"Is fat the villain?"
Kana asked while looking at a nutrition book.
Milia shook her head. "Misunderstood. Actually an excellent energy storage."
"Storage?"
Rei calculated. "Glucose is 4 kcal/g. Fat is 9 kcal/g."
"More than double!" Kana was surprised.
"That's why it's optimal as long-term energy source," Milia explained.
Kana looked at fatty acid structural formula. "So long..."
"12 to over 20 carbons linked," Rei pointed out.
"Energy is packed in this long chain."
Milia supplemented. "C-H bonds. The more of these, the higher the energy."
"Fatty acids come in saturated and unsaturated," Rei drew a diagram.
"Saturated?"
"All single bonds. Saturated with hydrogen."
Kana wrote in her notebook. "Unsaturated?"
"Has double bonds. C=C."
Milia showed a model. "This double bond bends the chain."
"Bends?"
"Can't rotate around the double bond. So it bends in a 'V' shape."
Rei explained. "Saturated fatty acids are linear. So they can be packed tightly."
"That's why butter is solid?" Kana understood.
"Correct. Unsaturated fatty acids are bent, creating gaps. More likely to be liquid."
Milia moved to the next theme. "Fatty acid breakdown, beta-oxidation."
"Beta?"
"Carbon position name. Oxidation occurs at the second carbon from the carboxyl group."
Rei drew a diagram. "Cut two carbons at a time."
"Palmitic acid has 16 carbons. Cut 8 times."
Kana imagined. "Like slicing salami."
"Good metaphor," Milia laughed. "Each cycle produces one acetyl-CoA."
"Which?"
"Enters the citric acid cycle. The one we learned before."
Rei calculated. "From one palmitic acid, theoretically 129 ATP."
"129!" Kana was surprised. "More than glucose."
"That's why marathon runners burn fat."
Milia added. "But oxygen is needed. Can't use in anaerobic exercise."
"Why?"
"Both beta-oxidation and electron transport chain require oxygen."
Rei explained. "Sprints use glucose. Long distance uses fat."
Kana asked. "What's the swaying of fatty acids?"
Milia spoke poetically. "Double bonds in unsaturated fatty acids. The chain sways there."
"Inside membranes, fatty acid tails constantly move. Creating fluidity."
Rei supplemented. "Cell membranes are phospholipid bilayers. Fatty acid parts sway."
"At low temperature?"
"If many saturated fatty acids, they solidify easily. So unsaturated fatty acids are mixed in."
Kana understood. "Balance is important."
"Right. Membranes can't be too rigid or too soft."
Milia continued. "Fish oil has many unsaturated fatty acids to keep membranes soft in cold water."
"Adaptation?"
"Yes. Changing fatty acid composition to suit the environment."
Rei summarized. "Fatty acids are energy sources, membrane components, and signaling molecules."
"Signaling?"
"Prostaglandins and such. Involved in inflammation and pain."
Kana murmured. "Fat is complex."
"Far from villain, essential for life," Milia said quietly.
"However, balance matters. Excessive intake is problematic."
Rei admitted. "Same for everything. Appropriate amount is important."
Kana stared at the fatty acid diagram. She felt the long chains swaying quietly.
"At night, fatty acids dance inside cells."
Milia smiled. "Poetic but also scientifically correct."
"Thermal motion of molecules. That's the identity of swaying."
The three imagined the invisible molecular dance. A quiet night where fatty acids sway.