Cellular Processes

### (a) Apoptosis and Necrosis

| Apoptosis | Necrosis |
|---|---|
| Programmed cell death | Traumatic, unplanned cell death |
| Highly controlled and energy-dependent | Occurs due to injury or toxic substances |
| Causes DNA fragmentation and blebbing of plasma membrane | Does not involve DNA fragmentation or blebbing |
| Cell breaks into small membrane-bound fragments that are phagocytosed | Cell contents are released into surroundings, causing inflammation |
| Useful in normal development and protection from disease | Leads to damage and inflammation |

### (b) Autocrine and Paracrine signaling

| Autocrine signaling | Paracrine signaling |
|---|---|
| The cell that secretes the ligand also has the receptor for it | Communication occurs between nearby cells over short distances |
| The signal acts on the same cell or same type of cell | The signal acts on neighbouring recipient cells |
| Example: cancer cells producing their own growth factors | Example: communication of neurons |

### (c) Anabolic and Catabolic pathways

| Anabolic pathways | Catabolic pathways |
|---|---|
| Build larger, complex molecules from smaller ones | Break down larger molecules into smaller ones |
| Endergonic: require energy | Exergonic: release energy |
| Examples: synthesis of glucose, fats, proteins, DNA | Produce reducing equivalents and ATP |
| Used for growth and synthesis | Provide useful energy for other processes |

### (d) Totipotent and Pluripotent cells

| Totipotent cells | Pluripotent cells |
|---|---|
| Can differentiate into all possible cell types of an organism | Can differentiate into most body tissues but not extraembryonic tissues |
| Can form embryonic and extraembryonic cells | Cannot form extraembryonic tissues like placenta or yolk sac |
| Highest differentiation potential | Lower potential than totipotent cells |
| Example: zygote | Example: cells derived from the inner cell mass |

Stem cells are undifferentiated cells and their ability to differentiate is called cell potency. They can divide to produce one stem cell and one differentiated cell.

Blood cells are already differentiated cells, so they have little or no potency. Among blood cells, haematopoietic stem cells are multipotent because they can differentiate into a limited range of blood cells such as RBCs, WBCs and platelets.

So, stem cells have higher potency, while blood cells are specialised and do not have such broad differentiation ability.

Each mitotic division doubles the number of cells.

Starting from 1 cell:
- after 1 division: $2$
- after 2 divisions: $4$
- after 3 divisions: $8$
- after 4 divisions: $16$
- after 5 divisions: $32$
- after 6 divisions: $64$

So, 6 mitotic divisions are needed.

Match each term with its correct description:

- Centromere → Holds the two sister chromatids together → (ii)
- Kinetochore → Site of attachment of chromosomes to spindle fibers → (vi)
- Metaphase → Assembly of homologous chromosomes on metaphase plate → (v)
- Zygotene → Pairing of homologous chromosomes → (iii)
- Pachytene → Crossing over between homologous chromosomes occurs → (vii)
- Meiosis I → Reductional division → (i)
- Meiosis II → Equational division → (iv)

Cell migration is important in embryo development because it helps in the formation of morphological characteristics. During gastrulation, large numbers of cells migrate to form the embryonic layers—endoderm, mesoderm, and ectoderm. It is also essential in organogenesis, where migrating cells reach their final positions and then differentiate to form different organs and limbs.

The citric acid cycle is significant because:

- It is the final common pathway for the oxidation of carbohydrates, lipids, and proteins.
- It oxidises the acetyl moiety of acetyl CoA to produce CO₂.
- It forms NADH and FADH₂, which are reoxidised through the electron transport chain to produce ATP.
- It also includes one step of substrate-level phosphorylation.

Thus, it is a major energy-producing pathway in aerobic organisms.