MPSC Notes - Cell Structure - Cell Organelles Parts - III

21. What are cytoplasmic inclusions?

Cytoplasmic inclusions are cytoplasmic molecular aggregates, such as pigments, organic polymers and crystals. They are not considered cell organelles.

Fat droplets and glycogen granules are examples of cytoplasmic inclusions.

22. Where in the cell can ribosomes be found? What is the main biological function of ribosomes?

Ribosomes can be found free in the cytoplasm, adhered to the outer side of the nuclear membrane or associated to the endoplasmic reticulum membrane defining the rough endoplasmic reticulum. Ribosomes are the structures where protein synthesis takes place.

23. What is the difference between smooth and rough endoplasmic reticulum?

The endoplasmic reticulum is a delicate membranous structure contiguous to the nuclear membrane and present in the cytoplasm. It forms an extensive net of channels throughout the cell and is classified into rough or smooth types.

The rough endoplasmic reticulum has a great number of ribosomes attached to the external side of its membrane. The smooth endoplasmic reticulum does not have ribosomes attached to its membrane.

The main functions of the rough endoplasmic reticulum are synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in the lipid synthesis and, in muscle cells, it is important in the conduction of the contraction stimulus.

24. A netlike membranous complex of superposed flat saccules with vesicles detaching from the extremities seen in electronic microscopy. What is the observed structure? What is its biological function?

What is being observed is the Golgi complex, or Golgi apparatus. This cytoplasmic organelle is associated with chemical processing and modification of proteins made by the cell and with storage and branding of these proteins for posterior use or secretion. Vesicles seen under the electronic microscope contain material already processed, ready to be exported (secreted) by the cell. The vesicles detach from the Golgi apparatus, travel across the cytoplasm and fuse with the plasma membrane then secreting their substances to the exterior.

25. On which organelle of the cell structure does intracellular digestion depend? What is the chemical content of those organelles?

Intracellular digestion occurs by the action of lysosomes. Lysosomes have digestive enzymes (hydrolases) that are made in the rough endoplasmic reticulum and stored in the Golgi apparatus. Lysosomes are hydrolase-containing vesicles that detach from the Golgi apparatus.

26. Why are lysosomes known as “the cleaners” of the cell waste?

Lysosomes carry out autophagic and heterophagic digestion: autophagic digestion by digesting residual substances from the cellular metabolism; heterophagic digestion by digesting substances that enter the cell. Lysosomes enfold the substances to be degraded forming digestive vacuoles, or residual vacuoles, that later migrate toward the plasma membrane fusing with it and liberating (exocytosis) the digested material to the exterior.

27. Which are the cell organelles that participate in cell division and in the formation of cillia and flagella of some eukaryotic cells?

The organelles that participate in the cell division and in the formation of cilia and flagella of some eukaryotic cells are the centrioles. Some cells have cillia (paramecium, the bronchial ciliated epithelium, etc.) or flagella (flagellate protists, sperm cells, etc.); these cell structures are composed of microtubules originated from the centrioles. Centrioles also make the aster microtubules that are very important for cell division.

28. What are the morphological, chemical and functional similarities and differences between lysosomes and peroxisomes?

Similarities: lysosomes and peroxisomes are small membranous vesicles that contain enzymes and enclose residual substances from internal or external origin degrading them. Differences: lysosomes have digestive enzymes (hydrolases) that break substances to be digested into small molecules; peroxisomes contain enzymes that degrade mainly long-chained fatty acids and amino acids and that inactivate toxic agents including ethanol; within peroxisomes there is the enzyme catalase, responsible for the oxidation of organic compounds by hydrogen peroxide (H2O2) and, when this substance is in excess, by the degradation of the peroxide into water and molecular oxygen.

29. What are mitochondria? What is the basic morphology of these organelles and in which cells can they be found?

Mitochondria are the organelles in which the most important part of the cellular respiration occurs: the ATP production.

Mitochondria are organelles delimited by two lipid membranes. The inner membrane invaginates to the interior of the organelle forming cristae that delimitate the internal space known as mitochondrial matrix and where mitochondrial DNA (mtDNA), mitochondrial RNA (mt RNA), mitochondrial ribosomes and respiratory enzymes can be found. Mitochondria are numerous in eukaryotic cells and they are even more abundant in those cells that use more energy, like muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self-replicate.

30. Why can mitochondria be considered the power plants of the aerobic cells?

Mitochondria are the “power plants” of aerobic cells because within them the final stages of the cellular respiration process occurs. Cellular respiration is the process of using organic molecule (mainly glucose) and oxygen to produce carbon dioxide and energy. The energy is stored in the form of ATP (adenosine triphosphate) molecules and later used in other cellular metabolic reactions. In mitochondria the two last steps of the cellular respiration take place: the Krebs cycle and the respiratory chain.

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