Notes: Calories, Caloric Density, ATP/ADP, and Cellular Energy

Calories

• Calorie is a unit of chemical energy used in the physical and biological sciences.
• A calorie ($c$) is the amount of energy required to raise the temperature of one gram of water by one degree Celsius ($^{ ext{o}}C$).
• The caloric content of food is measured by burning it completely to ashes under a container of water and measuring the increase in the water temperature.
• Only a handful of peanuts has enough chemical energy to boil more than a quart of water if the peanuts could be completely converted to heat.
• A bomb calorimeter is used by food scientists to measure the caloric content of foods.

Caloric Density

• The plates of kiwi fruit and M&Ms each contain about $ext{≈ }2 imes 10^{2}$ calories (about 200 Calories).
• Although certain foods may have about equal caloric content, they can differ substantially in caloric densities.

Caloric Accounting

• Caloric accounting is based on a person’s food and beverage intake, basal metabolic rate (BMR) for body functions, and physical activity.
• One pound of body weight equals approximately $1 ext{ lb} \approx 3{,}500$ calories.
• Caloric imbalances among these three factors can lead to weight gain or weight loss:
  • If calories in exceed calories out, a person would gain weight.
• Conceptual balance: $ext{CALORIES IN} - ext{CALORIES OUT}$ (illustrated as a balance between Food/Beverages/Body functions and Physical activity).

ATP and ADP

• ATP stands for adenosine triphosphate:
  • Contains adenine, ribose, and three phosphate groups (triphosphate).
  • It has a high-energy phosphate bond that stores energy for cellular work.
• ADP stands for adenosine diphosphate:
  • Contains adenine, ribose, and two phosphate groups (diphosphate).
• The energy storage in ATP is associated with the crowding of negative charges in the molecular tail (phosphate group chain).
  • This crowding is like a compressed spring; when a terminal phosphate is released, energy becomes available for cellular work.
• The release of the third phosphate from ATP yields energy for cellular work, leaving ADP.
• Phosphate transfer:
  • The released phosphate group can be transferred to other molecules.
  • This transfer enables cells to perform work (mechanical, chemical, or transport).

Cellular Respiration

• The chemical equation for aerobic cellular respiration is represented on a slide (not shown here).
• A key product of cellular respiration is ATP (adenosine triphosphate).
• The equation shown in the slide-deck represents a redox reaction (oxidation-reduction).
• Aerobic respiration requires oxygen; note that this is not the same as aerobic exercise.
• General representation of the aerobic respiration process is often summarized as glucose reacting with oxygen to produce carbon dioxide, water, and ATP (energy).
• For a common textbook form, a representative overall reaction is:
  $\mathrm{C<em>6H</em>{12}O<em>6} + 6\,\mathrm{O</em>2} \rightarrow 6\,\mathrm{CO<em>2} + 6\,\mathrm{H</em>2O} + \text{Energy (ATP)}$
• This illustrates that energy is released from glucose and captured in ATP during aerobic respiration.

ATP Cycle

• ATP is restored by adding a phosphate group to ADP using the chemical energy harvested from food molecules (such as carbohydrates and fats).
• This regeneration process is called the ATP cycle.
• Conceptual flow:
  • ATP stores chemical energy from food molecules.
  • The energy is used to perform cellular work, converting ATP to ADP and inorganic phosphate ($P_i$).
  • ADP + $P_i$ are reassembled into ATP using energy from metabolic processes.
• Visual summary (as described in the slides):
  • ATP ⇄ ADP + $P_i$ (with energy input from food molecules, primarily carbohydrates and fats)
  • The cycle continuously regenerates the usable energy currency of the cell.