Peroxisomes - Another Enzyme Package
There are many ways that peroxisomes are similar to lysosomes. They are small vesicles found around the cell. They have a single membrane that contains digestive enzymes for breaking down toxic materials in the cell. They differ from lysosomes in the type of enzyme they hold. Peroxisomes hold on to enzymes that require oxygen (oxidative enzymes). Lysosomes have enzymes that work in oxygen-poor areas and lower pH.
Peroxisomes absorb nutrients that the cell has acquired. They are very well known for digesting fatty acids. They also play a part in the way organisms digest alcohol(ethanol). Because they do that job, you would expect liver cells to have more peroxisomes than most other cells in a human body. They also play a role in cholesterol synthesis and the digestion of amino acids.
Creating Hydrogen Peroxide
Peroxisomes work in a very specific way. Their enzymes attack complex molecules and break them down into smaller molecules. One of the byproducts of the digestion is hydrogen peroxide (H2O2). Peroxisomes have developed to a point where they are able to contain that hydrogen peroxide and break it down into water (H2O) and oxygen (O2). The water is harmless to the cell and the oxygen can be used in the next digestive reaction.
Mysteries of the Peroxisome
Peroxisomes have a single membrane that surrounds the digestive enzymes and dangerous byproducts of their work (hydrogen peroxide). The protein enzymes are usually created by lysosomes floating in the cell. They then insert the proteins into the peroxisome bubble. Peroxisomes continue to grow until they split in two. Where does the membrane come from? Scientists are still researching that answer. It may come from the endoplasmic reticulum, but it may be created in a way different from lysosomes.
CELL ORGANELLES - QUIZ
Lysosomes - Little Enzyme Packages
You will find organelles called lysosomes in nearly every animal-like eukaryotic cell. Lysosomes hold enzymes that were created by the cell. The purpose of the lysosome is to digest things. They might be used to digest food or break down the cell when it dies. What creates a lysosome? You'll have to visit the Golgi complex for that answer.
A lysosome is basically a specialized vesicle that holds a variety of enzymes. The enzyme proteins are first created in the rough endoplasmic reticulum. Those proteins are packaged in a vesicle and sent to the Golgi apparatus. The Golgi then does its final work to create the digestive enzymes and pinches off a small, very specific vesicle. That vesicle is a lysosome. From there the lysosomes float in the cytoplasm until they are needed. Lysosomes are single-membrane organelles.
Lysosome Action
Since lysosomes are little digestion machines, they go to work when the cell absorbs or eats some food. Once the material is inside the cell, the lysosomes attach and release their enzymes. The enzymes break down complex molecules that can include complex sugars and proteins. But what if food is scarce and the cell is starving? The lysosomes go to work even if there is no food for the cell. When the signal is sent out, lysosomes will actually digest the cell organelles for nutrients.
Why Don't They Digest the Cell?
Here's something scientists are still trying to figure out. If the lysosome holds many types of enzymes, how can the lysosome survive? Lysosomes are designed to break down complex molecules and pieces of the cell. Why don't the enzymes break down the membrane that surrounds the lysosome?
Microtubules - Thick Protein Tubes
Microtubules are usually discussed with microfilaments. Although they are both proteins that help define cell structure and movement, they are very different molecules. While microfilaments are thin, microtubules are thick, strong spirals of thousands of subunits. Those subunits are made of the protein called tubulin. And yes, they got their name because they look like a tube.
Elements of the Cytoskeleton
All of the microfilaments and microtubules combine to form the cytoskeleton of the cell. The cytoskeleton is different from cytoplasm (cytosol). The cytoskeleton provides structure. Cytoplasm is just a fluid. The cytoskeleton connects to every organelle and every part of the cell membrane. Think about an amoeba. All of the pieces work together so that the foot might reach out towards the food. Then lysosomes and peroxisomes are sent to begin digestion. The movement of the cell membrane, organelles, and cytoplasm is all related to the tubules and filaments.
Moving Chromosomes
Microtubules have many more uses than just cell structure. They are also very important in cell division. They connect to chromosomes, help them with their first split, and then move to each new daughter cell. They are a part of a small pair of organelles called centrioles that have the specific purpose to help a cell divide. Once the cell has finished dividing, the microtubules are put to work in other places.
Moving Organisms
Beyond the role they play in internal cell movement, microtubules also work together to form larger structures that work on the outside of the cells. They can combine in very specific arrangements to form cilia and flagella. Cilia are little hairs you might see on the outside of a paramecium or other protists. They flap back and forth to help the cell move. Flagella are long, thick tails. They whip around and sometimes twirl, pushing the cell along.
Microfilaments - Stringy Proteins
You will find microfilaments in most cells. They are the partner of microtubules. They are long, thin, and stringy proteins (mainly actin) compared to the rounder, tube-shaped microtubules. We'd like to say you can find them here or there, but they are everywhere in a cell. They work with microtubules to form the structure that allows a cell to hold its shape, move itself, and move its organelles.
Making the Cytoskeleton
All of the microfilaments and microtubules combine to form the cytoskeleton of the cell. The cytoskeleton is different from cytoplasm (cytosol). The cytoskeleton provides structure. Cytoplasm is just a fluid. The cytoskeleton connects to every organelle and every part of the cell membrane. Think about an amoeba. All of the pieces work together so that the foot might reach out towards the food. Then lysosomes and peroxisomes are sent to begin digestion. The movement of the cell membrane, organelles, and cytoplasm is all related to the tubules and filaments.
You will also find many microfilaments in muscle tissue. They are called myofibrils when you find them in muscles. The two proteins myosin and actinwork together to help the muscle cells relax and contract. The two proteins need each other and together they are called actomyosin. Combine those protein threads with some ions in the muscle cell and you get a huge contraction. The groups of actomyosin contracting are called sarcomeres. All of the muscle cells work together to make a muscle contract.
A Role in Cell Movement
Cells move in a variety of ways. We just talked about the contraction of a muscle cell. That is an extreme example. When you learn about single-celled organisms, you will understand that they need to move. They may need to glide from one area to another. The microfilaments are often found anchored to proteins in the cell membrane. Sometimes microfilaments are found floating free and connected to other filaments and tubules. Those binding proteins allow the microfilaments to push and pull on the cell membrane to help the cell move.
Vacuoles - Storage Bins to the Cells
Vacuoles are storage bubbles found in cells. They are found in both animal and plant cells but are much larger in plant cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination. Eventually, those waste products would be sent out of the cell.
The structure of vacuoles is fairly simple. There is a membrane that surrounds a mass of fluid. In that fluid are nutrients or waste products. Plants may also use vacuoles to store water. Those tiny water bags help to support the plant. They are closely related to objects called vesicles that are found throughout the cell.
In plant cells, the vacuoles are much larger than in animal cells. When a plant cell has stopped growing, there is usually one very large vacuole. Sometimes that vacuole can take up more than half of the cell's volume. The vacuole holds large amounts of water or food. Don't forge that vacuoles can also hold the plant waste products. Those waste products are slowly broken into small pieces that cannot hurt the cell. Vacuoles hold onto things that the cell might need, just like a backpack.
Helping with Support
Vacuoles also play an important role in plant structure. Plants use cell walls to provide support and surround cells. The size of that cell may still increase or decrease depending on how much water is present. Plant cells do not shrink because of changes in the amount of cytoplasm. Most of a plant cell's volume depends on the material in vacuoles.
Those vacuoles gain and lose water depending on how much water is available to the plant. A drooping plant has lost much of its water and the vacuoles are shrinking. It still maintains its basic structure because of the cell walls. When the plant finds a new source of water, the vacuoles are refilled and the plant regains its structure.
