Photosynthesis & Cellular Respiration – Comprehensive Study Notes

Photosynthesis

• Two major phases

  • Light-dependent reactions (a.k.a. photophosphorylation)

  • Location : thylakoid membranes of the chloroplast.

  • Inputs : \text{H}2\text{O},\,\text{ADP},\,\text{P}i,\,\text{NADP}^+, photons.

  • Outputs :

    • \text{O}_2 (diffuses out of the plant and ultimately sustains aerobic life).
    • \text{ATP} (chemical energy, short-term storage).
    • \text{NADPH} (reducing power / high-energy electron carrier).

  • ATP & NADPH immediately feed the Calvin cycle.

  • Can run only in the light because the ETC requires photon-excited electrons, but ATP & NADPH produced in the day can be consumed in the dark by the Calvin cycle.

  • Light-independent reactions (Calvin cycle / “dark reactions”)

  • Location : stroma of chloroplast.

  • Main task : convert atmospheric \text{CO}_2 into triose-phosphate (G3P), which is later polymerised into glucose/starch.

  • Energy source : the ATP & NADPH generated by the light reactions.

  • Key point from transcript : "It can occur in the dark as long as ATP and NADPH are available. It does not occur only at night."

• Connections / Mnemonics

  • Review the Amoeba Sisters videos on photosynthesis for animated walk-throughs.
  • Remember location pairs: “Light-dependent → Lamella (thylakoid); Stroma → Sugar synthesis.”

Cellular Respiration

Big-picture pathway

1. Glycolysis
2. Pyruvate oxidation
3. Citric Acid Cycle (CAC) / Krebs cycle
4. Oxidative Phosphorylation (ETC + Chemiosmosis)

Quick mnemonic used in class: “Girl Please Carry On.”

1 Glycolysis

• Location : cytoplasm (happens in both prokaryotes & eukaryotes).
• Net equation :
  \text{Glucose} \; (C6H{12}O6) \;\xrightarrow{10\;steps}\; 2\,\text{Pyruvate}+2\,\text{ATP}{\text{net}}+2\,\text{NADH}
• Classification : catabolic (large → small; releases energy).
• Key class anecdote : glucose from bread is “cut in half.”

2 Pyruvate Oxidation (Link Reaction)

• Location : mitochondrial matrix (eukaryotes only).
• Each pyruvate (3 C) loses 1 C as \text{CO}_2 ➔ becomes acetyl-CoA (2 C).
• Products per glucose (2 pyruvates):
  • 2 \text{CO}_2
  • 2 NADH
  • 2 acetyl-CoA (enters CAC).
• Redox reminder : oxidation = loss of electrons/OIL (“Oxidation Is Loss”).

3 Citric Acid Cycle

• Location : mitochondrial matrix.
• Each acetyl-CoA drives 2 turns of the cycle per glucose.
• Per glucose (2 turns) output :
  • 6\,\text{NADH}
  • 2\,\text{FADH}_2
  • 4\,\text{CO}_2 (all remaining carbons from original glucose now released)
  • 2\,\text{ATP} (substrate-level phosphorylation)
• “Spinning wheel shreds the acetyl group apart.”

4 Oxidative Phosphorylation

• Sub-steps : (a) Electron Transport Chain (ETC) & (b) Chemiosmosis.
• Location : inner mitochondrial membrane.
• ETC
  • NADH & FADH$_2$ are re-oxidised → donate electrons to complexes I-IV.
  • Energy released pumps H^+ from matrix → inter-membrane space, storing potential energy as an electrochemical (proton) gradient.
  • Final e$^-$ acceptor : \text{O}2 ➔ \text{H}2\text{O}.
• Chemiosmosis
  • H^+ flows back into the matrix via ATP synthase.
  • Analogy from lecture : pressure on a wind turbine; proton flow "cranks" ATP synthase.
  • Yield : \approx28\text{–}34\,\text{ATP} per glucose (varies with shuttle efficiency).

ATP Accounting (typical textbook values)

Redox & Electron Carriers

• NAD$^+$ / NADH and FAD / FADH$_2$ are the major carriers.
• Oxidised forms : \text{NAD}^+,\,\text{FAD}.
• Reduced forms : \text{NADH},\,\text{FADH}_2 (they gain electrons).
• Phrase repeated in session : “OIL RIG” – Oxidation Is Loss, Reduction Is Gain.

Gibbs Free Energy (ΔG) & Reaction Spontaneity

• Definition : \Delta G = change in free energy (usable energy).
• Interpretations :
  • \Delta G < 0
  • Negative; energy released; spontaneous; exergonic.
  • Reaction can proceed without external energy input.
  • \Delta G > 0
  • Positive; energy required; non-spontaneous; endergonic.
  • \Delta G = 0 ➔ system at equilibrium; no net change.
• Class example : “If \Delta G = -7.7\,\text{kJ} the process will run on its own.”

Additional Concepts & Clarifications

• Catabolic vs Anabolic
  • Catabolic (e.g.
    glycolysis): break large → small, release energy.
  • Anabolic (e.g.
    Calvin cycle): build small → large, consume energy.
• Substrate-level phosphorylation – direct transfer of a phosphate group to ADP to form ATP (occurs in glycolysis & CAC).
• Oxidative phosphorylation – ATP formed indirectly via the proton gradient & ATP synthase.
• Prokaryotes vs Eukaryotes
  • Prokaryotes lack mitochondria; their ETC is embedded in the plasma membrane, but glycolysis is still universal.
• Practice advice (from instructor)
  • Re-watch Amoeba Sisters videos on both topics.
  • Use the provided study guide & quiz banks; retake quizzes to reinforce memory.
  • For unclear points, consult textbook, YouTube, or ChatGPT before exam opens.

Common Exam-Style Prompts (as hinted in session)

• Identify which step of respiration a given input/output pair belongs to.
• Recognise whether a \Delta G value denotes a spontaneous reaction.
• Distinguish oxidised vs reduced forms of NAD/FAD.
• Compare ATP yield of substrate-level phosphorylation vs oxidative phosphorylation.
• Recall exact locations of each photosynthetic & respiratory sub-process (thylakoid, stroma, cytoplasm, matrix, inner membrane).

Quick-Reference Summary Table

Key: PS = Photosynthesis