Question

(a) Why is bile juice considered to be very important for digestion, even though it doesn't contain any digestive enzymes?
(b) Name some substances present in initial filtrate and subsequently selectively reabsorbed in the tubules of nephron.

Hint:

Bile emulsifies fats (no enzymes needed). PCT reabsorbs essential nutrients (glucose, amino acids, ions, water). DCT and collecting duct fine-tune reabsorption under hormonal control.

Answer 1

Part (a): Importance of Bile Juice in Digestion
Step 1: Composition of bile juice.
Bile juice is a greenish-yellow fluid produced by the liver and stored in the gallbladder. It does not contain any digestive enzymes, but it contains bile salts, bile pigments (bilirubin and biliverdin), cholesterol, and inorganic salts.
Step 2: Role of bile in fat emulsification.
The primary importance of bile juice lies in its ability to emulsify fats. Bile salts (sodium glycocholate and sodium taurocholate) break down large fat globules into smaller, microscopic droplets. This process is called emulsification.
Step 3: Mechanism of emulsification.
Bile salts reduce the surface tension of fat droplets, causing them to break into smaller droplets. This increases the surface area of fats available for the action of lipase enzymes.
Step 4: Activation of lipase.
Bile juice also activates lipase enzymes (pancreatic lipase) and provides an alkaline medium (pH 7.7) required for their optimal activity.
Step 5: Absorption of fats and fat-soluble vitamins.
Bile salts help in the absorption of fatty acids, glycerol, and fat-soluble vitamins (A, D, E, K) by forming micelles. These micelles transport digested fats to the intestinal wall for absorption.
Step 6: Other functions.}
Bile juice also:

• Neutralizes the acidic chyme coming from the stomach
• Provides alkaline medium for intestinal and pancreatic enzymes
• Helps in excretion of waste products like bilirubin and cholesterol

Thus, despite lacking enzymes, bile juice is essential for digestion, particularly of fats. Part (b): Substances Present in Initial Filtrate and Selectively Reabsorbed
Step 1: Understanding initial filtrate.
The initial filtrate (glomerular filtrate) is formed in the Bowman's capsule of the nephron. It is essentially blood plasma without proteins and blood cells. It contains:

• Water
• Glucose
• Amino acids
• Urea
• Uric acid
• Electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻, Ca²⁺, PO₄³⁻)
• Vitamins
• Hormones

Step 2: Selective reabsorption in tubules.
As the filtrate passes through the renal tubules (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct), certain substances are selectively reabsorbed back into the blood to maintain homeostasis.
Step 3: Substances reabsorbed in proximal convoluted tubule (PCT).
The PCT is the major site of reabsorption:

• Glucose: Completely reabsorbed by active transport
• Amino acids: Completely reabsorbed
• Sodium ions (Na⁺): 65-70% reabsorbed actively
• Water: 65-70% reabsorbed passively by osmosis
• Chloride ions (Cl⁻): Reabsorbed passively
• Bicarbonate ions (HCO₃⁻): Reabsorbed
• Potassium ions (K⁺): Reabsorbed
• Calcium ions (Ca²⁺): Reabsorbed
• Phosphate ions (PO₄³⁻): Reabsorbed
• Urea: Partially reabsorbed
• Uric acid: Partially reabsorbed
• Vitamins: Reabsorbed

Step 4: Substances reabsorbed in loop of Henle.

• Water: Reabsorbed in descending limb
• Sodium and chloride ions: Reabsorbed in ascending limb

Step 5: Substances reabsorbed in distal convoluted tubule (DCT).

• Sodium ions: Reabsorbed under aldosterone influence
• Water: Reabsorbed under ADH influence
• Calcium ions: Reabsorbed under parathyroid hormone influence
• Bicarbonate ions: Reabsorbed

Step 6: Final answer.
\[ \boxed{\text{Glucose, amino acids, Na⁺, water, Cl⁻, HCO₃⁻, K⁺, Ca²⁺, vitamins, etc.}} \]