MCAT - Bio/Biochem Foundations:: Cell Structure, Transport, and Communication

Prokaryotic cells (bacteria and archaea) differ fundamentally from eukaryotic cells in that prokaryotes:

- **A)** Possess a membrane-bound nucleus, identical to eukaryotic cells
- **B)** Lack a membrane-bound nucleus (their DNA resides in a nucleoid region without an enclosing membrane) and generally lack membrane-bound organelles altogether
- **C)** Are always larger than eukaryotic cells
- **D)** Contain mitochondria, identical to eukaryotic cells

The mitochondrion, often called the 'powerhouse of the cell,' is the primary site of:

- **A)** Protein synthesis via ribosomes exclusively
- **B)** DNA replication for the entire cell
- **C)** Oxidative phosphorylation and the majority of ATP generation via aerobic cellular respiration, using the electron transport chain located on its inner membrane
- **D)** Lipid synthesis exclusively, with no role in energy production

The endosymbiotic theory, explaining the evolutionary origin of mitochondria (and chloroplasts in plants), proposes that these organelles originated from:

- **A)** Free-living prokaryotic cells that were engulfed by an ancestral host cell and eventually formed a stable, mutually beneficial endosymbiotic relationship, evolving into permanent organelles over evolutionary time
- **B)** Random self-assembly of membrane components with no prokaryotic ancestry
- **C)** A gradual internal budding process from the eukaryotic nucleus
- **D)** Viral infection of an ancestral eukaryotic cell

The rough endoplasmic reticulum (RER) is distinguished from the smooth endoplasmic reticulum (SER) by the presence of:

- **A)** Mitochondrial DNA studding its surface
- **B)** Chlorophyll pigment
- **C)** A double membrane, unlike the SER's single membrane
- **D)** Ribosomes attached to its cytoplasmic surface, giving it a 'rough' appearance under electron microscopy and making it the primary site of synthesis for proteins destined for secretion, insertion into membranes, or delivery to organelles of the endomembrane system

The smooth endoplasmic reticulum (SER), lacking ribosomes, is primarily involved in:

- **A)** Protein synthesis for secreted proteins
- **B)** Lipid synthesis, steroid hormone synthesis (prominent in steroidogenic endocrine cells), and detoxification of drugs/toxins (particularly abundant in liver hepatocytes, rich in cytochrome P450 enzymes)
- **C)** mRNA transcription
- **D)** Generating the cell's ATP supply

The Golgi apparatus functions in the cell's secretory pathway primarily to:

- **A)** Further modify, sort, and package proteins and lipids received from the ER (e.g., via glycosylation modifications), then direct them in vesicles to their appropriate final destinations (secretion, plasma membrane, lysosomes, etc.)
- **B)** Synthesize new mitochondrial DNA
- **C)** Directly translate mRNA into protein
- **D)** Serve as the primary site of ATP production

Lysosomes, membrane-bound organelles containing hydrolytic enzymes active at acidic pH, primarily function to:

- **A)** Synthesize new proteins for secretion
- **B)** Generate the cell's ATP supply, identical to mitochondria
- **C)** Digest and recycle cellular waste, damaged organelles (via autophagy), and material taken up by endocytosis/phagocytosis, using their acidic internal environment and array of hydrolytic enzymes (optimally active at that low pH, providing a safety margin should enzymes leak into the neutral-pH cytosol)
- **D)** Store excess genetic material

Peroxisomes, single-membrane-bound organelles distinct from lysosomes, are particularly important for:

- **A)** mRNA splicing
- **B)** Generating the bulk of cellular ATP, identical to mitochondria
- **C)** Synthesizing ribosomal RNA
- **D)** Beta-oxidation of very-long-chain fatty acids and detoxification reactions that generate hydrogen peroxide as a byproduct, which is then safely broken down by the peroxisomal enzyme catalase into water and oxygen