Cambridge IGCSE IGCSE & A-Level Sheet preview

Cellular Energetics

AP Biology · Topic 3

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Handout Vocabulary
3.1

Enzymes as Biological Catalysts
Syllabus
Big IdeaLearning ObjectiveEssential Knowledge

Big Idea 2 — Energetics
Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

3.1.A
Explain how enzymes affect the rate of biological reactions.

  • 3.1.A.1 The structure and function of enzymes contribute to the regulation of biological processes. Enzymes are proteins that are biological catalysts that facilitate chemical reactions in cells by lowering the activation energy.
  • 3.1.A.2 For an enzyme-mediated chemical reaction to occur, the shape and charge of the substrate must be compatible with the active site of the enzyme. This is illustrated by the enzyme-substrate complex model.

Source: College Board AP Course and Exam Description

An enzyme 酶 is a protein catalyst 催化剂 that speeds a reaction by lowering its activation energy 活化能, without being used up. Each enzyme has an active site 活性位点 that binds a specific substrate 底物 (the "lock and key" or induced fit), so enzymes are highly specific. They do not change whether a reaction is favorable – only how fast it goes.

An enzyme lowers the activation energy of a reaction An enzyme lowers the activation energy of a reaction

The lock-and-key and induced-fit models of enzyme action The lock-and-key and induced-fit models of enzyme action

Explore

Raise substrate and watch the rate saturate

An enzyme speeds a reaction by lowering activation energy. As substrate rises the rate climbs, then levels off once every active site is busy (saturation).

Vocabulary Train
English Chinese Pinyin
enzyme 酶 méi
catalyst 催化剂 cuī huà jì
activation energy 活化能 huó huà néng
active site 活性位点 huó xìng wèi diǎn
substrate 底物 dǐ wù
Exercise sheet
3.2

Environmental Impacts on Enzyme Function
Syllabus
Big IdeaLearning ObjectiveEssential Knowledge

Big Idea 2 — Energetics
Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

3.2.A
Explain how changes to the structure of an enzyme may affect its function.

  • 3.2.A.1 Change to the molecular structure of a component in an enzymatic system may result in a change to its function or efficiency.
    • i. Denaturation of proteins, such as enzymes, occurs when the protein structure is disrupted by a change in temperature, pH, or chemical environment, eliminating the ability to catalyze reactions.
    • ii. Environmental temperatures and pH outside the optimal range for a given enzyme will cause changes to its structure (by disrupting the hydrogen bonds), altering the efficiency with which it catalyzes reactions.
  • 3.2.A.2 In some cases, enzyme denaturation is reversible, allowing the enzyme to regain activity.

3.2.B
Explain how the cellular environment affects enzyme activity.

  • 3.2.B.1 The relative concentrations of substrates and products determine how efficiently an enzymatic reaction proceeds.

Source: College Board AP Course and Exam Description

Enzyme activity depends on conditions. Each enzyme has an optimal temperature and pH; beyond it, the protein denatures 变性 (loses shape) and stops working. Substrate concentration raises the rate until the enzyme saturates. Inhibitors 抑制剂 slow enzymes – competitive ones block the active site, noncompetitive ones bind elsewhere and change the shape.

Each enzyme has an optimum pH Each enzyme has an optimum pH

Rate rises to an optimum temperature, then falls as the enzyme denatures Rate rises to an optimum temperature, then falls as the enzyme denatures

Explore

Change temperature and watch enzyme activity

Each enzyme has an optimum temperature and pH. Too cold is slow; too hot denatures the enzyme so its active site loses shape and activity crashes.

Vocabulary Train
English Chinese Pinyin
denatures 变性 biàn xìng
Inhibitors 抑制剂 yì zhì jì
Exercise sheet
3.3

Cellular Energy and ATP
Syllabus
Big IdeaLearning ObjectiveEssential Knowledge

Big Idea 2 — Energetics
Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

3.3.A
Describe the role of energy in living organisms.

  • 3.3.A.1 All living systems require an input of energy.
  • 3.3.A.2 Life requires a highly ordered system and does not violate the first and second laws of thermodynamics.
    • i. Energy input must exceed energy loss to maintain order and to power cellular processes.
    • ii. Cellular processes that release energy may be coupled with cellular processes that require energy.
    • iii. Significant loss of order or energy flow results in death.
    • Exclusion statement: Students will need to understand the concept of energy, but the equation for Gibbs free energy is beyond the scope of the AP Exam.
  • 3.3.A.3 Energy-related pathways in biological systems are sequential to allow for a more controlled transfer of energy. A product of a reaction in a metabolic pathway is typically the reactant for the subsequent step in the pathway.

3.3.B
Explain how shared, conserved, and fundamental processes and features support the concept of common ancestry for all organisms.

  • 3.3.B.1 Core metabolic pathways (e.g., glycolysis, oxidative phosphorylation) are conserved across all currently recognized domains (Archaea, Bacteria, and Eukarya).

Source: College Board AP Course and Exam Description

ATP (adenosine triphosphate) is the cell's energy currency. Energy is stored in its phosphate bonds; breaking off a phosphate (ATP → ADP) releases energy to power cellular work, and reattaching one stores energy. Cells constantly recycle ATP, coupling energy-releasing reactions to energy-requiring ones.

The ATP-ADP cycle stores and releases energy The ATP-ADP cycle stores and releases energy

Exercise sheet
3.4

Photosynthesis
Syllabus
Big IdeaLearning ObjectiveEssential Knowledge

Big Idea 2 — Energetics
Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

3.4.A
Describe the photosynthetic processes and structural features of the chloroplast that allow organisms to capture and store energy.

  • 3.4.A.1 Photosynthesis is the series of reactions that use carbon dioxide $(\mathrm{CO_2})$, water $(\mathrm{H_2O})$, and light energy to make carbohydrates and oxygen $(\mathrm{O_2})$.
    • i. Photosynthetic organisms capture energy from the sun and produce sugars that can be used in biological processes or stored.
    • ii. Photosynthesis first evolved in prokaryotic organisms.
    • iii. Scientific evidence supports the claim that prokaryotic (cyanobacterial) photosynthesis was responsible for the production of an oxygenated atmosphere.
    • iv. Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.
    • Exclusion statement: Memorization of the steps in the Calvin cycle, the structure of the molecules, and the names of the enzymes involved, with the exception of ATP synthase, is beyond the scope of the AP Exam.
  • 3.4.A.2 Stroma and thylakoids are found within the chloroplast.
    • i. The stroma is the fluid within the inner chloroplast membrane and outside the thylakoid. The carbon fixation (Calvin cycle) reactions of photosynthesis occur in the stroma.
    • ii. The thylakoid membranes contain chlorophyll pigments organized into two photosystems, as well as electron transport proteins.
    • iii. Thylakoids are organized in stacks called grana. The light reactions of photosynthesis occur in the grana.
  • 3.4.A.3 The light reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture energy present in light to yield ATP and NADPH, which power the production of organic molecules in the Calvin cycle. This provides energy for metabolic processes.

3.4.B
Explain how cells capture energy from light and transfer it to biological molecules for storage and use.

  • 3.4.B.1 Electron transport chain (ETC) reactions occur in chloroplasts, in mitochondria, and across prokaryotic plasma membranes. In photosynthesis, electrons that pass through the thylakoid membrane are picked up and ultimately transferred to $\mathrm{NADP^+}$ reducing it to NADPH in photosystem I.
    • Exclusion statement: The full names of the specific electron carriers in the electron transport chain are beyond the scope of the AP Exam.
    • Exclusion statement: Specific steps, names of enzymes, and intermediates of the pathways for these processes are beyond the scope of this course and the AP Exam.
  • 3.4.B.2 During photosynthesis, chlorophylls absorb energy from light, boosting electrons to a higher energy level in photosystems I and II. Water then splits, supplying electrons to replace those lost from photosystem II.
  • 3.4.B.3 Photosystems I and II are embedded in the thylakoid membranes of chloroplasts and are connected by the transfer of electrons through an ETC.
  • 3.4.B.4 When electrons are transferred between molecules in a series of oxidation/reduction reactions as they pass through the ETC, an electrochemical gradient of protons (hydrogen ions) is established across the thylakoid membrane. The membrane separates a region of low proton concentration outside the thylakoid membrane from a region of high proton concentration inside the thylakoid membrane.
  • 3.4.B.5 The formation of the proton gradient is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase. The flow of protons back through membrane-bound ATP synthase by chemiosmosis drives the formation of ATP from ADP and inorganic phosphate; this is known as photophosphorylation.
  • 3.4.B.6 The energy captured in the light reactions and transferred to ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle. This occurs in the stroma of the chloroplast.

Source: College Board AP Course and Exam Description

Photosynthesis 光合作用 captures light energy to build sugar from $\text{CO}_2$ and water, releasing $\text{O}_2$. It has two stages:

The two stages of photosynthesis are linked by ATP and NADPH The two stages of photosynthesis are linked by ATP and NADPH

  • The light reactions (in the thylakoid membranes) use light to make ATP and NADPH and split water, releasing oxygen.
  • The Calvin cycle 卡尔文循环 (in the stroma) uses that ATP and NADPH to fix $\text{CO}_2$ into sugar.

So light energy becomes chemical energy stored in glucose.

Vocabulary Train
English Chinese Pinyin
Photosynthesis 光合作用 guāng hé zuò yòng
Calvin cycle 卡尔文循环 kǎ ěr wén xún huán
Exercise sheet
3.5

Cellular Respiration
Syllabus
Big IdeaLearning ObjectiveEssential Knowledge

Big Idea 2 — Energetics
Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

3.5.A
Describe the processes and structural features of mitochondria that allow organisms to use energy stored in biological macromolecules.

  • 3.5.A.1 Cellular respiration uses energy from biological macromolecules to synthesize ATP. Respiration and fermentation are characteristic of all forms of life.
  • 3.5.A.2 Aerobic cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that capture energy from biological macromolecules.
  • 3.5.A.3 The ETC transfers electrons in a series of oxidation-reduction reactions that establish an electrochemical gradient across membranes.
    • i. In cellular respiration, electrons delivered by NADH and $\mathrm{FADH_2}$ are passed to a series of electron acceptors as they move toward the terminal electron acceptor, oxygen. Aerobic prokaryotes use oxygen as a terminal electron acceptor, while anaerobic prokaryotes use other molecules.
    • ii. The transfer of electrons, through the ETC, is accompanied by the formation of a proton gradient across the inner mitochondrial membrane, with the membrane(s) separating a region of high proton concentration outside the membrane from a region of low proton concentration inside the membrane. The folding of the inner membrane increases the surface area, which allows for more ATP to be synthesized. In prokaryotes, the passage of electrons is accompanied by the movement of protons across the plasma membrane.
    • iii. The flow of protons back through membrane-bound ATP synthase by chemiosmosis drives the formation of ATP from ADP and inorganic phosphate. This is known as oxidative phosphorylation in aerobic cellular respiration.
    • iv. In aerobic cellular respiration, decoupling oxidative phosphorylation from electron transport generates heat. This heat can be used by endothermic organisms to regulate body temperature.
    • Exclusion statement: The full names of the specific electron carriers in the electron transport chain are beyond the scope of the AP Exam.
    • Exclusion statement: Specific steps, names of enzymes, and intermediates of the pathways for these processes are beyond the scope of this course and the AP Exam.

3.5.B
Explain how cells obtain energy from biological macromolecules in order to power cellular functions.

  • 3.5.B.1 Glycolysis is a biochemical pathway that releases the energy in glucose molecules to form ATP (from ADP and inorganic phosphate), NADH (from $\mathrm{NAD^+}$), and pyruvate.
  • 3.5.B.2 Pyruvate is transported from the cytosol to the mitochondrion where oxidation occurs. This process releases electrons during the Krebs (citric acid) cycle, reducing $\mathrm{NAD^+}$ to NADH and FAD to $\mathrm{FADH_2}$, and releasing $\mathrm{CO_2}$.
  • 3.5.B.3 The Krebs cycle takes place in the mitochondrial matrix. During the Krebs cycle, carbon dioxide is released from organic intermediates, ATP is synthesized from ADP and inorganic phosphate, and electrons are transferred by the coenzymes $\mathrm{NAD^+}$ and FAD.
  • 3.5.B.4 Electrons extracted in glycolysis and Krebs cycle reactions are transferred by NADH and $\mathrm{FADH_2}$ to the ETC in the inner mitochondrial membrane.
  • 3.5.B.5 When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of protons (hydrogen ions) across the inner mitochondrial membrane is established. The pH inside the mitochondrial matrix is higher than in the intermembrane space.
  • 3.5.B.6 Fermentation allows glycolysis to proceed in the absence of oxygen and produces organic molecules such as alcohol and lactic acid.
    • Exclusion statement: Memorization of the steps in glycolysis and the Krebs cycle, and of the structures of the molecules and the names of the enzymes involved, is beyond the scope of this course and the AP Exam.

Source: College Board AP Course and Exam Description

Cellular respiration 细胞呼吸 releases the energy in glucose to make ATP, mostly using oxygen. Its stages:

The stages of aerobic respiration and where in the cell they happen The stages of aerobic respiration and where in the cell they happen

  • Glycolysis 糖酵解 (in the cytoplasm) splits glucose, making a little ATP.
  • The Krebs cycle 克雷布斯循环 (mitochondrial matrix) releases $\text{CO}_2$ and loads electron carriers.
  • The electron transport chain 电子传递链 (inner membrane) uses those electrons to pump protons and make most of the ATP, with oxygen as the final electron acceptor.

Without oxygen, cells use fermentation 发酵 to keep glycolysis running, making far less ATP. Photosynthesis and respiration are complementary – the products of one are the reactants of the other.

Worked example. Aerobic respiration of one glucose nets about 2 ATP from glycolysis, 2 ATP from the Krebs cycle, and about 28 ATP from oxidative phosphorylation, for $\approx$ 32 ATP total. With no oxygen only glycolysis runs, so fermentation nets just 2 ATP per glucose — roughly 16 times less energy, which is why aerobic pathways dominate in oxygen-rich cells.

Vocabulary Train
English Chinese Pinyin
Cellular respiration 细胞呼吸 xì bāo hū xī
Glycolysis 糖酵解 táng jiào jiě
Krebs cycle 克雷布斯循环 kè léi bù sī xún huán
electron transport chain 电子传递链 diàn zi chuán dì liàn
fermentation 发酵 fā jiào
Exercise sheet
3.5

Exam tips
  • An enzyme lowers the activation energy and is not used up; its active site is specific to one substrate (lock and key).
  • Rate rises with temperature only up to the optimum — beyond it the enzyme denatures and the rate falls (unlike an ordinary reaction).
  • Know the ATP↔ADP cycle: breaking a phosphate releases energy to power the cell.
  • Write the overall equations: photosynthesis stores energy (builds glucose); respiration releases it (breaks glucose down) — they are opposites.
  • Match each stage to its location and whether it needs oxygen.

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