Unit 4

• Plasma Membrane Marker: $\text{Na}^+/\text{K}^+$-ATPase (sodium-potassium pump) is an integral transmembrane P-type ATPase found exclusively in the plasma membrane of animal cells, actively pumping $3\text{ Na}^+$ out and $2\text{ K}^+$ in per ATP hydrolyzed.
• Other Organelle Markers:
• Mitochondria: Succinate dehydrogenase (inner membrane) and Cytochrome c oxidase.
• Lysosomes: Acid phosphatase.
• Cytosol: Lactate dehydrogenase (LDH).
• Endoplasmic Reticulum: Glucose-6-phosphatase.
• Golgi Apparatus: Galactosyltransferase.

Correct option: (1) Plasma membrane

• Erythrocyte Glucose Transporter: GLUT1 is the primary, high-affinity facilitative glucose transporter expressed abundantly on erythrocyte (RBC) membranes ($\sim 5\%$ of total membrane protein) and brain microvessel endothelial cells.
• Major GLUT Transporters:
• GLUT1: RBCs, blood-brain barrier ($K_m \approx 1\text{--}2\text{ mM}$, basal uptake).
• GLUT2: Liver, pancreatic $\beta$-cells, basolateral membrane of renal tubules and enterocytes ($K_m \approx 15\text{--}20\text{ mM}$, high capacity, low affinity).
• GLUT3: Neurons and brain ($K_m \approx 1\text{ mM}$, high affinity).
• GLUT4: Skeletal muscle, cardiac muscle, and adipose tissue (insulin-dependent).
• GLUT5: Small intestine brush border and spermatozoa (specific for fructose).

Correct option: (1) GLUT1

Evaluating the Assertion and Reason concerning Blood group substances:

The correct answer is Option 1: Both (A) and (R) are correct and (R) is the correct explanation of (A)..