General Biology: Midterm Exam III Study Notes

CELL MEMBRANES & TRANSPORT

Structure and Function of Cell Membranes

The cell membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins. Its structure and composition determine the movement of substances into and out of the cell.

• Phospholipid Bilayer Fluidity: Increases with short, unsaturated fatty acid tails; decreases with long, saturated tails.

• Cholesterol: Modulates membrane fluidity and stability.

• Amphipathic Nature: Phospholipids have hydrophilic heads and hydrophobic tails.

Transport Mechanisms

• Passive Transport: Includes diffusion, facilitated diffusion, and osmosis. Moves substances down their concentration gradient without energy input.

• Active Transport: Requires energy (often ATP) to move substances against their concentration gradient.

• Example: Sodium-potassium pump actively transports Na+ and K+ ions across the membrane.

CELL COMMUNICATION

Signaling Pathways

Cells communicate through chemical signals that trigger specific responses via signal transduction pathways.

• Phases: Signal reception, transduction, and response.

• Types of Signals: Hormones, neurotransmitters, and growth factors.

• Second Messengers: Molecules like cAMP amplify the signal within the cell.

• G Protein-Coupled Receptors (GPCRs): Common membrane receptors involved in many signaling pathways.

• Example: Epinephrine binding to GPCRs triggers a cascade leading to glucose release.

METABOLISM & ENZYMES

Enzyme Function and Metabolic Pathways

Metabolism encompasses all chemical reactions in a cell, many of which are catalyzed by enzymes to increase reaction rates and lower activation energy.

• Activation Energy: The energy required to initiate a chemical reaction.

• Enzyme Specificity: Enzymes are highly specific for their substrates.

• Regulation: Enzyme activity can be regulated by inhibitors and activators.

• Example: ATP synthase catalyzes the formation of ATP from ADP and inorganic phosphate.

CELLULAR RESPIRATION

Stages and Energy Production

Cellular respiration is the process by which cells extract energy from glucose, primarily through aerobic pathways.

• Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate.

• Krebs Cycle (Citric Acid Cycle): Occurs in mitochondria; produces NADH and FADH2.

• Electron Transport Chain (ETC): Uses NADH and FADH2 to generate ATP via oxidative phosphorylation.

• Fermentation: Anaerobic process that regenerates NAD+ for glycolysis.

• Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

PHOTOSYNTHESIS

Light and Dark Reactions

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

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

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into glucose.

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

• Example: Chlorophyll absorbs light, initiating electron transport in photosystems.

GENE EXPRESSION & REGULATION

Transcription, Translation, and Regulation

Gene expression involves the transcription of DNA into RNA and the translation of RNA into proteins. Regulation ensures that genes are expressed at the right time and place.

• Transcription: DNA is copied into messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to synthesize proteins.

• Regulation: Involves promoters, enhancers, and repressors.

• Cell Differentiation: Results from differential gene expression.

• Example: The lac operon in bacteria regulates genes involved in lactose metabolism.