B2.2 Organelles and Compartmentalization PDF

B2.2 | Organelles and Compartmentalization Teacher’s Version B2.2 Organelles and Compartmentalization B2.2.1 CELL COMPARTMENTALIZATION Organelles as discrete subunits of...

B2.2 | Organelles and Compartmentalization Teacher’s Version B2.2 Organelles and Compartmentalization B2.2.1 CELL COMPARTMENTALIZATION Organelles as discrete subunits of cells that are adapted to perform specific functions Selectively permeable membranes play an important role in allowing the reactions/functions inside the cell to occur without interfering with one another. This isolation of reactions is referred to as cell compartmentalization Much of our research concerning the cell focuses on how cells work at the molecular level. The best way to do this is to reduce the cell to its component parts and study each part individually. This approach is known as reductionism Tools for cell research A process used in the study of the cell is called biochemical fractionation. It refers to the separation and isolation of specific chemicals and/or structures so that detailed research can be carried out Centrifugation or cell fractionation allows the extraction of organelles from cells. Ultracentrifuges are often used for this process. Cells are first mixed in a tube with substances that break down the cell membranes. The sample is then spun at high speeds to isolate the different components by size and shape. Larger and heavier cell components can be separated off at lower speeds. Once separated, larger and heavier organelles are found at the bottom of the tube. Chromatography is very effective at isolating pure substances such as amino acids, proteins, carbohydrates and plant pigments. A mixture of molecules is placed in a separating medium. The molecules separate out depending on their size and the speed with which they travel through the medium Gel electrophoresis separates molecules of different types by passing them through a gel using an electrical charge. 1 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version The molecules are separated based on properties such as size and charge. An example of the apparatus used in gel electrophoresis to isolate molecules by size and charge. Organelles: the compartments of the cell The organelles of the cell are separate structures within the cell that carry out specialized functions. To carry out these particular functions, each type of organelle has a unique structure. Organelles are separated from the rest of the cell by a protective barrier, sometimes involving two membranes Not all parts of a cell are considered organelles 2 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Component Organelle General function Encloses and protects plant cells Cell wall No Maintains cell shape, anchors organelles, facilitates cell Cytoskeleton No movement The region where most of the metabolic reactions in the cell Cytoplasm No occur Genetic control Nucleus Yes Storage and transport Vesicles Yes Protein synthesis Ribosomes Yes Regulates movement in and out of cell, transports materials Plasma membrane Yes to maintain the internal cell environment, cell recognition and communication Movement Cilia/flagella Yes Modifies and stores endoplasmic reticulum products, forms Golgi apparatus Yes lysosomes and transport vesicles Cellular energy (ATP) production Mitochondria Yes Conversion of light energy in to chemical energy Chloroplasts Yes Digest worn out organelles and debris, digest materials Lysosomes Yes brought into the cell by endocytosis 3 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version B2.2.2 THE NUCLEUS AND CYTOPLASM Advantage of the separation of the nucleus and cytoplasm into various compartments The development of the nucleus in eukaryotic cells was a huge advantage compared to prokaryotic cells because it allowed some of the important cell processes to take place more efficiently Transcription and translation are cell processes responsible for the production of proteins In transcription, a DNA strand serves as a template or copy strand for the formation of messenger RNA (mRNA). Translation occurs when ribosomes use the code carried by mRNA to produce a polypeptide/protein. Transcription happens in the nucleus of eukaryotic cells, while translation is carried out in the cytoplasm. The separation of these two important cellular processes allows post-transcriptional modification of mRNA to occur in the nucleus before translation happens in the cytoplasm. In prokaryotic cells there is no isolation of these two processes, and mRNA can immediately come into contact with ribosomes and initiate translation without any modification occurring B2.2.3 COMPARTMENTALIZATION OF THE CYTOPLASM Advantages of compartmentalization in the cytoplasm of cells All eukaryotic cells possess compartments or organelles that are involved with: o energy production o metabolism o biosynthesis o degradation The number and size of the compartments and organelles vary depending on the overall function of the cell in which they occur For example, certain types of pancreatic cells called acinar cells specialize in the secretion of digestive enzymes. They are essential to life in humans and have a greatly enlarged endoplasmic reticulum (ER), Golgi apparatus and granule storage compartments. 4 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Compartmentalization has allowed a division of labour within the cell, with specific tasks carried out by a single organelle or organelle-like structure. Enzymes can be kept in the areas where they will be most effective Keeping reactions separate in different parts of the cell means that the metabolites and enzymes for each particular process can be concentrated in a particular area. This ensures that pathways run smoothly, can be easily controlled and do not interfere with each other Lysosomes participate in the breakdown of wastes and cellular components that need to be replaced. This breakdown requires some potentially destructive enzymes that could cause severe damage to the cell if they were not isolated by a membrane When endocytosis occurs, the result is often a phagocytic vacuole. This vacuole is a means of protecting the cellular contents from potential damage when phagocytosis occurs 5 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Once formed, the phagocytic vacuole will move around in the cell until it contacts a lysosome. The vacuole then fuses with the acidic lysosome, allowing inactivation and digestion of the threat The very fact that each area or organelle carries out one specific function means that the cell must develop a means of integrating all the separate functions. To accomplish this, some organelles are connected in a functional series, allowing the chemical pathways important to the cell to take place B2.2.4 THE MITOCHONDRION (HL) Adaptations of the mitochondrion for production of ATP by aerobic respiration Cellular respiration is the process by which glucose is used to produce adenosine triphosphate (ATP) A labelled diagram of a mitochondrion, and a coloured transmission electron micrograph (TEM) of a mitochondrion. The overall equation for cellular respiration is: C6H12O6 + 6O2 à 6CO2 + 6H2O + energy All organisms must have the ability to produce ATP for energy, and therefore all organisms carry out respiration. 6 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Mitochondrion structure Description and function A membrane that separates the contents of the Outer mitochondrial membrane mitochondrion from the rest of the cell An internal cytoplasm-like substance that contains the enzymes for the first stages of respiration that take place Matrix in the mitochondria ( the link reaction and the Krebs cycle) Tubular regions surrounded by membranes that increase the surface area for reactions that take place Cristae towards the end of respiration (oxidative phosphorylation) A membrane that contains the carriers and enzymes for Inner mitochondrial membrane the final stages of respiration (electron transport chain and chemiosmosis) Space between inner and outer Some proteins span the membrane, providing membranes passageways for substances to be transported through. B2.2.5 THE CHLOROPLAST (HL) Adaptations of the chloroplast for photosynthesis Unlike respiration, where some of the steps occur outside the mitochondrion, all of the photosynthetic process occurs within the chloroplast Both mitochondria and chloroplast have an extra outer membrane (indicating a need for protection in a potentially hostile environment), their own DNA, and they are very near in size to a typical prokaryotic cell In photosynthesis, chemical bonds are made to produce carbon compounds. The raw materials of photosynthesis are carbon dioxide and water, with light providing energy. The overall equation is: 6CO2 + 12H2O + light à C6H12O6 + 6H2O + 6O2 7 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Water occurs on both sides of the equation. Twelve molecules are consumed, and six molecules are produced Whereas respiration is, in general, a catabolic process, photosynthesis is, in general, an anabolic process Photosynthesis occurs in organisms referred to as autotrophs. These organisms make their own food Description of chloroplast structure Function Extensive membrane sureface area of the Greater absorption of light by photosystems thylakoids Small space (lumen) and low volumes of Faster accumulation of protons to create a fluid within the thylakoids concentration gradient Stroma region similar to the cytoplasm of Provides a region where the enzymes necessary the cell and the matrix of the mitochondrion for the Calvin cycle can work Isolates the working parts and enzymes of the Double membrane on the outside chloroplast from the surrounding cytoplasm B2.2.6 THE DOUBLE MEMBRANE OF THE NUCLEUS Functional benefits of the double membrane of the nucleus The nucleus is bordered by a double membrane and is the region where DNA resides. This double membrane, referred to as the nuclear envelope, provides an area where DNA can carry out its functions without being affected by processes occurring in other parts of the cell The nuclear membrane has many pores that extend through both layers of the envelope. They allow ions and small molecules to diffuse between the nuclear material, the nucleoplasm, and the cytoplasm These pores also control the passage of mRNA, proteins and RNA-protein complexes into and out of the nucleus. These RNA-protein complexes often become ribosomes and are produced in a region of the nucleus called the nucleolus. The inner membrane of the nuclear envelope interacts with the inactive form of DNA called chromatin which occurs within the nucleus and maintains the shape of the nucleus. 8 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version B2.2.7 THE RIBOSOME (HL) Structure and function of free ribosomes and of the rough endoplasmic reticulum. Ribosomes are cytoplasmic organelles found in both prokaryotic and eukaryotic cells. Ribosomes are composed of proteins and a specific type of RNA known as ribosomal RNA (rRNA) There are specialized attachment sites located on the two subunits that allow multiple amino acids to be bonded into highly specific proteins. Ribosomes are responsible for protein synthesis and are found in two locations within cells. They may be attached to the ER, or they may be free in the cytoplasm of the cell. ER that has ribosomes attached to it is called rough ER (rER). 9 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Free ribosomes produce proteins that are used within the cell, such as in the supporting cytoskeleton of the cell. Proteins produced by free ribosomes are also used in the nucleus, by mitochondria, and in other organelles not derived from the cell's inner membrane system Ribosomes that are bound to membranes produce proteins that are transported through the ER and often are exported from the cell. Proteins called secretory proteins are produced by membrane-bound ribosomes and are sent to the Golgi apparatus, where they are properly packaged for cellular exit. B2.2.8 THE GOLGI APPARATUS (HL) Structure and function of the Golgi apparatus The Golgi apparatus usually consists of flattened sacs, especially numerous in cells engaged in producing and secreting substances The stacks of flattened sacs are called cisternae One side of the stack of flattened sacs is located near the ER (the cis side). The other side is directed towards the plasma membrane (the trans side). 10 | 慧 理 教 育 For You Education www.foryouedu.com.hk B2.2 | Organelles and Compartmentalization Teacher’s Version Protein- or lipid-filled transport vesicles are received on the cis side of the Golgi apparatus from either the rough ER or smooth ER As the protein or lipid moves through the cisternae, they are modified so that they can carry out the specific function needed at that time. Once modified the final product is then packaged into vesicles that depart on the trans side B2.2.9 CELLULAR VESICLES (HL) Structure and function of vesicles in cells Vesicles are small membrane-bound sacs in which various substances are transported or stored in the cell Some common examples of vesicles are: o peroxisomes, which contain enzymes used to break down fatty acids o lysosomes, which contain enzymes necessary for cellular digestion and also for destroying defective or damaged organelles o transport vesicles, which move molecules within the cell o secretory vesicles, which contain materials that are to be excreted from the cell, such as neurotransmitters and hormones Clathrins are proteins in the cell membrane that anchor certain proteins to specific sites, especially on the exterior plasma membrane in receptor-mediated endocytosis The clathrin proteins line coated pits, allowing the receptors to bind to specific molecules When an appropriate collection of molecules occurs in the lined pit, the pit deepens and will eventually seal off, forming a vesicle 11 | 慧 理 教 育 For You Education www.foryouedu.com.hk