Communication between cell organelles and compartments are essential for metabolic reactions, internal signaling, cellular homeostasis, cell survival or apoptosis, cell function, and defense against pathogens. In the previous pages we learned that there is an intense cell compartment communication mediated by vesicles. There are other organelles that do not participate, or at least not so much, in the vesicular trafficking. There are, however, other ways of exchange molecules between organelles like diffusion through cytosol and physical contact between organelle membrane (Figure 1).
The physical contacts between membranes are suggested as communication sites between different cell organelles, and new results are uncovering a wide variety of functions. Many physical contacts between membranes have been reported since the first days of transmission electron microscopy. The distance between the two membranes is as small as 30 nm, which permits interactions of their proteins. The contact sites are actually membrane microdomains with particular set of proteins and lipids. Generally, these close membranes do not fuse between each other.
Muscle triad (endoplasmic reticulum-T tubule - endoplasmic reticulum) is one example of membranes very close to each other. T tubules are invagination of plasma membrane. The interaction of plasma membrane and endoplasmic reticulum membranes have been observed in many other cells and it is a way for direct communication between endoplasmic reticulum and plasma membrane avoiding the Golgi apparatus. This interaction is mediated by tricalbin proteins, VAPs and Ist2, which are transmembrane proteins in the endoplasmic reticulum membrane that are able to recognize inositol phosphate (PI(4,5)P2) of the cytosolic monolayer of the plasma membrane. There are many other examples of physical membrane contacts. Those between endoplasmic reticulum and outer membrane of mitochondria are involved in lipid metabolism, calcium regulation, mitochondrial division, and cell homeostasis and cell survival/apoptosis. Endoplasmic reticulum also wraps peroxisomes. Internal proteins of peroxisomes come from the cytosol, but membrane lipids and proteins come from the endoplasmic reticulum. It has been suggested that peroxisome growing and division is mediated a flux of proteins and lipids coming from contact sites with endoplasmic reticulum membranes. Mitochondria and peroxisomes membranes have also been observed very close to each other, probably involved in lipid metabolism. Membrane interactions have also been reported for endosomes-peroxisomes, and chloroplasts-other cell compartments.
This section deals with peroxisomes, mitochondria, plastids, included chloroplasts, and lipid drops. These organelles are outside the main vesicular trafficking, that is they do not communicate with other organelles by vesicles. At least, not so much as those organelles mentioned in the previous section (endoplasmic reticulum, Golgi apparatus, and endosomes/ lysosomes).
Peroxisomes are membrane-bound organelles with high metabolic activity related to oxidative reactions. Mitochondria and plastids, chloroplasts included, are delimited by two membranes. They are major energy production centers in the eukaryotic cells. Oxidative phosphorylation is done in mitochondria to produce ATP, but other metabolic reactions are performed too. Chloroplasts are a type of plastids, organelles found in plant cells. Photosynthesis is carried out by chloroplasts. It transforms light into proton gradients to be used for ATP synthesis. Lipid drops, as well as some types of plastids, are storage organelles for lipids, proteins and carbohydrates. These organelles will be studied in the next pages.
Bibliography
Schrader M, Godinho LF, Costello JL, Islinger M. 2015. The different facets of organelle interplay—an overview of organelle interactions. Frontiers in cell and develomental biology. 254: 151-213. 
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