Topic B: CELL ORGANELLES Flashcards
What is the structure of the nucleus?
The nucleus is the largest organelle within the eukaryotic cell. It is easily observed under the light microscope and can be spherical or ovoid. It has a diameter of 10-20picom
The nucleus is bound by a nuclear envelope, which is made up of a double membrane, continuous with the endoplasmic reticulum.
Nuclear pores present in the nuclear envelope allow small molecules to move between the nucleus and the cytoplasm. mRNA synthesised in the nucleoplasm can move into the cytoplasm for translation.
Nucleoplasm is the semi-fluid matrix that fills the nucleus.
Most of the cells DNA is located within the nucleus, DNA is wound to histone proteins to exist as chromatin.
Nucleoli contain ribosomal RNA genes (rRNA) for the synthesis of rRNA.
What are the functions of the nucleus?
The nucleus controls the cell’s activities
* The nucleus contains the hereditary material of an organism in the form of DNA. Genes found on DNA contain information which are passed from one generation to another
* DNA replication occurs in the nucleus, followed by mitosis (nuclear divsion) Whch results in two genetically identical daughter nuclei as the parent. The nuclear envelope disintegrates during nuclear division and the nuclear envelope reforms after the replicated DNA has been divided equally.
* The nucleus directs protein synthesis by synthesizing mRNA from the instructions on DNA. Ribosomes and tRNA in the cytoplasm translate the genetic language on mRNA into a polypeptide.
* The genes in the chromatin are responsible for the synthesis of ribosomal RNA (rRNA), messenger RNA (mRNA) and transfer RNA (tRNA).
* The nucleolus is the site of rRNA synthesis. Ribosomal proteins imported from the cytoplasm are assembled with rRNA into large and small ribosomal Subunits in the nucleolus. These subunits exit the nucleolus through the nuclear pores, into the cytoplasm to assemble into ribosomes
What is the structure of the cytoplasm?
The cytoplasm is the semi-fluid mixture of the cytosol and organelles. The cytosol is the aqueous solution of ions, organic compournds excluding the the organelles
What is the structure of the ribosome?
They are roughly spherical about 20nm in diameter
They are non-membrane bound organelles
Ribosomes consist of two subunits a large subunit and a small subunit
Each subunit is made up of rRNA and ribosomal proteins Ribosomes exist either as free ribosomes or are attached to the rough endoplasmic reticulum (rER).
The ribosomes found in prokaryotes are known as 70S ribosome while in eukaryotes 80S ribosomes are present in the cytoplasm.
What is the function of the ribosomes?
Ribosomes are the sites of protein synthesis. Ribosomes translate the genetic message carried by messenger RNA (mRNA) into a polypeptide chain.
Free ribosomes are suspended in the cytosol and synthesise proteins that function within the cytosol
Ribosomes attached to rER synthesise proteins that are meant to be: packaged within organelles e.g. lysosomes
* secreted out of the cell
* included into the membranes
They are found extensively in cells that synthesise large quantities of proteins. For example, B-pancreatic cells which secrete insulin.
What is the endomembrane system?
The endomembrane system is the system of internal membranes within eukaryotic cells. The membranes of this system are related either through direct physical continuity or by transfer of membrane segments as tiny vesicles. Prokaryotes lack membrane bound organelles, thus they do not have an endomembrane system.
The endomembrane system is involved in: protein and lipid synthesis, transport of proteins and lipids to organelles or out of the cell or to be embedded into membranes.
The following organelles are part of the endomembrane system: Outer membrane of nuclear envelope Endoplasmic reticulum Golgi apparatus Vesicles and lysosomes
What is the structure of the RER?
The rER consists of a three-dimensional network of interconnecting flattened membrane-bound sacs called cisternae (singular: cisterma).
The rER is lined with a thin single membrane Ribosomes are attached to the cytoplasmic side of the ER membranes.
The membrane is continuous with the outer nuclear membrane
The fluid-filled ER lumen or cisternal space is interconnected.
What are the functions of the rER?
- Folding of polypeptides into proteins. Molecular chaperones are proteins present into the cytoplasm and the ER lumen. They bind and assist other polypps to fold into its 3D conformation. The rER also exerts quality control on protein folding. Incompletely folded proteins are targeted to be degraded in the cytoplasm.
- Transport vesicles containing protein pinch off from rough ER and transport to the Golgi A.
As such the rER forms part of an intracellular transport system, where substances may be transported from the nucelus to exterior of the cell. - Chemical modification of proteins. Glycosylation involves the addition of a short carbohydrate chain which is covalently bonded to the protein.
- As polypeptdes destined to be membrane proteins are synthesised by the ribosomes, they are inserted into the ER membrane and anchored by their hydrophobic regions Membrane proteins can be transported to other components of the endomembrane system in the form of transport vesicles.
What is the structure of the SER?
The sER consists of a three-dimensional network of interconnecting membrane-bound tubules called cisternae SER has a more tubular structure than rER,
The sER is lined with a thin single membrane
There are no ribosomes attached to the outer surface It is often not continuous with the nuclear membrane although it may connect with the rER.
The network of sER allows increased surface area for enzymatic reactions and storage of enzymes.
the golgi apparatus is also involved in glycosylation
What is the function of the sER?
- Enzymes in the sER are important to the synthesis of lipids including phospholipids and steroids e.g. cholesterol. Cells that synthesise and secrete steroid hormones such as cells of the testis and ovaries are rich in sER.
- Enzymes in the sER facilitate detoxification of poisons and drugs, especially in liver cells. It usually involves addition of hydroxyl groups to make these harmful substances more soluble, and easier to flush out of the body.
- Calcium storage Calcium ions are pumped into and stored within the lumen of sER. They are released from the sER in response to extracellular signals.
- Phospholipids are synthesised and added to the membrane of the En Membrane phospholipids can be transported to other components of endomembrane system in the form of transport vesicles.
- The final step of glycogenolysis (breakdown of glycogen to glucose) is catalysed by glucose-6-phosphatase in the liver. Glucose-6-phosphatase is an enzyme found in sER lumen which catalyses the conversion of glucose-6-phosphate to free glucose. Glucose molecules can exit the cell and enter the bloodstream increasing the level of blood glucose
What is the structure of the Golgi Apparatus?
The Golgi apparatus consists of one or several stacks of flattened, curved membrane-bound sacs called cisternae Golgi apparatus is bound by single-membrane
New cisternae are constantly formed at cis / receiving/forming face by fusing with transport vesicles that bud off from the rER and sER.
Golgi vesicles pinch off/ bud off frorm the trans / shipping / secreting face
The movement of Golgi vesicles is guided by cytoskeleton I microtubules to their target sites
What are the functions of the Golgi App?
There are more but this is most important
- The Golgi apparatus has a role in chemical modification of proteins and lipids transported from ER and sER respectively. Glycosylation involves the addition of short carbohydrate chain (oligosaccharide) to proteins and lipids to form glycoproteins and glycolipids respectively. Phosphorylation involves the adition of phosphate groups to protens hese modifications can serve as tags to direct proteins for transport to a specific location in the cell or to be secreted out of the cell. For example, mannose-6-phosphate tag is added to proteins or hydrolytic enzymes meant for lysosomes.
- Proteins that are formed in rER are transported in the form of transport vesicles to the Golgi apparatus where they are temporarily stored and concentrated. Proteins to be secreted are packaged into secretory vesicles. Secretory vesicles containing protein bud off / pinch off the Golgi apparatus and move through the cytosol via cytoskeleton towards the cell Surface membrane. As such, the Golgi apparatus also forms part of the intracellular transport system.
- rER synthesises lysosomal enzymes and are transported to the Golgi apparatus. The vesicles containing lysosomal enzymes pinch off the Golgi apparatus as lysosomes
Outline the process by which amino acids are processed in the cell.
1.Radioactive amino acids enter the cell through the cell surface membrane by facilitated diffusion or active transport. A polypeptide is synthesised by a ribosome attached to the endoplasmic reticulum (ER) using radioactive amino acids.
2. The polypeptide enters the lumen / cisternal space of rough ER and folds into its three dimensional conformation to form protein. Transport vesicle containing protein buds off pinches off from the rER.
3. Transport vesicle containing protein fuses with the cis face of Golgi apparatus
4. The protein / enzyme, trypsinogen is chemically modified and concentrated in the Golgi apparatus.
5. Secretory vesicle pinches off/ buds off from trans face of Golgi apparatus
6. Secretory vesicle containing trypsinogen move along the cytoskeleton towards the cell surface membrane
7. Membrane of secretory vesicle fuses with the cell surface membrane, releasing the trypsinogen into pancreatic duct by exocytosis.
What are the structure and function of vesicles?
Vesicles are tiny sacs bound by single membrane. They are formed when a small area of membrane buds off / pinches off to enclose the substance. They are formed from rough endoplasmic reticulum, smooth endoplasmic reticulum and Golgi apparatus. Vesicles transport substances from one membrane compartment to another. Vesicles can also be formed via endocytosis when the cell surface membrane pinches off to form a vesicle containing substances taken in from the exterior of the cell.
What is the structure of the lysosome?
Lysosomes are small, spherical vesicles found in most eukaryotic cells. They range from about 0.2-0.5 um in diameter.
They are bound by single membrane.
Lysosomes contain hydrolytic enzymes such as proteases, nucleases, lipases and acid phosphatases. The enzymes work optimally under acidic conditions of pH 5.
These enzymes are synthesised on the rER and transported to the Golgi apparatus. Golgi vesicles containing these hydrolytic enzymes bud off the Golgi apparatus as lysosomes. They are absent in plant cells.
