Cellular Respiration, Photosynthesis, Cell Signaling, and the Cell Cycle: Exam II Study Guide

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Cellular Respiration and Fermentation

Catabolic Pathways and ATP Production

Catabolic pathways break down organic molecules to release energy, which is used to produce ATP. These pathways include both fermentation and cellular respiration.

Fermentation: Partial degradation of sugars without oxygen.
- Alcoholic fermentation: Pyruvate is converted to acetaldehyde, then reduced by NADH to ethanol. Acetaldehyde is the final electron acceptor.
- Lactic acid fermentation: Pyruvate is directly reduced by NADH to lactate. Pyruvate is the final electron acceptor.
Respiration: Complete degradation of sugars using an electron transport chain.
- Aerobic respiration: Final electron acceptor is O2.
- Anaerobic respiration: Final electron acceptor is a molecule other than O2 (e.g., sulfate).
Overall equation for aerobic respiration:

Redox Reactions in Cellular Respiration

Oxidation: Loss of electrons (LEO: Loss of Electron is Oxidation).
Reduction: Gain of electrons (GER: Gain of Electron is Reduction).
Reducing agent: Electron donor; gets oxidized.
Oxidizing agent: Electron acceptor; gets reduced.
In respiration, glucose is oxidized and oxygen is reduced.
Carbohydrates and fats are efficient fuels due to many C–H bonds (rich in electrons).

Electron Transport Chain and Energy Harvest

Energy is harvested in steps, not all at once, to efficiently produce ATP.
NAD+ is reduced to NADH by dehydrogenase enzymes.
Electrons flow: glucose → NADH → electron transport chain → oxygen.
Electron carriers in the chain are increasingly electronegative; oxygen is the final acceptor, forming water.

Stages of Cellular Respiration

Glycolysis (in cytosol):
- Glucose is split into 2 pyruvate molecules.
- 2 ATP produced by substrate-level phosphorylation.
Pyruvate Oxidation & Citric Acid Cycle (in mitochondria):
- Pyruvate is converted to Acetyl-CoA (no ATP produced in this step).
- Acetyl-CoA enters the Citric Acid Cycle; electrons are transferred to NADH and FADH2.
- 2 ATP produced by substrate-level phosphorylation.
- Oxaloacetate is regenerated, making the cycle continuous.
Oxidative Phosphorylation (in inner mitochondrial membrane):
- NADH and FADH2 donate electrons to the electron transport chain (ETC).
- Electron flow powers H+ pumping, creating a gradient.
- ATP synthase uses the H+ gradient (chemiosmosis) to produce 26–28 ATP.
- Most ETC carriers are cytochromes (proteins with heme groups).
Versatility of Catabolism:
- Fats are broken down by beta oxidation; proteins by deamination.

Photosynthesis

Overview and Organisms

Photosynthesis converts light energy, water, and CO2 into organic sugars. It is essential for life on Earth.

Autotrophs: Organisms that make their own food from inorganic molecules (e.g., plants).
Heterotrophs: Organisms that obtain food and carbon from other living sources.

Leaf Anatomy and Chloroplast Structure

Mesophyll cells: Leaf interior cells rich in chloroplasts.
Stomata: Pores on leaf underside for gas exchange (CO2 in, O2 out).
Chloroplasts: Organelles of photosynthesis.
- Stroma: Dense fluid inside chloroplasts.
- Thylakoids: Membranous sacs containing chlorophyll; site of light reactions.
- Chlorophyll: Green pigment in thylakoids.

The Nature of Sunlight and Pigments

Light behaves as both waves and particles (photons).
Chlorophyll a: Main pigment for light capture.
Chlorophyll b: Accessory pigment, broadens light absorption.
Carotenoids: Accessory pigments, protect chlorophyll from damage.

Stages of Photosynthesis

Light Reactions (in thylakoid membrane):
- Sunlight excites electrons in chlorophyll, which are transferred through photosystems and electron transport chains.
- Photosystem II (PSII): Splits water, releases O2, pumps H+ for ATP synthesis.
- Photosystem I (PSI): Accepts electrons from PSII, reduces NADP+ to NADPH.
- Inputs: H2O, light, NADP+, ADP.
- Outputs: NADPH, ATP, O2 (waste).
Calvin Cycle (in stroma):
- Uses ATP and NADPH to fix CO2 into sugars.
- Three phases: Carbon fixation (by Rubisco), reduction (to G3P), regeneration of RuBP.
- For every 6 G3P produced, 5 are used to regenerate RuBP; 1 exits as sugar.
- Inputs: CO2, NADPH, ATP.
- Outputs: G3P (sugar), NADP+, ADP.

Adaptations to Dry Climates

Closing stomata conserves water but limits CO2 intake, leading to photorespiration (wasteful fixation of O2 by Rubisco).
C4 plants: Fix CO2 in mesophyll, transport 4-carbon compound to bundle-sheath cells for Calvin cycle (e.g., grasses).
CAM plants: Open stomata at night to fix CO2; Calvin cycle runs during the day with stomata closed.

Cell Signaling and Communication

Types of Cell Signaling

Local signaling:
- Paracrine: Signaling cell acts on nearby cells via local regulators (e.g., growth factors).
- Synaptic: Nerve cell releases neurotransmitters into a synapse to stimulate target cells.
Long-distance signaling:
- Endocrine: Hormones secreted into body fluids (e.g., blood) affect distant cells (e.g., sex hormones).

Stages of Cell Signaling

Reception: Ligand binds to receptor, causing a shape change.
- G-protein Coupled Receptor (GPCR): Ligand binding activates G-protein (GDP to GTP exchange), which activates enzymes like adenylyl cyclase.
- Ligand-gated ion channel: Ligand binding opens channel for ion flow (important in neurons).
- Intracellular receptors: Located in cytosol; respond to small, hydrophobic molecules (e.g., hormones).
Transduction: Signal is relayed and amplified through a cascade of proteins.
- Phosphorylation: Addition of phosphate group by kinases activates proteins.
- Phosphorylation cascade: Series of kinases activating each other.
- Dephosphorylation: Removal of phosphate by phosphatases turns off the pathway.
- Second messengers: Small molecules (e.g., cAMP, Ca2+) amplify the signal.
Response: Regulation of enzymes or gene expression.
- Transcription factors: Proteins that enter the nucleus to activate genes.
- Apoptosis: Programmed cell death as a possible response.

The Cell Cycle

Overview and Importance

The cell cycle is the sequence of events in the life of a cell, from its formation to its division into two daughter cells. It ensures the continuity of life and the faithful transmission of genetic material.

Genome: The complete set of genetic material in a cell.
Chromosomes: DNA molecules packaged with proteins (histones) into chromatin.
Somatic cells: Body cells (not gametes).
Gametes: Reproductive cells (sperm, eggs).
Sister chromatids: Duplicated copies of a chromosome, joined at the centromere.

Phases of the Cell Cycle

M Phase (Mitotic phase): Shortest phase; includes mitosis and cytokinesis.
- Prophase: Chromatin condenses, spindle forms, centrosomes move apart.
- Prometaphase: Nuclear envelope fragments, kinetochores form, microtubules attach.
- Metaphase: Chromosomes align at metaphase plate.
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: Nuclear envelopes reform, chromosomes decondense; followed by cytokinesis.
- Cytokinesis: Division of cytoplasm (cleavage furrow in animals, cell plate in plants).
Interphase: 90% of cell cycle; divided into G1 (growth), S (DNA synthesis), G2 (preparation for division).

Binary Fission in Prokaryotes

Prokaryotes divide by binary fission, not mitosis.
Replication begins at the origin of replication and proceeds around the circular DNA.

Cell Cycle Regulation

Controlled by a cell cycle control system (molecular signals).
Checkpoints: Control points where stop/go signals regulate progression (e.g., G1 checkpoint).
Cyclins: Regulatory proteins that control cell cycle progression.
Growth factors: Stimulate cell division.
Density-dependent inhibition: Crowded cells stop dividing.

Loss of Cell Cycle Control and Cancer

Cancer cells ignore normal regulatory signals.
They may divide uncontrollably, avoid apoptosis, and accumulate mutations.
Tumors: Masses of cancer cells; can be benign (localized) or metastatic (spread to other tissues).