ï»¿ Even if you're just studying, a lot of work is going on in your cells. Let's zoom down and check it out. We've arrived in the space between two cells. A sticky coat called the extracellular matrix holds the cells together. Each cell is surrounded by a flexible plasma membrane with an incredible number of projections, docking stations, and channels. Let's dive into one of these channels to enter a cell. Whoa-Look at this place! These girders and cables make up the cytoskeleton, the structural framework of the cell. They also serve as tracks for transporting cargo from one place to another. All this activity in the cell requires energy in the form of ATP molecules, which are made here in the mitochondrion. Notice the outer membrane and the inner membrane with its numerous infoldings. Many of the molecules involved in making ATP are built into the inner membrane. All those folds increase the inner surface area, enabling more ATP to be made. Moving towards the nucleus, we pass by layers of internal membranes. The nucleus is enclosed by a double membrane called the nuclear envelope. Let's enter the nucleus through a pore. The nucleus houses the genetic material of the cell-DNA, which carries the blueprints for making the cell's proteins. Almost two meters of DNA is crammed inside the nucleus. How does it all fit? The DNA is wrapped around proteins, like thread wrapped around spools. Look-- This section of the DNA has unwound and a different protein has attached to the DNA. DNA is being used as a template to make mRNA. mRNA molecules travel from the nucleus to the cytoplasm, carrying the instructions for making specific proteins. In the cytoplasm, a ribosome clamps onto a strand of mRNA. The ribosome ratchets along the mRNA, building a new protein. Some proteins stay in the cytoplasm. Others, like this one, are processed in special compartments within the cell. Protein processing and certain other metabolic activities occur in the endomembrane system, the cell's network of internal membranes. The endoplasmic reticulum, or ER, is part of the endomembrane system. There are two types of ER: rough and smooth. Rough ER is covered with ribosomes. Smooth ER lacks ribosomes. Lipids are made in the smooth ER. Let's go inside the rough ER. Note that you can still see the ribosome on the outside surface. The ribosome is manufacturing a new protein, which continues to grow inside the ER. Completed proteins move to the edge of the rough ER and depart in a vesicle that buds off from the ER membrane. Some vesicles fuse with the Golgi apparatus, another component of the endomembrane system. In the Golgi, proteins undergo further processing. Finished proteins are then packaged in vesicles that pinch off from the Golgi and are transported along cytoskeleton tracks. Some vesicles bind with the plasma membrane,secreting their contents outside the cell. Other vesicles, called lysosomes, contain digestive enzymes. Here, a lysosome fuses with a worn-out mitochondrion and breaks it down. Each of the trillions of cells in your body is a dynamo of activity requiring millions of ATPs every minute. But most people are unaware of all this activity in their cells.