Cell Biology Quiz - Organelles and Functions

Cell Biology Study Notes

• Revision 1, BMS111 covering Histology and Cell Biology, from Fall 2024-2025 at Galala University.

Cytoplasm

• Organelles: Living, permanent, essential, active components with vital functions.
• Inclusions: Non-living, temporary, not essential (except in some cases), inert, metabolic products.
• Cytosol: Semifluid cytoplasmic matrix.

Cell Organelles

• Membranous: Covered by membrane. Includes cell (plasma) membrane, mitochondria, rough endoplasmic reticulum (rER), smooth endoplasmic reticulum (SER), Golgi apparatus, lysosomes, and peroxisomes.
• Non-membranous: Uncovered by membrane. Includes ribosomes and proteosomes, and cytoskeleton.

Cell Membrane

• Definition: The limiting membrane of the cell; also called plasma membrane or plasmalemma.
• L.M.: Difficult to see with H&E stains; very thin (8-10nm). Special stains like Ag and PAS are used.
• E.M.: Trilaminar appearance: 2 electron-dense layers separated by an electron-lucent zone. A cell coat (glycocalyx) on the outer surface only.

Molecular Structure of Cell Membrane

• Lipid Component:
  • Phospholipid bilayer
  • Cholesterol
• Protein Component: (50%)
  • Peripheral proteins
  • Integral proteins
• Carbohydrate Component: Oligosaccharide chains linked to membrane proteins (glycoproteins) and phospholipids (glycolipids). Located only on the outer surface.

Phospholipid Bilayer

• Heads: Hydrophilic (attracted to water), charged, polar, and directed towards aqueous solutions.
• Tails: Hydrophobic (repelled by water), non-charged, non-polar, and directed inwards away from aqueous solutions.

Cholesterol

• Function:
  • Maintains membrane stability by restricting phospholipid movement.
  • Modulates membrane fluidity.

Membrane Proteins

• Peripheral (extrinsic): Small molecules, loosely attached to both surfaces of the cell membrane, outside the lipid bilayer.
• Integral (intrinsic): Larger molecules, firmly attached to the cell membrane and embedded within the lipid bilayer

Membrane Proteins (Integral)

• Some are partially embedded within the lipid bilayer.
• Others extend across the entire membrane (transmembrane proteins).
• Functions: Channels, carriers, and receptors.

Carbohydrate Component (Glycocalyx)

• Oligosaccharide chains linked to membrane proteins (glycoproteins) and phospholipids (glycolipids).
• Located on the outer surface of the plasma membrane only.
• Function in cell adhesion, cell identification, and as receptors for ligands.

Transport across cell membrane

• Endocytosis: (Active movement of substances INTO the cell)
  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis
• Exocytosis: (Active movement of substances OUT of the cell)

Receptor-Mediated Endocytosis

• Receptors (integral proteins) accumulate at the cell membrane to bind specific ligands.
• They associate with clathrin to form coated pits.
• Coated pits invaginate and pinch off, forming coated vesicles that contain ligands and their receptors.
• Clathrin is dissociated and recycled.
• Examples of uptake of hormones and uptake of LDL.

Medical Application: Dwarfism

• Caused by GH deficiency or non-functioning GH receptors at cell membrane of target cells.

Mitochondria

• Definition: Membranous organelles responsible for cell respiration and energy production (powerhouses of the cell) .
• Site: all cells except RBCs.
• Number: More in active cells like liver cells (1000/cell).
• Histological structure:
  • L.M: Cytoplasmic acidophilia (if abundant), large number of basic membrane proteins.
  • E.M: Oval or rounded vesicles; surrounded by two membranes (outer and inner). The inner membrane is folded into cristae which increases the surface area for ATP synthase (enzyme for oxidative phosphorylation).

Mitochondria (Continued)

• Matrix: Contains oxidative enzymes (citric acid cycle), mitochondrial DNA, RNA and few ribosomes. Dense granules rich in calcium (catalysts).
• Functions: Produce ATP; self-replicating organelles by simple fission.

Endoplasmic Reticulum (ER)

• Rough Endoplasmic Reticulum (rER): has attached ribosomes; Site of protein synthesis; initial glycosylation of proteins, transport proteins to Golgi complex.
• Smooth Endoplasmic Reticulum (sER): Does not have attached ribosomes; Site of lipid and glycogen synthesis, calcium release in muscle, detoxification of drugs, neonatal jaundice.

Golgi Complex

• E.M: Flattened, parallel curved saccules; interconnected and arranged one above the other, forming a stack.
• Surface: Has cis (immature) and trans (mature) faces.
• Functions:
  • Chemical modification of proteins and lipids (for lysosomes, secretion, and plasma membrane), by adding carbohydrates.
  • Packaging and sorting of products like hormones into secretory vesicles (exocytosis) and formation of lysosomes.

Lysosomes

• L.M: Containing hydrolytic enzymes (acid phosphatase).
• E.M:
  • Primary Lysosomes: Newly released from Golgi apparatus; enzymes are inactive; homogenous.
  • Secondary Lysosomes: Formed by fusion of primary lysosomes with vacuoles; enzymes are active; heterogeneous.

Secondary Lysosomes: Types

• Heterolysosomes: Primary Lysosome + phagosome
• Multivesicular Bodies: Primary Lysosome + pinocytic vesicle
• Autolysosomes: Primary lysosome + old organelle.
• Residual Bodies: Vesicles containing undigested materials; eliminated via exocytosis.

Peroxisomes

• Content: Membranous organelles containing oxidative enzymes (catalase and oxidase), produce and degrade hydrogen peroxide (H₂O₂).
• Importance:
  • Synthesis and degradation of hydrogen peroxide to kill microorganisms.
  • Liver detoxification of ethanol (alcohol).
  • β-oxidation of long-chain fatty acids.

Ribosomes

• Types: Free ribosomes and attached ribosomes.
• Structure: Composed of ribonucleic acid (rRNA) and protein; formed of large and small subunits.
• Free ribosomes: synthesize proteins used inside the cell (e.g., mitochondria, nucleus)
• Attached ribosomes: synthesize proteins packaged, modified and transported to Golgi complex, then to the destination (e.g., secretory proteins, cell membrane proteins), lysosomes.

Proteasomes

• Definition: Barrel-shaped organelle responsible for the active degradation of cytoplasmic proteins tagged with ubiquitin.
• Functions:
  • Removes excess enzymes and unnecessary proteins.
  • Removes proteins incorrectly folded.
  • Destroys viral proteins.

Cytoskeleton

• Microtubules:
  • Diameter: 24 nm.
  • Structure: Long hollow cylinders with walls of 13 parallel protofilaments formed by polymerization of α & β tubulin subunits.
  • Stability: Dynamic, each filament has 2 ends (+ and -).
  • Functions: Cell shape and support; intracellular transport of organelles, vesicles, and macromolecules; formation of mitotic spindle, centrioles, cilia, and flagella.
• Microfilaments: (Actin filaments)
  • Diameter: 5-7 nm.
  • Structure: Double helix of two filaments formed by polymerization of G-actin monomers.
  • Stability: Dynamic.
  • Functions: Cell support, cell shape changes (endocytosis, exocytosis, amoeboid movements), contractile ring during cell division, muscle contraction, and microvilli cores.
• Intermediate Filaments: (various types)
  • Diameter: 8-12 nm.
  • More structural support in cell, mainly forming a network around the nucleus

Centrioles

• Structure: Two cylindrical structures perpendicular to each other, surrounded by a matrix of tubulin(MTOC). The wall of each cylinder consists of 9 bundles (triplets) of microtubules (3 x 9 = 27 microtubules).
• Functions: Form mitotic spindle during cell division. Regulate growth of new microtubules in non-dividing cells. Share in the formation of cilia and flagella.

Cilia and Flagella

• Cilia: Finger-like cytoplasmic projections covered by cell membrane, consisting of 9 peripheral doublets + 2 central singlets formed by polymerization of microtubules.
• Flagella: Similar structure to cilia but longer, used for cell movement. In humans: tail of sperm for movement,.

Cell Inclusions

• A - Stored Food:
  • Carbohydrates: Glycogen granules in liver and muscle cells
  • Fats: Large globules in fat cells or small droplets in liver cells.
• B - Pigments:
  • Endogenous: Hemoglobin (RBCs), Melanin (skin and hair), Lipofuscin (residual bodies)
  • Exogenous: Carbon and dust particles, Carotene (carrots), tattoo dyes

Nucleus

• Definition: Largest membranous organelle inside the cell. Contains the cell's DNA in the form of chromosomes, site of transcription of 3 primary types of RNA.
• Components (by EM): Nuclear envelope, Chromatin, Nucleolus, Nuclear Sap.

Chromatin

• Definition: Genetic material consisting of DNA double helix + histones + non-histone proteins
• Forms:
  • Euchromatin: Extended (uncoiled) chromatin; transcriptionally active; electron lucent, predominates in active cells, visible in LM
  • Heterochromatin: Condensed (coiled) chromatin; transcriptionally inactive; electron dense, predominates in inactive cells like lymphocytes, visible in LM

Cell Cycle

• Interphase: The time between two successive cell divisions (90% of the cycle). Divided into 3 phases: Gap 1 (G1), Synthesis (S), Gap 2 (G2) phases.
• Mitosis: Cell division (M phase), has 4 main steps: Prophase, Metaphase, Anaphase, Telophase

Apoptosis

• Definition: Active programmed cell death
• Features: Chromatin condensation, nucleus breakdown, blebbing of plasma membrane, fragmented into apoptotic bodies which phagocytosed by macrophages

Stem Cells

• Definition: Undifferentiated cells capable of self-renewal and differentiation (production of different specialized cells).
• E.M. Features: The nucleus primarily contains euchromatin(active genes); The cytoplasm contains numerous polyribosomes and mitochondria
• Types: Embryonic stem cells and adult stem cells (bone marrow, adipose tissue, placenta, GIT, liver, skin, and blood vessels).

Cell Renewal

• Static: Cells that no longer divide (central nervous system, cardiac muscle).
• Stable: Cells that divide episodically and slowly to maintain normal tissue structure (Liver, smooth muscle, endothelial).
• Rapidly renewing: Cells that divide continuously (blood cells, epidermal cells, cells of the digestive tract).

Test your knowledge on cell biology concepts related to organelles, inclusions, and the plasma membrane. This quiz covers their structures, functions, and roles in cellular processes. Perfect for students studying cytology or cellular physiology.