Cellular Energetics

What you'll learn

Cellular energetics examines how cells capture, store, transfer and utilize energy to maintain life processes. This topic is central to AP Biology and connects photosynthesis, cellular respiration, and the laws of thermodynamics. You'll understand how ATP serves as the universal energy currency and how organisms convert light energy and chemical bonds into usable forms.

Key terms and definitions

ATP (Adenosine triphosphate) — The primary energy currency of cells, composed of adenine, ribose and three phosphate groups; hydrolysis of ATP releases approximately 30.5 kJ/mol under standard conditions.

Chemiosmosis — The process by which ATP is synthesized using the energy from a proton (H⁺) gradient across a membrane, coupling exergonic flow of protons to endergonic ATP synthesis.

Photophosphorylation — ATP synthesis in chloroplasts driven by light energy during photosynthesis, occurring in both cyclic and non-cyclic pathways.

Substrate-level phosphorylation — Direct transfer of a phosphate group from a substrate molecule to ADP to form ATP, occurring in glycolysis and the Krebs cycle.

Oxidative phosphorylation — ATP synthesis coupled to electron transport and chemiosmosis in mitochondria, producing the majority of ATP during aerobic respiration.

NAD⁺/NADH and FAD/FADH₂ — Electron carriers that shuttle high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain; reduced forms (NADH, FADH₂) carry electrons.

Autotroph — An organism that produces organic compounds from inorganic molecules using light energy (photoautotroph) or chemical energy (chemoautotroph).

Heterotroph — An organism that obtains organic molecules by consuming other organisms or their products.

Core concepts

Energy and metabolism fundamentals

The first and second laws of thermodynamics govern all cellular energy transformations. Energy cannot be created or destroyed, but every energy transfer increases the entropy (disorder) of the universe. Cells perform work through coupled reactions—exergonic reactions (ΔG < 0) release energy that drives endergonic reactions (ΔG > 0).

ATP hydrolysis is exergonic (ΔG°' = -30.5 kJ/mol) and powers most cellular work:

ATP + H₂O → ADP + Pᵢ + energy

Cells maintain ATP concentrations far from equilibrium. The actual free energy change (ΔG) in cells is typically -50 to -65 kJ/mol, making ATP hydrolysis highly favorable under physiological conditions.

Metabolic pathways are enzyme-catalyzed reaction sequences. Catabolic pathways break down complex molecules and release energy (e.g., cellular respiration). Anabolic pathways synthesize complex molecules and require energy input (e.g., photosynthesis).

Photosynthesis: capturing light energy

Photosynthesis converts light energy into chemical energy stored in glucose. The overall equation is:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Light-dependent reactions occur in thylakoid membranes:

• Photosystem II (P680) absorbs light, exciting electrons to a higher energy state
• Water photolysis (photolysis) splits H₂O, releasing O₂, H⁺ ions, and electrons that replace those lost from P680
• Electrons pass through the electron transport chain between PSII and PSI, pumping H⁺ from the stroma into the thylakoid lumen
• Photosystem I (P700) re-energizes electrons, which reduce NADP⁺ to NADPH
• Chemiosmosis occurs as H⁺ flows through ATP synthase from the lumen to the stroma, generating ATP (photophosphorylation)

Non-cyclic photophosphorylation produces both ATP and NADPH. Cyclic photophosphorylation involves only PSI and produces additional ATP without NADPH or O₂.

Light-independent reactions (Calvin cycle) occur in the stroma:

1. Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP, a 5-carbon sugar) catalyzed by rubisco, forming two 3-carbon molecules of 3-phosphoglycerate (3-PG)
2. Reduction: ATP and NADPH from light reactions reduce 3-PG to glyceraldehyde-3-phosphate (G3P)
3. Regeneration: Most G3P regenerates RuBP using ATP; one G3P molecule exits every three turns to synthesize glucose

Three CO₂ molecules fixed require 9 ATP and 6 NADPH to produce one G3P (net). Six turns of the cycle produce one glucose molecule.

Factors affecting photosynthesis rate:

• Light intensity (limiting at low levels)
• CO₂ concentration (limiting below ~0.4%)
• Temperature (affects enzyme activity)
• Water availability

Cellular respiration: releasing chemical energy

Cellular respiration extracts energy from glucose in four stages:

Glycolysis (cytoplasm):

• Glucose (6C) → 2 pyruvate (3C)
• Net products: 2 ATP (substrate-level phosphorylation), 2 NADH
• Occurs with or without oxygen (ancient metabolic pathway)
• Investment phase requires 2 ATP; payoff phase produces 4 ATP

Pyruvate oxidation (mitochondrial matrix):

• 2 pyruvate → 2 acetyl-CoA + 2 CO₂
• Products: 2 NADH
• Links glycolysis to the Krebs cycle

Krebs cycle (citric acid cycle/TCA cycle) (mitochondrial matrix):

• 2 acetyl-CoA (2C each) combine with oxaloacetate (4C) to form citrate (6C)
• Through decarboxylation and oxidation, regenerates oxaloacetate
• Products per glucose: 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂
• Generates reduced electron carriers for the ETC

Oxidative phosphorylation (inner mitochondrial membrane):

The electron transport chain comprises four protein complexes (I, II, III, IV):

• NADH donates electrons to Complex I; FADH₂ donates to Complex II
• Electrons transfer through the chain to progressively more electronegative carriers
• Energy released pumps H⁺ from the matrix to the intermembrane space (Complexes I, III, IV)
• Oxygen serves as the final electron acceptor, forming water

Chemiosmosis:

• H⁺ gradient creates proton-motive force (electrochemical gradient)
• ATP synthase allows H⁺ to flow back to the matrix, driving ATP synthesis
• Approximately 2.5 ATP per NADH; 1.5 ATP per FADH₂

Theoretical maximum ATP yield per glucose: ~32 ATP (accounting for transport costs)

• Glycolysis: 2 ATP + 2 NADH (→ 3-5 ATP)
• Pyruvate oxidation: 2 NADH (→ 5 ATP)
• Krebs cycle: 2 ATP + 6 NADH (→ 15 ATP) + 2 FADH₂ (→ 3 ATP)
• Total: ~30-32 ATP (actual yields vary)

Fermentation: anaerobic energy release

When oxygen is absent or limited, pyruvate does not enter mitochondria. Fermentation regenerates NAD⁺ from NADH, allowing glycolysis to continue producing 2 ATP per glucose.

Lactic acid fermentation (animals, some bacteria):

• Pyruvate + NADH → lactate + NAD⁺
• Occurs in muscle cells during intense exercise
• Lactate buildup causes muscle fatigue

Alcoholic fermentation (yeast, some plants):

• Pyruvate → acetaldehyde + CO₂
• Acetaldehyde + NADH → ethanol + NAD⁺
• Used in brewing and bread-making

Fermentation is far less efficient than aerobic respiration (2 ATP vs. ~32 ATP per glucose).

Metabolic diversity and alternative pathways

C₄ photosynthesis:

• Adaptation to hot, dry environments (e.g., maize, sugarcane)
• CO₂ initially fixed in mesophyll cells as 4-carbon oxaloacetate by PEP carboxylase
• Oxaloacetate converted to malate, transported to bundle-sheath cells
• CO₂ released near rubisco, concentrating CO₂ and minimizing photorespiration
• Spatial separation of initial fixation and Calvin cycle

CAM (Crassulacean Acid Metabolism) photosynthesis:

• Adaptation to arid conditions (e.g., cacti, pineapple)
• Temporal separation: stomata open at night to fix CO₂ as malate
• During day, stomata close (conserving water) and malate releases CO₂ for Calvin cycle
• Reduces water loss but slower growth rates

Photorespiration:

• Occurs when rubisco fixes O₂ instead of CO₂ (rubisco's dual specificity)
• Produces 2-phosphoglycolate, which cannot enter Calvin cycle
• Wasteful process consuming ATP without sugar synthesis
• More prevalent in hot, dry conditions when stomata close

Organisms metabolize other macromolecules when glucose is unavailable:

• Proteins: deaminated to amino acids, carbon skeletons enter at various points in glycolysis/Krebs cycle
• Lipids: beta-oxidation breaks fatty acids into acetyl-CoA units; glycerol enters glycolysis
• Lipids yield more ATP per gram than carbohydrates (more reduced carbons)

Regulation of cellular respiration and photosynthesis

Feedback inhibition regulates metabolic pathways. When ATP levels are high:

• Phosphofructokinase (PFK), the rate-limiting enzyme in glycolysis, is inhibited
• Citrate accumulation inhibits PFK
• AMP and ADP activate PFK (low energy signals)

Rubisco is the most abundant protein on Earth, yet relatively inefficient (3-10 reactions per second). Its regulation includes:

• Light activation (via changes in pH and Mg²⁺ concentration in stroma)
• Inhibition by carboxyarabinitol-1-phosphate at night

The ratio of NADPH to NADP⁺ and ATP to ADP regulates light reactions. High NADPH and ATP levels slow the Calvin cycle, backing up electron transport.

Worked examples

Example 1: Calculating ATP yield

Question: A researcher adds FADH₂ directly to isolated mitochondria with functioning electron transport chains and adequate ADP, Pᵢ, and O₂. The FADH₂ donates electrons at Complex II of the electron transport chain. Explain why FADH₂ produces fewer ATP molecules than NADH. (3 marks)

Answer:

• FADH₂ donates electrons at Complex II, bypassing Complex I (1 mark)
• Complex I pumps H⁺ across the inner membrane, but Complex II does not pump protons (1 mark)
• Therefore, FADH₂ contributes to a smaller proton gradient, resulting in fewer H⁺ ions available to drive ATP synthase and produce less ATP (~1.5 ATP vs. ~2.5 ATP for NADH) (1 mark)

Example 2: Photosynthesis limiting factors

Question: A student investigated the effect of light intensity on photosynthesis rate in pondweed (Elodea). At low CO₂ concentration (0.04%), increasing light intensity increased the rate of photosynthesis until 5000 lux, after which the rate plateaued. Explain this observation. (4 marks)

Answer:

• Below 5000 lux, light intensity is the limiting factor; more light increases the rate of light-dependent reactions (1 mark)
• More ATP and NADPH are produced, which increases the rate of the Calvin cycle and overall photosynthesis (1 mark)
• At 5000 lux and above, light is no longer limiting; another factor becomes limiting (1 mark)
• CO₂ concentration is now the limiting factor because insufficient CO₂ is available for carbon fixation in the Calvin cycle, regardless of ATP/NADPH availability (1 mark)

Example 3: Chemiosmosis comparison

Question: Compare and contrast chemiosmosis in chloroplasts and mitochondria. (5 marks)

Answer:

Similarities:

• Both use an electron transport chain to pump H⁺ across a membrane, creating a proton gradient (1 mark)
• Both use ATP synthase to catalyze ATP synthesis as H⁺ flows down its concentration gradient (1 mark)

Differences:

• In chloroplasts, H⁺ accumulates in the thylakoid lumen and flows to the stroma; in mitochondria, H⁺ is pumped from the matrix to the intermembrane space and flows back to the matrix (1 mark)
• Chloroplasts use light energy to energize electrons from water; mitochondria use chemical energy from NADH/FADH₂ derived from glucose breakdown (1 mark)
• In chloroplasts, the final electron acceptor is NADP⁺ (forming NADPH); in mitochondria, it is O₂ (forming H₂O) (1 mark)

Common mistakes and how to avoid them

• Confusing where processes occur: Glycolysis occurs in the cytoplasm, not mitochondria. The Calvin cycle occurs in the stroma, not the thylakoid membrane. Create a diagram showing the precise location of each process.

• Misunderstanding ATP yield: Students often memorize 38 ATP per glucose, an outdated theoretical maximum. The actual yield is ~30-32 ATP due to proton leak and transport costs. Always specify "approximately" and understand the assumptions.

• Mixing up electron carriers: NADH and NADPH are different molecules. NADH functions in respiration; NADPH in photosynthesis. Though chemically similar, they participate in different pathways and have distinct cellular roles.

• Forgetting oxygen's role: In photosynthesis, oxygen comes from water splitting (photolysis), not CO₂. In respiration, oxygen is the final electron acceptor in the ETC, not a reactant in glycolysis. Be precise about when and where O₂ is used or produced.

• Oversimplifying limiting factors: Multiple factors can limit photosynthesis simultaneously. At any given moment, the factor in shortest supply limits the rate (Blackman's Law of Limiting Factors). Temperature affects enzyme activity but isn't a resource like light or CO₂.

• Confusing substrate-level and oxidative phosphorylation: Substrate-level phosphorylation directly transfers phosphate to ADP (glycolysis and Krebs cycle produce 4 ATP total). Oxidative phosphorylation uses the proton gradient and produces the majority of ATP (~26-28 ATP).

Exam technique for "Cellular Energetics"

• Master command words: "Explain" requires reasoning and mechanisms (e.g., how chemiosmosis couples electron transport to ATP synthesis). "Describe" needs factual details without necessarily explaining why. "Compare" requires both similarities and differences.

• Use precise terminology: Write "proton gradient" or "electrochemical gradient," not "concentration difference." Specify "inner mitochondrial membrane" or "thylakoid membrane," not just "membrane." Examiners reward accurate biological vocabulary.

• Show calculation steps: When calculating ATP yield or analyzing data, show your working. Even if the final answer is incorrect, you can earn method marks. Include units and explain assumptions.

• Draw and annotate diagrams: For complex processes like the ETC or Calvin cycle, a labeled diagram can earn marks and clarify your written explanation. Practice drawing mitochondria and chloroplasts with correct structural features.

Quick revision summary

Cellular energetics describes energy flow through living systems. Photosynthesis captures light energy, converting CO₂ and H₂O into glucose and O₂ through light-dependent reactions (producing ATP and NADPH via chemiosmosis) and the Calvin cycle. Cellular respiration releases this stored energy through glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation, generating approximately 30-32 ATP per glucose. Fermentation provides an anaerobic alternative yielding only 2 ATP. Both photosynthesis and respiration rely on chemiosmosis—coupling proton gradients to ATP synthesis—demonstrating fundamental unity in energy transduction mechanisms across all life.