BMEG 245: Organelles & Vesicular Transport (Ch. 15)

What are membrane-enclosed organelles?

They are compartments within eukaryotic cells surrounded by lipid membranes that carry out specialized functions such as protein synthesis, degradation, or energy conversion.

List the major membrane-bound organelles in eukaryotic cells.

Nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, endosomes, peroxisomes, and mitochondria (plus chloroplasts in plants).

What is the function of the cytosol?

It contains many metabolic pathways, the cytoskeleton, and the machinery for protein synthesis.

What is the main function of the nucleus?

It stores the genome and is the site of DNA and RNA synthesis.

What does the endoplasmic reticulum (ER) do?

It synthesizes most lipids and proteins destined for secretion or other organelles.

What does the Golgi apparatus do?

It modifies, sorts, and packages proteins and lipids for secretion or delivery to another organelle.

What is the role of lysosomes?

They carry out intracellular degradation of worn-out organelles, macromolecules, and endocytosed particles.

What is the function of endosomes?

They sort endocytosed materials and recycle membrane components.

What is the role of peroxisomes?

They break down toxic molecules and fatty acids through oxidative reactions.

What is the function of mitochondria?

They generate ATP through oxidative phosphorylation and play roles in apoptosis and metabolism.

What is the function of chloroplasts in plant cells?

They perform photosynthesis to produce ATP and fix carbon.

What is the evolutionary origin of mitochondria and chloroplasts?

They evolved from endosymbiotic bacteria engulfed by early eukaryotic cells, retaining their own DNA and double membranes.

What is the endomembrane system?

A group of organelles (ER, Golgi, lysosomes, endosomes, and plasma membrane) that communicate via vesicular transport.

How is protein sorting achieved in cells?

Proteins contain signal sequences that direct them to their correct organelle or location.

What is a signal sequence?

A short amino acid sequence that directs a newly synthesized protein to a specific cellular compartment.

Give an example of a signal sequence for import into the ER.

An N-terminal hydrophobic sequence such as +H3N-Met-Met-Ser-Phe-Val-Ser-Leu-Leu-Leu-Val-Gly-Ile-Leu-Phe-Trp-Ala-Thr-Glu-Ala-Glu-Gln-Leu-Thr-Lys-Cys-Glu-Val-Phe-Gln.

Give an example of a signal sequence for import into the nucleus.

-Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val-

What is the nuclear pore complex (NPC)?

A large protein assembly that regulates the bidirectional movement of proteins and RNA between the nucleus and cytosol.

How are proteins imported into the nucleus?

Through nuclear pores using nuclear localization signals (NLS) recognized by nuclear import receptors.

What drives nuclear transport?

GTP hydrolysis by the Ran GTPase system; Ran-GTP and Ran-GDP gradients direct import/export.

What do GAP and GEF do in nuclear transport?

GAP (GTPase-activating protein) promotes GTP hydrolysis in the cytosol; GEF (guanine nucleotide exchange factor) exchanges GDP for GTP in the nucleus.

How do proteins enter mitochondria?

They are synthesized in the cytosol, unfolded, and imported through specialized translocator complexes (TOM/TIM) using a signal sequence and ATP.

How do proteins enter the ER?

They are translocated into the ER lumen while being synthesized by ribosomes bound to the ER membrane.

What directs a ribosome to the ER membrane?

A signal recognition particle (SRP) binds the ER signal sequence and guides the ribosome to the SRP receptor on the ER membrane.

What is the role of the translocation channel (translocon)?

It opens in the ER membrane to allow a growing polypeptide chain to cross into the ER lumen.

How are single-pass transmembrane proteins formed?

A hydrophobic stop-transfer sequence halts translocation, leaving one region in the membrane and the rest in the cytosol or lumen.

How are multi-pass transmembrane proteins formed?

Multiple start- and stop-transfer sequences insert alternating membrane-spanning segments into the bilayer.

What is vesicular transport?

A process in which membrane-enclosed vesicles bud from one compartment and fuse with another, transferring cargo and membrane components.

What are coated vesicles?

Vesicles covered by a protein coat (clathrin, COPI, or COPII) that shapes the membrane and selects cargo.

Describe the main types of coated vesicles and their origins/destinations.

Clathrin + adaptin 1: Golgi → lysosome; Clathrin + adaptin 2: plasma membrane → endosome; COPII: ER → Golgi; COPI: Golgi → ER.

What does clathrin do?

It forms a triskelion-shaped lattice that helps bend the membrane into a vesicle.

What does adaptin do?

It links clathrin to specific cargo receptors to ensure proper cargo selection.

What does dynamin do?

A GTPase that pinches off the budding vesicle from the donor membrane.

What happens after a vesicle buds off?

The clathrin coat is rapidly removed (uncoating), and the vesicle fuses with the target compartment.

How does vesicle docking occur?

Specificity is mediated by Rab proteins on the vesicle and tethering proteins on the target membrane.

What is the function of SNARE proteins?

v-SNAREs on vesicles and t-SNAREs on target membranes form tight complexes that bring membranes close for fusion.

How do SNAREs mediate membrane fusion?

The helical SNARE complexes act like a winch, forcing the two bilayers into proximity so they fuse.

What is glycosylation?

The covalent attachment of oligosaccharides to proteins in the ER and Golgi; it aids folding, stability, and targeting.

What are the three main functions of glycosylation?

Protect proteins from degradation, retain them in the ER until properly folded, and guide them to their correct organelle.

What is the unfolded protein response (UPR)?

A cellular reaction to ER stress that increases chaperone production and ER size to handle misfolded proteins.

What happens if misfolded proteins cannot be corrected?

They are exported to the cytosol for degradation by the proteasome.

What does the Golgi apparatus consist of?

A stack of flattened, membrane-enclosed sacs called cisternae, organized into cis, medial, and trans regions.

What are the functions of the Golgi?

Further modification (glycosylation, sulfation), sorting, and packaging of proteins and lipids for secretion or delivery.

How do proteins travel through the Golgi?

By vesicular transport between cisternae or by cisternal maturation from the cis to trans side.

What is the difference between constitutive and regulated exocytosis?

Constitutive: continuous secretion of materials (e.g. ECM proteins). Regulated: vesicles store cargo and release it upon a signal (e.g. hormones, neurotransmitters).

What triggers regulated secretion?

Signal molecules like hormones or nerve impulses that raise cytosolic Ca²⁺ cause vesicles to fuse with the plasma membrane.

Why do secretory proteins aggregate in the Golgi?

They aggregate under ionic conditions (acidic pH, high Ca²⁺) to form dense secretory granules for regulated release.

What is endocytosis?

The process by which cells take in materials through invagination of the plasma membrane and formation of vesicles.

What are the two main types of endocytosis?

Pinocytosis (nonspecific uptake of fluid/small molecules) and phagocytosis (engulfment of large particles by specialized cells).

What is receptor-mediated endocytosis?

A selective uptake mechanism where receptors on the plasma membrane bind ligands and internalize them in clathrin-coated vesicles.

Describe the steps of receptor-mediated endocytosis of LDL.

LDL binds to LDL receptors → clathrin-coated vesicle forms → uncoating → early endosome → lysosome → cholesterol released → receptor recycled.

What is LDL (Low-Density Lipoprotein)?

A particle that carries cholesterol and lipids through the bloodstream.

How do viruses exploit the endocytic pathway?

Many viruses enter cells through receptor-mediated endocytosis and then escape from endosomes into the cytosol.

What determines the fate of endocytosed receptors?

They can be recycled to the plasma membrane, sent to lysosomes for degradation, or moved to a different domain (transcytosis).

What is the function of lysosomal enzymes?

Hydrolytic enzymes that digest macromolecules at acidic pH inside lysosomes.

How is lysosomal pH maintained?

By an ATP-driven H⁺ pump that keeps the lumen acidic (pH ~5).

What are peroxisomes responsible for?

The oxidative breakdown of fatty acids and detoxification of harmful compounds like hydrogen peroxide.

What is the function of catalase in peroxisomes?

It breaks down hydrogen peroxide into water and oxygen, preventing oxidative damage.

How do peroxisomal proteins get imported?

They are synthesized in the cytosol and contain a C-terminal signal sequence (–Ser-Lys-Leu–) that directs them into the organelle.

What is the difference between the ER signal sequence and the peroxisomal signal sequence?

ER signals are N-terminal hydrophobic stretches; peroxisomal signals are short C-terminal tripeptides.

How do membrane-bound organelles communicate?

Through vesicle budding and fusion, maintaining orientation and content transfer across compartments.

Why is compartmentalization important for eukaryotic cells?

It allows different biochemical reactions to occur simultaneously without interference, increasing efficiency.