Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.
What Are Organelles?
Picture your town. What are the important buildings in the town? You might think of the post office, or the town hall, or even the recycling plant. All of these parts work together to make sure the town keeps functioning. Although a cell is much smaller than a town, it also has parts that work together to keep the whole healthy and functional. The compartments inside cells are called organelles, and each organelle has a specific job inside the cell.
One organelle, called the lysosome, functions like the recycling center of the cell. It takes in materials the cell no longer needs and breaks them apart into pieces the cell can use again. This process is called intracellular digestion, because it's like the digestion that occurs in our body, but instead it occurs inside the cell. Today, we're going to look at the parts of a lysosome that allow it to do its job, what jobs it actually does in the cell, and what can go wrong if there is a problem with your lysosomes.
Lysosomes are typically displayed as small circles floating around the cell. In reality, they come in many different shapes and sizes, so there isn't really a characteristic picture of a lysosome. However, there are some things that all lysosomes have in common. First, all lysosomes are enclosed by a phospholipid membrane made of a special type of fat called a 'phospholipid'. This membrane keeps the inside of the lysosome separate from the rest of the cell. The lysosomal membrane has lots of proteins that help move things in and out. This is important since the lysosome needs to be ready to accept material for digestion and to move digestive materials back into the cell for reuse.
Although it's thin, that lysosomal membrane serves an important purpose. Much like our stomach, the lysosome has a low pH and is filled with acids that help digest material. The pH of a lysosome is about 5, compared to 7.2 (neutral) in the rest of the cell. The acidic pH isn't just to break things apart. Inside the lysosome are enzymes, small proteins that speed up chemical reactions, and in the lysosome they help to break apart materials. These enzymes are only active at the low pH of the lysosome.
Lysosomal acid hydrolases are enzymes that specialize in breaking apart unwanted material for the cell. There are over 50 different types of acid hydrolases inside the lysosome. Some break down DNA and RNA, while others break down proteins, lipids, or carbohydrates. These enzymes are called acid hydrolases because they are only functional at a pH of 5. Why is this important?
Well, since these enzymes break apart important components of the cell, like proteins, carbohydrates, and even DNA, we wouldn't want them running around the cell out of control. The cell would digest itself. The first barrier to keeping these enzymes in is the lysosomal membrane, which we discussed. But the cell uses the acidic pH as a second barrier. Since the enzymes are only active at an acidic pH of 5, even if they were to escape they wouldn't be functional at the pH of 7.2 found in the rest of the cell.
Lysosomes are the recycling plants of the cell. They break down unwanted or worn out materials inside the cell, even other organelles. They also break apart extracellular debris, like dead cells, or pathogens invading the body.
Endocytosis occurs when small molecules are engulfed by the outer membrane of the cell. In this process, the molecules are captured in a little bubble (called a 'vesicle') and transported to the lysosome, where they fuse together. The materials inside the vesicle can then be digested.
Lysosomes also perform autophagocytosis, where they engulf entire organelles within the cell. This large scale digestion provides a way for the cell to get rid of old, worn out organelles and recycle the parts to make new ones. Contradictory to the static pictures of cells in text books, the internal parts of a cell are constantly moving, changing, and being replaced. The lysosome helps in that remodeling.
Specialized cells (such as our immune cells) use their lysosomes to get rid of pathogens in our bodies. Cells called 'macrophages' seek out invading pathogens and engulf them, as in endocytosis. However, this process occurs at a much larger scale. The vesicles bring the invaders to the lysosome where they await digestion.
Imagine if the recycling plant backed up in your town. Trash and recycling would pile up on the streets causing problems. Sometimes the lysosomes are defective too, and the effects are grave for the entire organism. Problems with lysosomes are called lysosomal storage disorders, which give rise to about 50 different diseases. Most lysosomal storage disorders affect the hydrolase enzymes that break down material. If the enzymes aren't working properly, waste materials build up in the lysosome. This can cause serious problems for the cell, as some of these materials are toxic. Tay-Sachs disease is a neurological disease that is caused by the buildup of a fatty substance in nervous tissue. A specific acid hydrolase isn't made correctly and doesn't do its job breaking down that substance, so it accumulates in the brain, causing severe and fatal neurological damage.
The lysosome is the recycling plant of the cell that does intracellular digestion. It breaks down materials brought into the cell (such as nutrients) during endocytosis, recycles old organelles during autophagocytosis, and kills bacteria during phagocytosis in immune cells. The lysosome is a membranous organelle that contains a highly acidic interior with acid hydrolases that only work around pH 5. Lysosomal storage disorders are a group of diseases where lysosomes cannot do their job, usually due to problems with the hydrolases. These diseases cause a buildup of toxic compounds and can be fatal.
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