B2.2 Organelles and compartmentalization

What route is used to export proteins from the cell?

A. Golgi apparatus → rough endoplasmic reticulum → plasma membrane

B. Rough endoplasmic reticulum → Golgi apparatus → plasma membrane

C. Golgi apparatus → lysosome → rough endoplasmic reticulum

D. Rough endoplasmic reticulum → lysosome → Golgi apparatus

Answer: B

A. Incorrect. Secreted proteins are synthesized first in the rough endoplasmic reticulum (RER), not in the Golgi apparatus. 

B. Correct. Proteins destined for secretion are synthesized on ribosomes attached to the RER, transported to the Golgi for processing and packaging, and finally sent in vesicles that fuse with the plasma membrane for exocytosis.

C. Incorrect. Lysosomes contain digestive enzymes and are not part of the protein export pathway.

D. Incorrect. Lysosomes are not intermediate stations in protein secretion.

Actin is a protein involved in muscle contraction. Where is actin produced?

A. At ribosomes bound to the endoplasmic reticulum

B. At free ribosomes 

C. Within the Golgi apparatus

D. In the sarcoplasmic reticulum

Answer: B

A. Incorrect. Bound ribosomes produce proteins for secretion, lysosomes, or membranes. Actin is used inside the muscle cell.

B. Correct. Free ribosomes in the cytoplasm synthesize proteins intended for use within the cell. Actin and myosin are produced by free ribosomes in muscle cells.

C. Incorrect. The Golgi modifies and packages proteins; it does not synthesize them.

D. Incorrect. The sarcoplasmic reticulum stores and releases Ca²⁺ for muscle contraction, not proteins.

What is the role of clathrin molecules in the formation of vesicles?

A. Facilitate transport of sodium and potassium ions 

B. Bind together to help the membrane become indented 

C. Adhere to the phospholipid bilayer to increase fluidity

D. Create a concentration gradient for uptake of substances into vesicles

Answer: B

A. Incorrect. Ion channels and pumps perform this role.

B. Correct. Clathrin is a triskelion-shaped protein that assembles into a network during endocytosis. This clathrin coat helps the plasma membrane invaginate and pinch off to form a vesicle.

C. Incorrect. Membrane fluidity is regulated by cholesterol, not clathrin.

D. Incorrect. Gradients are created by membrane pumps, not clathrin.

What cell component is found in eukaryotic cells but not in prokaryotic cells?

A. Mitochondria for respiration.

B. DNA containing genetic information.

C. Ribosomes for protein synthesis.

D. Cell wall to maintain shape.

Answer: A

A. Correct. Mitochondria are membrane-bound organelles present only in eukaryotic cells.

B. Incorrect. All living organisms have DNA; prokaryotes have a circular DNA molecule.

C. Incorrect. Ribosomes are universal to all cells.

D. Incorrect. Cell walls are found in many prokaryotes and also in plant and fungal eukaryotes.

Pancreatic gland cells produce and secrete large amounts of digestive enzymes. Which organelles would you expect to be present in higher than normal amounts in such cells?

A. Free ribosomes and Golgi apparatus 

B. Rough endoplasmic reticulum and lysosomes 

C. Rough endoplasmic reticulum and Golgi apparatus

D. Free ribosomes and lysosomes

Answer: C

A. Incorrect. Free ribosomes produce polypeptides that remain in the cytoplasm for intracellular use (e.g., enzymes for glycolysis). Digestive enzymes are secreted proteins, so they require ribosomes on the RER.

B. Incorrect. While RER is needed for enzyme synthesis, lysosomes mainly function in intracellular digestion, not secretion.  

C. Correct. RER synthesizes proteins destined for vesicular transport and secretion. Pancreatic cells are highly secretory, so they need abundant RER. The Golgi apparatus modifies, processes, and packages the proteins synthesized in the RER. Secretory cells like pancreatic cells typically have a prominent Golgi. 

D. Incorrect. Both free ribosomes (which synthesize intracellular proteins) and lysosomes (which carry out intracellular digestion) are not the primary organelles involved in secreting large amounts of protein out of the cell. 

What is a difference between eukaryotic and prokaryotic cells?

A. Cell walls are found only in prokaryotes. 

B. Naked DNA with histones is found only in prokaryotes. 

C. Compartmentalization is found only in eukaryotes.

D. Cilia and flagella are found only in prokaryotes.

Answer: C

A. Incorrect. Prokaryotes do have cell walls (peptidoglycan), but several eukaryotes also have cell walls (plants: cellulose; fungi: chitin).

B. Incorrect. Prokaryotes have naked DNA (not associated with histones). Eukaryotes have DNA associated with histones. he statement “naked DNA with histones” is self-contradictory.

C. Correct. Eukaryotic cells contain membrane-bound organelles that compartmentalize functions. Prokaryotes lack membrane-bound organelles and their cytoplasm is one uninterrupted chamber.

D. Incorrect. Both prokaryotes (e.g., E. coli) and some eukaryotes (e.g., Euglena, Paramecium) have cilia or flagella.

 Which statement distinguishes between the roles of free and bound ribosomes?

A. Free ribosomes synthesize proteins for use inside the cell, whereas bound ribosomes synthesize proteins mainly for export.

B. Free ribosomes synthesize proteins mainly for export, whereas bound ribosomes synthesize proteins for use inside the cell.

C. Free ribosomes synthesize proteins, whereas bound ribosomes do not.

D. Bound ribosomes synthesize proteins, whereas free ribosomes do not.

Answer: A

A. Correct. Both prokaryotes (e.g., E. coli) and some eukaryotes (e.g., Euglena, Paramecium) have cilia or flagella. Membrane-bound ribosomes (on the RER) synthesize proteins that enter the RER lumen for secretion or for transport to the Golgi.

B. Incorrect. This reverses the actual roles. 

C. Incorrect. Both types synthesize proteins.

D. Incorrect. Both types function in protein synthesis.  

Outline how cell organelles interact to produce and secrete proteins from cells. [4]

Any four of the following: 

a. organelles perform specialized functions that integrate/work together

OR

organelles are adapted to perform isolated tasks that coordinate;

b. nucleus makes the messenger RNA which codes for the protein;

c.ribosome reads the messenger RNA (mRNA) sequence / uses the mRNA to synthesize the protein / performs translation using mRNA;

d. (ribosomes embedded) rough Endoplasmic Reticulum/RER synthesizes proteins for secretion out of the cell;

e. RER transports proteins within the cell in vesicles/ forms vesicles around proteins for transport to golgi;

f. Golgi apparatus where proteins are processed/modified/packaged for secretion;

g. vesicles secrete proteins from the cell/fuse with membrane, release by exocytosis;

 Sample answer: 

Cells produce and secrete proteins through a coordinated interaction of specialized organelles. First, the nucleus produces messenger RNA (mRNA) that contains the code for the protein [1]. The mRNA then moves to the cytoplasm, where a ribosome reads the sequence and synthesizes the protein [1]. When the protein is destined for secretion, the ribosomes on the rough endoplasmic reticulum (RER) translate the mRNA and insert the growing protein into the RER [1], where it begins folding. The RER then packages the protein into transport vesicles and sends it to the Golgi apparatus [1], which modifies and prepares the protein for export. Finally, vesicles from the Golgi fuse with the cell membrane and release the protein through exocytosis [1].

Explain how the structure of a mitochondrion is adapted for its function. [2]

Any two of the following: 

a. large inner surface area of cristae for respiratory complexes/electron transport chains;

b. matrix contains/encloses DNA and ribosomes for protein (enzyme) synthesis / Krebs cycle enzymes;

c. (double) membrane(s) isolates metabolic processes from the rest of the cytoplasm;

d. small IM space between inner and outer membranes for accumulation of protons; 

Answers must clearly link a structure to a function for a mark

Sample answer:

The structure of the mitochondrion is specially adapted to optimize its primary function of producing adenosine triphosphate (ATP) through aerobic respiration. The large inner surface area of cristae for respiratory complexes [1] allows extensive anchoring of the ETC components and ATP synthase, thereby enhancing the efficiency of ATP generation via oxidative phosphorylation. In addition, the small intermembrane (IM) space between the inner and outer membranes for accumulation of protons [1]  enables protons (H⁺) pumped out by the ETC to build up rapidly, creating the electrochemical gradient necessary for chemiosmosis and ATP synthesis. The mitochondrial double membranes isolate metabolic processes from the rest of the cytoplasm, while the matrix contains/encloses DNA and ribosomes for protein (enzyme) synthesis as well as Krebs cycle enzymes [1], further supporting efficient energy production.

 Describe how the structure of the membrane allows the formation of vesicles. [2]

Any two of the following: 

a. fluidity of membrane allows change of shape/invagination/formation of vesicles;

b. phospholipids can move / phospholipid bilayer makes membrane fluid/flexible;

c. weak bonding between phospholipid tails;

d. bends/kinks in the phospholipid tails prevent close packing;

e. cholesterol affects membrane fluidity;

Sample answer: 

The membrane can form vesicles because its phospholipid bilayer is fluid and flexible. The fluidity of the membrane allows it to change shape and undergo invagination [1], so a region of the membrane can fold inward and pinch off to create a vesicle. This fluid behavior is possible because phospholipids can move within the bilayer [1], allowing the membrane to rearrange itself without breaking. Together, these properties enable sections of the membrane to bud off and form vesicles during processes such as endocytosis and exocytosis.