Biology I: Cellular Respiration, Photosynthesis, Cell Cycle, and Meiosis – Exam 3 Study Guide

Cellular Respiration and Fermentation

Overview and Function

Cellular respiration is a series of metabolic processes that break down organic molecules, such as glucose, to generate ATP, the main energy currency of the cell. This process allows cells to efficiently convert the energy stored in food into a usable form, minimizing energy loss as heat.

• Primary Function: To produce ATP by oxidizing glucose and other fuels.

• Comparison: Unlike burning fuel (which releases all energy at once), cellular respiration releases energy in controlled steps.

Redox Reactions in Metabolism

Redox (reduction-oxidation) reactions are central to energy transfer in cells. They involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Oxidizing Agent: Electron acceptor (becomes reduced).

• Reducing Agent: Electron donor (becomes oxidized).

• Example: In the reaction $Na + Cl \rightarrow Na^+ + Cl^-$, sodium is oxidized (reducing agent) and chlorine is reduced (oxidizing agent).

Coenzymes in Catabolism

• NAD+ and FAD: Main electron carriers derived from vitamins (niacin/B3 and riboflavin/B2, respectively).

• Reduction: NAD+ accepts 2 electrons and 1 proton to become NADH.

• Oxidation: NADH donates electrons and a proton to return to NAD+.

Phases of Cellular Respiration

Cellular respiration consists of four main phases: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, and Electron Transport Chain (ETC). Each phase occurs in a specific cellular location and has distinct inputs and outputs.

Phase	Location	Starting Materials	Final Products	NADH/FADH2	CO2 Produced?	ATP Produced	Main Accomplishment
Glycolysis	Cytosol	Glucose, 2 ATP, NAD+	2 Pyruvate, 2 NADH	2 NADH	No	2 Net	Split glucose
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate, NAD+, CoA	2 Acetyl CoA, 2 NADH, 2 CO2	2 NADH	Yes (2)	0	Prep pyruvate for cycle
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl CoA, NAD+, FAD	2 ATP, 6 NADH, 2 FADH2, 4 CO2	6 NADH, 2 FADH2	Yes (4)	2	Extract high-energy electrons
Electron Transport Chain	Inner Mitochondrial Membrane	NADH, FADH2, O2	ATP, H2O, NAD+, FAD	None (used up)	No	~26-28	Convert electron energy to ATP

Energy Transfer and Chemiosmosis

• Energy from glucose is transferred to NADH/FADH2, which donate electrons to the ETC.

• The ETC pumps H+ ions, creating a proton-motive force.

• ATP synthase uses this gradient to synthesize ATP from ADP and Pi (chemiosmosis).

Electron Transport Chain and ATP Synthesis

• Electrons lose energy as they move through the ETC.

• The energy is used to pump H+ ions, generating a gradient (proton-motive force).

• ATP synthase produces ATP as H+ flows back into the matrix.

• Oxygen is the final electron acceptor, forming water as a byproduct.

Phosphorylation Mechanisms

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP (in glycolysis and citric acid cycle).

• Oxidative phosphorylation: ATP synthesis powered by the ETC and chemiosmosis (in the inner mitochondrial membrane).

Fermentation and Anaerobic Respiration

• Fermentation: Allows glycolysis to continue by recycling NAD+ in the absence of oxygen. Yields 2 ATP per glucose.

• Types: Alcohol fermentation (yeast, produces ethanol and CO2), lactic acid fermentation (muscle cells, bacteria, produces lactate).

• Anaerobic respiration: Uses an ETC with a final electron acceptor other than oxygen (e.g., sulfate or nitrate).

Other Fuels and Biosynthesis

• Fats and proteins can enter cellular respiration at various points (e.g., fatty acids as acetyl CoA).

• Catabolic pathways provide carbon skeletons for anabolic pathways to synthesize biomolecules.

Photosynthesis

Overview and Function

Photosynthesis is the process by which autotrophs convert light energy into chemical energy, producing organic molecules and oxygen. It is essential for life on Earth, forming the base of the food web and maintaining atmospheric oxygen.

• Autotrophs: Producers that build organic molecules from CO2 (e.g., plants, algae, some bacteria).

• Heterotrophs: Consumers that obtain organic molecules from other organisms (e.g., animals, fungi).

• Carbon Cycle: Autotrophs fix CO2; both autotrophs and heterotrophs return CO2 via respiration.

Photosynthesis Equation and Reactant Fate

The summary equation for photosynthesis is:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• CO2: Reduced to form glucose.

• H2O: Oxidized to form O2.

Import/Export of Reactants and Products:

• CO2 (Import): Enters via stomata (microscopic pores).

• H2O (Import): Absorbed by roots, delivered via veins.

• O2 (Export): Exits through stomata.

• Sugar (Export): Transported from leaves to the rest of the plant via veins.

Light Reactions and Calvin Cycle

• Light Reactions: Occur in the thylakoid membranes; produce ATP and NADPH for the Calvin Cycle.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into G3P (a 3-carbon sugar).

Pigments and Light Absorption

• Pigments: Absorb specific wavelengths; reflected/transmitted wavelengths are seen as color (e.g., green for chlorophyll).

• Chlorophyll: Absorbs blue-violet and red; reflects green.

• Accessory Pigments: Broaden the spectrum of usable light (e.g., carotenoids).

Electron Flow in Light Reactions

• Photon excites pigment; energy transferred to reaction center.

• Electron transferred to primary acceptor; replaced by splitting water (releases O2).

• Electron passes through ETC, generating ATP (chemiosmosis), then re-excited in PS I and reduces NADP+ to NADPH.

Photorespiration and Adaptations

• Photorespiration: Occurs when RuBisCo fixes O2 instead of CO2 (hot, dry conditions).

• C4 Plants: Spatial separation of steps to maintain high CO2 for RuBisCo.

• CAM Plants: Temporal separation; fix CO2 at night, use it during the day.

The Cell Cycle

Phases and Purpose

• Cell Cycle: Interphase (G1, S, G2) and Mitotic (M) Phase (mitosis and cytokinesis).

• Purpose: Growth, development, repair (multicellular); reproduction (unicellular).

Stages of Mitosis

• Prophase: Chromosomes condense, spindle forms.

• Prometaphase: Nuclear envelope fragments, spindle attaches to kinetochores.

• Metaphase: Chromosomes align at metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclei reform, chromosomes decondense.

• Cytokinesis: Cytoplasm divides (cleavage furrow in animals, cell plate in plants).

Genetic Material Terminology

• DNA: Genetic information molecule.

• Gene: Unit of information within DNA.

• Genome: Complete set of DNA.

• Chromosome: DNA molecule with proteins.

• Replicated Chromosome: Two sister chromatids.

• Sister Chromatids: Identical copies, separated in anaphase.

• Chromatin: DNA-protein complex.

Cell Cycle Control and Cancer

• Checkpoints: G1 (before S), G2 (before M), M (before anaphase).

• Cyclins and Cdks: Cyclins regulate Cdks; MPF triggers mitosis.

• Cancer: Loss of cell cycle control; cells divide uncontrollably.

• Tumor: Mass of abnormal cells; Malignancy: Invades tissues; Metastasis: Spreads to other sites.

Meiosis and Sexual Life Cycles

Homologous Chromosomes and Genetic Variation

• Homologous Chromosomes: Pairs with genes for same traits; one from each parent.

• Alleles: Different versions of a gene.

• Autosomes: Non-sex chromosomes; Sex Chromosomes: X and Y in humans.

Somatic Cells vs. Gametes

Feature	Somatic Cells	Gametes
Ploidy	Diploid (2n)	Haploid (n)
Chromosome Number (humans)	46	23
Location	All body cells (except germ-line)	Gonads (testes/ovaries)

Meiosis: Process and Outcomes

• Meiosis I: Homologous chromosomes separate (Prophase I, Metaphase I, Anaphase I, Telophase I & Cytokinesis).

• Meiosis II: Sister chromatids separate (Prophase II, Metaphase II, Anaphase II, Telophase II & Cytokinesis).

• Results in four genetically unique haploid cells.

Comparison: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of cell divisions	One	Two
Number of daughter cells	Two	Four
Crossing over?	No	Yes (Prophase I)
Ploidy BEFORE	Diploid (2n)	Diploid (2n)
Ploidy AFTER	Diploid (2n)	Haploid (n)
Function in animals	Growth, repair, maintenance	Reproduction (gametes)

Sources of Genetic Diversity

• Mutations: Create new alleles.

• Independent Assortment: Random orientation of homologs in Metaphase I.

• Crossing Over: Exchange of DNA between nonsister chromatids in Prophase I.

• Random Fertilization: Any sperm can fertilize any egg, increasing combinations.