In this lesson, we'll discover how some cells can eat, drink, and digest their dinner through the process of endocytosis and a structure called the lysosome. In addition, we'll learn how a cell can throw out the leftovers across the cell membrane during exocytosis. Endocytosis Let's just imagine for a second that each one of your cells has a giant appetite. Perhaps one cell in particular has a chocolate addiction. Can you imagine what might happen if this cell saw a plate of brownies? What about a chocolate milkshake? Could a cell figure out a way to transport that across its cell membrane? Well, where there's a will, there's a way. If your cell is as much of a chocoholic as we're describing, it might be able to figure something out, even though it doesn't have a mouth like you and me. We've talked about active and passive transport mechanisms used to move substances across the cell membrane. In this lesson, we'll focus on additional methods of transportation that allow some cells to 'eat' or 'drink' their surroundings. In addition, we'll discuss how a cell digests these items and what happens to the leftover food that can't be digested at all. In order to be able to chow down on substances in its environment, a cell will actually engulf its surroundings, folding its cell membrane around whatever it wants to ingest as if it's taking a giant bite. This process is called endocytosis, or the import of molecules and particles into the cell by folding the cell membrane inwards. The prefix 'endo-' means 'within.' During endocytosis, vesicles, or circular buds comprised of membrane, are formed around the substance to be digested. These vesicles are then transported into the cytoplasm. Three Types of Endocytosis There are three types of endocytosis. First is phagocytosis. Phagocytosis comes from the Greek word for 'devour.' It is the process where large molecules or other cells are eaten by engulfing them. In phagocytosis, the cell membrane folds around the object to be eaten, creating a vesicle that can then be internalized. For example, a white blood cell in your immune system can eat a foreign bacterium whole - in one bite! During phagocytosis, the cell membrane folds around the object, engulfing it. Phagocytosis Diagram A second type of endocytosis is pinocytosis, which comes from the Greek word for 'drink.' It is the process where small, dissolved substances and liquids are taken up by a cell. Pinocytosis is similar to phagocytosis. Both of these processes allow cells to non-specifically take up substances from the surrounding by engulfing them with their membrane. However, in pinocytosis, smaller vesicles are taking up liquid and dissolved solutes in that liquid, such as the small squares here, rather than large molecules. For example, the cells surrounding your blood capillaries perform pinocytosis to 'drink' your blood while at the same time taking up any solutes that are dissolved inside of your blood. In a third type of endocytosis, the cell membrane also folds around a substrate to engulf and internalize it. However, in receptor-mediated endocytosis, this is a process that recognizes and targets a specific substrate through the help of integral membrane proteins on the cell membrane surface. Here integral membrane proteins called receptor proteins recognize and bind to a specific target, such as these stars. In this example, these receptor proteins would not recognize and bind to non-star objects, like the squares. When a receptor protein binds to a target, it signals the cell membrane to enclose the target and internalize it inside a specialized vesicle. Phagocytosis and pinocytosis are non-specific methods of endocytosis. They would eat any kind of chocolate. Receptor-mediated endocytosis is specific - it's picky. It knows exactly what it wants. All methods of endocytosis engulf a substrate in a vesicle Processes of Endocytosis Lysosomes Whether specific or non-specific, all methods of endocytosis internalize a substrate into a vesicle, which then enters the cytoplasm. So what happens next? Think about what happens when you eat or drink something. It travels down to your stomach, where it's then digested by enzymes in the acidic environment of your stomach. What happens after endocytosis is not that different. The vesicles fuse with structures called lysosomes. A lysosome is a structure within the cell that contains digestive enzymes. The lysosome is acidic compared to the rest of the cell because integral membrane proteins in the membrane surrounding the lysosome pump protons into this space. The digestive enzymes within the lysosome work optimally at this lower pH.

• published: 22 Mar 2016
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