Energy Transformation: ATP-ADP Cycle and Photosynthesis

ATP-ADP Cycle

• ATP (Adenosine Triphosphate): The primary energy currency of cells, composed of adenine, ribose, and three phosphate groups.
• Energy Release: Energy is released when a phosphate group is removed from ATP, converting it to ADP (Adenosine Diphosphate).
• Energy Storage: ADP is recharged into ATP when a phosphate group is added, a process requiring energy.
• ATP Yield by Molecule:
  • Carbohydrates: Most commonly broken down; yields up to $36$ ATP per glucose molecule ($4\,\text{calories/mg}$).
  • Lipids (Fats): Store the most energy ($80\%$ of body energy); yields about $146$ ATP per triglyceride ($9\,\text{calories/mg}$).
  • Proteins: Least likely to be used for energy; yields about $36$ ATP ($4\,\text{calories/mg}$).

Energy Flow and Metabolism

• Energy Source: The sun is the ultimate source of energy for all life on Earth.
• Autotrophs: Organisms like plants (photoautotrophs) that produce their own food using light energy to create glucose.
• Heterotrophs: Consumers that must eat plants or other animals to acquire energy indirectly from the sun.
• Energy Pyramid: Energy flows from Producers ($1,000\,\text{kcal}$) to Primary Consumers ($100\,\text{kcal}$), Secondary Consumers ($10\,\text{kcal}$), and Tertiary Consumers ($1\,\text{kcal}$).

Photosynthesis

• Definition: The process of converting light energy into the chemical energy of food, occurring mainly in the chloroplasts of leaves.
• Overall Equation:$6CO_2 + 6H_{2}O + \text{light} \rightarrow C_{6}H_{12}O_{6} + 6O_2$
• Reactants: Inorganic molecules (carbon dioxide and water), light energy, and chlorophyll.
• Products: Glucose ($C_{6}H_{12}O_{6}$) and oxygen ($O_2$).
• Five Steps:
  1. Roots absorb water.
  2. Leaves take in $CO_2$ via stomates.
  3. Chlorophyll captures sunlight.
  4. Glucose is produced.
  5. Oxygen is released.

Chloroplast Structure and Pigments

• Chloroplast Components:
  • Outer and Inner Membranes.
  • Stroma: Aqueous space containing DNA and ribosomes; site of the Dark Reaction (Calvin Cycle).
  • Thylakoids: Sacs piled into grana (singular: granum); membranes contain reaction centers for the Light Reaction.
• Photosynthetic Pigments:
  • Chlorophyll a: The main photosynthetic pigment ($P680$ and $P700$).
  • Chlorophyll b: An accessory pigment.
  • Absorption: Pigments primarily use blue and red light and reflect green light.

Stages of Photosynthesis

• Light Reaction (Light-Dependent):
  • Occurs in the thylakoid membranes.
  • Splits water to release oxygen and produces ATP and NADPH.
  • Employs Photosystem I ($P700$) and Photosystem II ($P680$) in a "Z scheme."
  • Photophosphorylation: Peter Mitchell ($1960$) proposed this works via a chemiosmotic mechanism using a proton motive force (PMF).
• Dark Reaction (Light-Independent/Calvin Cycle):
  • Occurs in the stroma.
  • Stages: Carboxylation ($CO_2$ input), Reduction, and Regeneration of Ribulose-1,5-bisphosphate.
  • Converts $CO_2$ into carbohydrates (sucrose, starch).

Translocation of Photosynthetic Products

• Source: Areas of supply where products are exported (e.g., mature leaves, storage roots).
• Sink: Areas of metabolism or storage (e.g., roots, tubers, developing fruits, immature leaves).
• Movement: Metabolites move from source to sink via the phloem.

Questions & Discussion

Learning Check

• Question: Where do plants get energy to produce their own food?
  • Response: Sun.
• Question: Consumers are also termed as?
  • Response: Heterotrophs.
• Question: When does ATP release its stored energy?
  • Response: When one of the phosphorus bonds is broken and released.
• Question: What molecule will be formed if 1 molecule of phosphate is released (from ATP)?
  • Response: ADP.
• Question: Which of the biomolecules possessed the greater amount of energy?
  • Response: Lipids/Fats.