 I am Dr. Miguel Hernández Prieto, an experienced researcher working on microbial photosynthesis. When we think of photosynthesis, we, our mental image, come to a plan. But cyanobacteria unicellular organisms can do exactly the same process. So why is cyanobacteria and plants used lie? Cyanobacteria cannot get to the size of a plant. Well, that's because plants are multicellular. Cyanobacteria is made of million cells, and cells is the smaller living form. To simplify things, we will represent cells as spheres. A spherical cell can be cut in half to visualize the inside. If we look from the side of the cell, we will see that the cell is surrounded by a plasma membrane. And this plasma membrane delimits the volume of the cell. While many substances and gases can permeate the plasma membrane, many molecules are transported through protein complexes within the membrane. This transport is essential for the input of nutrients into the cell, but also to get rid of toxic waste generated within the cell. Larger cells will need a larger surface to exchange substances. The problem is that when a cell becomes larger, the volume increases faster than the surface. This can be easily visualized if we imagine a perfect spherical cell of radius R. Both the area and the volume are proportional to the radius of the sphere. A cell with a radius of 5 nanometers will have an area of around 314 square nanometers and a volume of around 523 cubic nanometers and do an area per volume of 0.6. A cell with a radius of 15 nanometers will have a larger area of approximately 31,415 square nanometers and a volume of approximately 523,000 cubic nanometers, having an area to volume of approximately 0.06. The 10 times larger radius of the larger cell has resulted in a small surface area to volume ratio. Therefore, we have a bigger volume that requires more nutrients and produce more waste and a smaller surface to exchange it. And this is the main factor limiting the cell size. And we can see in this slide the smaller cells can get all the nutrients and release all the waste that need to thrive while the larger cells only can get part of the nutrients that need for the large volume and can also get rid only of a small portion of the waste. But not all cells are spherical. In plan, it is common to find cubic cells. Epithelial cells in the interest time contain folds called microbelies that increase the surface-to-volume ratio. And if you look through the microscope to see how bacteria cells you will be amazed of the variety in shape. Life in all its own is worth studying.