Cellular Respiration, Photosynthesis, and the Cell Cycle: Study Guide (Chapters 9, 10, 12)

Cellular Respiration (Chapter 9)

Redox Reactions in Cellular Respiration

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into ATP, involving redox (reduction-oxidation) reactions.

• Redox Reaction: A chemical reaction involving the transfer of electrons between two species. One substance is oxidized (loses electrons), and another is reduced (gains electrons).

• Oxidizing Agent: The molecule that accepts electrons (is reduced).

• Reducing Agent: The molecule that donates electrons (is oxidized).

• Electron Donor/Acceptor: The donor is oxidized; the acceptor is reduced.

• Net Equation of Respiration:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy\ (ATP)}$

• Glucose is oxidized; oxygen is reduced.

Role of NAD+ in Cellular Respiration

• NAD+ (Nicotinamide Adenine Dinucleotide): An electron carrier that cycles between oxidized (NAD+) and reduced (NADH) states.

• It accepts electrons during glycolysis and the Krebs cycle, transferring them to the electron transport chain.

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytosol; splits glucose into two pyruvate molecules. Has an investment phase (uses ATP) and a payoff phase (produces ATP and NADH).

• Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose, producing CO2, NADH, and FADH2.

• Oxidative Phosphorylation: Occurs at the inner mitochondrial membrane; includes the electron transport chain and chemiosmosis, generating most ATP.

Cellular Locations of Respiration Steps

• Glycolysis: Cytosol

• Pyruvate Oxidation & Citric Acid Cycle: Mitochondrial matrix

• Electron Transport Chain & Chemiosmosis: Inner mitochondrial membrane

Role of Oxygen

• Oxygen acts as the final electron acceptor in the electron transport chain, forming water.

• Without oxygen, the electron transport chain cannot function, and ATP production is severely limited.

Proton-Motive Force and ATP Generation

• Proton-Motive Force: The electrochemical gradient of protons (H+) across the inner mitochondrial membrane.

• Drives ATP synthesis via ATP synthase as protons flow back into the matrix.

• If protons cannot cross the membrane, ATP synthesis halts.

Fermentation

• Alcohol Fermentation: Converts pyruvate to ethanol and CO2; used by yeast.

• Lactic Acid Fermentation: Converts pyruvate to lactate; used by muscle cells and some bacteria.

• Both regenerate NAD+ for glycolysis but yield less ATP than aerobic respiration.

Aerobic vs. Anaerobic Respiration

• Aerobic Respiration: Uses oxygen as the final electron acceptor.

• Anaerobic Respiration: Uses other molecules (e.g., sulfate, nitrate) as final electron acceptors.

Feedback Mechanisms in Respiration

• Respiration is regulated by feedback inhibition.

• ATP and citrate are inhibitors; AMP is an activator of phosphofructokinase (key glycolytic enzyme).

Photosynthesis (Chapter 10)

Autotrophs vs. Heterotrophs

• Autotrophs: Organisms that produce their own food (e.g., plants, algae).

• Photoautotrophs: Use light energy to synthesize organic compounds.

• Heterotrophs: Obtain energy by consuming other organisms.

• Producers: Autotrophs; Consumers: Heterotrophs; Decomposers: Break down dead matter.

Chloroplast Structure and Function

• Chloroplasts: Organelles where photosynthesis occurs, mainly in leaf cells.

• Thylakoids: Membranous sacs containing chlorophyll; site of light reactions.

• Stroma: Fluid surrounding thylakoids; site of the Calvin cycle.

• Veins: Transport water and nutrients.

Net Equation of Photosynthesis

$\mathrm{6CO_2 + 6H_2O + Light\ Energy \rightarrow C_6H_{12}O_6 + 6O_2}$

• CO2 is reduced; H2O is oxidized.

• O2 is produced from water, not CO2.

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membranes; convert light energy to ATP and NADPH, releasing O2.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to convert CO2 into G3P (a sugar).

• NADP+: Electron carrier reduced to NADPH in light reactions.

• Photophosphorylation: ATP synthesis using the proton gradient generated by light-driven electron transport.

Calvin Cycle Details

• CO2 is fixed in the stroma; three phases: carbon fixation, reduction, regeneration of RuBP.

• Rubisco: Enzyme that fixes CO2.

• For every three CO2 molecules, one G3P is produced.

Pigments and Spectra

• Chlorophyll a: Main pigment; absorbs blue-violet and red light.

• Chlorophyll b: Accessory pigment; broadens absorption spectrum.

• Carotenoids: Accessory pigments; protect against photodamage.

• Absorption Spectrum: Wavelengths absorbed by pigments.

• Action Spectrum: Wavelengths most effective for photosynthesis.

Electron Flow in Light Reactions

• Linear Electron Flow: Involves both photosystems; produces ATP and NADPH.

• Cyclic Electron Flow: Involves only photosystem I; produces ATP but not NADPH or O2.

• Photosystem II functions first.

• ATP is generated via chemiosmosis; NADPH is produced at the end of the electron transport chain.

Chemiosmosis in Chloroplasts vs. Mitochondria

Feature	Chloroplasts	Mitochondria
Energy Source	Light	Food molecules
Location of Proton Gradient	Thylakoid space	Intermembrane space
ATP Synthesis Site	Stroma	Matrix

Stomata and Plant Types

• Stomata: Pores on leaf surfaces for gas exchange.

• C3 Plants: Use Calvin cycle directly; susceptible to photorespiration.

• C4 Plants: Minimize photorespiration by spatial separation of steps.

• CAM Plants: Minimize photorespiration by temporal separation (night/day) of steps.

• Photorespiration: Process where Rubisco adds O2 instead of CO2, reducing efficiency.

The Cell Cycle and Mitosis (Chapter 12)

Stages of Mitosis

• Prophase: Chromosomes condense; spindle forms.

• Prometaphase: Nuclear envelope fragments; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

Recognizing Cell Cycle Stages

• DNA content and chromosome appearance can indicate the cell cycle stage.

• G1, S, G2, and M phases have distinct DNA amounts and structures.

Cytokinesis

• Animals: Cleavage furrow forms, dividing the cytoplasm.

• Plants: Cell plate forms, leading to new cell wall formation.

Binary Fission

• Prokaryotic cell division; DNA replicates and cell splits into two.

Phases of the Cell Cycle

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for division.

• M Phase: Mitosis and cytokinesis occur.

• Checkpoints: Control points where the cell assesses readiness to proceed; failure can lead to uncontrolled division (cancer).

• Cyclins and CDKs: Regulatory proteins that control cell cycle progression.

Cancer and Cell Cycle Regulation

• Cancer: Uncontrolled cell division due to failed regulation.

• Density-Dependent Inhibition: Normal cells stop dividing when crowded; cancer cells do not.

• Anchorage-Dependent Inhibition: Normal cells require attachment to a substrate to divide; cancer cells can divide without anchorage.

Additional info: For visualizing mitosis, refer to 3D animations or diagrams showing chromosome and spindle behavior at each stage. Understanding checkpoints is crucial for grasping how mutations can lead to cancer.