Notes on Photosynthesis & Cellular Respiration
Photosynthesis & Cellular Respiration — Comprehensive Study Notes
Origins of the terms
Photosynthesis: from Greek roots
PHOTO = light
SYNTHESIS = putting together, to manufacture
Purpose: How plants obtain energy and raw materials to produce plant matter; how organisms feed and obtain energy
Big questions covered in the transcript
Where do plants obtain energy and raw materials to produce plant matter?
How do plants feed? How about humans and animals?
How energy flows between photosynthesis and cellular respiration in ecosystems
Where photosynthesis happens (organelles and structures)
Chloroplasts (main site in plant cells)
Key leaf/epidermal features for gas exchange (stomata) and water retention (cuticle)
Internal chloroplast substructures:
Thylakoid membranes (stacks called granum)
Stroma (fluid around thylakoids; site of Calvin cycle)
Additional internal components:
Cristae (mitochondrial folds)
Matrix (mitochondrial fluid)
Chloroplast vs mitochondrion: basic organization
Chloroplast components to know:
Outer membrane / Inner membrane
Intermembrane space
Thylakoid membranes inside the stroma; granum = stack of thylakoids
Stroma = fluid surrounding thylakoids; Calvin cycle occurs here
Mitochondria components to know:
Outer membrane / Inner membrane
Cristae (inward folds of inner membrane)
Matrix = interior space where mitochondrial enzymes reside
Ribosome (mitochondrial ribosomes) indicated in some diagrams
Overall chemical relationships (balanced equations)
Photosynthesis (overall, in leaves in light):
Reactants to products:
6\,{\rm CO}2 + 6\,{\rm H2O} \rightarrow \mathrm{C6H{12}O6} + 6\,{\rm O2}
Cellular respiration (overall, aerobic):
Reactants to products:
\mathrm{C6H{12}O6} + 6\,{\rm O2} \rightarrow 6\,{\rm CO}2 + 6\,{\rm H2O} + \text{ATP}
Typical net ATP yield per glucose in eukaryotic cells: ~30–32 ATP (depends on shuttle systems and conditions)
Photosynthesis: two major stages
Light-dependent stage (occurs in thylakoid membranes)
Light-independent stage (Calvin cycle, occurs in the stroma)
Synonym: Calvin cycle = light-independent reactions
Related process names: Aerobic respiration, Anaerobic respiration (contrast with photosynthesis)
Light-dependent reactions: steps and outputs
Step 1: Sunlight is absorbed by chlorophyll in the thylakoid membranes
Step 2: Water (H₂O) is split (photolysis) to yield H⁺, electrons (e⁻), and O₂
Step 3: Excited electrons move through a chain of proteins (electron transport chain)
Step 4: Protons and electrons drive synthesis of energy carriers
Step 5: ATP and NADPH are produced and exported to the Calvin cycle for the next stage
Net products used in the Calvin cycle:
\text{ATP} and \text{NADPH} produced for carbon fixation
Calvin Cycle (Light-independent reactions): overview
Location: Stroma of the chloroplast
Key enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
Core inputs: CO₂, ATP, NADPH
Core outputs: G3P (glyceraldehyde-3-phosphate) and RuBP regeneration
Main steps:
CO₂ fixation: RuBP + CO₂ → 2 × 3-PGA (3-phosphoglycerate) (catalyzed by RuBisCO)
Reduction: 3-PGA → G3P using ATP and NADPH
Regeneration: Most G3P is recycled to regenerate RuBP (uses ATP); some G3P exits cycle to synthesize sugars (glucose and other carbohydrates)
Key intermediates:
RuBP (Ribulose-1,5-bisphosphate)
3-PGA (3-phosphoglycerate)
G3P (glyceraldehyde-3-phosphate)
Net input/output idea:
CO₂ fixation builds carbohydrate framework from inorganic carbon using energy from ATP and reducing power from NADPH
Calivin Cycle: specific notations and components from the transcript
Important players: RuBisCO, RuBP, ATP, NADPH
Cycle dynamics: 6-turns of the cycle typically required to form one molecule of glucose (since 6 CO₂ fixed supply enough carbon for one sugar) (details shown in diagrams with RuBP, 3-PGA, G3P, NADP⁺, ADP, Pi, etc.)
Typical stoichiometry per turn: CO₂ is fixed, 3-PGA is produced, ATP and NADPH are consumed to form G3P, which is used to regenerate RuBP
Light-dependent vs Calvin cycle: recap
Light-dependent stage inputs/outputs:
Inputs: light energy, H₂O
Outputs: O₂, ATP, NADPH (and reduced NADPH used in Calvin cycle)
Calvin cycle inputs/outputs:
Inputs: CO₂, ATP, NADPH
Outputs: G3P (precursor to glucose and other carbohydrates); Regenerated RuBP
Cellular respiration: overview and stages
Overall purpose: extract chemical energy from glucose to form ATP
Stages (in order):
Glycolysis (occurs in cytosol; anaerobic-friendly)
Pyruvate oxidation (to acetyl-CoA) inside mitochondria
Krebs (Citric Acid) Cycle
Electron Transport Chain (ETC) & Oxidative Phosphorylation
Optional anaerobic pathways when oxygen is limited: Alcoholic Fermentation and Lactic Acid Fermentation
Mitochondrial structure and its role in respiration
Cristae: folds of the inner mitochondrial membrane that increase surface area for ETC
Matrix: central mitochondrial space where the Krebs cycle enzymes reside
Ribosome: mitochondrial ribosomes for some protein synthesis
Glycolysis (location, inputs, outputs)
Location: cytosol
Key steps (summary):
One glucose molecule is energized by consuming 2 ATP
Glucose splits into two molecules of glyceraldehyde-3-phosphate (G3P)
Through a series of steps, energy is harvested to form 2 pyruvate, 2 NADH, and a net of 2 ATP
Net equation (summary):
\text{Glucose} + 2\,\mathrm{NAD}^+ + 2\,\mathrm{ADP} + 2\,\mathrm{Pi} \rightarrow 2\,\text{pyruvate} + 2\,\mathrm{NADH} + 2\,\mathrm{ATP} + 2\,\mathrm{H2O} + 2\,\mathrm{H}^+
Important derivatives: NADH produced feeds into ETC; 2 ATP net produced per glucose (consumed 2 ATP upfront)
Pyruvate oxidation (link between glycolysis and Krebs cycle)
Location: mitochondrial matrix (pyruvate enters mitochondria)
Process: Pyruvate is converted to Acetyl-CoA while producing NADH and releasing CO₂
Net per pyruvate:
\text{Pyruvate} + \mathrm{CoA} + \mathrm{NAD}^+ \rightarrow \mathrm{Acetyl\text{-}CoA} + \mathrm{CO_2} + \mathrm{NADH}
Per glucose (2 pyruvate per glucose): yields 2 Acetyl-CoA and 2 NADH and 2 CO₂
Krebs Cycle (Citric Acid Cycle)
Location: mitochondrial matrix
Key reaction: Acetyl-CoA combines with oxaloacetate to form citrate; cycles regenerate oxaloacetate
Per Acetyl-CoA turn yields:
3 NADH, 1 FADH₂, 1 GTP (equivalent to ATP), and 2 CO₂
Per glucose (two turns, since 2 acetyl-CoA enter):
6 NADH, 2 FADH₂, 2 ATP (as GTP), 4 CO₂
Common notations used in the transcript visuals include: NADH, NAD⁺, FADH₂, FAD, GTP, GDP, Pi, oxaloacetate, citrate
Electron Transport Chain (ETC) and oxidative phosphorylation
Location: inner mitochondrial membrane
Mechanism: high-energy electrons from NADH and FADH₂ are passed along a chain of proteins, pumping protons across the membrane to create a proton-m motive force
End result: protons flow back through ATP synthase to generate ATP
Final electron acceptor: O₂, forming water (H₂O)
Typical ATP yield considerations (numbers vary by organism and shuttle systems):
NADH yields about 2.5 ATP per molecule
FADH₂ yields about 1.5 ATP per molecule
Overall approximate net ATP from full respiratory = ~30–34 ATP per glucose under optimal conditions
Fermentation (anaerobic respiration alternatives when O₂ is limited)
Alcoholic fermentation
Purpose: regenerate NAD⁺ to allow glycolysis to continue in absence of oxygen
Pathway (summary):
Pyruvate is decarboxylated to acetaldehyde + CO₂
Acetaldehyde is reduced to ethanol using electrons from NADH, regenerating NAD⁺
Net products per glucose (from glycolysis):
2 ATP (net from glycolysis), 2 NAD⁺ regenerated, 2 CO₂, 2 ethanol
Representative sequence: glucose → 2 pyruvate → 2 acetaldehyde + 2 CO₂ → 2 ethanol + 2 NAD⁺
Lactic acid fermentation
Purpose: regenerate NAD⁺ to sustain glycolysis without O₂
Pathway (summary): Pyruvate is reduced to lactate using electrons from NADH, regenerating NAD⁺
Net products per glucose (from glycolysis):
2 ATP (net from glycolysis), 2 NAD⁺ regenerated, 2 lactate
Connections, significance, and real-world relevance
Energy flow: Light energy captured by chloroplasts stores energy in chemical bonds of glucose; mitochondria release this energy as ATP to power cell activities
Carbon cycling: CO₂ fixed during photosynthesis and released during respiration; balance maintains atmospheric CO₂ levels
Food and industry: Fermentation (alcoholic and lactic acid) underpins bread baking, brewing, wine production, yogurt and other fermented foods
Health and physiology: Aerobic respiration dominates in well-oxygenated tissues; anaerobic pathways cope with short bursts of high energy demand but are less efficient
Philosophical/practical implications: energy transformations are central to life; the efficiency of energy capture and use influences ecological balance and human technology (bioenergy, agriculture, etc.)
Quick reference: key formulas and yields
Photosynthesis (overall):
6\,{\rm CO}2 + 6\,{\rm H2O} \rightarrow \mathrm{C6H{12}O6} + 6\,{\rm O2}
Cellular respiration (overall, aerobic):
\mathrm{C6H{12}O6} + 6\,{\rm O2} \rightarrow 6\,{\rm CO}2 + 6\,{\rm H2O} + \text{ATP}
Net ATP per glucose: ~30–32 ATP (varies by shuttle and conditions)
Glycolysis (net):
\text{Glucose} + 2\,\mathrm{NAD}^+ + 2\,\mathrm{ADP} + 2\,\mathrm{Pi} \rightarrow 2\,\text{pyruvate} + 2\,\mathrm{NADH} + 2\,\mathrm{ATP} + 2\,\mathrm{H2O} + 2\,\mathrm{H}^+
Pyruvate oxidation:
\text{Pyruvate} + \mathrm{CoA} + \mathrm{NAD}^+ \rightarrow \mathrm{Acetyl\text{-}CoA} + \mathrm{CO_2} + \mathrm{NADH}
Krebs cycle (per acetyl-CoA turn):
\mathrm{Acetyl\text{-}CoA} + 3\,\mathrm{NAD}^+ + \mathrm{FAD} + \mathrm{GDP} + \mathrm{Pi} \rightarrow 2\,\mathrm{CO2} + 3\,\mathrm{NADH} + \mathrm{FADH_2} + \mathrm{GTP} + \mathrm{CoA}
Per glucose (two turns): 6\,\mathrm{NADH} + 2\,\mathrm{FADH2} + 2\,\mathrm{GTP} + 4\,\mathrm{CO2}
ETC energy yield: NADH ≈ 2.5 ATP, FADH₂ ≈ 1.5 ATP
Fermentation outcomes:
Alcoholic fermentation (per glucose): 2 ATP, 2 CO₂, 2 ethanol, NAD⁺ regenerated
Lactic acid fermentation (per glucose): 2 ATP, 2 lactate, NAD⁺ regenerated
Notes on terminology and cross-references
Stroma vs stoma: Stroma is the soluble medium inside the chloroplast where the Calvin cycle occurs; stomata are leaf pores that regulate gas exchange with the atmosphere
Thylakoids form granum; grana are stacks connected by lamellae within chloroplasts
Cristae and matrix are the key structural features of mitochondria relevant to respiration
RuBisCO is a key enzyme in carbon fixation; CO₂ fixation begins the Calvin cycle
Summary takeaway
Photosynthesis stores energy in sugars using light energy; cellular respiration releases that energy as ATP to power cellular processes
The two processes are complementary and interconnected through shared intermediates (ATP, NADPH/NADH) and the exchange of gases (CO₂ and O₂)