Gahh, The Society For The Protection Of Plants Are Coming!

Today's post is on Biology, kids.

We all know that plants photosynthesise, which involved water, which we know is sucked up through the roots. We also know that plants need CO2 to photosynthesise, and that they release Oxygen as a product of photosynthesis. But, plants don't have mouths or lungs to breathe, and they don't have tongues and stomachs to suck up water as if through a straw.

So how do plants do it? Break them open and they're just leafy-green all the way through. The greatest puzzle of mankind? Of course not, it call all be explained with some other stuff we already know.

Take the process of diffusion. Here's the law, just in case;

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration (AKA, down a concentration gradient).

So, we know its possible for CO2 and Oxygen to diffuse. If only plants had cells that let that happen...

Oh, would you just look at that? That ^ is a cross section of a leaf. And see the thing labelled 'Stoma'? That's basically a tiny hole in the skin of the leaf. Conveniently, it's big enough to let Oxygen and CO2 pass in and out.

No prizes for guessing this is how gas exchange happens in a plant.

So lets make things nice and clear.

To start off, there is a high concentration of CO2 in the air, so it diffuses into the leaf. Its then used to photosynthesise and is turned into Oxygen. This means there's a higher concentration of Oxygen in the plant than outside it, so it diffuses out. This happens continuously.

So, if that's how plants get CO2 for photosynthesis, how do they get the water?

Yep, through the roots.

But, for the water to get from the roots to the leaves, it would pretty much have to defy gravity. How? By sucking. Yeah, I know plants don't have mouths or stomachs. But they can suck, because just the same as gases can be diffused out of the leaves, so can water. So water vapour left over from photosynthesising is diffused out, leaving a little gap for more water to fill. So all the water in the plant moves up a little bit, leading to a tiny bit being sucked up through the roots.

Lovely. That whole process of the water moving is known as Transpiration. A complex name for a pretty simple theory.

Now, there a few other little things I'll need to know.

And those are how the rate of transpiration is affected.

Light is a huge factor. Generally, the brighter the light, the greater the transpiration rate, because the plant is photosynthesising faster. But the plants do need to keep some water to keep them hydrated, which is why all the stomata are on the bottom of the leaf, where it's cooler and darker. The stomata also start to close in the dark, because photosynthesising can't happen in the dark.

Another factor is temperature. When it's hotter, the rate increases because the particles have more energy to move and diffuse.

Air movement also has a big role. Diffusion (and subsequently transpiration) happens more quickly when it's windy because the water vapour is blown away. But when it's not windy, all the water vapour stays around the plant, so there is a higher concentration of water outside the plant.

Finally, the humidity of the air around the plant affects transpiration. Think about it, if there's more water in the air, the concentration gradient will be less, to diffusion will be slower.

Tah-dah, there's a perfectly adequate lesson on transfusion. And damn, my teacher could't explain it in a whole hour. Tut tut, I'm appalled.

Thank me later, people of the world.