<P> Another large class of protein compartments are bacterial microcompartments, which are made of a protein shell that encapsulates various enzymes . These compartments are typically about 100 - 200 nanometres across and made of interlocking proteins . A well - understood example is the carboxysome, which contains enzymes involved in carbon fixation such as RuBisCO . </P> <P> Although the cytoskeleton is not part of the cytosol, the presence of this network of filaments restricts the diffusion of large particles in the cell . For example, in several studies tracer particles larger than about 25 nanometres (about the size of a ribosome) were excluded from parts of the cytosol around the edges of the cell and next to the nucleus . These "excluding compartments" may contain a much denser meshwork of actin fibres than the remainder of the cytosol . These microdomains could influence the distribution of large structures such as ribosomes and organelles within the cytosol by excluding them from some areas and concentrating them in others . </P> <P> The cytosol has no single function and is instead the site of multiple cell processes . Examples of these processes include signal transduction from the cell membrane to sites within the cell, such as the cell nucleus, or organelles . This compartment is also the site of many of the processes of cytokinesis, after the breakdown of the nuclear membrane in mitosis . Another major function of cytosol is to transport metabolites from their site of production to where they are used . This is relatively simple for water - soluble molecules, such as amino acids, which can diffuse rapidly through the cytosol . However, hydrophobic molecules, such as fatty acids or sterols, can be transported through the cytosol by specific binding proteins, which shuttle these molecules between cell membranes . Molecules taken into the cell by endocytosis or on their way to be secreted can also be transported through the cytosol inside vesicles, which are small spheres of lipids that are moved along the cytoskeleton by motor proteins . </P> <P> The cytosol is the site of most metabolism in prokaryotes, and a large proportion of the metabolism of eukaryotes . For instance, in mammals about half of the proteins in the cell are localized to the cytosol . The most complete data are available in yeast, where metabolic reconstructions indicate that the majority of both metabolic processes and metabolites occur in the cytosol . Major metabolic pathways that occur in the cytosol in animals are protein biosynthesis, the pentose phosphate pathway, glycolysis and gluconeogenesis . The localization of pathways can be different in other organisms, for instance fatty acid synthesis occurs in chloroplasts in plants and in apicoplasts in apicomplexa . </P>

Where does most atp production take place within a cell (points 1)