The Fundamental Unit of Life

Given: We need to compare plant cells and animal cells.

Differences between Plant Cells and Animal Cells:

| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (made of cellulose) | Absent |
| Chloroplasts | Present (for photosynthesis) | Absent |
| Vacuole | Large central vacuole present | Small or absent |
| Centrosome/Centrioles | Generally absent | Present |
| Shape | Usually fixed, rectangular | Irregular or round |
| Plastids | Present (chloroplasts, chromoplasts, leucoplasts) | Absent |
| Lysosomes | Rarely present | Commonly present |
| Energy storage | Starch granules | Glycogen granules |

Conclusion: Plant cells have a cell wall, chloroplasts, and a large central vacuole which are absent in animal cells, while animal cells have centrioles and lysosomes which are generally absent in plant cells.

Given: We need to distinguish between prokaryotic and eukaryotic cells.

Differences between Prokaryotic and Eukaryotic Cells:

| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nuclear membrane | Absent (no true nucleus) | Present (well-defined nucleus) |
| Size | Generally smaller (1–10 µm) | Generally larger (10–100 µm) |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Chromosome number | Single chromosome (circular DNA) | Multiple chromosomes |
| Histone proteins | Absent | Present with DNA |
| Ribosomes | 70S (smaller) | 80S (larger) |
| Cell division | Binary fission | Mitosis/Meiosis |
| Examples | Bacteria, Blue-green algae | Plant cells, Animal cells, Fungi |

Conclusion: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells possess both.

Given: The plasma membrane is the outermost boundary of the cell (in animal cells) that controls the movement of substances in and out of the cell.

Concept: The plasma membrane is selectively permeable and maintains the internal environment of the cell.

What would happen:

1. The cell would lose its ability to regulate the entry and exit of substances — essential molecules like glucose, ions, and water would leak out.
2. The cytoplasm would spill out into the external environment, disrupting all cellular activities.
3. Harmful substances from outside would freely enter the cell, causing damage.
4. The cell would lose its shape and structural integrity.
5. Ultimately, the cell would die because it cannot maintain homeostasis.

Conclusion: Rupture of the plasma membrane would lead to the death of the cell as it can no longer control its internal environment.

Given: The Golgi apparatus (Golgi complex) is a membrane-bound organelle involved in processing, packaging, and dispatching materials synthesised in the cell.

Functions of Golgi apparatus:
- It receives proteins and lipids from the Endoplasmic Reticulum (ER), modifies them, packages them, and sends them to their destinations inside or outside the cell.
- It is involved in the formation of lysosomes.
- It helps in the synthesis of cell wall material in plant cells.

What would happen without Golgi apparatus:

1. Proteins and lipids synthesised by the ER would not be processed or packaged properly.
2. Secretion of enzymes, hormones, and other substances would stop — the cell could not communicate or function properly.
3. Lysosomes would not be formed, so intracellular digestion would be disrupted.
4. Waste materials would accumulate inside the cell, leading to cell death.
5. In plant cells, cell wall formation would be impaired.

Conclusion: Without the Golgi apparatus, the cell's secretory and packaging functions would collapse, eventually leading to cell dysfunction and death.

Answer: Mitochondria is known as the powerhouse of the cell.

Reason:

- Mitochondria are the sites of cellular respiration, the process by which energy is released from food (glucose) in the presence of oxygen.
- The energy released is stored in the form of ATP (Adenosine Triphosphate) molecules.
- ATP is the energy currency of the cell — it provides energy for all cellular activities such as muscle contraction, protein synthesis, active transport, etc.
- The process occurs in two stages:
- Glycolysis in the cytoplasm
- Krebs cycle and oxidative phosphorylation in the mitochondria

$$\text{Glucose} + \text{O}_2 \xrightarrow{\text{Mitochondria}} \text{CO}_2 + \text{H}_2\text{O} + \text{ATP (Energy)}$$

Conclusion: Since mitochondria generate most of the cell's supply of ATP (energy), they are called the powerhouse of the cell.