AP Biology Comprehensive Study Guide

Cellular Respiration & Photosynthesis

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PART ONE: CELLULAR RESPIRATION

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Chapter 1: Overview & Foundational Concepts

What Is Cellular Respiration?

Cellular respiration is the process by which cells break down organic molecules (primarily glucose) to produce ATP (adenosine triphosphate), the universal energy currency of living cells. It is fundamentally a controlled oxidation process—electrons are systematically stripped from glucose and used to drive ATP synthesis.

Overall Equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat)

Key Terminology You Must Know

Term	Definition
Oxidation	Loss of electrons (LEO — Lose Electrons = Oxidized)
Reduction	Gain of electrons (GER — Gain Electrons = Reduced)
NAD⁺/NADH	Electron carrier; oxidized form/reduced form
FAD/FADH₂	Electron carrier; oxidized form/reduced form
Substrate-level phosphorylation	ATP made by direct transfer of a phosphate group to ADP
Oxidative phosphorylation	ATP made using energy from electron transport chain
Chemiosmosis	ATP synthesis driven by proton (H⁺) gradient across membrane

Energy Accounting Preview

Stage	ATP Yield (approx.)	Location
Glycolysis	2 net ATP	Cytoplasm
Pyruvate Oxidation	0 ATP	Mitochondrial matrix
Krebs Cycle	2 ATP	Mitochondrial matrix
Oxidative Phosphorylation	~30–32 ATP	Inner mitochondrial membrane
Total	~36–38 ATP

⚠️ AP Exam Note: Modern estimates place the total at ~30–32 ATP per glucose. Know both the older (~36–38) and modern estimates, and understand why the actual yield is lower (proton leakage, varied shuttle systems, variable H⁺/ATP ratios).

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Chapter 2: Glycolysis

Location

Cytoplasm (cytosol) — does NOT require mitochondria; occurs in ALL living cells

Overview

Glycolysis literally means "glucose splitting." One 6-carbon glucose molecule is split into two 3-carbon pyruvate molecules through a series of 10 enzymatic reactions.

The Two Phases

Phase 1: Energy Investment (Steps 1–5)

• Uses 2 ATP (phosphorylation of glucose and fructose-6-phosphate)
• Glucose is destabilized and split into two G3P (glyceraldehyde-3-phosphate) molecules
• Key enzyme: phosphofructokinase (PFK) — the main regulatory enzyme, inhibited by ATP, activated by AMP

Phase 2: Energy Payoff (Steps 6–10)

• Produces 4 ATP (substrate-level phosphorylation) and 2 NADH
• G3P is oxidized and converted to pyruvate

Net Products of Glycolysis (per glucose)

Glucose → 2 Pyruvate
         + 2 Net ATP (4 produced − 2 invested)
         + 2 NADH
         + 2 H₂O

Regulation of Glycolysis

• High ATP → inhibits PFK → slows glycolysis (cell doesn't need more energy)
• High AMP → activates PFK → speeds glycolysis (cell needs energy)
• High citrate → inhibits PFK (signals Krebs cycle is backed up)
• This is an example of allosteric regulation and feedback inhibition

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Chapter 3: Pyruvate Oxidation (Pyruvate Decarboxylation)

Location

Mitochondrial matrix (pyruvate is transported across the inner mitochondrial membrane)

What Happens

Each pyruvate (3C) undergoes oxidative decarboxylation:

1. A carboxyl group is removed as CO₂ (decarboxylation)
2. The remaining 2-carbon group is oxidized, and NAD⁺ is reduced to NADH
3. The 2-carbon acetyl group attaches to Coenzyme A → forming acetyl-CoA

Net Products (per glucose = 2 pyruvates)

2 Pyruvate + 2 CoA + 2 NAD⁺ → 2 Acetyl-CoA + 2 CO₂ + 2 NADH

Key Enzyme

Pyruvate dehydrogenase complex — a massive multi-enzyme complex embedded in the mitochondrial matrix

💡 Remember: No ATP is produced here, but the NADH produced will yield significant ATP later in oxidative phosphorylation.

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Chapter 4: The Krebs Cycle (Citric Acid Cycle)

Location

Mitochondrial matrix

Overview

Acetyl-CoA (2C) enters the cycle by combining with oxaloacetate (4C) to form citrate (6C). The cycle turns twice per glucose (once per acetyl-CoA).

Step-by-Step Summary

Acetyl-CoA (2C) + Oxaloacetate (4C)
         ↓
      Citrate (6C)
         ↓ [CO₂ released, NADH made]
    α-Ketoglutarate (5C)
         ↓ [CO₂ released, NADH made]
     Succinyl-CoA (4C)
         ↓ [ATP made via substrate-level phosphorylation]
      Succinate (4C)
         ↓ [FADH₂ made]
      Fumarate (4C)
         ↓ [H₂O added]
       Malate (4C)
         ↓ [NADH made]
   Oxaloacetate (4C) ← regenerated, ready for next cycle

Net Products Per Turn (per acetyl-CoA)

Product	Amount
CO₂	2
ATP (or GTP)	1
NADH	3
FADH₂	1

Net Products Per Glucose (2 turns)

Product	Amount
CO₂	4
ATP	2
NADH	6
FADH₂	2

🧠 Memory Trick: "Can I Keep Selling Stuff For Money?" = Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate (cycle intermediates in order)

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Chapter 5: Oxidative Phosphorylation

Location

Inner mitochondrial membrane

This stage has two components: the Electron Transport Chain (ETC) and ATP Synthase (Chemiosmosis)

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5A: The Electron Transport Chain (ETC)

The ETC is a series of protein complexes (I–IV) embedded in the inner mitochondrial membrane.

NADH → Complex I → CoQ → Complex III → Cytochrome c → Complex IV → O₂ → H₂O
FADH₂ → Complex II → CoQ → Complex III → ...

The Players:

Complex	Name	Function
Complex I	NADH dehydrogenase	Accepts e⁻ from NADH; pumps H⁺
Complex II	Succinate dehydrogenase	Accepts e⁻ from FADH₂; does NOT pump H⁺
Complex III	Cytochrome bc₁ complex	Passes e⁻; pumps H⁺
Complex IV	Cytochrome c oxidase	Passes e⁻ to O₂; pumps H⁺
CoQ (Ubiquinone)	Mobile carrier	Shuttles e⁻ between complexes
Cytochrome c	Mobile carrier	Shuttles e⁻ between complexes III and IV

Critical Points:

• NADH enters at Complex I → drives pumping of more H⁺ → yields ~2.5 ATP
• FADH₂ enters at Complex II → bypasses Complex I → yields only ~1.5 ATP (fewer H⁺ pumped)
• O₂ is the final electron acceptor — without oxygen, the chain stops (aerobic requirement)
• Electrons flow from higher potential energy to lower potential energy (NADH → O₂)

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5B: Chemiosmosis and ATP Synthase

The Proton Gradient:

• As electrons move through the ETC, H⁺ ions are pumped from the matrix → intermembrane space
• This creates an electrochemical gradient (proton-motive force) across the inner membrane:
  • Intermembrane space: HIGH H⁺ concentration, positive charge
  • Matrix: LOW H⁺ concentration, negative charge

ATP Synthase:

• H⁺ ions flow DOWN their gradient back into the matrix through ATP synthase (Complex V)
• This flow drives the rotation of ATP synthase's rotor — it works like a molecular turbine
• Each rotation synthesizes ATP from ADP + Pᵢ
• Approximately 3 H⁺ must flow through ATP synthase to make 1 ATP

Peter Mitchell proposed this chemiosmotic hypothesis in 1961 and won the Nobel Prize in Chemistry in 1978.

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5C: Why Oxygen Matters

Without oxygen:

• Electrons cannot be passed to the final acceptor
• ETC backs up and stops
• NADH cannot be re-oxidized to NAD⁺
• Krebs cycle and pyruvate oxidation stop
• Cell must rely on fermentation

This is why organisms that need oxygen are called obligate aerobes.

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5D: Summary of ATP Yield

Electron Carrier	Number	ATP per carrier	ATP Subtotal
NADH (glycolysis)	2	~1.5–2.0*	~3–4
NADH (pyruvate ox.)	2	~2.5	~5
NADH (Krebs)	6	~2.5	~15
FADH₂ (Krebs)	2	~1.5	~3
Direct ATP (glycolysis + Krebs)	4	1	4
Total			~30–32

*NADH from glycolysis uses membrane shuttles that vary in efficiency (malate-aspartate vs. glycerol-3-phosphate shuttle)

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Chapter 6: Fermentation

When Does Fermentation Occur?

When oxygen is absent or limited (anaerobic conditions), cells cannot use the ETC. Fermentation allows glycolysis to continue by regenerating NAD⁺ from NADH.

⚠️ Fermentation does NOT produce additional ATP beyond what glycolysis provides (2 net ATP).

Types of Fermentation

1. Lactic Acid Fermentation

Pyruvate + NADH → Lactate + NAD⁺

• Occurs in: muscle cells during intense exercise, red blood cells, many bacteria
• Products: lactate (lactic acid)
• Used in: yogurt, cheese, sauerkraut production
• Lactate accumulation causes the "burn" in exercising muscles

2. Alcoholic Fermentation

Pyruvate → Acetaldehyde + CO₂ (via pyruvate decarboxylase)
Acetaldehyde + NADH → Ethanol + NAD⁺ (via alcohol dehydrogenase)

• Occurs in: yeast, some bacteria
• Products: ethanol + CO₂
• Used in: bread, beer, wine production

Comparing Fermentation vs. Aerobic Respiration

Feature	Fermentation	Aerobic Respiration
O₂ required	No	Yes
ATP yield	2	~30–32
Final e⁻ acceptor	Pyruvate/acetaldehyde	O₂
Glucose fully oxidized?	No	Yes
Products	Lactate or ethanol + CO₂	CO₂ + H₂O

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PART TWO: PHOTOSYNTHESIS

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Chapter 7: Overview & Foundational Concepts

What Is Photosynthesis?

Photosynthesis is the process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored in glucose. It is essentially the reverse of cellular respiration in terms of energy transformation.

Overall Equation:

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

Where Does Photosynthesis Occur?

Chloroplasts — double-membrane organelles found primarily in plant leaves

Key Structures:

Chloroplast Anatomy:
├── Outer membrane
├── Inner membrane
├── Intermembrane space
├── Stroma (fluid-filled space inside inner membrane)
│   └── Where Calvin Cycle occurs
└── Thylakoids (membranous sacs)
    ├── Organized into stacks called GRANA
    ├── Connected by LAMELLAE
    ├── Thylakoid membrane → where light reactions occur
    └── Thylakoid lumen (inside of thylakoid)

The Two Main Stages

Stage	Location	Inputs	Outputs
Light Reactions	Thylakoid membrane	Light, H₂O, ADP, NADP⁺	ATP, NADPH, O₂
Calvin Cycle	Stroma	CO₂, ATP, NADPH	G3P (glucose precursor)

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Chapter 8: Pigments and Light Absorption

Photosynthetic Pigments

Pigment	Color Absorbed	Color Reflected	Location
Chlorophyll a	Red (680–700 nm) & Blue (430–450 nm)	Green	Reaction center
Chlorophyll b	Red-orange & Blue	Yellow-green	Antenna complex
Carotenoids (β-carotene)	Blue-violet	Orange-yellow	Antenna complex
Xanthophylls	Blue-violet	Yellow	Antenna complex

Why are leaves green? Chlorophyll absorbs red and blue light but reflects green light — this reflected green is what we see.

Absorption vs. Action Spectrum

• Absorption spectrum: Graph showing which wavelengths a pigment absorbs
• Action spectrum: Graph showing the rate of photosynthesis at different wavelengths
• They closely match — confirming that absorbed light drives photosynthesis
• Engelmann's experiment (1883) demonstrated this using algae and aerobic bacteria

Photosystems

Photosystem II (P680) — absorption peak at 680 nm Photosystem I (P700) — absorption peak at 700 nm

Each photosystem contains:

• Antenna complex: Cluster of pigment molecules that absorb light and funnel energy to the reaction center
• Reaction center: Contains a special pair of chlorophyll a molecules where light energy is converted to chemical energy

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Chapter 9: The Light Reactions

Location

Thylakoid membrane

Overview of Electron Flow (Non-Cyclic Photophosphorylation)

H₂O → [PSII] → Plastoquinone (PQ) → Cytochrome b6f → Plastocyanin (PC) → [PSI] → Ferredoxin → NADP⁺ reductase → NADPH
         ↑                ↑
     O₂ released     H⁺ pumped
                   (builds gradient)

Step-by-Step Light Reactions

Step 1: PSII absorbs light

• Light excites electrons in P680 chlorophyll to a higher energy state
• These high-energy electrons are passed to the primary electron acceptor

Step 2: Water splitting (Photolysis)

• PSII is "oxidized" (lost electrons) — it replaces them by splitting water
• 2H₂O → 4H⁺ + 4e⁻ + O₂
• Oxygen is released as a byproduct → this is the source of all atmospheric O₂ from photosynthesis

Step 3: Electron Transport (PSII → PSI)

• Electrons flow through: Plastoquinone → Cytochrome b₆f complex → Plastocyanin → PSI
• As electrons move through cytochrome b₆f, H⁺ ions are pumped into the thylakoid lumen
• This builds the proton gradient used for ATP synthesis

Step 4: ATP synthesis (Photophosphorylation)

• H⁺ flows from lumen → stroma through ATP synthase (similar to mitochondria)
• Drives synthesis of ATP from ADP + Pᵢ

Step 5: PSI absorbs light

• PSI (P700) absorbs light, further energizing the electrons
• These electrons reduce NADP⁺ → NADPH via ferredoxin and NADP⁺ reductase
• NADPH will be used in the Calvin Cycle

Products of Light Reactions (per 2 H₂O split)

2 H₂O + 2 NADP⁺ + 3 ADP + 3 Pᵢ + Light →
O₂ + 2 NADPH + 3 ATP

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Cyclic Photophosphorylation

• Only PSI is involved
• Electrons from PSI flow back to the cytochrome b₆f complex instead of reducing NADP⁺
• Produces ATP but NOT NADPH and NOT O₂
• Occurs when: NADPH is in excess, the cell needs more ATP
• Provides additional ATP for the Calvin Cycle when needed

Comparing Mitochondria and Chloroplasts

Feature	Mitochondria	Chloroplast
Electron donor	NADH, FADH₂	H₂O (light-driven)
Electron acceptor	O₂	NADP⁺
Direction of H⁺ pumping	Matrix → Intermembrane space	Stroma → Thylakoid lumen
ATP made by	ATP synthase	ATP synthase
Energy source	Chemical (glucose)	Light
Purpose	Cellular work	Carbon fixation

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Chapter 10: The Calvin Cycle (Light-Independent Reactions)

Location

Stroma of the chloroplast

Overview

The Calvin Cycle uses the ATP and NADPH from the light reactions to convert CO₂ into G3P (glyceraldehyde-3-phosphate), which is used to build glucose and other organic compounds.

⚠️ "Dark reactions" is a misleading term — the Calvin Cycle does not require darkness; it simply does not use light directly. It runs day and night as long as ATP and NADPH are available.

The Three Stages

Stage 1: Carbon Fixation

• Enzyme: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)
  • Most abundant enzyme on Earth!
• CO₂ combines with RuBP (ribulose-1,5-bisphosphate), a 5-carbon molecule
• Product: unstable 6C compound that immediately splits into two 3-carbon molecules of 3-PGA (3-phosphoglycerate)

CO₂ + RuBP (5C) → [unstable 6C] → 2 × 3-PGA (3C each)

Stage 2: Reduction

• 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate)
• Requires: ATP and NADPH (from light reactions)
• This is the actual "making" of organic carbon

3-PGA + ATP + NADPH → G3P + ADP + NADP⁺ + Pᵢ

Stage 3: Regeneration of RuBP

• Most G3P (5 out of 6 produced) is used to regenerate RuBP
• Requires ATP
• The enzyme phosphoribulokinase converts RuMP to RuBP

5 G3P + 3 ATP → 3 RuBP

Accounting for One Turn of the Calvin Cycle

To fix 1 CO₂:

• 3 ATP used
• 2 NADPH used
• 1/6 G3P produced (net)

To make 1 molecule of G3P (net):

• 3 CO₂ fixed
• 9 ATP used
• 6 NADPH used

To synthesize 1 glucose molecule:

• 6 CO₂ fixed
• 18 ATP consumed
• 12 NADPH consumed
• 2 G3P → glucose

What Happens to G3P?

• ~⅙ exits the cycle → used to build glucose, amino acids, fatty acids
• ~⅚ remains → regenerates RuBP to continue the cycle

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Chapter 11: Photorespiration and Alternative Pathways

Photorespiration: The Problem with RuBisCO

RuBisCO has a "design flaw" — it can bind either CO₂ OR O₂ at its active site.

When O₂ binds (in hot, dry, high-light conditions):

• O₂ + RuBP → 3-PGA + 2-phosphoglycolate (a 2C waste product)
• This process is called photorespiration or the C2 cycle
• Does NOT produce ATP or NADPH
• Actually CONSUMES ATP and releases CO₂
• Drastically reduces photosynthetic efficiency

Why does this happen?

• In hot, dry conditions, plants close stomata to prevent water loss
• CO₂ depletes inside the leaf while O₂ builds up
• High O₂/CO₂ ratio favors the oxygenase reaction

Solutions: C4 and CAM Plants

C3 Plants (most common)

• First stable product of carbon fixation is 3-carbon 3-PGA
• Examples: wheat, rice, soybeans, most trees
• Vulnerable to photorespiration in hot climates

C4 Plants

Adaptation: Separate CO₂ fixation spatially

• Mesophyll cells: CO₂ + PEP → oxaloacetate (4C) → malate (4C) [enzyme: PEP carboxylase — very high affinity for CO₂, does NOT bind O₂]
• Bundle sheath cells: Malate breaks down → releases CO₂ → enters Calvin Cycle
• CO₂ is concentrated around RuBisCO → minimizes photorespiration
• More efficient in hot, high-light environments
• Examples: corn (maize), sugarcane, sorghum

C4 Pathway Summary:
Mesophyll: CO₂ → 4C compound
                 ↓ (transported to bundle sheath)
Bundle Sheath: 4C compound → CO₂ + 3C (returned to mesophyll)
                              ↓
                          Calvin Cycle

CAM Plants (Crassulacean Acid Metabolism)

Adaptation: Separate CO₂ fixation temporally

• Night: Stomata open → CO₂ is fixed into malate (stored in vacuoles)
• Day: Stomata close (prevents water loss) → Malate releases CO₂ for Calvin Cycle
• Most water-efficient photosynthetic strategy
• Examples: cacti, agaves, pineapples, jade plants

Comparison Table

Feature	C3	C4	CAM
CO₂ acceptor	RuBP	PEP	PEP
First stable product	3-PGA (3C)	Oxaloacetate (4C)	Oxaloacetate (4C)
Calvin Cycle cell	Mesophyll	Bundle sheath	Mesophyll (day)
Stomata open	Day	Day	Night only
Photorespiration	High when hot	Minimal	Minimal
Water efficiency	Low	Medium	High
Examples	Wheat, rice	Corn, sugarcane	Cacti, agave

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Chapter 12: Connecting Photosynthesis and Cellular Respiration

The Grand Connection

These two processes are metabolically linked — products of one are reactants of the other:

Photosynthesis:    6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂
                                              ↕
Cellular Resp.:    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Shared Molecules

• G3P from Calvin Cycle can become pyruvate → enter respiration
• Acetyl-CoA is the hub connecting carbohydrate, fat, and protein metabolism
• ATP/ADP cycle connects energy production and consumption
• NADH/NAD⁺ and NADPH/NADP⁺ are related but functionally distinct (NADPH powers biosynthesis; NADH powers ATP production)

Metabolic Flexibility

• Plants perform BOTH photosynthesis and cellular respiration simultaneously
• At the light compensation point, photosynthesis rate = respiration rate (net CO₂ exchange = 0)
• Above this point, net photosynthesis stores energy
• Below this point, plants are net consumers of stored energy

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PART THREE: PRACTICE QUESTIONS WITH DETAILED EXPLANATIONS

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Section A: Multiple Choice Questions

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Question 1

Which of the following correctly identifies the location of the electron transport chain in eukaryotic cells?

A) Cytoplasm
B) Outer mitochondrial membrane
C) Inner mitochondrial membrane
D) Mitochondrial matrix

✅ Answer: C) Inner mitochondrial membrane

Detailed Explanation: The electron transport chain (ETC) is embedded in the inner mitochondrial membrane. This location is essential for its function because:

1. The protein complexes (I–IV) need to be in a membrane to pump H⁺ ions across it
2. The inner membrane creates two distinct compartments — the matrix (inside) and intermembrane space (outside) — separated by the membrane
3. The impermeability of the inner membrane to H⁺ is critical — it prevents protons from flowing back except through ATP synthase
4. This compartmentalization is what allows the proton gradient (proton-motive force) to build up and drive ATP synthesis

Why the others are wrong:

• A (Cytoplasm): Glycolysis occurs in the cytoplasm, not the ETC
• B (Outer mitochondrial membrane): The outer membrane is relatively permeable (contains porins) and has no role in the ETC
• D (Mitochondrial matrix): The Krebs cycle and pyruvate oxidation occur in the matrix. While the matrix face of the ETC faces the matrix, the chain itself is in the inner membrane

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Question 2

During vigorous exercise, muscle cells switch from aerobic respiration to lactic acid fermentation. Which of the following BEST explains why this switch occurs?

A) Glucose becomes depleted and pyruvate must serve as an alternative fuel
B) Oxygen becomes limiting, preventing the electron transport chain from functioning
C) The pH drops, inhibiting the enzymes of the Krebs cycle
D) NADH accumulates to levels that directly inhibit glycolysis

✅ Answer: B) Oxygen becomes limiting, preventing the electron transport chain from functioning

Detailed Explanation: This question tests understanding of WHY fermentation occurs and what purpose it serves.

During intense exercise:

1. Muscles consume O₂ faster than the circulatory system can deliver it
2. Without O₂, electrons cannot be passed to the final electron acceptor in Complex IV
3. The ETC stalls → proton pumping stops → no more ATP from oxidative phosphorylation
4. NADH produced by glycolysis can no longer be re-oxidized to NAD⁺ by the ETC
5. Without NAD⁺, glycolysis itself would halt (Step 6 of glycolysis requires NAD⁺)
6. Solution: Lactic acid fermentation uses pyruvate as the electron acceptor instead, regenerating NAD⁺ and allowing glycolysis to continue producing 2 ATP per glucose

The key insight: Fermentation's PRIMARY purpose is to regenerate NAD⁺, not to produce additional ATP.

Why the others are wrong:

• A: Glucose is not the limiting factor here — oxygen is. Fermentation uses pyruvate as an electron acceptor, not as fuel
• C: While pH does drop during intense exercise, this is an effect of lactate accumulation, not the trigger for switching to fermentation
• D: NADH accumulation IS related to the problem, but the reason NADH accumulates is because the ETC (which requires O₂) can't process it. Answer B identifies the root cause

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Question 3

A student adds a drug that makes the inner mitochondrial membrane freely permeable to H⁺ ions. Which of the following predicts the most likely outcome?

A) ATP production increases because H⁺ can flow more freely
B) ATP production decreases, but cellular respiration still continues
C) Both the Krebs cycle and glycolysis immediately stop
D) Oxygen consumption decreases because less energy is needed

✅ Answer: B) ATP production decreases, but cellular respiration still continues

Detailed Explanation: This question is a classic "proton uncoupler" scenario. The drug described is similar to 2,4-dinitrophenol (DNP), a known uncoupling agent (once disastrously used as a weight-loss drug).

What happens:

1. H⁺ ions leak back into the matrix without passing through ATP synthase
2. The proton gradient collapses — no gradient means no driving force for ATP synthase
3. ATP synthesis via oxidative phosphorylation dramatically decreases
4. However, the ETC can still function — electrons can still flow and O₂ is still reduced to H₂O
5. In fact, the ETC may run FASTER because the proton gradient (which would normally slow it down through "back pressure") is eliminated
6. Energy is released as heat instead of being captured as ATP
7. Glycolysis and the Krebs cycle can also continue (they don't directly require the proton gradient)

Real-world connection: DNP was used as a "diet pill" in the 1930s — it caused rapid weight loss because metabolic energy was being wasted as heat, but it also caused fatal hyperthermia. Brown adipose tissue (in infants and hibernating animals) uses a controlled version of this with thermogenin (UCP1) to generate body heat.

Why the others are wrong:

• A: Making the membrane permeable collapses the gradient → LESS ATP, not more
• C: The Krebs cycle (in the matrix) and glycolysis (in the cytoplasm) are not directly stopped by a leaky inner membrane. They lack the proton gradient component
• D: O₂ consumption would likely INCREASE because the ETC runs faster

AP Biology Study Guide: Photosynthesis and Cellular Respiration

Overview and Equations

Photosynthesis and cellular respiration are interconnected processes that cycle carbon, oxygen, and energy in ecosystems. Photosynthesis converts light energy into chemical energy stored in glucose: $6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$[1][2][5]. Cellular respiration releases this energy: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$[1][2][3][5]. Autotrophs (e.g., plants) perform photosynthesis; all organisms, including plants, perform respiration via mitochondria[1][2][5].

Photosynthesis

Occurs in chloroplasts of plant cells. Divided into light-dependent reactions (thylakoid membranes) and light-independent reactions (Calvin cycle in stroma)[1][2][6].

Light-Dependent Reactions

• Light excites electrons in photosystems II and I (chlorophyll pigments).
• Electrons pass through an electron transport chain (ETC), pumping H⁺ into thylakoid lumen, creating a proton gradient.
• Protons flow through ATP synthase (chemiosmosis), producing ATP. Water splits (photolysis), releasing O₂ and providing electrons[1][2].
• Products: ATP, NADPH (electron carrier)[1][2].

Calvin Cycle (Light-Independent)

• RuBP (5-carbon sugar) + CO₂ → unstable 6-carbon intermediate → 6 G3P (3-carbon sugars) via Rubisco enzyme[1].
• ATP and NADPH reduce G3P; one G3P exits to form glucose, others regenerate RuBP[1][2].
• Net: Fixes 6 CO₂ into glucose[1][5].

Photorespiration (inefficiency): Rubisco binds O₂ instead of CO₂, wasting energy[8].

Cellular Respiration

Occurs in mitochondria (eukaryotes). Breaks glucose into ATP (~30-38 ATP net). Aerobic process; anaerobic alternatives: fermentation[1][3].

1. Glycolysis (Cytoplasm)

• Glucose → 2 pyruvate + 2 ATP (net) + 2 NADH.
• Anaerobic; no O₂ needed[1][3][6].

2. Pyruvate Oxidation (Mitochondrial Matrix)

• Pyruvate → acetyl CoA + CO₂ + NADH[1].

3. Krebs Cycle (Citric Acid Cycle, Matrix)

• Acetyl CoA → CO₂ + 2 ATP + NADH/FADH₂ (electron carriers)[1][3].

4. Oxidative Phosphorylation (Inner Mitochondrial Membrane)

• NADH/FADH₂ donate electrons to ETC, pumping H⁺ into intermembrane space.
• Proton gradient drives ATP synthase (chemiosmosis): ~32-34 ATP[1][3].

Key Comparisons

Feature	Photosynthesis	Cellular Respiration
Location	Chloroplast (thylakoid/stroma)	Cytoplasm/mitochondria
Reactants	CO₂, H₂O, light	Glucose, O₂
Products	Glucose, O₂	CO₂, H₂O, ATP
ETC Role	Produces ATP/NADPH	Produces ATP
Membrane Process	Thylakoid (H⁺ inside)	Inner mito. (H⁺ outside matrix)[1][2][3]

Plants do both: chloroplasts for photosynthesis, mitochondria for respiration[5].

Practice Questions

Multiple-choice and short-answer styled for AP Biology. Answers with explanations follow.

Multiple-Choice

1. In the light-dependent reactions, protons accumulate in the
   a) stroma
   b) thylakoid lumen
   c) cytoplasm
   d) mitochondrial matrix

2. Glycolysis produces a net yield of
   a) 2 ATP, 2 NADH
   b) 36 ATP
   c) 2 pyruvate only
   d) acetyl CoA

3. The Calvin cycle enzyme Rubisco initially binds
   a) O₂ to produce ATP
   b) RuBP and CO₂
   c) glucose and NADPH
   d) pyruvate

4. In cellular respiration, most ATP forms via
   a) glycolysis
   b) Krebs cycle
   c) chemiosmosis
   d) pyruvate oxidation

5. Photorespiration occurs when Rubisco binds
   a) CO₂ efficiently
   b) O₂ instead of CO₂
   c) H₂O
   d) ATP

Short-Answer

6. Explain how chemiosmosis links to ATP production in both processes.
7. Why do plants respire despite photosynthesizing?
8. Compare electron carriers: NADPH vs. NADH.

Answers and Explanations

1. b) thylakoid lumen. Electrons in ETC pump H⁺ into thylakoid space, creating gradient for ATP synthase[1][2].

2. a) 2 ATP, 2 NADH. Glucose splits to 2 pyruvate in cytoplasm; invests 2 ATP, gains 4 (net 2), produces 2 NADH[1][3].

3. b) RuBP and CO₂. Forms unstable 6C compound splitting to G3P; ATP/NADPH regenerate RuBP[1].

4. c) chemiosmosis. ETC protons flow through ATP synthase in oxidative phosphorylation (~32 ATP)[1][3].

5. b) O₂ instead of CO₂. Releases CO₂ without sugar gain, reducing efficiency[8].

6. In photosynthesis, H⁺ gradient across thylakoid drives ATP synthase (ATP/NADPH). In respiration, across mitochondrial inner membrane drives ATP (~34 from ETC)[1][3]. Both use proton motive force.

7. Plants store energy as glucose in photosynthesis but need ATP for growth/metabolism; respiration breaks it down in mitochondria[5].

8. NADPH (photosynthesis): carries H⁺/e⁻ from light reactions to Calvin cycle for reduction[1][2]. NADH (respiration): from glycolysis/Krebs to ETC for oxidative phosphorylation[1][3]. Both e⁻ carriers but NADPH has extra phosphate.[1]