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Which Organelle Contains Enzymes for Intracellular Digestion?

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be learning about the lysosome, which is responsible for intracellular digestion. We'll learn the structure and function of this unique organelle, the specific enzymes involved, and its role in human health.

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.

Lysosome in an animal cell
lysosome

Lysosome Structure

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.

Structure of a lysosome
lysosomal structure

Acid Hydrolases

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.

Lysosome Function

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.

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