Movement of substances by diffusion is known as passive and it is transferred from one cell to the other cell. Diffusion also involves transfer of substances in the intercellular space of the leaf to its outside. Passive diffusion does not involve any energy. The molecules in a diffusion process move from higher concentration region to that of lower concentration in a random fashion in the case of passive diffusion. Diffusion takes place very slowly and it is found to occur in the living tissue. Diffusion is very important in plants as it is the only way to gaseous drive inside the plant. The rate of diffusion is influenced by the factors like concentration gradient, pressure, temperature and permeability of the membrane.
A gradient has to be present for diffusion to occur. The rate of diffusion depends on the substance size and the smaller sized substances will move quickly. The diffusion of the substances through the membrane is based on the solubility of the substance in the lipids, which are the important ingredients of the membrane. The substance that is able to get dissolved in lipids will be able to transfer through the membrane rapidly. The hydrophilic molecules which are able to dissolve properly in water will not be able to get diffused through membrane properly. Therefore, the transfer of water soluble substances has to be facilitated through the membrane as the membrane is made of lipids. The proteins present in the membrane facilitate the molecules to pass across the membrane. Even for the facilitated diffusion, there should be a concentration gradient that exists across the membrane.
In the case of facilitated diffusion, the proteins inside the membrane aid in the transfer of substances and the transport process does not involve energy. The transport of molecules from their lower concentration to their higher concentration cannot be classified into facilitated diffusion and it will not need energy input. The transport of molecules will become maximized when all the proteins in the membrane are used up.
The facilitated diffusion is very specific. Specific substances are chosen by the cell for its uptake in this form of diffusion. The inhibitors that stop the proteins to transport the substances will also show the impact on facilitated diffusion. The proteins are found to create channels in the membrane which are small while some are larger. A few of the protein channels works by controlling mechanisms. The channels called prions are present in the membranes of mitochondria, plastids and bacteria which are known to allow larger molecules to pass through them. The aquaporins are protein molecules present in the membrane that will rotate and leave the molecules inside the cell.