"They swell at night?"
Kana stared at the microscope image. White spheres inside cells.
"Lipid droplets," Milia answered. "They grow larger hours after dinner."
"Why at night?"
"Because they store excess energy. Daytime consumption, nighttime storage."
Toma showed interest. "What are lipid droplets made of?"
"Neutral fat. Triacylglycerol."
Kana wrote in her notebook. "Triacylglycerol?"
"Three fatty acids bonded to glycerol," Milia drew a diagram.
"Why three?"
"Because glycerol has three hydroxyl groups. Each forms ester bond with fatty acid."
Toma asked. "What are fatty acids?"
"Long hydrocarbon chain with carboxyl group. Highly hydrophobic."
"Hydrophobic?"
"Water-repelling. So they cluster in aqueous solution."
Kana understood. "That's why they form droplets?"
"Exactly. Cytoplasm is aqueous. Lipids don't dissolve in water, so form separate phase."
Milia continued. "Lipid droplet surface has protein membrane."
"Membrane?"
"Perilipin. Stabilizes lipid droplets."
Toma asked. "How do they swell?"
"Fatty acid synthesis. From acetyl-CoA, extending two carbons at a time."
"Two at a time?"
"Fatty acid synthase extends chain using malonyl-CoA."
Kana was confused. "Acetyl-CoA?"
"Breakdown product of sugars, lipids, amino acids. Center of metabolism," Milia explained.
"Center?"
"Many pathways go through acetyl-CoA. Also entrance to citric acid cycle."
Toma got excited. "So sugars become fat?"
"Exactly. Excess sugar is converted to fatty acids and stored."
Kana murmured. "That's why eating too much makes you gain weight..."
"Biologically correct strategy. To prepare for famine."
Milia moved to another topic. "Conversely, lipid droplets shrink sometimes."
"When?"
"During hunger. Break down fatty acids to obtain energy."
"How?"
"Lipase hydrolyzes neutral fat. Separates into fatty acids and glycerol."
Toma asked. "How are fatty acids used?"
"Beta-oxidation. In mitochondria, cleaving two carbons at a time."
"Two at a time again?"
"Yes. Produces acetyl-CoA. This enters citric acid cycle."
Kana was moved. "Both synthesis and breakdown, two-carbon units?"
"There's symmetry. Efficient system."
Milia emphasized. "Lipids are most efficient energy storage form."
"Why?"
"9 kilocalories per gram. Carbohydrates and proteins are 4 kilocalories."
"More than double!"
"And being hydrophobic, they don't hydrate. High energy density per weight."
Toma murmured. "That's why migratory birds store fat."
"For long-distance flight. Maximum energy with minimum weight."
Kana looked at the microscope image again. "These tiny droplets have huge energy?"
"With tens of thousands, it's considerable amount."
Milia added. "Brown adipose tissue is different. Produces heat."
"Heat?"
"In mitochondria, burns fatty acids converting to heat. Abundant in babies and hibernating animals."
Toma laughed. "Lipid droplets aren't just storage."
"Key to metabolism. Also releases signal molecules."
Kana was impressed. "There's meaning to swelling at night."
"Life's rhythm. Circadian clock controls lipid metabolism too."
Milia said quietly. "Lipid droplets are energy banks."
"Deposits and withdrawals?"
"Yes. Life's economic system."
Toma looked out the window. Night was deepening.
"Even now, lipid droplets are swelling throughout my body?"
"Certainly," Milia smiled.
"Night of lipid droplets," Kana murmured.
The three felt the invisible rhythm of metabolism.