AP Biology Study Guide: Photosynthesis and Cellular Respiration

Photosynthesis Overview

Photosynthesis converts light energy into chemical energy, producing glucose and oxygen from carbon dioxide, water, and light.[1][2] The overall equation is $6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$.[3] It occurs in chloroplasts, with light-dependent reactions in thylakoid membranes producing ATP, NADPH, and O2, and the light-independent Calvin cycle in the stroma using those to fix CO2 into glucose.[1][2]

Light-Dependent Reactions

• Occur in thylakoid membranes containing chlorophyll pigments that absorb light.[1]
• Photosystem II (PSII): Light excites electrons; photolysis splits water (H2O → 2H+ + 1/2 O2 + 2e-), providing electrons and protons.[1][2]
• Electrons move through electron transport chain (ETC), pumping H+ into thylakoid lumen, creating a proton gradient.[1]
• Photosystem I (PSI): Electrons re-excited, reduce NADP+ to NADPH via NADP+ reductase.[1][2]
• Chemiosmosis: H+ flows through ATP synthase, driving photophosphorylation (ADP + Pi → ATP).[1]
• Linear flow: Produces ATP, NADPH, O2; cyclic flow: Produces only ATP (electrons cycle around PSI).[1]

Calvin Cycle (Light-Independent Reactions)

• Fixes 3 CO2 per turn using RuBP (5-carbon sugar) catalyzed by Rubisco, forming unstable 6-carbon intermediate that splits into 6 G3P (3-carbon).[2]
• 6 G3P → glucose; uses 9 ATP and 6 NADPH.[1][2]
• Regeneration: 5 G3P reform 3 RuBP using ATP.[2]

Key Structures: Chloroplasts (thylakoids for light reactions, stroma for Calvin cycle); chlorophyll in thylakoid membranes enables proton gradient.[1]

Cellular Respiration Overview

Cellular respiration converts glucose into ATP, CO2, and H2O, primarily aerobically.[1][2][3] Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$ (reverse of photosynthesis).[3] Occurs in mitochondria (except glycolysis in cytoplasm); yields ~30-32 ATP per glucose.[2] Stages: glycolysis, pyruvate oxidation/Krebs cycle, oxidative phosphorylation (ETC + chemiosmosis).[2][4]

Glycolysis

• Cytoplasm; anaerobic; glucose → 2 pyruvate + 2 ATP (net) + 2 NADH.[2]
• 10-step linear pathway.[2]

Pyruvate Oxidation and Krebs Cycle (Citric Acid Cycle)

• Pyruvate → acetyl CoA (matrix); releases CO2, produces NADH.[2]
• Krebs: Acetyl CoA + oxaloacetate → citrate (6C) → regenerates oxaloacetate; per glucose: 2 ATP, 6 NADH, 2 FADH2, 4 CO2.[2][4]

Oxidative Phosphorylation

• ETC (inner mitochondrial membrane): NADH/FADH2 donate electrons; pumps H+ into intermembrane space.[2]
• Chemiosmosis: H+ gradient drives ATP synthase (~26-28 ATP).[2][4]
• O2 final electron acceptor (H2O formed).[2]

Fermentation (anaerobic): Regenerates NAD+ from glycolysis; lactic acid (animals) or ethanol (yeast); 2 ATP total.[1][4]

Comparison Table: Photosynthesis vs. Cellular Respiration

Feature	Photosynthesis	Cellular Respiration
Location	Chloroplasts (thylakoids/stroma) [1]	Cytoplasm/mitochondria [2]
Reactants	CO2, H2O, light [1][3]	Glucose, O2 [1][3]
Products	Glucose, O2 [1][3]	CO2, H2O, ATP [1][3]
ETC Electron Donor/Acceptor	H2O/O2 (different from respiration) [1]	NADH/FADH2 to O2 [2]
ATP Production	Photophosphorylation [1]	Substrate-level + oxidative [2][4]
Organisms	Autotrophs [3]	Heterotrophs/autotrophs [3]

Practice Questions

Multiple Choice

1. In the light-dependent reactions of photosynthesis, the proton gradient is established across which membrane?
   A) Plasma membrane
   B) Inner mitochondrial membrane
   C) Thylakoid membrane
   D) Nuclear membrane

   Answer: C. The ETC in the thylakoid membrane pumps H+ into the lumen, creating the gradient for chemiosmosis and ATP synthesis.[1][2]

2. Which process produces the most ATP in cellular respiration?
   A) Glycolysis
   B) Krebs cycle
   C) Electron transport chain/chemiosmosis
   D) Pyruvate oxidation

   Answer: C. Oxidative phosphorylation via ETC and chemiosmosis yields ~26-28 ATP per glucose, far more than glycolysis (2 ATP) or Krebs (2 ATP).[2][4]

3. What is the role of photolysis in photosynthesis?
   A) Fix CO2 into RuBP
   B) Split water to provide electrons for PSII
   C) Regenerate NAD+
   D) Pump H+ in stroma

   Answer: B. Photolysis at PSII (H2O → 2H+ + 1/2 O2 + 2e-) replaces electrons excited from chlorophyll, enabling linear electron flow; without it, no ATP/NADPH.[1]

Short Answer

4. Compare linear and cyclic electron flow in photosynthesis.

   Answer: Linear flow (PSII → ETC → PSI → NADPH) produces ATP, NADPH, and O2 via photolysis; used for Calvin cycle. Cyclic flow (PSI → ETC → PSI) produces only ATP, no NADPH/O2; balances ATP/NADPH when excess NADPH present.[1]

5. Explain why fermentation is less efficient than aerobic respiration.

   Answer: Fermentation (e.g., glycolysis + lactic acid/ethanol) yields only 2 ATP/glucose, regenerating NAD+ anaerobically without ETC/oxidative phosphorylation (~30 ATP). No O2 means no final electron acceptor, halting ETC.[1][2][4]

Diagram-Based (Conceptual)

6. If a poison blocks ATP synthase in mitochondria, predict the effect on cellular respiration stages.

   Answer: Krebs cycle/ETC would slow/stop as H+ gradient builds up (no outflow), halting proton pumping/electron flow; NADH/FADH2 accumulate, backing up glycolysis/Krebs. Net ATP drops drastically despite glycolysis continuing initially.[2] (Similar to photosynthesis thylakoids.[1])

AP Biology Study Guide: Cellular Energetics (Unit 3)

Topic: Photosynthesis & Cellular Respiration

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I. Introduction: The Flow of Energy

Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

• The Core Concept: Life requires energy. The sun provides the initial energy input, which is converted into chemical energy (glucose) via photosynthesis. This chemical energy is then harvested to create usable cellular energy (ATP) via cellular respiration.
• Key Thermodynamic Principle: Energy cannot be created or destroyed, only transformed. These processes involve redox reactions (transfer of electrons).
  • Oxidation: Loss of electrons (often associated with loss of H+).
  • Reduction: Gain of electrons (often associated with gain of H+).
  • Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

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II. Photosynthesis

Goal: Convert light energy into chemical energy stored in glucose. Location: Chloroplasts (in plants and algae). Equation: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$

A. Light-Dependent Reactions

• Location: Thylakoid Membranes.
• Input: Light, $H_2O$, $NADP^+$, $ADP$.
• Output: $O_2$, $NADPH$, $ATP$.
• Key Steps:
  1. Photosystem II (PSII): Photons hit pigment molecules; energy excites electrons. Water is split (photolysis) to replace these electrons, releasing $O_2$ and $H^+$.
  2. Electron Transport Chain (ETC): Excited electrons move down the chain, losing energy. This energy pumps $H^+$ from the stroma into the thylakoid space.
  3. Photosystem I (PSI): Electrons are re-energized by light and passed to $NADP^+$ reductase, creating NADPH.
  4. Chemiosmosis: The high concentration of $H^+$ in the thylakoid space flows back into the stroma through ATP Synthase, generating ATP (Photophosphorylation).

B. The Calvin Cycle (Light-Independent Reactions)

• Location: Stroma.
• Input: $CO_2$, $ATP$, $NADPH$.
• Output: G3P (sugar precursor), $ADP$, $NADP^+$.
• Key Steps:
  1. Carbon Fixation: Enzyme RuBisCO attaches $CO_2$ to RuBP (5-carbon molecule).
  2. Reduction: ATP and NADPH are used to convert the intermediate into G3P.
  3. Regeneration: Some G3P exits to make glucose; most is used to regenerate RuBP so the cycle can continue.

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III. Cellular Respiration

Goal: Break down glucose to produce ATP. Location: Cytoplasm (Glycolysis) and Mitochondria (Krebs & ETC). Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP + Heat)}$

A. Glycolysis

• Location: Cytoplasm.
• Process: Glucose (6C) is split into two Pyruvate (3C).
• Yield: Net 2 ATP (substrate-level phosphorylation), 2 NADH.
• Oxygen: Not required (Anaerobic).

B. Pyruvate Oxidation

• Location: Mitochondrial Matrix.
• Process: Pyruvate enters mitochondrion, loses a carbon (released as $CO_2$), and becomes Acetyl-CoA. Produces 1 NADH per pyruvate.

C. Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial Matrix.
• Process: Acetyl-CoA combines with oxaloacetate. Carbon atoms are fully oxidized to $CO_2$.
• Yield (per glucose): 2 ATP, 6 NADH, 2 $FADH_2$.

D. Oxidative Phosphorylation (ETC & Chemiosmosis)

• Location: Inner Mitochondrial Membrane (Cristae).
• Process:
  1. ETC: NADH and $FADH_2$ donate electrons. Energy from electron flow pumps $H^+$ into the intermembrane space.
  2. Final Electron Acceptor: Oxygen accepts electrons and $H^+$ to form water ($H_2O$).
  3. Chemiosmosis: $H^+$ flows back into the matrix through ATP Synthase, producing ~32-34 ATP.

E. Fermentation (Anaerobic)

• Occurs if no Oxygen is present.
• Goal: Regenerate $NAD^+$ so Glycolysis can continue.
• Types: Lactic Acid Fermentation (muscles/bacteria) or Alcohol Fermentation (yeast).

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IV. Critical Connections (AP Focus)

1. Chemiosmosis: Both processes use an electron transport chain to create a proton ($H^+$) gradient to drive ATP Synthase.
   • Photosynthesis: $H^+$ pumped into thylakoid space.
   • Respiration: $H^+$ pumped into intermembrane space.
2. Electron Carriers:
   • Photosynthesis uses NADP+/NADPH.
   • Respiration uses NAD+/NADH and FAD/$FADH_2$.
3. Evolution: Mitochondria and Chloroplasts likely originated from free-living prokaryotes (Endosymbiotic Theory), evidenced by their own DNA, ribosomes, and double membranes.

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V. Practice Questions

Multiple Choice Questions

1. During the light-dependent reactions of photosynthesis, the synthesis of ATP is directly driven by: A) The absorption of photons by chlorophyll. B) The flow of electrons through Photosystem I. C) The movement of protons ($H^+$) down their electrochemical gradient through ATP synthase. D) The splitting of water molecules by the oxygen-evolving complex.

2. Cyanide is a poison that binds to Complex IV of the mitochondrial electron transport chain, preventing the transfer of electrons to oxygen. What is the immediate effect of cyanide poisoning on cellular respiration? A) Glycolysis will stop immediately. B) The proton gradient across the inner mitochondrial membrane will dissipate, halting ATP synthesis. C) The Krebs cycle will speed up to compensate for the lack of ATP. D) Carbon dioxide production will increase significantly.

3. Which of the following best describes the role of NADH and $FADH_2$ in cellular respiration? A) They catalyze the breakdown of glucose during glycolysis. B) They act as final electron acceptors in the absence of oxygen. C) They transport high-energy electrons to the electron transport chain. D) They are converted directly into ATP within the mitochondrial matrix.

4. A plant is placed in a chamber with radioactive carbon dioxide ($^{14}CO_2$). After exposure to light, where will the radioactive carbon first appear? A) In ATP molecules within the thylakoid. B) In G3P (glyceraldehyde-3-phosphate) within the stroma. C) In water molecules within the vascular tissue. D) In Oxygen gas released from the stomata.

5. Which statement accurately compares the location of the electron transport chains in photosynthesis and cellular respiration? A) Both occur in the inner membrane of the mitochondria. B) Photosynthesis occurs in the thylakoid membrane; respiration occurs in the inner mitochondrial membrane. C) Photosynthesis occurs in the stroma; respiration occurs in the matrix. D) Both occur in the cytoplasm of the cell.

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Short Free Response Question (FRQ)

6. Experimental Analysis: Scientists are studying the rate of photosynthesis in aquatic plants. They measure the rate by counting oxygen bubbles produced per minute. They vary the light intensity while keeping temperature and $CO_2$ constant.

• Part A: Predict the relationship between light intensity and oxygen production.
• Part B: Explain the biological mechanism behind your prediction using the light-dependent reactions.
• Part C: If the light intensity continues to increase indefinitely, will the oxygen production continue to increase indefinitely? Justify your answer.

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VI. Answer Key & Detailed Explanations

Multiple Choice Explanations

1. Correct Answer: C

• Explanation: This question tests understanding of chemiosmosis. While light absorption (A) initiates the process, and electron flow (B) builds the gradient, the direct driver of ATP synthase is the flow of protons ($H^+$) down their gradient. Water splitting (D) provides electrons and protons but does not directly spin ATP synthase.
• AP Tip: Distinguish between the source of energy (light) and the mechanism of phosphorylation (proton gradient).

2. Correct Answer: B

• Explanation: If Complex IV is blocked, electrons cannot be passed to Oxygen. The ETC backs up, and proton pumping stops. Without active pumping, the existing proton gradient dissipates (leaks back), and ATP Synthase stops working. Glycolysis (A) can continue temporarily (via fermentation). The Krebs cycle (C) will stop, not speed up, because NADH cannot be oxidized back to $NAD^+$. $CO_2$ production (D) decreases because the Krebs cycle halts.
• AP Tip: Understand the dependency of the Krebs cycle on the ETC to recycle electron carriers ($NAD^+$).

3. Correct Answer: C

• Explanation: NADH and $FADH_2$ are oxidized electron carriers. Their primary function is to drop off high-energy electrons at the ETC (Complex I and II, respectively). They do not catalyze reactions (A), they are not final acceptors (Oxygen is) (B), and they are not converted into ATP; their energy is used to make ATP (D).
• AP Tip: Remember that "reduced" carriers (NADH) hold energy; "oxidized" carriers ($NAD^+$) are empty containers ready to pick up electrons.

4. Correct Answer: B

• Explanation: This tests the Calvin Cycle. $CO_2$ is fixed during the light-independent reactions in the stroma. RuBisCO attaches $CO_2$ to RuBP, eventually forming G3P. Carbon does not go into ATP (A) or Water (C). Oxygen (D) comes from the splitting of water, not $CO_2$.
• AP Tip: A classic AP trap is thinking the $O_2$ released comes from $CO_2$. It comes from $H_2O$ (proven by isotope tracing experiments).

5. Correct Answer: B

• Explanation: This tests cellular organelle structure. Photosynthesis ETC is embedded in the thylakoid membrane of chloroplasts. Respiration ETC is embedded in the inner mitochondrial membrane. Stroma (C) and Matrix (C) are the fluid-filled spaces where the Calvin Cycle and Krebs Cycle occur, respectively.
• AP Tip: Memorize the specific sub-compartments of mitochondria and chloroplasts.

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FRQ Scoring Guidelines & Explanation

6. Experimental Analysis

• Part A (1 Point):

  • Prediction: As light intensity increases, oxygen production will increase initially.
  • Reasoning: More photons allow more excitation of electrons in the photosystems.

• Part B (1 Point):

  • Explanation: In the light-dependent reactions, light energy splits water (photolysis), releasing oxygen as a byproduct. Higher light intensity provides more energy to split more water molecules per unit of time, increasing the rate of electron flow and consequently oxygen release.

• Part C (1 Point):

  • Answer: No, it will not increase indefinitely.
  • Justification: The rate will plateau (level off). This is due to limiting factors. Eventually, the enzymes (like RuBisCO) become saturated, or another factor (like $CO_2$ concentration or temperature) becomes the limiting reagent. Additionally, too much light can damage chlorophyll (photoinhibition).

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VII. AP Exam Study Tips for Energetics

1. Master the Diagrams: Be able to label a chloroplast and mitochondrion and draw where protons accumulate vs. where they flow.
2. Understand "Uncouplers": AP loves questions about chemicals (like DNP) that make membranes leaky to protons. If protons leak, the gradient is lost, ATP production stops, but the ETC keeps running (producing heat).
3. Know the Inputs/Outputs: Don't just memorize the summary equation. Know what enters/exits each stage (e.g., What goes into Krebs? What comes out?).
4. Connect to Evolution: Be ready to explain how the similarities in ETC and ATP Synthase support the Endosymbiotic Theory.
5. Math & Graphs: Be prepared to calculate $Q_{10}$ (temperature coefficient) or analyze a graph where a variable (like light wavelength) is changed. Remember: Green light is least effective for photosynthesis (it is reflected).

VIII. Recommended Review Activities

• Draw the Cycle: Draw the Krebs cycle and Calvin cycle from memory, labeling carbon counts and energy carriers.
• Compare/Contrast Table: Create a table comparing Photosynthesis and Respiration (Location, Electron Source, Final Acceptor, Energy Transformation).
• Flashcards: Create cards for key enzymes (RuBisCO, ATP Synthase, PFK) and their specific roles.