Organelles - Study Notes

ORGANELLES Organelles or small organs inside are a structure that has a separating membrane from the surrounding matrix. Every one Organelles have a unique function to ensure order and integrity within cells are maintained. a. The nucleus The nucleus or cell nucleus is the largest organelle in the cell, found in all eukaryotic cells (cells whose core material is encased in a membrane). In general, cells contain one nucleus, but in some organisms there are more than one. The shape of the cell nucleus is usually round, oval or irregular, with a diameter of ± 10 nm and a length of ± 20 nm, located around the center of the cell. The nucleus is covered by two layers of lipoprotein membrane, each separated by a gap of 20 - 30 nm. The outer membrane bordering the cytoplasm is directly connected to the endoplasmic reticulum and ultimately to the plasma membrane. In the nuclear membrane there are pores that allow the exchange of substances between the nucleus and the cytoplasm, for example RNA and protein. The matrix in the nucleus is called the nucleoplasm, it contains various enzymes proteins, chromosomes, and the nucleolus (nucleus). The main components that make u chromosomes are basic proteins called histones and DNA (dioxy ribonucleic acid). DNA is a genetic substance that is capable of replicating (doubling) when cells reproduce (divide). When cells are not dividing, chromosomes look like long, fine threads called chromatin. The number of chromosomes in cells of various species varies, but the number tends to be constant in each species. The nucleolus (nucleus) is round, found in the nucleoplasm and only appears after the end of the cell division process. The nucleolus has a role in the synthesis of RNA, ribosomes which will be released from the nucleus into the cytoplasm. b. Endoplasmic Reticulum The endoplasmic reticulum (ER) is composed of a stack of sacs (cavities) of parallel membranes and forms a system that is spread like a net throughout the cytoplasm. T membrane structure is the same as the plasma membrane which forms tubules (channels), sisternae (flat cavities), and vesicles (bubbles). So that you better understand the structure of the endoplasmic reticulum The spaces formed between parallel membranes cause the ER system to look like complicated channels and in some parts are connected to the plasma membrane or nuclear membrane. The endoplasmic reticulum is divided into: 1) Rough Endoplasmic Reticulum (rough ER), is the endoplasmic reticulum to which a number of ribosomes are attached to the surface of its membrane, which is an organelle that plays a role in protein synthesis. Proteins synthesized by ribosomes enter the cisterns and are transported (via rough ER) to other organelles, for example the Golgi apparatus. 2) Smooth Endoplasmic Reticulum (Smooth ER), the surface of the membrane is no attached to ribosomes and plays a role in the synthesis of fats and steroids. The synthesis results are then secreted into the endoplasmic reticulum to be transported to other parts of the cell, for example to the Golgi apparatus, vacuole or removed from the cell by exocytosis. The endoplasmic reticulum is only found in eukaryotic cells, both animal and plant cells. c. Ribosome Ribosomes are round organelles composed of nucleoproteins, namely protein compounds with RNA. In prokaryotic cells ribosomes are ± 20 nm in size, but in eukaryotic cells they are smaller. Some ribosomes are attached to the endoplasmic reticulum membrane to form groups called polysomes, the rest are scattered in the cytosol, and are even found in other organelles such as mitochondria and chloroplasts in plant cells. It has been stated above that ribosomes play a role in protein synthesis. d. Micro Bodies Microbodies are round organelles, composed of a layer of membrane, have no internal structure with a diameter of 0.5-1.5 nm. Microbodies are organelles produced from the endoplasmic reticulum. Micro bodies are divided into two, namely peroxisomes and glyoxysomes. 1) Peroxisomes are organelles found in plant photosynthetic tissue (chloroplasts), while in animals they are often found in liver and kidney cells. Peroxisomes produce several a – l metabolic enzymes. The oxidizing glycolic acid enzyme plays a role in the process of oxidizing glycolate into glyoxylic acid and H2O2 , which is one of a series of photorespiration processes in plants. Hydrogen peroxide (H2 O2 ) is produced from several biochemical reactions in plant cells or animal cells which is toxic. Next, it will be broken down by the catalase enzyme which is also found in peroxisomes into non-toxic compounds. 2) Glyoxysomes, produce enzymes which function to break down fat molecules into carbohydrates during germination, in this reaction H2 O2 is also produced which will later be broken down by the catalase enzyme. The action of the catalase enzyme is very fast, this can be demonstrated by dripping H2O2 on fresh liver. e. Golgi Body The Golgi Body was discovered by Camillio Golgi in 1898. It is composed of a pile of flat sacs (sisterna) and at the edges there are bubbles (vesicles). Golgi bodies are spread throughout the cytoplasm, measuring 1 – 3 ÿ in length and 0.5 ÿ in width, in glandular cells and nerve cells their size is relatively large compared to those in brain cells. Bubbles (vesicles) from the Golgi apparatus can escape and move to the cell surface to secrete their contents outside the cell. The Golgi body in animal cells produces digestive enzymes from the digestive glands. This organelle also synthesizes certain polysaccharides which are added to proteins originating from ribosomes, forming glycoproteins in the form of mucin (mucus). In plant cells, the Golgi apparatus contains polysaccharides and proteins to form cell walls, besides that it also produces mucus, for example to lubricate the tips of roots when they penetrate the soil. Mucus, wax in percha (rubber) plants, and sticky secretions produced by the Golgi apparatus, sometimes even play a role in fat transport. In plants, the Golgi bodies are called dictyosomes. f. Lysosomes Lysosomes are very small organelles compared to other organelles. Generally measuring 0.2 – 0.5 ÿm. Lysosomes are produced by the Golgi apparatus (derived from loose Golgi apparatus vesicles) and are distributed in the cytoplasm in large numbers. Lysosomes are found in almost all eukaryotic cells. In lysosomes there are various hydrolytic enzymes, such as protease, lipase, and phosphatase which play a role in intracellular digestion, breaking down various substances in the cell. Some of these substances come from outside, such as polysaccharides, fats and proteins, including bacteria captured by phagocytosis. In summary, the functions of lysosomes are as follows. 1) Digesting substances taken in by endocytosis, for example in cells white blood that eats bacteria. 2) Autophagocytes, a process of dissolving unwanted structures in cells, for example destroying other organelles that no longer function. 3) Exocytosis. 4) Autolysis, namely self-destruction of the cell by freeing all the contents of the lysosomes in the cell, for example destroying the tail of a tadpole when it reaches adulthood by reabsorbing its tail. g. Mitochondria Mitochondria are round, oval or rod shaped with a diameter of 0.5-0.1 ÿm and a length of 1-4 ÿm. It is present in all aerobic eukaryotic cells and varies in number. In cells with high activity, such as nerve cells, brain cells, liver cells and spermatozoid cells, there are hundreds or even thousands of mitochondria. Mitochondria have two separate membrane layers, the structure of the outer membrane is similar to the plasma membrane. The inner membrane has inward folds called cristae. With the presence of cristae, the surface of the inner membrane becomes wider so that the cellular respiration process is more effective. The respiration process occurs in the cristae and matrix. In the matrix there are DNA, RNA, ribosomes, and various enzymes that control the reaction stages of cellular respiration. The respiratory reaction that takes place in mitochondria is oxidative decarboxylation of the Krebs cycle following electron transfer. h. Microtubules Microtubules are found in animal cells and plant cells, are cylindrical or tubular in shape and do not have branches. Its length reaches several micrometers with an outer diameter of ± 25 ÿm and an inner diameter of 12 ÿm. Microtubules are composed of tubulin protein molecules arranged in a helical (twisted) arrangement to form hollow cylindrical walls. This organelle is rigid, and acts as a cell skeleton (cytoskeleton). which is able to maintain cell shape in order to remain. Function of microtubules Another is to help transport substances, which is the main component constituent of cilia, flagellum, centrioles and spindle threads cell division is taking place. i. Microfilaments Microfilaments are fine threads with varying diameters between 5-7 ÿm. These threads are composed of actin and protein myosin in small amounts. Microfilaments have a role as a framework cells (cytoskeleton) and plays a role in the processes of endocytosis and exocytosis. In addition, microfilaments have an important role in muscle contraction. j. Centrosome Centrosomes are only found in animal cells and play a role in the process cell division, namely regulating chromosome movement. This organelle consists of: two centrioles and surrounded by cytoplasm called the centrosphere. Centrioles cylindrical shape with a diameter of ± 0.2 ÿm which is composed of microtubules. Centrioles are located near the cell nucleus. When the cell is dividing the centrioles duplicate to form 2 pairs. Next, each pair separates from each other towards opposite poles and forms spindle threads.