Biology Chapter 1: Golgi Apparatus & Lysosomes PDF

Golgi apparatus The Golgi apparatus is a stack of flattened sacs called cisternae (Figure 1.26). More than one Golgi apparatus may be present in a cell. The stack is constantly being formed at one end from vesicles which bud off from the ER, and are broken down again at the other end to form Golgi v...

Golgi apparatus The Golgi apparatus is a stack of flattened sacs called cisternae (Figure 1.26). More than one Golgi apparatus may be present in a cell. The stack is constantly being formed at one end from vesicles which bud off from the ER, and are broken down again at the other end to form Golgi vesicles. The stack of sacs together with the associated vesicles is referred to as the Golgi apparatus or Golgi complex. The Golgi apparatus collects and processes molecules, particularly proteins from the RER. It contains hundreds of enzymes for this purpose. After processing, the molecules can be transported in Golgi vesicles to other parts of the cell or out of the cell. Releasing molecules from the cell is called secretion and the pathway followed by the molecules is called the secretory pathway. These are some examples of the functions of the Golgi apparatus Figure 1.26: TEM of a Golgi apparatus. A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green). These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (Chapter 4). Golgi vesicles are used to make lysosomes. Sugars are added to proteins to make molecules known as glycoproteins. Sugars are added to lipids to make glycolipids. Glycoproteins and glycolipids are important components of membranes (Chapter 4, Section 4.2, Structure of membranes) and are important molecules in cell signalling (Chapter 4, Section 4.4, Cell signalling). During plant cell division, Golgi enzymes are involved in the synthesis of new cell walls. In the gut and the gas exchange system, cells called goblet cells release a substance called mucin from the Golgi apparatus (Chapter 9, Section 9.4, Warming and cleaning the air). Mucin is one of the main components of mucus. Lysosomes Lysosomes are simple sacs, surrounded by a single membrane. In animal cells they are usually 0.1–0.5 μm in diameter (Figure 1.27). In plant cells the large central vacuole may act as a lysosome, although lysosomes similar to those in animal cells are also seen in the cytoplasm. lysosome: a spherical organelle found in eukaryotic cells; it contains digestive (hydrolytic) enzymes and has a variety of destructive functions, such as removal of old cell organelles Lysosomes contain digestive enzymes. The enzymes are called hydrolases because they carry out hydrolysis reactions. These enzymes must be kept separate from the rest of the cell to prevent damage. Lysosomes are responsible for the breakdown (digestion) of unwanted substances and structures such as old organelles or even whole cells. Hydrolysis works fastest in an acid environment so the contents of lysosomes are acidic, pH 4–5 compared with 6.5–7.0 in the surrounding cytoplasm. Among the 60+ enzymes contained in lysosomes are proteases, lipases and nucleases which break down proteins, lipids and nucleic acids respectively. The enzymes are synthesised on RER and delivered to lysosomes via the Golgi apparatus. The activities of lysosomes can be split into the four categories discussed below. Getting rid of unwanted cell components Lysosomes can engulf and destroy unwanted cell components, such as molecules or organelles, that are located inside the cell. Endocytosis Endocytosis is described in more detail in Chapter 4 (Section 4.5, Movement of substances across membranes). Material may be taken into the cell by endocytosis, for example when white blood cells engulf bacteria. Lysosomes may fuse with the endocytic  vacuoles formed and release their enzymes to digest the contents. Exocytosis Lysosomal enzymes may be released from the cell for extracellular digestion. An example is the replacement of cartilage by bone during development. The heads of sperms contain a special lysosome, the acrosome, for digesting a path through the layers of cells surrounding the egg just before fertilisation. Self-digestion The contents of lysosomes are sometimes released into the cytoplasm. This results in the whole cell being digested (a process called autolysis). This may be part of normal development, as when a tadpole tail is reabsorbed during metamorphosis or when a uterus is restored to its normal size after pregnancy. It also occurs after the death of an individual as membranes lose their partial permeability. Microtubules and microtubule organising centres (MTOCs) Microtubules are long, rigid, hollow tubes found in the cytoplasm. They are very small, about 25 nm in diameter. Together with actin filaments and intermediate filaments (not discussed in this book), they make up the cytoskeleton, an essential structural component of cells which helps to determine cell shape. Microtubules are made of a protein called tubulin. Tubulin has two forms, α-tubulin (alpha-tubulin) and β-tubulin (beta-tubulin). α- and β-tubulin molecules combine to form dimers (double molecules). These dimers are then joined end to end to form long ‘protofilaments’. This is an example of polymerisation, the process by which giant molecules are made by joining together many identical subunits. Thirteen protofilaments line up alongside each other in a ring to form a cylinder with a hollow centre. This cylinder is the microtubule. Figure 1.29a shows the helical pattern formed by neighbouring α- and β-tubulin molecules. Apart from their mechanical function of support, microtubules have a number of other functions. Secretory vesicles and other organelles and cell components can be moved along the outside surfaces of the microtubules, forming an intracellular transport system, as in the movement of Golgi vesicles during exocytosis. During nuclear division (Chapter 5), a spindle made of microtubules is used for the separation of chromatids or chromosomes. Microtubules form part of the structure of centrioles. Microtubules form an essential part of the mechanism involved in the beating movements of cilia and flagella. The assembly of microtubules from tubulin molecules is controlled by special locations in cells called microtubule organising centres (MTOCs). These are discussed further in the following section on centrioles. Because of their simple construction, microtubules can be formed and broken down very easily at the MTOCs, according to need. ADP (adenosine diphosphate): the molecule that is converted to ATP by addition of phosphate (a reaction known as phosphorylation) during cell respiration; the enzyme responsible is ATP synthase; the reaction requires energy microtubules: tiny tubes made of a protein called tubulin and found in most eukaryotic cells; microtubules have a large variety of functions, including cell support and determining cell shape; the ‘spindle’ on which chromatids and chromosomes separate during nuclear division is made of microtubules ribosome: a tiny organelle found in large numbers in all cells; prokaryotic ribosomes are about 20 nm in diameter while eukaryotic ribosomes are about 25 nm in diameter Golgi apparatus (Golgi body, Golgi complex): an organelle found in eukaryotic cells; the Golgi apparatus consists of a stack of flattened sacs, constantly forming at one end and breaking up into Golgi vesicles at the other end Golgi vesicles: carry their contents to other parts of the cell, often to the cell surface membrane for secretion; the Golgi apparatus chemically modifies the molecules it transports, e.g. sugars may be added to proteins to make glycoproteins Centrioles Under the light microscope the centriole appears as a small structure close to the nucleus (Figure 1.4). Centrioles are discussed later in this chapter. Microvilli (singular: microvillus) are finger-like extensions of the cell surface membrane. They are typical of certain animal cells, such as epithelial cells. Epithelial cells cover the surfaces of structures. The microvilli greatly increase the surface area of the cell surface membrane, as shown in Figure 1.19. This is useful, for example, for reabsorption in the proximal convoluted tubules of the kidney and for absorption of digested food into cells lining the gut. Centrioles and centrosomes The extra resolution of the electron microscope reveals that just outside the nucleus of animal cells there are really two centrioles and not one as it appears under the light microscope (compare Figures 1.4 and 1.19). They lie close together and at right angles to each other in a region known as the centrosome. Centrioles and the centrosome are absent from most plant cells. A centriole is a hollow cylinder about 500 nm long, formed from a ring of short microtubules. Each centriole contains nine triplets of microtubules (Figures 1.30 and 1.31). Until recently, it was believed that centrioles acted as MTOCs for the assembly of the microtubules that make up the spindle during nuclear division (Chapter 5). It is now known that this is done by the centrosome, but does not involve the centrioles. However, centrioles are needed for the production of cilia. Centrioles are found at the bases of cilia and flagella, where they are known as basal bodies. The centrioles act as MTOCs. The microtubules that extend from the basal bodies into the cilia and flagella are essential for the beating movements of these organelles. At the base of each cilium and flagellum is a structure called the basal body which is identical in structure to the centriole. We now know that centrioles replicate themselves to produce these basal bodies, and that cilia and flagella grow from basal bodies. Figure 1.33 is a scanning electron micrograph of cilia in the respiratory tract. Figure 1.33: Scanning electron micrograph of cilia in the respiratory tract Beating mechanism The beating motion of cilia and flagella is caused by the dynein (protein) arms making contact with, and moving along, neighbouring microtubules. This produces the force needed for cilia to beat. As neighbouring MTDs slide past each other, the sliding motion is converted into bending by other parts of the cilium. Functions If the cell is attached to something so that it cannot move, fluid will move past the cell. If the cell is not attached, the cell will swim through the fluid. Single- celled organisms can therefore use the action of cilia and flagella for locomotion. You will easily be able to find videos of such motion on the internet. In vertebrates, beating cilia are found on some epithelial cells, such as those lining the airways (Chapter 9). Here more than 10 million cilia may be found per mm2. They maintain a flow of mucus which removes debris such as dust and bacteria from the respiratory tract.