Tutorial Answers

Tutorial 2: Lecture Unit 1 – 24-28 February 2025

1) If you place sterile broth that contains no cells in a bowl out on a table overnight, by morning it will contain a growing population of bacterial cells.

a) How could this be explained according to cell theory?

- This phenomenon can be explained by contamination from airborne microorganisms. While the broth was initially sterile, bacterial spores or cells from the environment could have landed in the broth, where they found nutrients and multiplied.

b) Which principle of cell theory explains the phenomenon?

- The third principle of cell theory, which states that all cells arise from pre-existing cells, explains this phenomenon. The bacteria that appeared in the broth must have originated from other bacteria present in the environment.

2) Glucose Transport and Surface Area Calculations

Given:

- Glucose requirement = 100,000 molecules per µm³ per second

- Each Glutr transports = 400 molecules per second

- Each Glutr occupies = 12 nm² = 12 × 10⁻⁶ µm²

- Formulas for a sphere:

- Surface Area (SA) = 4πr²

- Volume (V) = (4/3)πr³

- Use π ≈ 3

a) What percentage of each cell’s surface area will be occupied by Glutr?

We need to calculate:

1. Total glucose requirement per second (volume × 100,000)

2. Number of Glutr needed (total glucose required ÷ 400)

3. Total surface area occupied by Glutr (number of Glutr × 12 × 10⁻⁶ µm²)

4. Percentage of total cell surface occupied (Glutr area ÷ total SA × 100)

b) If phospholipids occupy at least 25% of the membrane surface, which cells, if any, would be unable to meet their cellular need for glucose?

- If Glutr occupies more than 75% of the membrane (since 25% is reserved for phospholipids), the cell cannot meet its glucose demand.

- We compare the percentage calculated in part (a) to see if it exceeds 75%.

(We can calculate these values explicitly if needed.)

3) A cell cannot produce actin. Describe all the consequences for the cell.

Actin is a major cytoskeletal protein. If a cell cannot produce actin, it will experience:

1. Loss of Cell Shape and Structural Integrity

- Actin filaments provide mechanical support. Without actin, the cell may become fragile and lose its proper shape.

2. Defective Cell Motility

- Cells that move using actin-based structures (like lamellipodia and filopodia) would be immobile.

- Important for immune cells like macrophages.

3. Failure in Intracellular Transport

- Actin helps move vesicles and organelles inside the cell.

- Organelles like mitochondria may not be properly positioned.

4. Impaired Cytokinesis

- During cell division, actin forms the contractile ring for cytokinesis.

- Without actin, the cell cannot properly divide.

5. Endocytosis and Exocytosis Malfunction

- Actin is involved in forming vesicles during these processes.

- Nutrient uptake and secretion would be impaired.

4) Lysosome vs. Vesicle: Origin and Function

Lysosome Formation:

1. Enzymes are synthesized in the rough ER.

2. Packaged into vesicles by the Golgi apparatus.

3. These vesicles mature into lysosomes, which contain hydrolytic enzymes.

Vesicle Formation:

- Vesicles can form from various sources:

- Endocytosis: Plasma membrane invaginates to form a vesicle.

- Exocytosis: Golgi vesicles fuse with the membrane to release contents.

Why are both part of the endomembrane system?

- Both lysosomes and vesicles originate from the Golgi and ER and are involved in intracellular transport, degradation, and secretion.

5) Prokaryotic vs. Eukaryotic Cells: Three Key Differences

To determine whether a cell is prokaryotic or eukaryotic, check for:

1. Presence of a Nucleus

- Eukaryotes: Have a true nucleus with a nuclear membrane.

- Prokaryotes: Have nucleoid regions instead of a nucleus.

2. Membrane-bound Organelles

- Eukaryotes: Have mitochondria, ER, Golgi, lysosomes, etc.

- Prokaryotes: Lack these organelles.

3. Cell Size and Complexity

- Eukaryotic cells are larger and more complex.

- Prokaryotic cells are smaller and simpler, often with a cell wall.

6) Zebrafish Pigment Movement Using Molecular Motors

Zebrafish adjust their pigmentation based on light levels. This happens through vesicle transport using molecular motors:

- In bright light (to become transparent):

- Pigment vesicles move toward the cell center.

- Transported along microtubules by dynein (a motor protein moving toward the cell's center).

- In low light (to appear darker):

- Pigment vesicles move outward toward the cell periphery.

- Transported by kinesin (a motor protein moving toward the cell membrane).

- This mechanism helps zebrafish blend into their environment by concentrating or dispersing pigment granules inside their cells.

7) Infection Strategy of a Pathogen Secreting a Collagen-Degrading Enzyme

- Collagen is a major component of connective tissues.

- If a pathogen secretes a collagen-degrading enzyme, it can break down the extracellular matrix (ECM), which holds tissues together.

- This allows the pathogen to:

1. Penetrate deeper into tissues and avoid immune detection.

2. Enter blood vessels, facilitating systemic infection (spreading through the circulatory system).

3. Destroy host defenses, making infection more