Exercises: Types of cells: prokaryotic and eukaryotic

All cells, whatever their type, share three basic components. State them and explain the function of each one.

In the diagram of an animal eukaryotic cell, number 1 points to a large, spherical structure located in the central area, while number 2 points to an oval organelle with internal folds or cristae. State which number points to the nucleus and which number points to a mitochondrion, and explain the function of both structures.

Explain the fundamental difference between a prokaryotic cell and a eukaryotic cell, and give an example of a living thing with each cell type.

Explain what the cell nucleus is, what it contains, and which types of cells have one.

What is the function of the cell membrane? Explain what would happen if the cell lost it, comparing it with what losing your skin would mean for you.

What are mitochondria for? Reason in which cells of your body you think they will be very abundant.

When observing a sample of your cheek cells and one of onion under the microscope, differences can be seen. Point out two differences in shape and structure between them and explain which type of eukaryotic cell each one belongs to.

Which organelle makes the green parts of plants green? Explain which process takes place in it and why it is so important for life on Earth.

In a plant cell, state the function of the cell wall, a chloroplast, and the central vacuole, and explain why these three structures are not found in an animal cell.

Draw up a comparative table of animal and plant cells with these rows: cell wall, chloroplasts, vacuoles, and typical cell shape.

Where is the genetic material (DNA) found in a eukaryotic cell and in a prokaryotic cell? What consequences does this difference have on the internal organisation of each cell type?

Plant cells have a rigid wall that animal cells do not have. Explain what advantage the wall gives a plant that has no skeleton, and how animals make up for its absence.

The central vacuole of a plant cell can occupy most of its interior. Explain what it contains, what functions it performs (storing water and substances, keeping the cell firm), and what happens to a plant when its vacuoles lose water.

Make two schematic drawings, one of an animal cell and one of a plant cell, with at least four labels on each; then explain in one sentence the most important difference between the two drawings.

Bacteria do not have mitochondria and yet they also obtain energy from food. Explain how it is possible for a cell without membrane-bound organelles to carry out the same life processes as a eukaryotic cell, and say where you think those processes take place inside it.

If the nucleus were the "management office" of a factory and the cytoplasm the "workshop", explain what this analogy gets right (instructions, information, work) and one important limitation.

In a laboratory, three unlabelled cell samples (A, B, and C) from three different tissues have been mixed up: the wing muscle of a sparrow, the white petal of a daisy, and a green spinach leaf.

When observed under a microscope, these characteristics are recorded:

• Sample A: elongated cells, with very high energy consumption, lacking cell walls and chloroplasts, and packed with mitochondria.

• Sample B: cells with a rigid cell wall, a large central vacuole, and hundreds of chloroplasts.

• Sample C: plant cells with developed walls and vacuoles, but lacking a single chloroplast and with scarce mitochondria.

1. a) Reasonably identify which tissue and living being each sample belongs to, justifying your decision based on the presence, absence, and abundance of each organelle.
2. b) Predict what would happen to the cells of sample B if the plant were kept in a dark room for three weeks: would they be able to continue producing glucose? Would their mitochondria immediately stop functioning? Justify your answer.

A bacterium is about $2\ \mu\text{m}$ long and an animal cell is about $20\ \mu\text{m}$ in diameter. Calculate how many bacteria in a row would fit across the diameter of an animal cell. Assuming that both were spherical, also calculate the surface area of each cell's membrane using the formula $S = 4\pi r^2$ and work out how many times larger the animal cell's membrane surface area is. Use both results to explain why eukaryotic cells can afford to be much larger and what problem size poses when exchanging substances with their surroundings.

Unlike humans (whose mature red blood cells lose their nucleus), amphibians and birds have red blood cells that retain their nucleus throughout their lives.

1. a) When comparing two red blood cells of the same volume, one human and one from a frog, the human red blood cell is able to hold a significantly larger amount of hemoglobin (the oxygen-carrying protein). Justify what causes this functional advantage at the cellular level.
2. b) In a laboratory experiment, both types of red blood cells are exposed to a substance that slowly damages the proteins in their cell membrane. The frog red blood cells manage to synthesize new proteins to repair the membrane and survive, while the human ones cannot repair themselves and die after a few days without being able to divide. Explain the biological cause of this fact by relating the function of the nucleus and DNA to the capacity for cellular self-repair and reproduction.

You observe a drop of stagnant water under an optical microscope. The circular field of view you see through the eyepiece measures $1\ \text{mm}$ in diameter ($1000\ \mu\text{m}$). In it, you distinguish a single-celled amoeba of about $200\ \mu\text{m}$ and several elongated bacteria of about $2\ \mu\text{m}$.

1. a) Calculate how many bacteria lined up in a row would be needed to cross the diameter of the microscope's field of view from one side to the other, and how many amoebas would be needed to do the same.
2. b) The resolution limit of the human eye (the minimum distance between two points to distinguish them separately) is approximately $0.1\ \text{mm}$ ($100\ \mu\text{m}$). Justify with numerical data why a person with good eyesight might just perceive the amoeba with the naked eye as a tiny speck, whereas to see the bacterium, a microscope is strictly essential.
3. c) If an amoeba and a bacterium are both single-celled organisms, why would it be very difficult for a typical prokaryotic bacterium to grow to the size of a $200\ \mu\text{m}$ amoeba? Reason what problem a giant bacterium would have in exchanging and distributing nutrients toward its centre without organelles or internal transport systems.

A farmer carries out an experiment with two groups of bean plants:

• Group 1: grown in pots with traditional black soil. Their roots are completely white and their leaves are green.

• Group 2: grown in transparent glass tubes with water and dissolved mineral salts (hydroponics), so that the roots receive continuous sunlight. After two weeks, these roots have turned light green and contain functional chloroplasts.

1. a) Knowing that all cells of the same plant share the exact same DNA, justify why roots that grow underground do not waste energy forming chloroplasts and what stimulus has triggered them to appear in group 2.
2. b) A classmate claims: «Since normal roots in group 1 do not have chloroplasts, they do not need to respire nor do they consume oxygen; cellular respiration is exclusive to leaves». Refute this claim by explaining where the root gets the energy to absorb water and salts, and predict what would happen to the plant if the soil is completely flooded with water for many days, preventing the entry of air.

In the laboratory you have an optical microscope whose resolution limit is $0.2\ \mu\text{m}$ ($200\ \text{nm}$). You observe onion epidermis cells and consult these average size data:

• Onion cell: $150\ \mu\text{m}$

• Nucleus: $10\ \mu\text{m}$

• Mitochondrion: $2\ \mu\text{m}$

• Ribosome: $0.025\ \mu\text{m}$ ($25\ \text{nm}$)

• Plasma membrane thickness: $0.008\ \mu\text{m}$ ($8\ \text{nm}$)

1. a) Analyze the data and deduce, with justification, which of these structures you will be able to observe clearly with your optical microscope and which will be impossible to see with it, no matter how much you magnify the image.
2. b) A classmate submits his lab notebook with a drawing of the onion cell seen under the microscope in which he has drawn in great detail dozens of ribosomes floating in the cytoplasm and the lipid bilayer of the membrane. Justify why the teacher tells him that this drawing is not the result of real observation.
3. c) What type of microscope had to be invented to discover and photograph ribosomes? Explain how it mainly differs from the optical microscope to achieve such a high level of detail.

In a class debate, a student defends the following idea: «Since all eukaryotic cells in our body possess the same types of organelles, all cells have exactly the same proportion of them and perform a similar metabolic effort».

To refute their statement, you have information about three specialized human cells:

• Cell A (cardiac muscle fiber): beats and pumps blood restlessly throughout life.

• Cell B (pancreatic cell): manufactures and secretes into the digestive tract thousands of digestive proteins per minute, packaging them into vesicles.

• Cell C (macrophage, a white blood cell): patrols tissues engulfing bacteria and destroying them through internal cellular digestion.

1. a) Deduce and justify which specific organelle (mitochondria, endoplasmic reticulum/Golgi apparatus, or lysosomes) must be especially hyperdeveloped in each of these three cells so that they can fulfill their function.
2. b) Predict what would happen to a patient's heart if a toxin selectively blocked the function of the mitochondria in type A cells.

A regenerative medicine team wants to repair the heart muscle tissue of a patient who has suffered a heart attack, where thousands of contractile cells have died. To try to rebuild the tissue, three possible cell types from the patient themselves are analyzed:

1. Mature red blood cells.

2. Mature skin cells (epithelial cells).

3. Pluripotent stem cells.

1. a) Justify why mature red blood cells are completely ruled out for this treatment, taking into account their structural limitations and their ability to divide.
2. b) Skin cells do have a nucleus and divide easily. Why would it not work to inject them directly into the heart to restore the heartbeat either? Relate your answer to the concept of cell differentiation and specialization.
3. c) Explain why stem cells are the only valid alternative. What fundamental changes must they undergo in their cytoplasm (which organelles must they multiply in large numbers and what function must they take on) to end up behaving like functional cardiac cells?