy or has more energy. So hyper means more. Now let's look at the same example of osmosis and movement of water, how it applies to a plant cell. So we have our same hypertonic, isotonic, hypotonic conditions, in other words, the solution that the cell is in. So remember, this is 10% salt, this is 0.9%, and this is straight water, no salt. Now the difference is that a plant cell actually has a cell wall. It has a plasma membrane also, but it does have a cell wall, which is much more rigid. So when you put the cell in a hypertonic solution, meaning there's more solute outside the cell, right in the solution, same thing, you're gonna have a net movement of water out of the cell

one thing that you'll recall from our information that we've learned on plant cells is that plant cells hold most of their water in this large central vacuole right. And you see that when the water is removed, you have a much smaller central vacuole. It doesn't take up as much space, therefore it shrinks in the cytoplasm shrinks in, and you don't have any osmotic force really pressing out on the rigid cell wall. So in plants, we call this plasmolysis, and this is sort of what your plant looks like when it gets really wilty and it doesn't have a nice rigid structure. Now with isotonic condition, remember you have no net movement of water because you have equal solute concentrations. Now hypotonic because we water our plants with water, not with salt water. So this is a healthy typical way that a plant cell should look. You have a net movement of water in, you notice that the central vacuole is full, it's pressing out.

against the cytoplasm, which presses out against the cell wall, and this causes the cell to be turgid, okay, or have turgor pressure. And this is part of what gives non-woody plants their structure. So if you've ever had a houseplant that you forgot to water for a few days or a week, and it starts to really get wilty and kind of not look so healthy, and if you haven't let it go too long, you water it, wait awhile, and it begins to stand up and have shape and structure again. And that's because you've restored the turgor pressure of the cell.

Tonicity: Hypotonic, Isotonic, and Hypertonic

Natalie Russell

The Process of Diffusion and its Relation to Plasma Membranes

Diffusion is the movement of molecules from high to low concentration without energy input.

Plasma membranes are made up of phospholipids with hydrophilic heads and hydrophobic tails, creating a semi-permeable barrier.

This allows small and uncharged molecules to directly diffuse across the membrane, while larger or charged molecules require energy or protein carriers to cross.

Solute molecules can freely cross the membrane through diffusion, leading to an equal concentration inside and outside the cell.

Osmosis, on the other hand, involves the movement of water to balance the solute concentration difference, especially when the solute molecules are unable to cross the membrane.

This process is crucial in scenarios such as the movement of salt in and out of red blood cells.

Understanding Hypertonic, Hypotonic, and Isotonic Solutions

A hypertonic solution has a higher salt concentration than the cell, causing water to move out of the cell, leading to cell shrinkage.

A hypotonic solution has a lower salt concentration than the cell, causing water to move into the cell, resulting in cell swelling.

An isotonic solution has the same salt concentration as the cell, resulting in no net movement of water.

The Role of Osmosis in Red Blood Cells and Plant Cells

Osmosis can cause red blood cells to burst through a process called hemolysis, which is the result of a net movement of water into the cells.

In plant cells, osmosis affects the central vacuole, leading to plasmolysis in hypertonic conditions and turgor pressure in hypotonic conditions, influencing the overall structure of the plant.

Isotonic conditions result in no net movement of water, maintaining a stable state for the cells.

The Importance of Turgor Pressure in Plants

Turgor pressure is caused by the cytoplasm pressing against the cell wall, providing structure to non-woody plants.

When plants lack water, they become wilted, but watering them restores turgor pressure, allowing them to regain shape and structure.