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Unit 2 Study Guide: Cellular Respiration, Photosynthesis, Cell Structure, Mitosis, and DNA Synthesis

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Cellular Respiration

Overview and Key Concepts

Cellular respiration is the process by which cells convert biochemical energy from nutrients into ATP, the energy currency of the cell. It involves a series of redox reactions, energy transformations, and the participation of biological macromolecules and membranes.

  • Redox Reactions: Involve the transfer of electrons; oxidation is loss, reduction is gain.

  • Enzymes: Catalyze reactions, regulate activity, and are essential for efficient energy conversion.

  • Transport Proteins: Facilitate movement of molecules across membranes, crucial in processes like the electron transport chain.

General Equation for Cellular Respiration

The overall equation for aerobic cellular respiration is:

  • Glucose is oxidized; oxygen is reduced.

  • CO2 and H2O are produced as waste products.

  • ATP is generated for cellular work.

Stages of Cellular Respiration

  • Glycolysis: Occurs in cytosol; glucose is split into two pyruvate molecules, producing ATP and NADH.

  • Pyruvate Processing: Pyruvate is converted to acetyl CoA, producing NADH and CO2.

  • Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized, generating NADH, FADH2, ATP, and CO2.

  • Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 donate electrons; O2 is the final electron acceptor; H+ gradient drives ATP synthesis.

Energy Transformations

  • Energy in glucose is transferred to ATP via substrate-level and oxidative phosphorylation.

  • NADH and FADH2 carry high-energy electrons to the ETC.

Substrate-Level vs. Oxidative Phosphorylation

Type

Location

Mechanism

When Occurs

Substrate-Level

Glycolysis, Citric Acid Cycle

Direct transfer of phosphate to ADP

Early stages

Oxidative

ETC (mitochondria)

ATP synthase uses H+ gradient

Final stage

Fermentation

  • Function: Enables ATP production without oxygen.

  • Lactic Acid Fermentation: Used by animals and some bacteria; pyruvate is reduced to lactate.

  • Ethanol Fermentation: Used by yeast and some plants; pyruvate is converted to ethanol and CO2.

  • ATP Yield: Lower than aerobic respiration due to incomplete oxidation of glucose.

Connections to Other Metabolic Pathways

  • Intermediates from cellular respiration are used in biosynthetic pathways (e.g., amino acid synthesis).

  • Fatty acids and proteins can enter cellular respiration at various points.

Photosynthesis

Overview and Key Concepts

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

  • Redox Reactions: Water is oxidized, CO2 is reduced.

  • Energy Transformations: Light energy is converted to chemical energy.

  • Biological Macromolecules: Chlorophyll and other pigments absorb light.

General Equation for Photosynthesis

  • CO2 is fixed into glucose.

  • O2 is released as a byproduct.

Stages of Photosynthesis

  • Light Reactions: Occur in thylakoid membranes; produce ATP and NADPH.

  • Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into sugars.

Energy Transformations

  • Light energy is absorbed by chlorophyll, exciting electrons.

  • ATP and NADPH are produced via electron transport and chemiosmosis.

  • ATP and NADPH are used in the Calvin cycle to synthesize glucose.

Comparison: Cellular Respiration vs. Photosynthesis

Process

Reactants

Products

Energy Source

Cellular Respiration

Glucose, O2

CO2, H2O, ATP

Chemical (glucose)

Photosynthesis

CO2, H2O, light

Glucose, O2

Light

ATP Formation: Light Reactions vs. Cellular Respiration

  • Both use chemiosmosis and ATP synthase.

  • Source of electrons differs: water in photosynthesis, NADH/FADH2 in respiration.

C3, C4, and CAM Photosynthesis

Type

CO2 Fixation

Adaptation

C3

Directly via Calvin cycle

Most plants; less efficient in dry environments

C4

CO2 fixed in mesophyll, then Calvin cycle in bundle sheath

Reduces photorespiration; better in hot, dry climates

CAM

CO2 fixed at night, Calvin cycle during day

Conserves water; ideal for arid environments

Plant Biomass

  • Most dry biomass comes from carbon fixed from atmospheric CO2.

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic Cells: Lack membrane-bound organelles; have cell wall, plasma membrane, nucleoid, ribosomes.

  • Eukaryotic Cells: Have nucleus and membrane-bound organelles (mitochondria, ER, Golgi, etc.).

Plant vs. Animal Cells

Feature

Plant Cell

Animal Cell

Cell Wall

Present

Absent

Chloroplasts

Present

Absent

Central Vacuole

Large

Small/absent

Lysosomes

Rare

Common

Endomembrane System

  • Includes ER, Golgi apparatus, lysosomes, vesicles.

  • Proteins and lipids are synthesized, modified, and transported to specific destinations.

Cell Function and Organelles

  • Abundance of certain organelles reflects cell function (e.g., muscle cells have many mitochondria).

  • Alteration of components affects cell function (e.g., defective lysosomes lead to storage diseases).

Mitosis and the Cell Cycle

Function of Mitosis

  • Enables growth, repair, and asexual reproduction in multicellular organisms.

  • Allows reproduction in unicellular organisms.

Cell Cycle Phases

  • Interphase: G1 (growth), S (DNA synthesis), G2 (preparation).

  • M Phase: Mitosis (division of nucleus) and cytokinesis (division of cytoplasm).

Phases of Mitosis

  • Prophase: Chromosomes condense, spindle forms.

  • Metaphase: Chromosomes align at cell equator.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear envelope reforms, chromosomes decondense.

Chromosome Movement

  • Microtubules attach to kinetochores and pull chromatids apart.

Cell Cycle Regulation

  • Checkpoints (G1, G2, M) ensure proper division.

  • Regulatory proteins (cyclins, CDKs) control progression.

Consequences of Errors

  • Uncontrolled division can lead to cancer.

  • Errors in mitosis can cause aneuploidy or cell death.

DNA Synthesis and Repair

DNA as Hereditary Material

  • Experiments (e.g., Avery, Hershey-Chase) demonstrated DNA carries genetic information.

  • DNA replication is semi-conservative: each new molecule has one old and one new strand.

DNA Replication Process

  • Origin of Replication: Site where replication begins.

  • Replication Fork: Y-shaped region where DNA is unwound.

  • Enzymes: DNA pol I & III (synthesis), ligase (joins fragments), primase (RNA primers), helicase (unwinds), topoisomerase (relieves tension), sliding clamps (stabilize), ssBPs (single-strand binding proteins).

Leading vs. Lagging Strand

  • Leading Strand: Synthesized continuously toward replication fork.

  • Lagging Strand: Synthesized discontinuously as Okazaki fragments, away from fork.

Telomeres and Replication

  • Telomeres: Repetitive DNA at chromosome ends; protected by telomerase in germ cells.

Source of Energy for DNA Polymerization

Energy comes from hydrolysis of nucleotide triphosphates:

Error Correction and Repair

  • DNA pol III proofreads and corrects errors.

  • Excision repair removes damaged bases (e.g., UV-induced lesions).

  • Failure in repair (e.g., xeroderma pigmentosum) leads to increased mutation rates.

Replication and Cell Division

  • Replication produces two identical copies of chromosomes for mitosis.

  • Essential for growth and reproduction.

Connections to Macromolecules, Cells, and Organelles

  • DNA structure and function are linked to proteins (enzymes), cell structure, and organelle function.

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