Exercises: Cell organelles

Match each organelle with its basic function: mitochondrion, chloroplast, ribosome, Golgi apparatus, lysosome, endoplasmic reticulum, vacuole and nucleus.

Indicate which organelles and structures are characteristic of the plant cell and absent in the animal cell, and which are shared by both cell types.

Classify the organelles and structures of the eukaryotic cell into two groups: membrane-bound and non-membrane-bound. Give at least two examples of each group.

In a cross-section of an animal eukaryotic cell, six characteristic organelles or structures can be distinguished: 1) mitochondrion, 2) rough endoplasmic reticulum, 3) Golgi apparatus, 4) lysosome, 5) nucleus, and 6) centrosome. Describe the basic function of each of them in one sentence.

Observe the image of a sectioned eukaryotic plant cell. Identify the structures marked with numbers 1 to 6 and explain, comparing it with the animal cell, what role the cell wall and the large vacuole play in the shape and rigidity of plant cells.

Locate in which organelle or cellular structure each of these processes occurs and briefly justify your answer: aerobic respiration, photosynthesis, synthesis of proteins destined for secretion and intracellular digestion of aged materials.

In a pulse-chase experiment using tritiated leucine (³H-Leu) in insulin-synthesizing pancreatic beta cells, the localization of radioactivity is quantified at 5, 20, and 60 minutes of chase.

A) Predict in which organelles or cellular compartments the radioactive signal will predominantly concentrate at each of the three time points, describing what transformations the protein undergoes in each organelle.

B) If before adding the leucine pulse the cells are treated with brefeldin A (a drug that, by inhibiting ARF activation and COP I vesicle formation, blocks vesicular traffic from the RER to the Golgi), predict where the radioactive insulin will accumulate at 60 minutes and what impact this will have on the organism's blood glucose levels.

C) Where would the insulin be directed if a mutation deleted its N-terminal hydrophobic signal sequence? Justify your answer.

Consider two artificially induced cellular alterations:

1. A mutation in the MICOS proteins that maintain the curvature of the inner mitochondrial membrane leads to the complete disappearance of the cristae; the inner membrane turns into a smooth vesicle concentric to the outer membrane.

2. A toxin selectively disrupts the continuity of the chloroplast thylakoid membranes, such that their lipid and protein components disperse into open sheets that directly connect the thylakoid lumen with the stroma.

Predict and justify the impact of each alteration on the net synthesis of ATP associated with mitochondrial respiration and the light-dependent reactions of photosynthesis, respectively. Base your arguments on Mitchell's chemiosmotic hypothesis, proton compartmentalization, and the surface-area-to-volume ratio of energy-transducing membranes.

Morphometric electron microscopy analysis of three unknown human cell lines (A, B, and C) provides the following data regarding the percentage of cell volume occupied by each organelle:

• Type A: Mitochondria: 38 %; Smooth endoplasmic reticulum: 17 %; RER: 5 %; Golgi apparatus: 2 %; Secretory granules: < 1 %.

• Type B: Mitochondria: 6 %; Smooth endoplasmic reticulum: 2 %; RER: 34 %; Golgi apparatus: 20 %; Secretory granules: 15 %.

• Type C: Mitochondria: 5 %; Smooth endoplasmic reticulum: 2 %; RER: 4 %; Golgi apparatus: 3 %; Lysosomes and phagosomes: 26 %.

Reasonably assign to each cell type one of the following physiological functions: mechanical contraction and calcium homeostasis (skeletal muscle fiber), massive protein secretion (pancreatic acinar cell), or destruction of phagocytosed pathogens (macrophage). Justify each assignment by relating the assigned function to the abundance of the corresponding organelles.

Unit challenge. As a future biomedical researcher, you must analyze real cell samples from electron micrographs. Your objective is to prepare a comparative technical datasheet demonstrating cell diversity and organelle function.

1. Data collection: Access the public scientific repository Cell Image Library (cellimagelibrary.org) and select two micrographs with a visible scale bar: one bacterium and one eukaryotic cell (animal or plant) where its organelles are clearly distinguishable.

2. Measurement and calibration: Measure the scale bar and cellular dimensions with a ruler. Using the relationship $\text{Magnification} = \frac{\text{Observed size}}{\text{Actual size}}$, calculate the actual size of both cells and of two representative organelles (such as a mitochondrion or chloroplast), expressing the values in $\mu\text{m}$ (e.g., $1,5\ \mu\text{m}$).

3. Reasoned report: Design a comparative table contrasting prokaryotes and eukaryotes, discussing the function of the measured organelles and explaining how they support the modern tenets of cell theory and the endosymbiotic hypothesis.

Evaluation criteria:

• Accuracy in calculating actual cell sizes and correct use of scale in $\mu\text{m}$.

• Morphological identification and appropriate functional justification of the organelles.

• Conceptual rigor in linking observations to cell theory and organizational models.