Photosynthesis and diffusion

There's not much light at this time of year and only a few hardy plants retain their leaves and continue to do photosynthesis. In order to do photosynthesis, they need to get carbon dioxide into the leaf. They do this by the process of DIFFUSION which is actually a bit of Physics. In a gas or a liquid, the particles are in constant random motion. If there is a group of particles all together, the random motion will tend to spread them apart. Thus diffusion is when gases or liquids move from where they are most concentrated to where they are least concentrated. Look at my box diagram of the leaf. There are far more black dots representing carbon dioxide in the air underneath the leaf than there are in the spongy mesophyll layer. Carbon dioxide moves from where it is most concentrated underneath the leaf to where it is least concentrated inside the leaf.