Cellular Respiration, Photosynthesis, Cell Communication, and Cell Division: Study Guide

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from carbohydrates, proteins, and fats to produce ATP. It involves a series of metabolic pathways that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products.

• Chemical Equation: The overall equation for cellular respiration using glucose is: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy\ (ATP)}$

• Redox Reactions: During cellular respiration, glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

• Oxidation vs. Reduction: Oxidation is the loss of electrons; reduction is the gain of electrons. The reducing agent donates electrons, while the oxidizing agent accepts electrons.

• Phosphorylation: Substrate-level phosphorylation directly transfers a phosphate group to ADP to form ATP. Oxidative phosphorylation uses energy from the electron transport chain to drive ATP synthesis.

Stages of Cellular Respiration

1. Glycolysis

   • Occurs in the cytoplasm.

   • Divided into two phases:

     • Energy investment phase: ATP is consumed to phosphorylate glucose.

     • Energy payoff phase: ATP and NADH are produced.

   • Net yields per glucose: 2 ATP, 2 NADH, 2 pyruvate.

   • Key enzymes: Five major enzymes are involved, including phosphofructokinase (PFK) at step 3, which is the commitment step and a key regulatory point.

2. Pyruvate Oxidation

   • Pyruvate is transported into the mitochondrial matrix and converted to acetyl CoA.

   • Net yield per pyruvate: 1 NADH, 1 acetyl CoA.

3. Krebs Cycle (Citric Acid Cycle)

   • Occurs in the mitochondrial matrix.

   • Acetyl CoA combines with oxaloacetate to form citrate, which is metabolized back to oxaloacetate.

   • Net yields per acetyl CoA: 1 ATP, 3 NADH, 1 FADH2.

   • Key enzymes: Five major enzymes, including those responsible for substrate-level phosphorylation and redox reactions.

4. Oxidative Phosphorylation

   • Occurs at the inner mitochondrial membrane.

   • Consists of the electron transport chain (ETC) and chemiosmosis.

   • Electron Transport Chain:

     • Complex I: NADH donates electrons; H+ pumped to intermembrane space (IMS).

     • Complex II: FADH2 donates electrons.

     • Complex III: Electrons passed down; H+ pumped to IMS.

     • Complex IV: Electrons transferred to O2 (final electron acceptor), forming H2O; H+ pumped to IMS.

   • Purpose: Electron transfer drives proton pumping, creating a proton gradient.

5. Chemiosmosis and ATP Synthesis

   • Protons flow back into the matrix through ATP synthase, driving ATP production.

   • Total ATP yield per glucose: About 30–32 ATP.

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+.

• Alcohol Fermentation: Glucose is converted to ethanol; NAD+ is regenerated. Occurs in yeast and some bacteria. Anaerobic process.

• Lactic Acid Fermentation: Glucose is converted to lactate; NAD+ is regenerated. Occurs in muscle cells and some bacteria. Anaerobic process.

Comparison: Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses oxygen as the final electron acceptor; produces more ATP.

• Anaerobic respiration: Uses other molecules as final electron acceptors; less ATP produced.

Obligate vs. Facultative Anaerobes

• Obligate anaerobes: Cannot survive in the presence of oxygen.

• Facultative anaerobes: Can survive with or without oxygen.

Versatility of Catabolism

• Carbohydrates, proteins, and fats can all be used as fuel for cellular respiration.

Regulation of Cellular Respiration

• Stimulators: AMP stimulates phosphofructokinase (PFK).

• Inhibitors: Citrate and ATP inhibit PFK.

Photosynthesis

Overview of Photosynthesis

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

• Chemical Equation: $\mathrm{6CO_2 + 6H_2O + Light\ energy \rightarrow C_6H_{12}O_6 + 6O_2}$

• Redox Reactions: CO2 is reduced to glucose; H2O is oxidized to O2.

Autotrophs vs. Heterotrophs

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

• Heterotrophs: Organisms that consume other organisms for energy.

Chloroplast Structure

• Mesophyll: Leaf tissue where most chloroplasts are found.

• Stomata: Pores for gas exchange.

• Stroma: Fluid-filled space inside chloroplasts.

• Thylakoid: Membranous sacs containing chlorophyll.

• Grana: Stacks of thylakoids.

• Chlorophyll: Main photosynthetic pigment.

Stages of Photosynthesis

1. Light Reactions

   • Occur in the thylakoid membranes.

   • Convert light energy and water into ATP, NADPH, and O2.

   • Photosynthetic Pigments: Chlorophyll a, chlorophyll b, and carotenoids absorb light at different wavelengths.

   • Light and Pigments: Light is part of the electromagnetic spectrum; photons are absorbed by pigments.

   • Spectrophotometer: Measures pigment absorption at various wavelengths.

   • Photosystem Structure: Consists of a light-harvesting complex and a reaction center.

   • Electron Flow:

     • Linear electron flow: Produces ATP and NADPH.

     • Cyclic electron flow: Produces ATP only.

   • Chemiosmosis Comparison: Both mitochondria and chloroplasts use chemiosmosis to generate ATP, but the direction of proton flow and energy sources differ.

2. Calvin Cycle

   • Occurs in the stroma.

   • Uses ATP and NADPH to convert CO2 into glucose.

   • Three phases: carbon fixation, reduction, regeneration of RuBP.

Cell Communication (Cell Signaling)

Overview of Cell Signaling

Cells communicate through signaling molecules that bind to receptors, triggering a series of events leading to a cellular response.

• Local signaling: Autocrine, paracrine, and synaptic signaling.

• Long-distance signaling: Hormones.

Three Stages of Cell Signaling

1. Signal Reception: Signal molecule binds to receptor.

2. Signal Transduction: Relay of signals through a cascade of molecular interactions.

3. Cellular Response: Activation of cellular processes.

Types of Receptors

• G-Protein Coupled Receptors (GPCRs): Seven transmembrane alpha helices; activate G-proteins (active with GTP, inactive with GDP).

• Receptor Tyrosine Kinases (RTKs): Ligand binding site, tyrosine domains; autophosphorylation triggers signaling.

• Ligand-Gated Ion Channels: Open in response to ligand binding, allowing ions to pass through.

• Intracellular Receptors: Located inside the cell; activated by small or hydrophobic chemical messengers.

Signal Transduction Pathways

• Protein Phosphorylation: Protein kinases add phosphate groups; protein phosphatases remove them.

• Second Messengers: Small molecules like cAMP, Ca2+, and IP3 relay signals inside the cell.

• Pathways: cAMP and Ca2+ act as secondary messengers to amplify and propagate signals.

• Cellular Responses: Can regulate enzymes, proteins, or gene expression.

Regulation of Cell Signaling

• Amplification: One signal molecule can cause a large cellular response.

• Coordination: Branching, cross-talk, and specificity ensure appropriate responses.

• Scaffolding Proteins: Increase efficiency by grouping pathway proteins.

• Termination: Signal ends when ligand unbinds from receptor.

Apoptosis

• Definition: Programmed cell death, a controlled process to eliminate damaged or unnecessary cells.

Cell Division

Organization of Genetic Material

• Chromosomes: Structures containing DNA and proteins.

• Somatic cells: Diploid (2n); all body cells except gametes.

• Gametes: Haploid (n); sperm and egg cells.

Cell Division Types

• Binary Fission: Prokaryotic cell division; circular DNA replicates and cell splits.

• Mitosis: Eukaryotic cell division for growth and repair; produces two identical diploid cells.

• Meiosis: Eukaryotic cell division for sexual reproduction; produces four non-identical haploid cells.

Mitosis Phases

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

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelope reforms.

• Cytokinesis: Cytoplasm divides.

Cell Cycle Control

• Checkpoints: G1, G2, and M checkpoints regulate progression.

• Internal/External Factors: Signals that promote or inhibit cell division.

Cancer

• Benign Tumors: Non-invasive growths.

• Malignant Tumors: Invasive, can metastasize (spread).

• Transformation: Normal cell becomes cancerous.

Meiosis and Heredity

Genetics and Chromosomes

• Heredity: Transmission of traits from parents to offspring.

• Genes: Units of heredity located at specific loci on chromosomes.

• Karyotype: Display of an individual's chromosomes.

• Homologous Chromosomes: Chromosome pairs with the same genes.

• Sex Chromosomes: Determine sex (X and Y); Autosomes: All other chromosomes.

• Fertilization: Fusion of gametes to form a zygote.

Meiosis I and II

• Meiosis I:

  • Prophase I: Chromosomes condense, synapsis and crossing over occur.

  • Metaphase I: Homologous pairs align at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate.

  • Telophase I and Cytokinesis: Two haploid cells form.

• Meiosis II:

  • Prophase II: Chromosomes condense.

  • Metaphase II: Chromosomes align at the metaphase plate.

  • Anaphase II: Sister chromatids separate.

  • Telophase II and Cytokinesis: Four haploid cells result.

Unique Events in Meiosis

• Synapsis and crossing over (exchange of genetic material).

• Homologous chromosomes align and separate.

• Reduction of chromosome number (diploid to haploid).

Sources of Genetic Variation

• Crossing Over: Exchange of genetic material between homologous chromosomes.

• Independent Assortment: Random orientation of homologous pairs during metaphase I.

• Random Fertilization: Any sperm can fertilize any egg.

Process	Location	ATP Yield	Key Products
Glycolysis	Cytoplasm	2	2 NADH, 2 Pyruvate
Pyruvate Oxidation	Mitochondrial Matrix	0	2 NADH, 2 Acetyl CoA
Krebs Cycle	Mitochondrial Matrix	2	6 NADH, 2 FADH2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	~26-28	H2O

Additional info: For enzyme details and specific pathway diagrams, refer to class announcements and textbook summary figures as indicated in the study guide.