Lecture 4- A Tour in the Cell-Part 2.pdf

General Cell And Developmental Biology [BIO 102] A Tour Inside The Cell-II By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  02 Lyso...

General Cell And Developmental Biology [BIO 102] A Tour Inside The Cell-II By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  02 Lysosomes, Vacuoles and Peroxisomes Are Cellular Digestion Centers Biology  Lysosomes ▪ Organelles containing enzymes that digest food particles, captured bacteria, break up and recycle worn-out organelles, and debris. ▪ The hydrolytic enzymes inside lysosomes originate in the rough ER. ▪ The Golgi apparatus detects these enzymes by recognizing a sugar attached to them, and then packages them into vesicles that eventually become lysosomes. ▪ The lysosomes, in turn, fuse with transport vesicles carrying debris from outside or from within the cell. Biology  ▪ The lysosome’s enzymes break down the large organic molecules into smaller subunits by hydrolysis, releasing them into the cytosol for the cell to use. ▪ Lysosomes also use their hydrolytic enzymes to recycle the cell’s own organic material, a process called Autophagy. ▪ During autophagy, a damaged organelle becomes surrounded by a double membrane (of unknown origin), and a lysosome fuses with the outer membrane of this vesicle. Biology  [ Phagocytosis and Autophagy] ▪ Amoebas and many other unicellular eukaryotes eat by engulfing smaller organisms or food particles, a process called phagocytosis. ▪ The food vacuole formed in this way then fuses with a lysosome, whose enzymes digest the food. ▪ Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. ▪ Some human cells also carry out phagocytosis (Macrophages). Biology  ▪ With the help of lysosomes, the cell continually renews itself. A human liver cell, for example, recycles half of its macromolecules each week. ▪ Some cells have more lysosomes than others (e.g., WBCs , Liver Cells). ▪ White blood cells, for example, have many lysosomes because these cells engulf and dispose debris and damaged bacteria. Biology Biology  What keeps a lysosome from digesting the entire cell? The lysosome’s membrane maintains the pH of the organelle’s interior at about 4.8, much more acidic than the neutral pH of the rest of the cytoplasm. If one lysosome was to burst, the liberated enzymes would no longer be at their optimum pH, so they could not digest the rest of the cell. Biology ▪ Malfunctioning lysosomes can cause illness: for example; Tay-Sachs disease. ▪ In Tay-Sachs disease: o A defective lysosomal enzyme allows fatty substances called Gangliosides to accumulate up to toxic levels in nerve cells of the brain. o The nervous system deteriorates. o An affected person finally becomes unable to see, hear, or move. o In the most severe forms of the illness, death usually occurs by age of 5. Biology  Vacuoles ▪ Most plant cells lack lysosomes, but they do have an organelle that serves a similar function. ▪ In mature plant cells, the large central vacuole contains a watery solution of enzymes that degrade and recycle molecules and organelles. Biology ❖ The vacuole also has other roles: ▪ Most of the growth of a plant cell comes from an increase in the volume of its vacuole (In some plant cells, the vacuole occupies up to 90% of the cell’s volume). ▪ As the vacuole acquires water, it exerts Turgor Pressure against the cell membrane. This pressure helps plants stay rigid and upright. ▪ Besides water and enzymes, the vacuole also contains a variety of salts, sugars, and weak acids. Therefore, the pH of the vacuole’s solution is usually somewhat acidic. Biology  ▪ In citrus fruits, the solution is very acidic, producing the tart taste of lemons and oranges. ▪ Water-soluble pigments also reside in the vacuole, producing blue, purple, and magenta colors in some leaves, flowers, and fruits. ▪ Some protists have vacuoles, although their function is different from that in plants: ✓ The contractile vacuole in Paramecium, for example, pumps excess water out of the cell. ✓ In Amoeba, food vacuoles digest nutrients. Biology  Peroxisomes ▪ They are organelles that contain several types of enzymes that get rid of toxic substances. ▪ All eukaryotic cells contain peroxisomes. ▪ Although they resemble lysosomes in size and function, peroxisomes originate at the ER (not the Golgi) and contain different enzymes. ▪ In some peroxisomes, the concentration of enzymes reaches such high levels that they condense into easily recognized crystals. Biology ▪ Peroxisomes in liver and kidney cells help remove toxins from the blood. ▪ In addition, they carry out oxidation reactions that break down fatty acids and amino acids. Biology ▪ In a disease called adrenoleukodystrophy (ALD), a faulty peroxisomal enzyme causes saturated, very long chain fatty acids (VLCFA) to accumulate up to toxic levels in the nervous system (myelin sheath that covers nerve cells in the brain and spinal cord) and adrenal cortex (the largest part of an adrenal gland) causing severe brain damage and eventually death.  Mitochondria 1. They are the powerhouses or energy factories of the cell. 2. They use a process called cellular respiration to extract from food the needed energy for growth, cell division, protein production, secretion, and many chemical reactions in the cytoplasm. Biology  3. With the exception of a few types of protists, all eukaryotic cells have mitochondria. 4. A mitochondrion is oval- shaped with two membrane layers: an outer membrane and an intricately folded inner membrane that encloses the mitochondrial matrix. 5. Within the matrix is DNA that encodes proteins essential for mitochondrial function; ribosomes occupy the matrix as well. 6. Cristae are the folds of the inner membrane. The cristae add tremendous surface area to the inner membrane, which houses the enzymes that catalyze the reactions of cellular respiration. Biology 7. In most mammals, mitochondria are inherited from the female parent only (This is because the mitochondria in a sperm cell degenerate after fertilization).  ▪ Mitochondrial DNA is therefore useful for tracking inheritance through female lines in a family. ▪ Also, genetic mutations that cause defective mitochondria also pass only from mother to offspring. ▪ Mitochondrial illnesses are most serious when they affect the muscles or brain, because these energy hungry organs depend on the functioning of many thousands of mitochondria in every cell. Biology Chloroplasts ▪ It is the site of photosynthesis in eukaryotes. ▪ Plants and many protists carry out photosynthesis, a process that uses energy from sunlight to produce glucose and other food molecules. ▪ They contain the green pigment chlorophyll in addition to the enzymes and other molecules that function in the photosynthetic production of sugar. ▪ These lens-shaped organelles are found in leaves and other green organs of plants and in algae.  ▪ Structure: 1. The contents of a chloroplast are partitioned from the cytosol by an envelope consisting of two membranes separated by a very narrow inter-membrane space. 2. Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids. 3. In some regions, thylakoids are stacked like poker chips; each stack is called a granum. 4. The fluid outside the thylakoids is the stroma, which contains the chloroplast DNA and ribosomes as well as many enzymes. Biology  5. The membranes of the chloroplast divide the chloroplast space into three compartments: the inter-membrane space, the stroma, and the thylakoid space (This compartmental organization enables the chloroplast to convert light energy to chemical energy during photosynthesis). Biology  ▪ A chloroplast is one representative of a larger category of plant organelles called plastids. ▪ Some plastids [Chromoplasts] synthesize lipid-soluble red, orange, and yellow carotenoid pigments, such as those found in carrots and ripe tomatoes. ▪ Plastids that assemble starch molecules [Amyloplasts] are important in cells specialized for food storage, such as those in potatoes. ▪ Any plastid can convert into any other type. As a tomato ripens, for example, its green chloroplasts change into plastids that store red carotenoid pigments. Biology  ▪ Like mitochondria, all plastids (including chloroplasts) contain DNA and ribosomes. ▪ The genetic material encodes proteins unique to plastid structure and function, including some of the enzymes required for photosynthesis. Biology  Extracellular Components and Connections between Cells help Coordinate Cellular Activities ▪ The plasma membrane is usually regarded as the boundary of the living cell, but most cells synthesize and secrete materials extracellularly. ▪ Although these materials and the structures they form are outside the cell, their study is important to cell biology because they are involved in many essential cellular functions. Biology  Cell Walls: ▪ It is not just a barrier that outlines the cell. Cell walls impart shape, regulate cell volume, and prevent bursting when a cell takes in too much water. ▪ Surround the cell membranes of nearly all bacteria, archaea, fungi, algae, and plants. ▪ The exact chemical composition of the wall varies from species to species and even from one cell type to another in the same organism as in plant, but the basic design of the wall is consistent. Biology  ▪ While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose, a polysaccharide made up of long, straight chains of glucose units. ▪ In Plants: ✓ The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. ✓ Plant cell walls are much thicker than the plasma membrane ranging from 0.1 μm to several micrometers. Biology a) Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose synthase. b) Then secreted to the extracellular space, where they become embedded in a matrix of other polysaccharides and proteins. Biology  c) A young plant cell first secretes a relatively thin and flexible wall called the primary cell wall. d) The middle lamella is between primary walls of adjacent cells. ✓ It is a thin layer rich in sticky polysaccharides called Pectins. ✓ The middle lamella glues adjacent cells together. e) When the cell matures and stops growing, it strengthens its wall: ✓ Some plant cells do this simply by secreting hardening substances into the primary wall. ✓ Other cells add a secondary cell wall between the plasma membrane and the primary wall. Biology ✓ The secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support.  f) Plant cell walls are usually perforated by channels between adjacent cells called Plasmodesmata: ▪ They are channels that connect plant cells. ▪ Cytosol passing through the plasmodesmata joins the internal chemical environments of adjacent cells. ▪ The plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous. ▪ Water and small solutes can pass freely from cell to cell, and in some circumstances, certain proteins and RNA molecules Biology  ▪ The macromolecules transported to neighboring cells appear to reach the plasmodesmata by moving along fibers of the cytoskeleton. Biology  Cell Junctions: ▪ Cells in an animal or plant are organized into tissues, organs, and organ systems. ▪ Neighboring cells often adhere, interact, and communicate via sites of direct physical contact: Plasmodesmata in Plant Cells Tight Junctions, Desmosomes, and Gap Biology Junctions in Animal Cells.  Cell Junctions: ▪ In animals, there are three main types of cell junctions: tight junctions, desmosomes, and gap junctions. o Gap junctions are most like the plasmodesmata of plants, although gap junction pores are not lined with membrane o All three types of cell junctions are especially common in epithelial tissue, which lines the external and internal surfaces of the body. Biology  ❑ Tight Junction ✓ Fuses animal cells together, forming an impermeable barrier between them. ✓ Proteins anchored in membranes connect to actin in the cytoskeleton and join cells into sheets, such as those lining the inside of the digestive tract. ✓ These connections allow the body to control where biochemicals move, since fluids cannot leak between the joined cells. Biology  ❑ Desmosomes (Anchoring/ Adhering Junction) ✓ They connect an animal cell to its neighbors. ✓ Function like rivets. ✓ Proteins at each anchoring junction span the cell membrane and link to each cell’s cytoskeleton. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. ✓ These junctions hold skin cells in place by anchoring them to one another and to the extracellular matrix. ✓ Desmosomes attach muscle cells to each other in a muscle. ✓ Some muscle tears involve the rupture of desmosomes. Biology Biology  ❑ Gap Junction ✓ A protein channel that links the cytoplasm of adjacent animal cells. ✓ Allowing exchange of ions, nutrients, and other small molecules. ✓ It is therefore analogous to plasmodesmata in plants. Biology Biology Biology  The Extracellular Matrix (ECM) of Animal Cells o Animal cells lack cell walls. Instead, many animal cells secrete a complex extracellular matrix. o The main ingredients of the ECM are glycoproteins and other carbohydrate- containing molecules.  o The most abundant glycoprotein in the ECM of most animal cells is collagen, which forms strong fibers outside the cells. o In fact, collagen accounts for about 40% of the total protein in the human body. The collagen fibers are embedded in a network woven out of proteoglycans secreted by cells. Biology  o Some cells are attached to the ECM by glycoproteins found in ECM such as fibronectin. o Fibronectin and other ECM proteins bind to cell-surface receptor proteins called integrins. o Integrins span the membrane and bind on their cytoplasmic side to associated proteins attached to microfilaments of the cytoskeleton. o Integrins can transmit signals between the ECM and the cytoskeleton and thus to integrate changes occurring outside and inside the cell. Biology THANK YOU