Cell Biology Quiz on Organelles and Functions

Cell Organelles

•  Cellular components are divided into nucleus and cytoplasm
•  Cytoplasm contains organelles and cytosol
•  Organelles are complexes of macromolecules, carrying out specific functions, some are membrane-bound and others are not
•  Membrane-bound organelles concentrate enzymes and reactants, increase biochemical efficiency and isolate harmful proteins and molecules from the rest of the cell
•  Non-membranous organelles include microtubules, centrioles, and nucleoli
•  Cytosol is a gel-like substance containing dissolved macromolecules, organic compounds & ions, cytoskeleton (microtubules, actin filaments and intermediate filaments), which organizes organelles and provides mechanical support. Also contains inclusions—products of metabolic activity and insoluble substances, such as glycogen, lipid droplets, lipofuscin, hemosiderin, and crystals

Learning Objectives

•  Describe location, function and staining characteristics of cytoplasmic organelles
•  Differentiate between membrane-bound organelles (rough ER, smooth ER, Golgi complex, mitochondria, lysosomes, and peroxisomes) and explain their functions
•  Differentiate between free polyribosomes and membrane-bound polyribosomes and identify their roles in protein sorting
•  Determine the trafficking of different classes of protein to their destination
•  Describe autophagy and cytoplasmic protein degradation
•  Explain how faulty protein degradation leads to Alzheimer's disease
•  Describe proteasome-mediated protein degradation
•  Describe the functions of peroxisomes
•  Identify all cell organelles in a given electron micrograph, indicating the cell types in which they are abundant

Mitochondria

•  Double membrane-bound organelle
• Provides energy to the cell (generating & expending energy) abundant in heart muscle, liver cells, and some kidney cells
•  Absent in erythrocytes and terminal keratinocytes/corneocytes
•  Spherical or filamentous, acidophilic structures (0.5-1µm wide and 10µm long)
•  Constantly turning over, lifespan of 1-10 days (varies)

Mitochondria Structure

• Possesses two membranes separated by intermembrane space
•  Outer membrane contains large channels (e.g., porins), permits ions & small molecules to enter the intermembrane space.
•  Inner membrane is impermeable to ions but projects into the matrix to form numerous folds (cristae), increasing the surface area, and is responsible for oxidative phosphorylation and ATP synthesis
•  Attached to the inner membrane are ATP synthase particles (elementary particles)
•  Matrix contains soluble enzymes of the Krebs cycle, enzymes for fatty acid oxidation, electron-dense matrix granules that store Ca++, mitochondrial DNA, ribosomes and tRNA

Mitochondria cont. (Intermembrane Space)

• Contains specific enzymes: creatine kinase, adenylate kinase, and cytochrome C.
• When cytochrome C is discharged into the cytoplasm, it initiates apoptosis (programmes cell death)
• Cytochrome C activates caspase, a key enzyme in cell death or apoptosis

Mitochondria cont. (Matrix)

• Surrounded by the inner membrane
• Contains soluble enzymes of the Krebs cycle and enzymes for fatty acid oxidation; electron dense matrix granules store Ca++ and mitochondrial DNA, ribosomes and tRNA

Mitochondria cont. (Cristae)

• Two types of cristae: tubular or vesicular and plate-like
• Most cells contain mitochondria with plate-like cristae. Steroid hormone-secreting cells (e.g., adrenal cortex cells, gonadal cells) have tubulo-vesicular cristae

Mitochondrial Enzymes System

• Responsible for ATP generation in various metabolic pathways (oxidative phosphorylation, citric acid cycle, beta oxidation of fatty acids)
• Mitochondrial DNA synthesizes only 1% of mitochondrial proteins
• The remaining proteins are synthesized by free ribosomes in the cytoplasm and transported into the mitochondrial matrix through protein complexes called translocases (TOM and TIM)
• Absence of translocases affects mitochondrial functions

Endoplasmic Reticulum (ER)

• Mesh-like interconnected maze of branching tubes and flattened lamellae (cisternae)
• Two types: rough (rER) and smooth (sER)
• Rough ER: ribosomes attach to the outer surface and is involved in protein synthesis, post-translational modification, proper protein folding and initial glycosylation.
• Smooth ER: outer surface ribosomes-free, involved in synthesis of steroid hormones, membrane lipids, breakdown of toxins & drugs, detoxification, synthesis of triglycerides from monoglycerides in intestinal epithelium, and calcium sequestration in striated muscle (sarcoplasmic reticulum)

Ribosomes

•  In the cytosol, ribosomes are connected by a strand of mRNA (polysomes)
•  Found in two forms: free polysomes (free in the cytosol) and bound polysomes (attached to the outer surface of rER cisternae)
• Synthesize different classes of proteins
•  Ribosomes are responsible for cytoplasmic basophilia in Light Microscopy (LM)

Golgi Complex (Apparatus)

• Group of flattened sacs with vesicles around the margins.
• From one to hundreds in a cell, depending on the cell type.
• Prominent in protein-secreting cells.
• At the transmission electron microscopy (TEM) level, it appears to consist of three defined regions: cis, medial, and trans.

Golgi Complex cont.

• Cis face: receiving/forming/entry face; substances enter from the endoplasmic reticulum for processing
• Trans face: exit face; matured proteins exit in the form of small detached vesicles.
• Involved in concentrating and maturing proteins. The process of protein maturation involves movement through the cis, medial, and trans regions. Each region has its unique enzymes involved in protein maturation. Mature proteins are then secreted in vesicles.

Golgi Complex functions

•  Posttranslational modifications of proteins (O-glycosylation)
•  Sorting and packaging of proteins

Lysosomes

• Membrane-bound organelles containing around 40 different types of enzymes working in an acidic pH.
• They contain acid hydrolases, inactive in the cytosol if they leak out (cytosol pH is 7.2).
• Sites of intracellular digestion and turnover of cellular components.
• Found in all cell types, abundant in phagocytic cells (e.g., neutrophils and macrophages)

Biogenesis of Lysosomes

• Lysosomal enzymes synthesised in rER, modified, sorted and packaged in Golgi
• Mannose-6-phosphate added to lysosomal enzymes in Golgi
• Material for degradation brought to lysosomes by heterophagy (endocytosis & phagocytosis) and autophagy (degradation of cell’s senescent organelles and large denatured proteins)
• Degraded materials either released into cytoplasm or removed by exocytosis

Lysosomes degrade cytoplasmic proteins and organelles via autophagy

• Three autophagic pathways: macroautophagy, microautophagy, and chaperone-mediated autophagy.

Residual Bodies

• Digested materials inside lysosomes are recycled or released to the outside.
• Indigestible compounds stored are called residual bodies.
• Appear as yellowish brown pigments around the nucleus (lipofuscins).
• The unreleased indigestible compounds in long-living cells are called aging pigments; present in neurons and cardiac muscle cells

Lysosomal Storage Diseases

• Absence of certain lysosomal enzymes leads to lysosomal storage diseases which is a pathological condition.
• Caused by mutations in genes encoding lysosomal proteins.
• Results in accumulation of undigested products.
• Examples include Tay-Sachs disease (absence of lysosomal galactosidase), and Gaucher's disease/ Pompe disease (lack of enzyme that breaks down glycolipids / lack of acid maltase).

Peroxisomes (Microbodies)

• Membrane-bound spherical or slightly ovoid structures (0.3-1.5 µm)
• Regulate hydrogen peroxide levels in cells and contain catalase enzymes that break down hydrogen peroxide.
• Involved in beta-oxidation of long-chain fatty acids, formation of bile acids and cholesterol. Synthesizes plasmalogen in nerve cells.
• Abundant in liver cells (hepatocytes).
• Peroxisomes differ from lysosomes in their self-replicating ability.

Peroxisome malfunction (Zellweger Syndrome)

• Leads to over-accumulation of very long-chain fatty acids, affecting brain (hypotonia), liver (jaundice) and kidneys (polycystic kidney disease).
• An example of a peroxisome-related disorder.

Proteasome-Mediated Protein Degradation

• Lysosomes are not involved, but proteasomes are
• Proteins must be tagged by ubiquitin.
• Used by cells to destroy abnormal proteins (e.g., misfolded, denatured, or abnormal), cell cycle progression proteins (e.g., cyclins, transcription factors), and tumor suppressors/tumor progressors.
• Protein degradation malfunction can lead to decreased protein degradation in some diseases (e.g., Alzheimer's and Angelman syndrome) or accelerated protein degradation in certain conditions (e.g. human papillomavirus infection).

Other Topics

•  Includes a summary of Golgi complex functions
•  Includes a summary of smooth ER functions
•  Includes pictures of various organelles, cell types and tissues

Test your knowledge on the roles and structures of organelles in eukaryotic cells. This quiz covers non-membranous organelles, membrane functions, and protein modifications essential for cellular processes. Dive deep into the complexities of cell biology and enhance your understanding of cellular efficiency and organization.