Lecture 5. Cytoskeleton.pdf

General Cell And Developmental Biology [BIO 102]  The Cytoskeleton Supports Eukaryotic Cells By Dr. Maha M Salah & Dr. Mariam Gamaleldin Assistant Professors, Biotechnology School, Nile University ...

General Cell And Developmental Biology [BIO 102]  The Cytoskeleton Supports Eukaryotic Cells By Dr. Maha M Salah & Dr. Mariam Gamaleldin Assistant Professors, Biotechnology School, Nile University  02 The cytosol of a eukaryotic cell contains a cytoskeleton, a complex network of protein “tracks” and tubules extending throughout the cytoplasm. Biology  02 Bacterial cells also have fibers that form a type of cytoskeleton, constructed of proteins similar to eukaryotic ones. Biology  ❑ Functions of The Cytoskeleton 1. Give mechanical support to the cell and provide the physical support necessary to maintain the cell’s characteristic three-dimensional shape. This is especially important for animal cells, which lack walls. 2. It is a transportation 4. It helps connect cells system. to one another. 3. It aids in cell 5. The cytoskeleton also enables division. cells—or parts of a cell—to move. Biology  ❑ Structure of the Cytoskeleton ▪ The remarkable strength and resilience of the cytoskeleton are based on its architecture. ▪ The cytoskeleton includes three major components: microfilaments, intermediate filaments, and microtubules. Biology  ▪ They are distinguished by protein type, diameter, and how they aggregate into larger structures. ▪ Other proteins connect these components to one another, creating an intricate meshwork. Biology  Motor Proteins ▪ Motor proteins are impressive molecular machines that can use ATP to move organelles along the cytoskeletal filaments or move the cytoskeletal filaments themselves. (Think of trucks transferring cargo on the road inside the city) ▪ There are 3 classes of motor proteins: 1. Myosin 2. Kinesin 3. Dynein Biology  Motor Proteins Biology  Microfilaments = Actin Filaments ▪ Thin solid rods; each microfilament is only about 7 nanometers in diameter. ▪ They are built from molecules of actin, a globular protein. ▪ Actin microfilament has a plus end and minus end. ▪ So, a microfilament is a twisted double chain of actin subunits. ▪ Actin microfilament networks are part of nearly all eukaryotic cells. Biology ❑ Functions: 1- Provide strength for cells to survive stretching and compression. 2- Help to anchor one cell to another. 3- It forms a three-dimensional network just inside the plasma membrane (cortical microfilaments) and this helps support the cell’s shape. ✓ This network gives the outer cytoplasmic layer of a cell, called the Cortex, the semisolid consistency of a gel, in contrast with the more fluid state of the interior cytoplasm. 4- In some kinds of animal cells, such as nutrient-absorbing intestinal cells, bundles of microfilaments make up the core of microvilli ✓ These are delicate projections that increase the cell’s surface area. 5- Microfilaments are well known for their role in cell motility. HOW? ✓ Thousands of actin filaments and thicker filaments made of a protein called myosin interact to cause contraction of muscle cells. ✓ In the unicellular eukaryote Amoeba and some of our white blood cells, localized contractions, brought about by actin and myosin, are involved in the amoeboid (crawling) movement of the cells. The cell crawls along a surface by extending cellular extensions called pseudopodia and moving toward them. Biology  ✓ In plant cells, actin-protein interactions contribute to cytoplasmic streaming, a circular flow of cytoplasm within cells. This movement, which is especially common in large plant cells speeds the movement of organelles and the distribution of materials within the cell. Biology  How do actin + myosin generate muscle contraction? ❑ Myosin heads “pull” actin fibers from minus end to plus end, pushing the fibers towards each other leading to muscle contraction. This requires ATP.  Intermediate Filaments ▪ Intermediate filaments are named for their diameter (10 nanometers), (which is larger than the diameter of microfilaments but smaller than that of microtubules). ▪ While microtubules and microfilaments are found in all eukaryotic cells, Intermediate filaments are only found in the cells of some animals, including vertebrates. ▪ They are diverse class of cytoskeletal elements; Each type is constructed from a particular molecular subunit belonging to a family of proteins whose members include the keratins. ▪ Microtubules and microfilaments, in contrast, are consistent in diameter and composition in all eukaryotic cells. ▪ Each Keratin filament is made of a type I monomer and type II monomer forming a dimer. ▪ Dimers stack together to form tetramers. ▪ Tetramers stack together to form filaments. Biology  ❑ Functions: 1. They are specialized for bearing tension (like microfilaments). 2. They maintain a cell’s shape by forming an internal scaffold in the cytosol, thus reinforcing the shape of a cell and fixing the position of certain organelles. 3. Keratins provide toughness to hair and nails. Biology  4. Intermediate filaments also help bind some cells together (Recall Desmosomes) Biology  Diseases caused by defective keratin ▪ Genetically defective keratin can cause skin blisters from just rubbing the skin, because keratin is too weak to keep the cells attached to Epidermolysis Bullosa the basal lamina Simplex  Microtubules ▪ All eukaryotic cells have microtubules which are hollow rods (23 nanometers) constructed from globular proteins called tubulins. ▪ Each tubulin protein is a dimer, made up of two slightly different polypeptides; α tubulin and β tubulin.  ▪ Like actin, tubulin also has a minus end and a plus end. ▪ Microtubules grow in length by adding tubulin dimers; they can also be dis-assembled and their tubulins used to build microtubules elsewhere in the cell. Biology Biology ❑ Functions: 1. They Shape and Support the cell. 2. Serve as Tracks along which organelles equipped with motor proteins (kinesin and dynein) can move. 3. They Guide vesicles from the ER to the Golgi apparatus and from the Golgi to the plasma membrane. 4. Involved in the separation of chromosomes during cell division. 5. Responsible for the beating of Cilia and Flagella.  How do kinesin and dynein transport cargo (vesicles) on microtubules? Kinesin heads “walk” along microtubules towards the plus end. Dynein moves towards the minus end. ▪ In animal cells, structures called centrosomes (region often located near nucleus) organize the microtubules. ▪ Plants typically lack centrosomes and assemble microtubules at sites scattered throughout the cell. ▪ The centrosome contains two centrioles, each composed of nine sets of triplet microtubules arranged in a ring. ▪ The centrioles also indirectly produce the extensions that enable some cells to move; cilia and flagella ❖ Cilia and Flagella ▪ Though different in length, number per cell, and beating pattern, motile cilia and flagella share a common structure. Both are microtubule-containing extensions that project from some cells. ▪ Each motile cilium or flagellum has a group of microtubules sheathed in an extension of the plasma membrane. ▪ Motile cilia usually occur in large numbers on the cell surface. ▪ Flagella are usually limited to just one or a few per cell, and they are longer than cilia. ▪ Flagella and cilia differ in their beating patterns: ✓ A flagellum has a propeller like motion like the tail of a fish. ✓ In contrast, cilia have alternating power and recovery strokes. ▪ Role of Cilia and Flagella: ✓ Many unicellular eukaryotes are propelled through water by cilia or flagella that act as locomotor appendages. ✓ The sperm of animals, algae, and some plants have flagella. ✓ When cilia extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. i. The ciliated lining of the trachea (sweeps mucus containing trapped debris out of the lungs) ii. The cilia lining the oviducts in a woman’s reproductive tract (help move an egg toward the uterus). Biology  ✓ A cilium may also act as a signal-receiving “antenna” for the cell. Cilia that have this function are generally non-motile, and there is only one per cell. o In vertebrate animals, it appears that almost all cells have such a cilium, which is called a primary cilium. o Membrane proteins on this kind of cilium transmit molecular signals from the cell’s environment to its interior, triggering signaling pathways that may lead to changes in the cell’s activities. Biology o Cilium-based signaling appears to be crucial to brain function and to embryonic development. Eukaryotic motile cilium  Eukaryotic flagellum: 1-axoneme, 2-cell membrane, 3-IFT Cross section of an axoneme in a flagellum (intra-flagellar transport), 4-basal body, 5-cross section of flagellum, 6-triplets of microtubules of basal body.  Animal versus Plant Cells Despite their fundamental similarities, there are some striking differences between animal and plant cells. ▪ Animal cells have centrioles, centrosomes, and lysosomes, whereas plant cells do not. ▪ Plant cells have a cell wall, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not. Biology THANK YOU