Cell Biology Quiz: Transport and Organelles

Plasma Membrane

• The plasma membrane is a lipid bilayer structure, visible under transmission electron microscopy.
• It has a thickness of 8 to 10 nm.
• Molecules are arranged asymmetrically.
  • Complex sugars are on the outer portion.
  • 40-50% are proteins.
  • 30-40% are phospholipids (with different residues).
  • Cholesterol is present.
• The plasma membrane is a selective barrier regulating cell-to-cell adhesion.
• The fluid mosaic model describes the membrane's amphipathic lipid bilayer with embedded integral membrane proteins and peripheral proteins attached to its surfaces.
• Phospholipids and proteins move within the bilayer, making it fluid.
• Temperature is directly proportional to fluidity; fatty acid chain length, protein number, and unsaturated fatty acid amount are inversely proportional to fluidity. Cholesterol makes the bilayer less deformable, decreasing its permeability and preventing fatty acid crystallization.
• Lipids can move laterally and rarely flip-flop between layers.
• Lipid rafts are localised regions containing high concentrations of cholesterol and glycosphingolipids, offering distinct structural and functional properties.
• Lipid rafts are dynamic platforms for cell signaling, with close proximity of interacting proteins enabling rapid and efficient signal transduction.
• Rafts act as platforms separating specific signaling molecules.

Rough Endoplasmic Reticulum (RER)

• RER is a network of interconnected tubules delimited by a membrane.
• Its membrane is continuous with the nuclear envelope, which is important for mRNA exit.
• RER is abundant in cells with high protein synthesis needs.
• Ribosomes on the RER membrane synthesize proteins destined for the lumen of the RER, beginning translation there.
• RER is involved in post-translational modifications of proteins (proteolysis, glycosylation, phosphorylation).

Smooth Endoplasmic Reticulum (SER)

• SER is a network of interconnected tubules.
• It lacks ribosomes.
• SER plays roles in lipid metabolism, synthesizing and secreting steroids, particularly in hepatocytes (where it detoxifies ingested alcohol).
• SER functions in lipid and steroid metabolism, glycogen metabolism, membrane formation, and recycling activities.
• SER contains enzymes involved in detoxification and conjugation of noxious substances.

Golgi Apparatus

• The Golgi apparatus is a stack of flattened, membrane-limited sacs (cisternae) and tubular extensions near the microtubule-organising center.
• Two faces are cis-Golgi network (CGN) (closer to RER), and trans-Golgi network (TGN) (further from RER).
• Golgi functions include protein and lipid modification (glycosylation), activation of peptides, complex carbohydrate synthesis (glycosaminoglycans), and selection of enzymes for lysosomes.
• Golgi produces micro-vesicles.

Vesicular Transport

• Vesicular transport maintains plasma membrane integrity and helps in transferring molecules between cellular compartments.
• This involves vesicle budding (formation) and fusion with other membranes.
• Endocytosis brings substances into the cell, playing a role in nutrient uptake, cell signalling, and shape changes.
• Exocytosis carries substances out of the cell.
• Different kinds of vesicles are characterized by specific coatings (COPs), used for moving substances from RER to Golgi and vice versa, and clathrin, for transporting materials in cell.
• Rab-GTPases provide specific addressing for vesicle transport, directing vesicles to their target destinations.
• SNARE proteins (v-SNARE and t-SNARE) ensure accurate fusion.

Receptor-mediated Endocytosis

• Allows entry of specific molecules (cargo) via receptors accumulating in specific regions of the membrane (lipid rafts).
• Cargo receptors bind specific molecules.
• Clathrin molecules build a basket-like cage around them, triggering membrane invagination to form a vesicle.
• Adaptin aids in selecting cargo.
• Dynamin, a mechanoenzyme GTPase, mediates vesicle formation.

Pinocytosis

• A non-specific process engulfing fluid and small proteins by small vesicles (below 150 nm).
• Continuously happens in most cells.
• Often associated with caveolin and flotillin proteins in lipid rafts (not clathrin).

Phagocytosis

• Ingestion of large particles (cell debris, bacteria) by large vesicles (phagosomes).
• Generally involves receptor-mediated recognition and is triggered by pathogen-associated molecular patterns (PAMPs).

Lysosomes

• Digesting organelles containing hydrolytic enzymes.
• Involved in degradation of macromolecules and organelles
• Acidic internal environment protects the cell
• Contain transport proteins to release digested products.
• Lysosomal disorders arise from abnormal accumulation of substances.

Peroxisomes

• Small organelles with oxidative enzymes (catalase).
• Important for detoxification (especially in liver cells).
• Involved in beta-oxidation of fatty acids.

Mitochondria

• Double-membrane organelles with an outer membrane (lipid-rich) and inner membrane (with cristae).
• Contain their own DNA, ribosomes, and tRNAs (maternally inherited).
• Site of ATP synthesis (cellular respiration).
• Involved in lipid and steroid synthesis, ion storage, and heat production.

Cell Cycle

• Sequence of events from one cell division to the next.
• Consists of phases: G1, S, G2, and M (mitosis).
• G1 and G2 are growth and preparation phases, respectively.
• S phase stands for DNA synthesis.
• Mitosis involves division and separation of replicated genetic material.
• Prophase, Metaphase, Anaphase, Telophase are the four steps of mitosis

Meiosis

• Two sequential nuclear divisions producing gametes with half the chromosome number.
• Two cell divisions (meiosis I, II).
• Meiosis I is characterised by homologous chromosomes pairing up, crossing over, and separation.
• Meiosis II is similar to mitosis, resulting in four haploid gametes.

Apoptosis

• Programmed cell death.
• Active process involving DNA fragmentation, cytoplasmic shrinkage, and membrane blebbing.
• Apoptotic bodies are engulfed to avoid inflammation/damage.

Necrosis

• Accidental cell death triggered by various physical or chemical stressors
• Process characterized by cellular swelling, membrane rupture, and release of harmful cellular content, which leads to inflammation at the surrounding tissue.

Cytoskeleton

• A network of protein filaments in the cytoplasm.
• Maintains cell structure, provides support, drives vesicle movement, allows for cell movement, allows for chromosome separation during cell division and allowing neurones to maintain shape and spatial conformation;

Centrosomes

• The microtubule organizing centre (MTOC) of animal cells.
• Composed of two centrioles, orthogonally arranged.
• Involved in spindle formation during cell division.
• Centrioles are responsible for forming basal bodies for cilia and flagella.

Nucleus

• Membrane-bound organelle containing the genetic material (chromatin).
• Site of DNA replication and RNA transcription.
• The nucleus is enveloped by a double membrane (nuclear envelope).
• The inner and outer membrane touch at points called nuclear pores allowing for material flow in and out.
• The nucleolus is a structure within the nucleus involved in rRNA synthesis.