Histology of Cellular Organelles (LEC 2)

What is the plasma membrane?

The plasma membrane is the outermost organelle that keeps everything together and surrounds most organelles.

What is cytoplasm?

Cytoplasm refers to everything on the intracellular aspect of the plasma membrane.

What is the cytoplasmic matrix (cytosol)?

The cytoplasmic matrix, or cytosol, is an aqueous gel containing molecules of different sizes where the physiological processes of the cell occur.

What is the function of the cytoskeleton?

The cytoskeleton comprises polymerized proteins that provide a framework for cell structure and organization, allowing for movement within the cell and influencing fluidity and cell division.

What are organelles?

Organelles are small structures within a cell that perform specific functions, often referred to as 'little organs.'

What are inclusions in cells?

Inclusions are accumulations in cells that are byproducts of metabolism.

What are organelles in eukaryotic cells?

Organelles are functionally distinct intracellular locations where processes necessary for life occur, most of which are membrane enclosed.

How are organelles arranged in the cell?

Organelles are arranged in the cytosol in a very particular way to facilitate their function and are interconnected and joined by the cytoskeleton, not floating freely.

What is common about the membranes of organelles?

The membranes of organelles are very similar to the plasma membrane, with the exclusion of cholesterol.

What is a key characteristic of organelles' functions?

Each organelle has a specific function, but they often work together to complete complex tasks necessary for the cell.

Which organelles are necessary for gene expression and protein synthesis?

The nucleus, ribosomes, endoplasmic reticulum, and Golgi complex are necessary for gene expression and protein synthesis in various forms.

What organelles are involved in energy production?

Mitochondria are the organelles involved in energy production within the cell.

Which organelles are responsible for digestion and detoxification?

Lysosomes and peroxisomes are the organelles responsible for digestion and detoxification in the cell.

What is the nucleus?

The nucleus is the largest organelle in the cell, constituting approximately 10% of the cell's volume.

What does the nucleus contain?

The nucleus contains genetic information and the machinery for DNA replication and RNA transcription.

What structures are found in the nucleus of non-dividing cells?

In non-dividing (interphase) cells, the nucleus contains a nuclear envelope (membrane), nuclear pores, nuclear lamina, nucleoplasm, chromatin (DNA and proteins), and a nucleolus.

What is the nuclear envelope?

The nuclear envelope is a membrane that encloses the nucleus, consisting of an inner and outer nuclear membrane.

What are the characteristics of the inner nuclear membrane?

The inner nuclear membrane faces the nucleoplasm and is in contact with the nuclear lamina.

What are the characteristics of the outer nuclear membrane?

The outer nuclear membrane faces the cytoplasm, is continuous with the endoplasmic reticulum (ER), and has ribosomes attached to its cytoplasmic surface.

What is the nuclear lamina?

The nuclear lamina is a thin, sheet-like meshwork composed of Lamins A and B, serving as a scaffold for the nuclear envelope, chromatin, and nuclear pores.

What happens to the nuclear lamina during mitosis?

The nuclear lamina disassembles during mitosis.

What is the Nuclear Pore Complex (NPC)?

The Nuclear Pore Complex (NPC) allows for selective transport between the nucleus and cytoplasm.

What size of ions and molecules can diffuse freely through the NPC?

Ions and molecules less than 9 nm can diffuse freely through the NPC.

How are molecules larger than 9 nm transported through the NPC?

Molecules larger than 9 nm must be transported by active transport through the NPC.

How do NPCs relate to nuclear membranes?

Nuclear Pore Complexes span the inner and outer nuclear membranes.

What is the diameter of a Nuclear Pore Complex?

The Nuclear Pore Complex measures 80-100 nm in diameter.

What are Nuclear Pore Complexes made of?

NPCs are made up of more than 50 proteins, known as nucleoporins.

What is chromatin?

Chromatin is DNA folded in an organized manner within the nucleus.

What is the measurement of human DNA when stretched out?

Human DNA consists of 2.85 billion base pairs and measures approximately 1.8 meters when stretched out, which is 100,000 times the diameter of the average nucleus.

What are the two types of chromatin?

The two types of chromatin are euchromatin, which appears as dispersed areas (lightly stained), and heterochromatin, which appears as highly condensed (darkly stained) areas.

What is a nucleosome?

The smallest folded unit of chromatin is called a nucleosome, which consists of 2 loops of DNA wrapped around a core of 8 histone proteins.

How is the structure of nucleosomes described?

Nucleosomes are often described as having a 'beads on a string' formation, where the beads represent the nucleosomes and the string is the DNA between them.

What characterizes the chromatin pattern in most cells?

Most cells have a heterogeneous chromatin pattern, displaying a mix of euchromatin and heterochromatin areas.

What forms when 6 nucleosomes are combined?

6 nucleosomes form a chromatin fibril, which then coils.

How are coils of chromatin held together?

The coils of chromatin are looped and held together by nonhistone proteins.

What happens to chromatin during cell division?

During cell division, chromatin condenses into chromosomes.

What are chromatids joined at?

Chromatids are joined at the centromere.

How many chromosomes does each cell contain, excluding gametes?

Each cell (excluding the gametes) contains 23 pairs of chromosomes: 22 autosomes and 2 sex chromosomes.

What are the ends of the chromosomes called?

The ends of the chromosomes are called telomeres, which shorten with each cell division.

What does telomere length indicate?

Telomere length is an indication of the lifespan of the cell.

What is the nucleolus?

The nucleolus is a non-membranous region of the nucleus that is the primary site for ribosomal production and assembly.

What does the nucleolus contain?

The nucleolus contains transcriptionally active rRNA genes.

How does the size of the nucleolus vary?

The nucleolus varies in size but is well developed in cells involved actively in protein synthesis.

Can the nucleolus be duplicated?

Yes, the nucleolus may be duplicated.

What are the three morphologically distinct regions of the nucleolus?

Fibrillar center (FC), Fibrillar material (F), Granular material (G)

Fibrillar center (FC)

pale staining, containing DNA loops of specific chromosomes, rRNA genes, RNA polymerase I, and transcription factors.

Fibrillar material (F)

dark staining, containing ribosomal genes in active transcription and rRNA

Granular material (G)

in between staining, involved in initial ribosomal assembly

What is the primary function of ribosomes?

Ribosomes make proteins.

What do ribosomes consist of?

Ribosomes consist of 2 ribosomal subunits and associated proteins.

What process do ribosomes perform to synthesize proteins?

Ribosomes assemble to synthesize a protein and disassemble after the process is complete.

How do ribosomes interact with mRNA?

Ribosomes bind mRNA and translate it to assemble the protein.

What is a polyribosome?

A polyribosome is a structure where many ribosomes are bound to a single mRNA molecule.

What are two types of ribosome locations?

Ribosomes may be membrane-bound or free in the cytoplasm.

What are the two pathways for protein synthesis?

There are two pathways for protein synthesis: one involves free ribosomes, and the other involves membrane-bound ribosomes.

What are ribosomopathies?

Ribosomopathies are disorders caused by defects in ribosome function, an example being Treacher

What are membrane-bound ribosomes?

Membrane-bound ribosomes are attached to the cytoplasmic surface of the ER membrane, specifically associated with rough ER (RER). They require an ER signal sequence on mRNA to synthesize proteins that are being translocated into the ER lumen.

What are free ribosomes?

Free ribosomes are not attached to any membrane and their mRNA does not have an ER signal sequence. They synthesize all other proteins that are not directed into the ER.

What is the structure of the Endoplasmic Reticulum (ER)?

The Endoplasmic Reticulum (ER) is an interconnected network of branching tubules and flattened sacs distributed throughout the cytoplasm.

How is the ER membrane related to the nuclear envelope?

At certain sites, the ER membrane is continuous with the outer nuclear envelope membrane.

What are the two regions of the ER and their functions?

The two regions of the ER are:

1. Smooth ER: functions in the synthesis of lipids and detoxification.

2. Rough ER: functions in the synthesis of proteins destined for the plasma membrane, lysosomes, or secretion.

What is the ER lumen?

The ER lumen is a single, continuous internal space inside the ER, separated from the cytoplasm by the ER membrane.

What is the Rough Endoplasmic Reticulum (rER)?

The rER is a region with ribosomes attached to the cytoplasmic surface of the ER.

What type of mRNA molecules bind to rER membranes?

Only mRNA molecules with an ER signal sequence bind to rER membranes.

In which type of cells is the Rough ER abundant?

The Rough ER is abundant in cells specialized in protein synthesis.

What type of cells are secretory cells, and what do they do?

Secretory cells produce proteins for extracellular export.

What functions are associated with the Rough ER?

The Rough ER is the site of initial post-translational modifications and folding.

What is the Smooth Endoplasmic Reticulum (sER)?

The sER is a region of the ER that lacks bound ribosomes.

In which type of cells is the Smooth ER abundant?

The Smooth ER is abundant in cells specialized in lipid metabolism.

What is the role of the Smooth ER in steroid synthesis?

The Smooth ER is well-developed in cells that synthesize and secrete steroids.

What major role does the Smooth ER play in detoxification?

The Smooth ER plays a major role in detoxification, particularly through the cytochrome P450 system.

How does the Smooth ER function in muscle cells?

The Smooth ER functions to sequester calcium in muscle cells, specifically known as the sarcoplasmic reticulum.

What is the Golgi Apparatus?

The Golgi Apparatus makes vesicles and is especially well-developed in secretory cells.

Where is the Golgi Apparatus usually located?

The Golgi Apparatus is usually located next to the nucleus and centrosome.

What is the structure of the Golgi Apparatus?

The Golgi Apparatus is a complex of flattened, membrane-enclosed cisternae.

What are the two faces of each Golgi stack?

Each Golgi stack has two faces: the Cis Golgi Network (CGN), which is close to the rough ER, and the Trans Golgi Network (TGN), which is away from the rough ER and is involved in the exit of mature vesicles.

What is the transport direction between the Rough Endoplasmic Reticulum (rER) and the Golgi Apparatus?

The transport is bidirectional, consisting of anterograde transport (from rER to cis-Golgi network) and retrograde transport (from cis-Golgi network to rER).

What mediates the transport between the rER and Golgi?

The transport is mediated by coatomers (COPs).

Which coatomers mediate retrograde transport?

COP-I mediates retrograde transport from the cis-Golgi network to the rER.

Which coatomers mediate anterograde transport?

COP-II mediates anterograde transport from the rER to the cis-Golgi network.

How do vesicles travel within the Golgi Apparatus?

Vesicles travel from cisterna to cisterna as they bud off and fuse with the next stack in the required direction.

What is a key function of the Golgi Apparatus related to proteins?

The Golgi Apparatus is involved in post-translational modification, where each region is responsible for specific modifications depending on the protein's destiny.

What are two essential processes performed by the Golgi Apparatus?

The Golgi Apparatus is responsible for sorting and packaging proteins and lipids for transport to their destinations.

What does the Golgi Apparatus produce for lysosomes?

The Golgi Apparatus produces the precursor vesicles for lysosomes.

What is the Constitutive Secretory Pathway?

The Constitutive Secretory Pathway involves vesicles produced for continuous secretion, which can be directed to either basolateral or apical surfaces.

What is the Regulated Secretory Pathway?

The Regulated Secretory Pathway involves vesicles that are stored until needed, requiring a stimulus for release.

What is the Lysosomal Pathway?

The Lysosomal Pathway involves phosphorylation on attached mannose (mannose-6-phosphate), leading to the release of vesicles as clathrin-coated transport vesicles.

What is the role of peroxisomes in fat metabolism?

Peroxisomes play an important role in fat metabolism, especially in the beta-oxidation of very long chain fatty acids (VLCFA).

What do peroxisomes catalyze in the formation of plasmalogens?

Peroxisomes catalyze the initial reactions in the formation of plasmalogens, which are a class of phospholipids abundant in myelin.

What can deficiencies or abnormal locations of peroxisomes cause?

Deficiencies or abnormal locations of peroxisomes can lead to profound abnormalities in the myelination of nerve cells, such as in Zellweger’s syndrome.

How are peroxisomal proteins synthesized?

Peroxisomal proteins are synthesized by free (cytoplasmic) ribosomes and must have a peroxisomal targeting signal.

What are peroxisomes and their dimensions?

Peroxisomes are small organelles (microbodies) specialized to compartmentalize and degrade toxic reactive oxygen molecules, measuring approximately 0.5 micrometers in diameter.

What enzymes do peroxisomes contain and what is their function?

Peroxisomes contain catalase and other peroxidases, which convert hydrogen peroxide into O2 and water, playing a crucial role in detoxification, especially in hepatocytes for the detoxification of ingested alcohol.

What are endosomes?

Endosomes are membrane-bounded compartments that form part of endocytotic pathways.

What do endosomes contain?

Endosomes may contain membrane proteins, fluids, signaling molecules, etc.

What is the function of endosomes related to proteins?

Endosomes function to sort and recycle proteins, redirecting them to different cellular compartments.

How do endosomes communicate with the rough Endoplasmic Reticulum (rER)?

Endosomes communicate with the rER for continuous production of hydrolases.

What happens when endosomes fuse with a mannose-6-phosphate containing vesicle?

When endosomes fuse with a mannose-6-phosphate containing vesicle, they form a lysosome.

How are lysosomes formed?

Lysosomes are formed from the fusion of an endosome or phagosome with a mannose-6-phosphate (M6P) containing transport vesicle.