Cellular Processes — NCERT Solutions

Cell Migration in Embryonic Development:

Cell migration is the directed movement of cells from one location to another within the developing embryo. It is a fundamental process in embryogenesis and is important in the following ways:

1. Gastrulation: During gastrulation, cells migrate to form the three primary germ layers — ectoderm, mesoderm, and endoderm. This establishes the basic body plan of the organism.

2. Organogenesis: Migrating cells reach specific locations where they differentiate into specialized tissues and organs. For example, neural crest cells migrate extensively to form structures such as peripheral neurons, craniofacial cartilage, and pigment cells.

3. Formation of the nervous system: Neuronal precursor cells migrate to their correct positions in the developing brain and spinal cord, which is essential for proper neural circuit formation.

4. Vascularization: Endothelial cell precursors (angioblasts) migrate to form blood vessels throughout the embryo, ensuring proper blood supply to developing tissues.

5. Immune system development: Precursors of immune cells migrate from the bone marrow to the thymus and other lymphoid organs.

6. Wound healing and tissue repair: Cell migration is also critical postnatally for repair processes.

Conclusion: Cell migration is indispensable for the correct spatial organization of cells during embryonic development. Defects in cell migration can lead to developmental abnormalities and congenital disorders.

Significance of the Citric Acid Cycle (Krebs Cycle / TCA Cycle):

The citric acid cycle (also called the Tricarboxylic Acid cycle or Krebs cycle) is a central metabolic pathway that takes place in the mitochondrial matrix. Its significance includes:

1. Energy Production:

• One turn of the cycle produces:
• $3$ NADH
• $1$ FADH₂
• $1$ GTP (or ATP)
• $2$ CO₂
• NADH and FADH₂ feed into the electron transport chain (oxidative phosphorylation) to produce a large amount of ATP.
• Net ATP yield per acetyl-CoA: approximately 10 ATP (via oxidative phosphorylation).

2. Central Metabolic Hub:

• The cycle connects carbohydrate, fat, and protein metabolism.
• Acetyl-CoA derived from glucose (glycolysis), fatty acids (β-oxidation), and amino acids all enter the cycle.

3. Biosynthetic (Anabolic) Role — Amphibolic Pathway:

• Intermediates of the cycle serve as precursors for biosynthesis:
• α-ketoglutarate → amino acids (glutamate)
• Oxaloacetate → amino acids (aspartate), gluconeogenesis
• Succinyl-CoA → heme synthesis
• Citrate → fatty acid synthesis (exported to cytoplasm)

4. CO₂ Release:

• Two molecules of CO₂ are released per turn, completing the oxidation of the acetyl group.

Conclusion: The citric acid cycle is an amphibolic pathway (both anabolic and catabolic) and is the central hub of cellular metabolism, playing a critical role in energy generation and biosynthesis.

Correct Answer: (d) Hexokinase

Justification:
The first step of glycolysis is the phosphorylation of glucose to glucose-6-phosphate:
$$\text{Glucose} + \text{ATP} \xrightarrow{\text{Hexokinase}} \text{Glucose-6-phosphate} + \text{ADP}$$
This reaction is catalysed by Hexokinase (in most tissues). Glucokinase performs the same reaction but is specific to the liver and pancreatic β-cells and has a lower affinity for glucose. Phosphoglycerate kinase acts at step 7, and Phosphofructokinase acts at step 3 of glycolysis.

Correct Answer: (c) Ligands bind to receptors in a non-specific manner.

Justification:
This statement is NOT true. Ligand–receptor binding is highly specific — each ligand binds to its complementary receptor with high specificity, similar to a lock-and-key mechanism. This specificity is fundamental to cell signaling and ensures that only the correct target cells respond to a given signal.

• Option (a) is true: In endocrine signaling, hormones (ligands) are secreted into the bloodstream and travel to distant target cells.
• Option (b) is true: Synaptic signaling involves neurotransmitters acting on adjacent (postsynaptic) cells — a form of paracrine signaling.
• Option (d) is true: Cancer cells often secrete growth factors that stimulate their own receptors (autocrine signaling), promoting uncontrolled proliferation.

Correct Answer: (a) It occurs independent of light energy

Justification:
The dark reactions (Calvin cycle / carbon fixation reactions) are called 'dark reactions' not because they occur in the dark or are inhibited by light, but because they do not directly require light energy. They use the ATP and NADPH produced during the light reactions to fix CO₂ into carbohydrates. These reactions can occur both in the presence and absence of light, as long as ATP and NADPH are available. Therefore, options (b) and (c) are incorrect, making (d) also incorrect.

Cyclic vs. Non-Cyclic Photophosphorylation:

Summary:

• Cyclic photophosphorylation produces only ATP and involves only PS I with electrons cycling back.
• Non-cyclic photophosphorylation involves both photosystems, produces ATP + NADPH, and results in the evolution of oxygen from water splitting.

Calvin Cycle (Dark Reactions / C₃ Pathway):

The Calvin cycle, also called the C₃ pathway or dark reactions, occurs in the stroma of the chloroplast. It was discovered by Melvin Calvin. It uses the ATP and NADPH produced during the light reactions to fix atmospheric CO₂ into organic molecules (sugars).

The cycle has three main stages:

Stage 1: Carbon Fixation

• CO₂ from the atmosphere combines with a 5-carbon acceptor molecule, Ribulose-1,5-bisphosphate (RuBP), catalysed by the enzyme RuBisCO (Ribulose bisphosphate carboxylase/oxygenase).
• This produces an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA) (a 3-carbon compound — hence C₃ pathway).

$$\text{CO}_2 + \text{RuBP} \xrightarrow{\text{RuBisCO}} 2 \times \text{3-PGA}$$

Stage 2: Reduction

• 3-PGA is reduced using ATP and NADPH (from light reactions) to form glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.

$$\text{3-PGA} + \text{ATP} + \text{NADPH} \rightarrow \text{G3P}$$

• Some G3P molecules are used to synthesize glucose and other organic compounds.

Stage 3: Regeneration of RuBP

• The remaining G3P molecules are used to regenerate RuBP using ATP, so the cycle can continue.

$$\text{G3P} + \text{ATP} \rightarrow \text{RuBP}$$

Overall equation for one turn (fixing one CO₂):
$$3\text{CO}_2 + 9\text{ATP} + 6\text{NADPH} \rightarrow \text{G3P} + 9\text{ADP} + 8\text{P}_i + 6\text{NADP}^+$$

To synthesize one molecule of glucose (6C), the cycle must turn 6 times, consuming:

• 18 ATP
• 12 NADPH
• 6 CO₂

Significance: The Calvin cycle is the primary pathway for carbon fixation in photosynthetic organisms and is responsible for converting inorganic carbon (CO₂) into organic biomolecules that form the basis of food for all life on Earth.