Cell Organelles
Cell organelles are specialized structures within cells that perform various functions essential to cellular survival and activity. Examples include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and chloroplasts. These organs perform functions such as DNA storage, energy production, protein synthesis, and waste disposal. Each organelle plays an important role in maintaining cell homeostasis and ensuring proper cellular functioning. Their coordinated activities contribute to the overall health and functionality of the organism at the microscopic level.
GOLGI BODIES or Dictyosomes in plants
Golgi bodies were identified by Camillo Golgi in 1989 in plants and lower invertebrates it is also called dictyosome. It is also considered as the Golgi complex. the Golgi Complex contains parallelly arranged. flattened sac-like structures, which lack ribosomes. Golgi complex or Golgi bodies occur in all cells except the prokaryotic cells and eukaryotic cells of certain fungi, sperm cells of bryophytes and pteridophytes, cells of mature sieve tubes of plants and mature sperm, and red blood cells of animals their number in plant cells may vary from serval hundred to a single organelle in some algae Golgi bodies of plant cells are about 1-3 μ in length and 0.5 μ height. It is composed of flattened membrane-bound sacs (cisternae) and associated versicles. Each Golgi complex usually contains about 3-7 flat tubular cisternae. Insects have up to 30 cisternae. The Golgi body has a distinct polarity in both structure and function. The face of the Golgi body faces the Endoplasmicreticuluni and is usually convex and is called the cis-face or the forming phase. The proteins from the Endoplasmic reticulum enter the Golgi body from this side. This opposite side which is concave is called the trans face or the maturing face from which the proteins exit. The Golgi Complex Consists of three functioning Distinct compartments. The cis-Golgi network, the Golgi stack, and the trans-Golgi network. Transport vesicles carry the proteins between these networks. In addition, membrane tubules may also connect the Golgi compartment. various cell organelles found in cells (only some important ones given)
(a) granular endoplasmic reticulum
(b) Smooth endoplasmic reticulum
(c) Golgi complex
.jpg "Diagram of golgi body") |
| Diagram of Golgi body |
Plant cell Golgi bodies serve as a site where complex polysaccharides are synthesized. They are used in cell wall formation. Hemicellulose of pectin is synthesized in the Golgi bodies and Transferred to the cell surface by transport vesicles. proteins, Lipids, and polysaccharides are transported from the Golgi body to their final destinations, through the secretory pathway. Proteins are carried to the plasma membrane by bulk flow.
In yeasts and plant cells, proteins are transported from the Golgi body to the vacuole as they lack lysosomes. Golgi body contains various enzymes viz., phospholipase ADpase, ATPase, GTPase. Thiamine pyrophosphate, Galactose transferase, Glucose-6-Phosphotase, etc.
Holocrine, apocrine, and merocrine-like cellular secretions are caused due to Golgi Bodies.
ENDOPLASMIC RETICULUM (ER)
The name Endoplasmic Reticulum was coined by Porter (1953) after observing liver cells. It is a network of tubules and sacs that extend from the nuclear membrane throughout the cytoplasm. Cells involved in protein synthesis and storage contain large amounts of ER, whereas erythrocytes, (RBC) eggs and embryonic cells lack ER.
ER consists of cisternae, vesicles, and tubules.
a) Cisternae are long flattened sac-like structures with 40-50 m u in diameter. They are arranged in bundles parallelly. They occur commonly in cells involved in the synthesis of various substances. Ex. brain, notochord, and pancreas.
b) Tubules are reticulately branched systems that occur almost in all cells with a diameter of 50-500m.
c) Vesicles are oval membrane-bound vascular structures with a diameter of 25-500 mμ they remain isolated in the cytoplasm and are abundantly found in the pancreas.
Structurally, it is a trilaminar 50-60 A thick membrane having continuation with the nuclear membrane Golgi body and plasma membrane. Depending upon their surface morphology, E.R. is of two types viz., Rough E.R. and Smooth E.R
The rough E.R. is coated with ribosomes whereas
the smooth E.R. is devoid of ribosomes. (AGRANULAR) (SER)
SMOOTH ENDOPLASMIC RETICULUM
Cells involved in lipid synthesis are rich in S.E.R. Also it is found in glycogen-storing liver cells, adipose cells, interstitial cells, leucocytes, and spermatocytes. Muscle cells also possess SER, here it is called the sarcoplasmic reticulum. Refianal cells possess them as tightly packed vesicles and hence are called myeloid bodies.
Functions:
(1) Synthesis of fats: Besides providing mechanical support to the cell the SER is actively Involved in lipid synthesis As the fats are Highly hydrophobic, they are synthesized in the cytosol. Although some lipids are synthesized in association with other membranes, most of them are synthesized at SER and are transported through the ER to their destination.
Eukaryotic membranes are basically composed of phospholipids, glycolipids, and cholesterol. Phospholipids are mainly derived from glycerol. They are synthesized on the cytosolic side of the E.R. Membrane with the involvement of the enzyme glycerol-3- phosphate and coenzyme A.
(2) Glycogenolysis: The smooth ER. is involved in the breakdown of glycogen, through the action of glucose-6-phosphatase. Glycogenolysis by smooth E.R. takes place only in the cells of fasted animals.
(3) Detoxification: Smooth E.R. performs an important role in detoxification in liver cells. The detoxification reactions also handle the various metabolic waste products like fatty acids, bile salts, and steroids, and have recovered from hemoglobin breakdown.
(4) Cholesterol metabolism: S.E.R. also plays a role in cholesterol and lipid metabolism in the liver and secretion of CL ions in the HCL-secreting cells of the stomach, secretion of triglycerides in the intestinal absorptive cells, synthesis and secretions of steroids hormones by mammalian gonads. The pigments of the retinal cells are also synthesized by S.E.R. from vitamin A.
GRANULAR OR ROUGH ENDOPLASMIC
RETICULUM (RER) Endoplasmic reticulum coated with ribosomes is called the rough E.R. for being sorted and secreted.
The m-RNA'S to be translated on membrane-bound ribosomes contain a set of unique codons just 3' of the initiation site and translation of these codons yields a unique sequence at the amino terminal of the growing polypeptide called the signal sequence, which triggers attachment of the ribosome to the membrane.
Protein processing is related to the E.R. and the 20 amino acid sequence is not necessary for the functioning of the protein and is involved in tagging the polypeptide to the E.R.
Transmembrane proteins are translocated partially into the lumen of E.R. Further translocation is stopped by a stop-transfer sequence on the polypeptide. The lumen of E.R. contains various enzymes and proteins which are involved in the proper folding and functioning of proteins.
GRANULAR OR ROUGH ENDOPLASMIC
RETICULUM (RER) Endoplasmic reticulum coated with ribosomes is called the rough E.R. for being sorted and secreted.
The m-RNA'S to be translated on membrane-bound
ribosomes contain a set of unique codons just 3' of the
initiation site and translation of these codons yield a unique sequence at the amino terminal of the growing polypeptide called the signal sequence, which triggers the attachment of the ribosome to the membrane.
Protein processing is related to the E.R. and the 20 amino acid sequence is not necessary for the functioning of the protein and is involved in tagging the polypeptide to the E.R.
Transmembrane proteins are translocated partially into the lumen of E.R. Further translocation is stopped by a stop-transfer sequence on the polypeptide. The lumen of E.R. contains various enzymes and proteins which are involved in the proper folding and functioning of proteins.
(2) Glycosylation of proteins: Proteins which en- ter E.R are glycosylated on specific aspargine residues on the protein, while their translation is still in process. Glycosylation involves the addition of an oligosaccharide to the aspargine residues. A membrane-bound enzyme called oligosaccharide transferase transfers the oligosaccharide to the Asparagine. Further synthesis of glycerides, fatty acids, and fatty acids biosynthesis takes place in the E.R. membrane due to the involvement of various enzymes.
NUCLEUS
The nucleus was discovered by Robert Brown in 1831. The nucleus is considered to be the core of the cell. It commands all metabolic events. Miescher (1869) Discovered nucleoproteins and nucleic acids and suggested that it is an important component of the nucleus, in 1888. Waldeyer observed and designated chromo- somes.
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| Nucleus labeled diagram |
The number of nuclei per cell may vary from one to many. Generally, cells contain a single nucleus, and such cells are called uninucleate cells. Paramaecium protozoan contains 2 nuclei and is called a Binucleate. Some cells possess 3 or more nuclei and are called Polynucleate. Polynucleate cells of plants are called co-enocytic cells while those of animals are called syncytial cells. The nucleus may be elliptical or discoid in shape; it is covered with a nuclear envelope that contains nuclear pores involved in the transport of molecules across the nuclear membrane. The nuclear membrane separates the nucleus from the cytoplasm and peroxides the structural framework of the nucleus.
Nuclear membrane: is derived from the membrane system of the cell. it is a double-layered structure, the outer layer is continuous with the endoplasmic reticulum. Sev- eral nuclear pore complex exists on the membrane. The nucleus contains a number of proteins, such as RNA polymerase, DNA polymerase, several types of cytochrome, and histones; all are synthesized in the cytoplasm of the cell and transported into the nucleus.
NUCLEAR MATRIX
Within the nuclear envelope, the nucleus body possesses a dense jelly-like mass composed of karyolymaphs and chromatin. Karyolymph is rich in proteins with a small percentage of DNA, RNA, and Phospholipids. In this matrix DNA replication, Transcription, and transport of sub-stances take place. The Nuclear Matrix has a network of the interchromatin matrix.
There is a nucleolus in the nucleus; its number may vary in different cells. Cells lacking nuclei show little or no protein synthesis. Nucleolus or nucleoli are situated along certain chromosomes. This morphology of the nucleus undergoes change during the cell cycle. During prophase, it disappears and during Metaphase it reappears.
FUNCTIONS OF NUCLEOLUS
It is the most important structure in the nucleus, being the site of TRNA transcription and processing of ribosome assembly.
FUNCTIONS OF THE NUCLEUS
(1) Nucleus carries the genetic material, i.e. DNA.
(2) DNA also helps in the regulation of gene expression by selective import of proteins into the nucleus. Proteins responsible for genome structure and organization are all imported into the nucleus selectively, They include histones. DNA polymerases, RNA polymerases, transcription factors, and splicing factors. These proteins are targeted to the nucleus by a specific amino acid sequence called nuclear localization signals. Their signals direct their transport through the nuclear pore complex.
(3) Nuclear matrix provides an ultra-skeletal structure for the nucleus and acts as a support for various reactions and domains.
The nuclear region contains a cluster of rRNA gens.
They are very actively transcribed by RNA Polymerase I.
Let's have some questions on Golgi body and nucleus:
1. Which of the following is not considered as a cell organelle?
a) nucleus
b) Golgi apparatus
c) endoplasmic reticulum
d) ribosome
Answer: d) Ribosome
2. What is the function of Golgi body in plant cells?
a) energy production
b) protein synthesis
c) DNA storage
d) synthesis of complex polysaccharides
Answer: d) Synthesis of complex polysaccharides
3. Which type of endoplasmic reticulum is involved in lipid synthesis?
a) smooth endoplasmic reticulum
b) granular endoplasmic reticulum
c) Golgi Complex
d) nucleus
Answer: a) smooth endoplasmic reticulum
4. Which organelle is responsible for the breakdown of glycogen?
a) nucleus
b) Golgi apparatus
c) rough endoplasmic reticulum
d) smooth endoplasmic reticulum
Answer: d) smooth endoplasmic reticulum
5. What is the function of the nucleus?
a) DNA storage
b) protein synthesis
c) transcription of tRNA
d) Energy production
Answer: c) Transcription of tRNA
6. What is the role of the nuclear matrix?
a) energy production
b) Support for different responses and domains
c) DNA storage
d) protein synthesis
Answer: B) Support for different responses and domains
7. Which cell organelle carries the genetic material (DNA)?
a) nucleus
b) Golgi apparatus
c) endoplasmic reticulum
d) ribosome
Answer: A) Nucleus
Reference:
- Bsc 2nd year botany 3 dots.
