Key Biological Processes

Why This Matters

Every question about how organisms survive, grow, reproduce, and respond to their environment traces back to these fundamental biological processes. You're being tested on your ability to connect the dots—understanding how energy transformation, information flow, and cellular regulation work together to sustain life. These aren't isolated facts; they form an interconnected web where photosynthesis feeds into cellular respiration, DNA replication enables cell division, and cell signaling maintains homeostasis.

Don't just memorize definitions—know what each process accomplishes, where it occurs, and how it connects to other processes. When an exam asks about energy flow in ecosystems, you need to link photosynthesis and cellular respiration. When it asks about genetic continuity, you need to connect DNA replication, protein synthesis, and cell division. Master the mechanisms, and the facts will stick.

---

Energy Transformation Processes

Life requires constant energy conversion. These processes capture, store, and release energy in forms cells can actually use—primarily ATP. The flow of energy through living systems depends on these complementary reactions.

Photosynthesis

• Converts light energy into chemical energy—glucose molecules store this energy in their chemical bonds for later use
• Occurs in chloroplasts using chlorophyll pigments that absorb specific wavelengths of light, primarily in the thylakoid membranes
• Two-stage process: light-dependent reactions (in thylakoids) produce ATP and NADPH; the Calvin cycle (in stroma) uses these to fix $CO_2$ into glucose

Cellular Respiration

• Breaks down glucose to produce ATP—the universal energy currency that powers virtually all cellular work
• Three main stages: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and electron transport chain (inner mitochondrial membrane)
• Yields approximately 36-38 ATP per glucose molecule—far more efficient than fermentation, which produces only 2 ATP

Metabolism

• The sum of all chemical reactions in a cell—includes catabolism (breaking down molecules to release energy) and anabolism (building molecules using energy)
• Metabolic pathways are interconnected—intermediates from one pathway often feed into others, creating flexibility in how cells use nutrients
• Regulated by enzymes and hormones—ensuring energy production matches cellular demand and preventing wasteful reactions

---

Information Storage and Expression

Life depends on storing genetic instructions and accurately converting them into functional molecules. The central dogma—DNA → RNA → Protein—describes this information flow.

DNA Replication

• Semi-conservative process—each new DNA molecule contains one original strand and one newly synthesized strand, ensuring accuracy
• Requires multiple enzymes: helicase unwinds the double helix, DNA polymerase synthesizes new strands, ligase joins fragments
• High fidelity with proofreading—error rate of approximately $10^{-9}$ per base pair, critical for genetic stability across generations

Protein Synthesis (Transcription and Translation)

• Transcription occurs in the nucleus—RNA polymerase reads DNA and produces mRNA, which carries the genetic message to ribosomes
• Translation occurs at ribosomes—mRNA codons are matched with tRNA anticodons, assembling amino acids into polypeptide chains
• Proteins determine phenotype—they serve as enzymes, structural components, signaling molecules, and regulators of gene expression

---

Cell Division and Reproduction

Organisms grow, repair damage, and reproduce through controlled cell division. The type of division determines whether daughter cells are identical or genetically unique.

Cell Division (Mitosis and Meiosis)

• Mitosis produces two identical diploid cells—used for growth, repair, and asexual reproduction; maintains chromosome number
• Meiosis produces four haploid gametes—involves two divisions and crossing over, creating genetic variation essential for evolution
• Checkpoints regulate both processes—ensuring DNA is replicated correctly and chromosomes align properly before division proceeds

---

Cellular Regulation and Communication

Cells must respond to changing conditions and coordinate with other cells. Feedback loops and signaling pathways allow precise control over biological processes.

Enzyme Function and Regulation

• Enzymes lower activation energy—they're biological catalysts that speed reactions without being consumed, often by millions of times
• Specificity determined by shape—the active site binds only specific substrates, following the induced fit model
• Regulated by multiple factors—temperature, pH, inhibitors (competitive and noncompetitive), and allosteric modulators control enzyme activity

Cell Signaling

• Three stages: reception (signal binds receptor), transduction (signal is amplified through cascades), response (cellular change occurs)
• Signaling molecules include hormones and neurotransmitters—these can act locally (paracrine) or throughout the body (endocrine)
• Signal transduction often involves second messengers—molecules like cAMP amplify signals, allowing small amounts of hormone to produce large effects

Homeostasis

• Maintains stable internal conditions—despite external fluctuations in temperature, pH, glucose levels, and other variables
• Negative feedback loops dominate—a change triggers a response that reverses the change, like a thermostat maintaining temperature
• Positive feedback amplifies change—used in specific situations like blood clotting and childbirth, where rapid escalation is beneficial

---

Transport and Exchange

Cells must move materials across membranes to obtain nutrients, remove waste, and maintain proper concentrations. The selective permeability of membranes makes life possible.

Membrane Transport

• Passive transport requires no energy—substances move down their concentration gradient via diffusion, osmosis (water), or facilitated diffusion (using proteins)
• Active transport requires ATP—moves substances against their gradient using pumps like the sodium-potassium pump ($Na^+/K^+$-ATPase)
• Bulk transport moves large particles—endocytosis brings materials in; exocytosis releases materials out, both using vesicles

---

Quick Reference Table

---

Self-Check Questions

1. Which two processes are essentially reverse reactions of each other, and what molecules do they exchange?

2. How does meiosis generate genetic variation in ways that mitosis cannot? Identify at least two mechanisms.

3. Compare and contrast competitive and noncompetitive enzyme inhibition—how does each affect the active site and reaction rate?

4. If an FRQ describes a cell responding to a hormone that triggers a cascade of reactions inside the cell, what three stages of cell signaling should you discuss?

5. A cell needs to move glucose from an area of low concentration to high concentration. What type of transport is required, what cellular structure provides the energy, and what would happen if that structure were disabled?