Organelles of the Cell

Organelles of the Cell Cells are the Starting Point All living organisms on Earth are divided in pieces called cells. There are smaller pieces to cells that include proteins and organelles. There are also larger pieces called tissues and systems. Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth. A main purpose of a cell is to organize. Cells hold a variety of pieces and each cell has a different set of functions. It is easier for an organism to grow and survive when cells are present. If you were only made of one cell, you would only be able to grow to a certain size. You don't find single cells that are as large as a cow. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your life possible. Prokaryotes – Missing a Nucleus Prokaryotes do not have an organized nucleus. Their DNA is kind of floating around the cell. It's clumped up, but not inside of a nucleus. Archaea and Bacteria. Eukaryotes – Cell with Parts Eukaryotes are what you think of when you think of a classic "cell." The possibilities are endless. Eukaryotes are cells that can do anything. They are the cells that have helped organisms advance to new levels of specialization beyond imagination. (1) Eukaryotic cells have an organized nucleus with a nuclear envelope. They have a "brain" for the cell. They have a discreet area where they keep their DNA. It is also said that they have a "true nucleus." Can we say it any other way? (2) Eukaryotic cells usually have organelles. They might have mitochondria, maybe a chloroplast, or some endoplasmic reticulum. They have parts that work to make the cell a self-sufficient organism. (3) Although limited in size by the physics of diffusion, eukaryotic cells can get very large. (4) Eukaryotic cells have extra stuff going on and extra parts attached ORGANELLES of the CELL Cytoplasm is the fluid that fills a cell. Scientists used to call the fluid protoplasm. Early on, they didn't know about the many different types of fluids in the cell. There is special fluid in the mitochondria, endoplasmic reticulum, Golgi apparatus, and nucleus. The only two 'plasms' left are cytoplasm (the fluid in the cell also called cytosol) and nucleoplasm (the fluid in the nucleus). Each of those fluids has a very different composition. The cell organelles are suspended in the cytosol. You will learn that the microfilaments and microtubules set up a "skeleton" of the cell and the cytosol fills the spaces. The cytoplasm has many different molecules dissolved in solution. You'll find enzymes, fatty acids, sugars, and amino acids that are used to keep the cell working. Waste products are also dissolved before they are taken in by vacuoles or sent out of the cell. Special Fluids in the Nucleus Nucleoplasm has a little different composition. Nucleoplasm can only be found inside of the nucleus. It doesn't have big organelles in suspension. The nucleoplasm is the suspension fluid that holds the cell's chromatin and nucleolus. It is not always present in the nucleus. When the cell divides, the nuclear membrane dissolves and the nucleoplasm is released. After the cell nucleus has reformed, the nucleoplasm fills the space again. More than Filling The cytoplasm (cytosol) in a cell does more than just suspends the organelles. It uses its dissolved enzymes to break down all of those larger molecules. The products can then be used by the organelles of the cell. Glucose may exist in the cytosol but the mitochondria can't use it for fuel. The cytosol has enzymes that break glucose down into pyruvate molecules that are then sent to the mitochondria. Cell Membrane We have been talking about cells being a unit of organization in biology. Let's look at the cell membrane and see how that membrane keeps all of the pieces inside. When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell. Flexible Containers The cell membrane is not one solid piece. Everything in life is made of smaller pieces and a membrane is no different. Compounds called proteins and phospholipids make up most of the cell membrane. The phospholipids make the basic bag. The proteins are found around the holes and help move molecules in and out of the cell. Scientists describe the organization of the phospholipids and proteins with the fluid mosaic model. That model shows that the phospholipids are in a shape like a head and a tail. The heads like water (hydrophilic) and the tails do not like water (hydrophobic). The tails bump up against each other and the heads are out facing the watery area surrounding the cell. The two layers of cells are called the bilayer. Ingrained in the Membrane What about the membrane proteins? Scientists have shown that the proteins float in that bilayer. Some of them are found on the inside of the cell and some on the outside. Other proteins cross the bilayer with one end outside of the cell and one end inside. Those proteins that cross the layer are very important in the active transport of ions and small molecules. Many Membranes As you learn more about the organelles inside of the cell, you will find that most have a membrane. They do not have the same chemical makeup as the cell membrane. Each membrane is unique to the organelle. The membrane that surrounds a lysosome is different from the membrane around the endoplasmic reticulum. They are both different from the cell membrane. Some organelles have two membranes. A mitochondrion has an outer and inner membrane. The outer membrane contains the mitochondrion parts. The inner molecule holds digestive enzymes that break down food. While we talk about membranes all the time, you should remember they all use a basic phospholipid bilayer, but have many other different parts. Nucleus - Commanding the Cell The cell nucleus acts like the brain of the cell. It helps control eating, movement, and reproduction. If it happens in a cell, chances are the nucleus knows about it. The nucleus is not always in the center of the cell. It will be a big dark spot somewhere in the middle of all of the cytoplasm (cytosol). You probably won't find it near the edge of a cell because that might be a dangerous place for the nucleus to be. If you don't remember, the cytoplasm is the fluid that fills cells. Important Materials in the Envelope The things that make a eukaryotic cell are a defined nucleus and other organelles. The nuclear envelope surrounds the nucleus and all of its contents. The nuclear envelope is a membrane similar to the cell membrane around the whole cell. There are pores and spaces for RNA and proteins to pass through while the nuclear envelope keeps all of the chromatin and nucleolus inside. When the cell is in a resting state there is something called chromatin in the nucleus. Chromatin is made of DNA, RNA, and nuclear proteins. DNA and RNA are the nucleic acids inside of the cell. When the cell is going to divide, the chromatin becomes very compact. It condenses. When the chromatin comes together, you can see the chromosomes. You will also find the nucleolus inside of the nucleus. When you look through a microscope, it looks like a nucleus inside of the nucleus. It is made of RNA and protein. Chromosomes are the things that make organisms what they are. They carry all of the information used to help a cell grow, thrive, and reproduce. Chromosomes are made up of DNA. Segments of DNA in specific patterns are called genes. Your genes make you who you are. You will find the chromosomes and genetic material in the nucleus of a cell. In prokaryotes, DNA floats in the cytoplasm in an area called the nucleoid. Wound Up Loose and Tight Chromosomes are not always visible. They usually sit around uncoiled and as loose strands called chromatin. When it is time for the cell to reproduce, they condense and wrap up very tightly. The tightly wound DNA is the chromosome. Chromosomes look kind of like long, limp, white hot dogs. They are usually found in pairs. Completing the Sets Scientists count individual strands of chromosomes. They count individuals not every organism has pairs. You probably have 46 chromosomes (23 pairs). Peas only have 12. A dog has 78. The number of chromosomes is NOT related to the intelligence or complexity of the creature. There is a crayfish with 200 chromosomes. Does that make a crayfish five times smarter or more complex than you are? No. There are even organisms of the same species with different numbers of chromosomes. You will often find plants of the same species with multiple sets of chromosomes. Chromosomes work with other nucleic acids in the cell to build proteins and help in cell division. You will most likely find mRNA in the nucleus with the DNA. tRNA is found outside of the nucleus in the cytosol. When the chromosomes are visible, cells with two complete sets of chromosomes are called diploids (46 in a human). Most cells are diploid. Cells with only one set (23 in a human) are called haploid cells. Haploids are most often found in cells involved in sexual reproduction such as a sperm or an egg. Haploid cells are created in cell division termed meiosis. Cell Wall - What's it for? Cell membranes surround every cell you will study. Cell walls made of cellulose are only found around plant cells and a few other organisms. Cellulose is a specialized sugar that is classified as a structural carbohydrate and not used for energy. If a plant cell is like a water balloon, the cell wall is like a cardboard box that protects the balloon. The balloon is protected from the outside world by a structure that provides protection and support. While many sugars, such as glucose, can dissolve in water (H20), cellulose will not dissolve in water and can form long chains to support plants. When you eat plant material, you can’t even digest and break down cellulose for energy. Cows and other herbivores have special bacteria in their stomachs to digest the cellulose polymers. While cell walls protect the cells, they also allow plants to grow to great heights. You have a skeleton to hold you up. A 100-foot-tall redwood tree does not. It uses the strong cell walls to maintain its shape. For overall support, dense cells in the core of the trunk can let a tree grow very high. Cell walls are slightly elastic for smaller plants, leaves, and thin branches. Winds can push them from side to side and they bounce back. Big redwoods need strength in high winds and sway very little (except at the top). Another Hole in the Wall A cell wall is not an impenetrable fortress around the delicate plant cell. There are small holes, called plasmodesmata, in the cell walls between plant cells. The cell membranes of neighboring cells are able to connect through these holes. The connections allow the transfer of nutrients, waste, and ions (simplistic pathways). Molecules can also pass through the spaces within the cell walls, avoiding the cells completely (apoplectic pathways). It is great that nutrients can move from cell to cell, but there is also a problem with all the holes. Cells can lose water. Plants lose large amounts of water in the middle of the day or on very hot days. When the air heats up and the water vapor pressure decreases, plants lose water through the process of transpiration. The water escapes through pores on the surface of the plant called stomata. Even when the plant cells lose water, the basic shape is maintained by the cell walls. The plant may droop or wilt, but it can recover when water returns to the system. It will look just the same as when it started. Cell Walls in Other Species You may hear about cell walls in other species. Bacteria have a structure called a cell wall. Fungi and some ptotozoa also have cell walls. They are not the same as the plant cell walls made of cellulose. The other walls might be made from proteins or a substance called chitin. Chitin is another structural carbohydrate. They all serve the same purpose of protecting and maintaining structure, but they are very different molecules. NucleolusThe nucleolus is the largest structure in the nucleus of eukaryotic cells where it primarily serves as the site of ribosome synthesis and assembly. Nucleoli also have other important functions like assembly of signal recognition particles and playing a role in the cell's response to stress. Nucleoli are made of proteins and RNA and form around specific chromosomal regions. Malfunction of nucleoli can be the cause of several human diseases. Centrioles - Organizing Chromosomes Every animal-like cell has two small organelles called centrioles. They are there to help the cell when it comes time to divide. They are put to work in both the process of mitosis and the process of meiosis. You will usually find them near the nucleus but they cannot be seen when the cell is not dividing. And what are centrioles made of? Microtubules. You will not see well-defined centrioles when the cell is not dividing. You will see a condensed and darker area of the cytoplasm called the centrosome. When the time comes for cell division, the centrioles will appear and move to opposite ends of the nucleus. During division you will see four centrioles. One pair moves in each direction. Interphase is the time when the cell is at rest. When it comes time for a cell to divide, the centrioles duplicate. During prophase, the centrioles move to opposite ends of the nucleus and a mitotic spindle of threads begins to appear. Those threads then connect to the now apparent chromosomes. During anaphase, the chromosomes are split and pulled towards each centriole. Once the entire cell begins to split in telophase, the chromosomes begin to unravel and new nuclear envelopes begin to appear. The centrioles have done their job. Ribosomes - Protein Construction Teams Cells need to make proteins. Proteins are made up of subunits called AMINO ACIDS. Those proteins might be used as enzymes or as support for other cell functions. Ribosomes are the protein builders or the protein synthesizers of the cell. They are like construction guys who connect one amino acid at a time and build long chains. Ribosomes are found in many places around the cell. You might find them floating in the cytoplasm (cytosol). Those floating ribosomes make proteins that will be used inside of the cell. Other ribosomes are found on the endoplasmic reticulum. Endoplasmic reticulum with attached ribosomes is called rough. It looks bumpy under a microscope. Those attached ribosomes make proteins that will be used inside the cell and proteins made for export out of the cell. Two Pieces Make the Whole A ribosome is not just one piece. There are two pieces or subunits. Scientists named them 60S (large) and 40-S (small). When the cell needs to make protein, mRNA is created in the nucleus. The mRNA is then sent into the cell and the ribosomes. When it is time to make the protein, the two subunits come together and combine with the mRNA. The subunits lock onto the mRNA and start the protein synthesis. The 60-S/ 40-S model works fine for eukaryotic cells. Prokaryotic cells have ribosomes made of 50-S and 30-S subunits. It's a small difference, but one of many you will find in the two different types of cells. Scientists have used this difference in ribosome size to develop drugs that can kill prokaryotic microorganisms that cause disease. Amino acids are the chemical units or "building blocks," that make up proteins. Amino acids contain about 16 percent nitrogen. Chemically, this is what distinguishes them from the other two basic nutrients, sugars and fatty acids, which do not contain nitrogen. To understand how vital amino acids are, you must understand how essential proteins are to life. It is protein that provides the structure for all living things. Every living organism, from the largest animal to the tiniest microbe, is composed of protein. And in its various forms, protein participates in the vital chemical processes that sustain life. There are 20 Amino Acids which make all proteins. Proteins are a necessary part of every living cell in the body. Next to water, protein makes up the greatest portion of our body weight. In the human body, protein substances make up the muscles, ligaments, tendons, organs, glands, nails, hair, and many vital body fluids, and are essential for the growth of bones. The enzymes and hormones that catalyze and regulate all bodily processes are proteins. Proteins help to regulate the body's water balance and maintain the proper internal pH. They assist in the exchange of nutrients between the intercellular fluids and the tissues, blood, and lymph. A deficiency of protein can upset the body's fluid balance. Proteins form the structural basis of chromosomes, through which genetic information is passed from parents to offspring. The genetic "code" contained in each cell's DNA is actually information for how to make that cell's proteins. Proteins are chains of amino acids linked together by what are called peptide bonds. Each individual type of protein is composed of a specific group of amino acids in a specific chemical arrangement. It is the particular amino acids present and the way in which they are linked together in sequence that gives the proteins that make up the various tissues their unique functions and characters. Each protein in the body is tailored for a specific need; proteins are not interchangeable. The proteins that make up the human body are not obtained directly from the diet. Rather, dietary protein is broken down into its constituent amino acids, which the body then uses to build the specific proteins it needs. Thus, it is the amino acids rather than protein that are the essential nutrients. Twenty Amino Acids There are twenty amino acids required for human life to exist. Adults need nine essential amino acids that they cannot synthesize (make) and must get from food. The other eleven can be produced within our bodies. In addition to these amino acids, there are some others found in nature (and some very small amounts in us). These twenty are the biggies to our species and defined as the standard amino acids. Alanine, Arginine, Asparagine, Aspartic Acid, Cysteine, Glutamic Acid, Glutamine, Glycine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Proline, Serine, Threonine, Tryptophan, Tyrosine, and Valine. (Essential amino acids are in bold) Mixing and Matching Amino Acids The process of making proteins is quite simple. Remember that mRNA is made in the nucleus and sent into the cell. The mRNA then combines with the ribosome subunits. Another nucleic acid lives in the cell - tRNA, which stands for transfer RNA. tRNA is bonded to the amino acids floating around the cell. With the mRNA offering instructions, the ribosome connects to a tRNA and pulls off one amino acid. Slowly the ribosome makes a long amino acid chain that will be part of a larger protein. This process is called PROTEIN SYNTHESIS. Mitochondria - Turning on the Powerhouse Mitochondria are known as the powerhouses of the cell. They are organelles that act like a system that takes in nutrients, breaks them down, and creates energy for the cell. The process of creating cell energy is known as cellular respiration. Most of the chemical reactions involved in cellular respiration happen in the mitochondria. Mitochondria are very small organelles. You might find cells with several thousand mitochondria. The number depends on what the cell needs to do. If the purpose of the cell is to transmit nerve impulses, there will be fewer mitochondria than in a muscle cell that needs loads of energy. If the cell feels it is not getting enough energy to survive, more mitochondria can be created. Sometimes they can even grow, move, and combine with other mitochondria, depending on the cell's needs. Mitochondria Structure Mitochondria have two membranes (not one as in other organelles). The outer membrane covers the organelle and contains it. The inner membrane folds over many times (cristae). That folding increases the surface area inside the organelle. Many of the chemical reactions happen on the inner membrane of the mitochondria. The increased surface area allows the small organelle to do as much work as possible. If you have more room to work, you can get more work done. Similar surface area strategies are used by microvilli in your intestinal cells. The fluid inside of the mitochondria is called the matrix. Using Oxygen to Release Energy The matrix is filled with water (H2O) and proteins (enzymes). Those proteins take food molecules and combine them with oxygen (O2). The mitochondria are the only place in the cell where oxygen can be combined with the food molecules. After the oxygen is added, the material can be digested. They are working organelles that keep the cell full of energy. A mitochondrion may also be involved in controlling the concentration of calcium (Ca) within the cell. Chloroplasts – Chloroplasts are the food producers of the cell. They are only found in plant cells and some protists. Animal cells do not have chloroplasts. Every green plant you see is working to convert the energy of the sun into sugars. Plants are the basis of all life on Earth. They create sugars, and the byproduct of that process is the oxygen that we breathe. That process happens in the chloroplast. Mitochondria work in the opposite direction and break down the sugars and nutrients that the cell receives. Special Structures Two membranes contain and protect the inner parts of the chloroplast. The stroma is an area inside of the chloroplast where reactions occur and starches (sugars) are created. One thylakoid stack is called a granum. The thylakoids have chlorophyll molecules on their surface. That chlorophyll uses sunlight to create sugars. The stacks of sacs are connected by stromal lamellae. The lamellae act like the skeleton of the chloroplast, keeping all of the sacs a safe distance from each other and maximizing the efficiency of the organelle. Making Food The purpose of the chloroplast is to make sugars and starches. They use a process called photosynthesis to get the job done. Photosynthesis is the process of a plant taking energy from the Sun and creating sugars. When the energy from the Sun hits a chloroplast, chlorophyll uses that energy to combine carbon dioxide (CO2) and water (H2O). The molecular reactions create sugar and oxygen (O2). Plants and animals then use the sugars (glucose) for food and energy. Animals also use the oxygen to breathe. Different Chlorophyll Molecules We said that chlorophyll molecules sit on the outside of the thylakoid sacs. Not all chlorophyll is the same. Three types of chlorophyll can complete photosynthesis. There are even molecules other than chlorophyll that are photosynthetic. One day you might hear about carotenoids, phycocyanin(bacteria), phycoerythrin (algae), and fucoxanthin (brown algae). While those compounds might complete photosynthesis, they are not all green or the same structure as chlorophyll. Endoplasmic Reticulum - Wrapping it Up While the function of the nucleus is to act as the cell brain, the endoplasmic reticulum (ER) functions as a packaging system. ER does not work alone. The ER works closely with the Golgi apparatus, ribosomes, RNA, mRNA, and tRNA. It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell's function. There are two forms of endoplasmic reticulum found within the cell: rough endoplasmic reticulum and smooth endoplasmic reticulum (rough ER and smooth ER) Rough and Smooth Endoplasmic Reticulum There are rough ER and smooth ER. They both have the same types of membranes but they have different shapes and rough ER has ribosomes attached. Rough ER looks like sheets of bumpy membranes while smooth ER looks more like tubes. Sometimes the ER looks like a flat balloon. Sacs of the ER called cisternae store the complex molecules. Smooth ER has its purpose in the cell. It acts as a storage organelle. It is important in the creation and storage of steroids. It also stores ions in solution that the cell may need at a later time. Steroids are a type of ringed organic molecule used for many purposes in an organism. They are not always about building muscle mass like a weight lifter. The ion storage is important because sometimes a cell needs ions fast. It might not want to search the environment for ions, so it is easier to have them stored in a pack for easy use. Rough ER was mentioned in the section on ribosomes. They are very important in the synthesis and packaging of proteins. Some of those proteins might be used in the cell and some are sent out. The ribosomes are attached to the membrane of the ER. As the ribosome builds the amino acid chain, the chain is pushed into the ER. When the protein is complete, the rough ER pinches off a vesicle. That vesicle, a small membrane bubble, can move to the cell membrane or the Golgi apparatus. Vacuoles-Storage bins to the cell 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. 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. Golgi Apparatus- Packing Things Up The Golgi apparatus or Golgi complex is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist. It is pronounced GOL-JI in the same way you would say squee-gie, as soft a "G" sound. While layers of membranes may look like the rough ER, they have a very different function. Foundation of Vesicles The Golgi complex gathers simple molecules and combines them to make molecules that are more complex. It then takes those big molecules, packages them in vesicles, and either stores them for later use or sends them out of the cell. It is also the organelle that builds lysosomes (cell digestion machines). Golgi complexes in the plant may also create complex sugars and send them off in secretory vesicles. The vesicles are created in the same way the ER does it. The vesicles are pinched off the membranes and float through the cell. The Golgi complex is a series of membranes shaped like pancakes. The single membrane is similar to the cell membrane in that it has two layers. The membrane surrounds an area of fluid where the complex molecules (proteins, sugars, enzymes) are stored and changed. Because the Golgi complex absorbs vesicles from the rough ER, you will also find ribosomes in those pancake stacks. Working with the Rough ER The Golgi complex works closely with the rough ER. When a protein is made in the ER, something called a transition vesicle is made. This vesicle or sac floats through the cytoplasm to the Golgi apparatus and is absorbed. After the Golgi does its work on the molecules inside the sac, a secretory vesicle is created and released into the cytoplasm. From there, the vesicle moves to the cell membrane and the molecules are released out of the cell. 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. 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? 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. 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. 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. THIS IS EXTRA INFORMATION-NOT REQUIRED-FOR YOUR INFO ONLY Types of Plastids: Chromoplasts and Leucoplasts The two important types of Plastids are as follows: The plastids contain the pigments which may be of various types. Sometimes pigments are absent. On the basis of presence of pigments, the plastids are of two types: (i) the chromoplasts (chromatophores) having pigment, and (ii) the leucoplasts which are colorless plastids. Chromoplasts: The chromoplasts may be further divided on the basis of colour of the pigment and these are of the following types — 1. Chloroplasts: It is the most common plastid (Gr. Chlor, green, plast, and living) which contains chlorophyll a and b pigments, DNA and RNA. Chloroplasts are found mainly in the cells of the leaves of higher plants and algae. It is the most biologically important plastid. By the process of photosynthesis, they produce oxygen and the most of the chemical energy used by living organisms. 2. Phaeoplast: These are yellow or brown plastids found in brown algae, diatoms and dinoflagellates. Fucoxanthin is a carotenoid pigment which masks the color of chlorophyll a, which is also present. It also absorbs light and transfer the energy to chlorophyll a. 3. Rhodoplasts: These are red colored plastids. It is found in red algae and its red colour is due to phycoerythrin. It also absorbs light. 4. Chromatophores: These are present in the blue-green algae. The term chromatophore is used instead of plastid, since the pigments are not organized within a discrete plastid body but are often arranged on lamellar structures in concentric rings or plates within algal cell. Blue-green color of this algae is due to phycocyanin and phycobilins. These accessory pigments do not participate in photosynthesis. 5. Purple photosynthetic bacteria have a chlorophyll, bacteriochlorophyll. Their purple color is due to special carotenoids. Non-photosynthetic chromoplasts: A variety of accessory pigments is also found which do not appear to be directly involved in photosynthesis or energy transfer. Chromoplasts may develop from chloroplasts by accumulation of non--photosynthetic pigments, e.g., red carotenoid, lycopene in tomatoes. Genes for synthesis of pigments lie in the nucleus. 1. Carotenoids and xanthophylls: Carotene protects against photo-oxidation. These are redyellow or brown in color. These give color to flowers and fruits. 2. Leucoplasts: These plastids are devoid of pigment and are membranous structures. They serve to store starches, oils and proteins. These are of the following types— (a) Amyloplasts: These are food storage cells and store starch. These are generally found in storage tubers, cotyledons and endosperm. These are found in regions of little or no illumination. Amyloplasts have nucleoids and ribosomes. (b) Elaioplasts: These are found in certain monocotyledons and their function is to store oils. (c) Proteinoplasts (aleuroneplasts): These are found in seeds of Ricinus and Brazilnut, and store proteins. Epidermal cells of Helleborus also possess proteinoplasts. Plastid differentiation depends upon the metabolic requirements of the cell. The chloroplasts may develop from leucoplasts, and chromoplasts, which are considered end forms of plastid differentiation, may develop from either leucoplasts or chloroplasts. Piroplasmids can differentiate into one of three types of plastids and since, in certain cases, one type of plastid can differentiate into another, it has been generally assumed that all plastids are essentially the same in structure, having the ability to differentiate in various ways, depending upon the requirements of the cells. Eukaryote Prokaryote Nucleus Golgi Apparatus Neuron Nucleus Endoplasmic Reticulum Mitochondria Mitochondria Chloroplast Central Vacuole in a Plant Cell Cell Membrane PLANT CELL vs. ANIMAL CELL