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

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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 primary energy currency of the cell. This process allows cells to efficiently convert the energy stored in food into a usable form, preventing energy loss as heat.

Primary Function: To convert chemical energy from glucose into ATP.
Comparison to Combustion: Both are redox processes that oxidize fuel and release energy. However, combustion releases all energy at once, while cellular respiration releases energy in controlled steps.

Redox Reactions in Metabolism

Redox (reduction-oxidation) reactions are central to metabolism, involving the transfer of electrons between molecules.

Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Oxidizing Agent: Electron acceptor (gets reduced).
Reducing Agent: Electron donor (gets oxidized).
Example: In the reaction , sodium is oxidized (reducing agent) and chlorine is reduced (oxidizing agent).

Example of redox reaction: Na + Cl → Na⁺ + Cl⁻

Coenzymes in Catabolism

NAD+ and FAD: Main electron carriers derived from vitamins (NAD+ from niacin/B3, FAD from riboflavin/B2).
Reduction: NAD+ accepts 2 electrons and 1 proton to become NADH.
Oxidation: NADH loses electrons and a proton to return to NAD+.

Phases of Cellular Respiration

Cellular respiration consists of four main phases, each with distinct inputs, outputs, and cellular locations.

Phase	Location	Starting Materials	Final Products	NADH/FADH2 Produced	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 electron transport chain (ETC).
The ETC pumps H+ ions, creating a proton-motive force.
ATP synthase uses this gradient to phosphorylate ADP, forming ATP (chemiosmosis).

Electron Transport Chain and ATP Synthase

Electrons lose energy as they move through the ETC.
The energy is used to pump H+ ions into the intermembrane space, generating a gradient.
ATP synthase allows H+ to flow back, spinning the enzyme and synthesizing ATP.

Role of Oxygen and Water

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

Phosphorylation Types

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 final stage).

Fermentation and Anaerobic Respiration

Fermentation: Regenerates NAD+ to keep glycolysis running; yields 2 ATP per glucose.
Types: Alcohol fermentation (yeast), lactic acid fermentation (muscle cells, bacteria).
Anaerobic respiration: Uses an ETC with a final electron acceptor other than oxygen.

Catabolic Pathways and Biosynthesis

Fats and proteins can enter cellular respiration at various points.
Catabolic pathways provide carbon skeletons for anabolic (biosynthetic) pathways.

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 the energy needs of plants and the biosphere.

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 into organic form; both autotrophs and heterotrophs return CO2 via respiration.

Photosynthetic Organisms

Photosynthesis occurs in plants, multicellular algae, unicellular eukaryotes, and some bacteria.
Animals and fungi do not perform photosynthesis.
Plants perform both photosynthesis and cellular respiration.

Photosynthesis Equation and Reactant Fate

The summary equation for photosynthesis is:

CO2: Reduced to form glucose.
H2O: Oxidized to form O2.

Import/Export:

CO2 (Import): Enters via stomata.
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.

Import and export of gases and sugars in plant leaves

Light Reactions and Calvin Cycle

Light Reactions: Occur in thylakoid membranes; produce ATP and NADPH for the Calvin Cycle.
Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 and synthesize G3P (a 3-carbon sugar).

Pigments and Light Absorption

Pigments: Absorb specific wavelengths; reflected/transmitted wavelengths are seen as color.
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.
Water is split, providing electrons and releasing O2.
Electrons move through ETC, generating ATP and reducing NADP+ to NADPH.

Cyclic Electron Flow

Involves only Photosystem I; produces ATP but not NADPH or O2.

Comparison: Photosynthesis vs. Cellular Respiration

Feature	Photosynthesis	Cellular Respiration
Phosphorylation	Photophosphorylation (light energy)	Oxidative (food energy)
Cycle	Calvin Cycle (anabolic)	Citric Acid Cycle (catabolic)
H+ Gradient	Stroma → Thylakoid Space	Matrix → Intermembrane Space

Photorespiration and Adaptations

Photorespiration: RuBisCo adds O2 instead of CO2 in hot, dry conditions, reducing efficiency.
C4 Plants: Spatially separate carbon fixation to maintain high CO2 for RuBisCo.
CAM Plants: Temporally separate carbon fixation (night vs. day).

The Cell Cycle

Reproduction and Cell Division

Asexual Reproduction: Single parent, genetically identical offspring.
Sexual Reproduction: Involves meiosis, produces genetically diverse gametes.

Phases of the Cell Cycle

Interphase: G1 (growth), S (DNA replication), G2 (preparation for division).
M Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

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, forming two daughter cells.

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.

Mitotic Spindle and Cytokinesis

Mitotic Spindle: Microtubule structure that moves chromosomes.
Cytokinesis: Cleavage furrow in animals; cell plate in plants.

Cell Cycle Control

Checkpoints: G1, G2, and M checkpoints regulate progression.
Cyclins and Cdks: Cyclins regulate Cdks, which trigger cell cycle transitions (e.g., MPF for mitosis).

Cancer and Cell Cycle Regulation

Cancer: Loss of cell cycle control, uncontrolled division.
Tumor: Mass of abnormal cells.
Malignancy: Invasive tumor.
Metastasis: Spread of cancer cells.
Normal vs. Cancer Cells: Cancer cells lack density-dependent inhibition and anchorage dependence.
Treatments: Surgery, radiation, chemotherapy, immunotherapy.

Meiosis and Sexual Life Cycles

Homologous Chromosomes and Genetic Variation

Homologous Chromosomes: Pairs with genes for the same traits; one from each parent.
Alleles: Different versions of a gene.
Separation: Homologs separate in Meiosis I; sister chromatids in Meiosis II.

Autosomes vs. Sex Chromosomes

Autosomes: Non-sex chromosomes (22 pairs in humans).
Sex Chromosomes: Determine sex (XX in females, XY in males).

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)

Animal Sexual Life Cycle

Diploid adults produce haploid gametes via meiosis.
Gametes fuse during fertilization to form a diploid zygote.
Zygote undergoes mitosis to become a multicellular adult.

Meiosis: Phases and Genetic Diversity

Meiosis I: Homologous chromosomes separate (Prophase I, Metaphase I, Anaphase I, Telophase I).
Meiosis II: Sister chromatids separate (Prophase II, Metaphase II, Anaphase II, Telophase II).
Cells become haploid after Meiosis I.
Crossing Over: Exchange of DNA between nonsister chromatids during Prophase I.
Independent Assortment: Random orientation of homologs during Metaphase I.
Random Fertilization: Any sperm can fuse with any egg, increasing genetic diversity.
Mutations: Original source of genetic variation.

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 (gamete production)