Osmosis

What Does this Mean?

Well, let's go make a cup of tea to explain it. I think I've used this example somewhere else, but it's the same principle.

You have a glass (so you can see what's happening), and you fill it with hot water. In it, you then place your tea bag (or strainer, if you're using one of them instead), and you will begin to notice that the tea starts to diffuse, as it should do because you're making a cup of tea. The once clear water turns brown, and the tea bag starts to fill with water.

This is the process from a high concentration to a low concentration. Within the tea bag is a load of tea leaves (we shall call them the solute). While the solute cannot breach the barrier of the tea bag (we shall this the semi-permeable membrane), the water molecules can go through the membrane and into the tea bag, thus making the process of osmosis.

Here is a YouTube video from Don't Memorise explaining Osmosis:

What Does this have to do with Plants?

Well, the process is where a plant gains water from the roots, and then it transports it up through the plant to other parts, such as the leaves, or the stem, and even the flower. So, this process helps the plant to stay healthy by doing this.

In a plant, the solute is the sugars and amino acids in the plant that also help it to grow. These cannot move through osmosis, but are capable of being transported by the Xylem and Phloem, and this is called translocation.

Interesting fact: osmosis doesn't just occur in plants. It can also happen in animal and human cells. It is the main reason for active transport in eukaryotic cells.

Examples

A single plant cell, when taken from the plant, and placed into or near water, will 'ingest' the water using the vacuole of the cell. But the opposite happens if there is no water about. A leaf, for example, will not have a lot of water inside it if it has not rained for a few days and the plant is dry.

When a plant cell has plenty of water inside, it is considered to be turgid. This is considered to be as hard as the cell gets, with everything inside it 'bloated' to the point of breaking, and not being able to be move.

When a cell has no water inside it, it is considered flaccid, or limp. The other organelles within the cell have plenty of room to move about.

Aquatic plants that have water evaporated by the sun, would also lose water through the process of osmosis.

Any plant that uses the process has a thick cell wall to enable it to be rigid or limp. Animal cells also use this process, but they do not have the thick cell wall, and so will change shape when they take on water, or lose it.

Some cells, like blood plasma or tissue fluid are meant to be kept within strict limits, as if they don't have this they will not work efficiently.

Some Things to Note

Osmosis and Diffusion often get mixed up. This is because they both equalise the concentration of two solutions. They are also both passive transport systems, requiring no energy to process.

Where they differ is that diffusion can occur in any mixture. Osmosis only occurs in a mixture that requires a semi-permeable membrane. The other difference is that osmosis only allows the water molecules or solvent molecules to pass through the membrane. It is a special case of diffusion.

So, Osmosis is usually described as a passive transport that moves molecules between a semi-permeable membrane only.