General Biology Study Guide: Membranes, Energy, Cellular Respiration, Photosynthesis, and Cell Communication (Chapters 7–11)

Chapter 7: Membranes & Transport

Membrane Structure

The plasma membrane is a selectively permeable barrier that separates the cell from its environment. Its structure is primarily a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a semi-permeable boundary.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and other molecules floating in or on the fluid lipid bilayer.

• Membrane Proteins: Integral (span the membrane) and peripheral (attached to the surface) proteins perform functions such as transport, signaling, and cell recognition.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), involved in cell-cell recognition.

• Cholesterol: Modulates membrane fluidity and stability.

Example: Glycoproteins on red blood cells determine blood type.

Membrane Transport

Transport across membranes is essential for maintaining homeostasis. It occurs via passive or active mechanisms.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Transport of substances via membrane proteins (channels or carriers).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Bulk Transport: Endocytosis (into the cell) and exocytosis (out of the cell) for large molecules.

Example: Sodium-potassium pump ($Na^+/K^+$ ATPase) maintains electrochemical gradients in animal cells.

Chapter 8: Energy & Metabolism

Metabolism Overview

Metabolism encompasses all chemical reactions in an organism, divided into catabolic (breakdown) and anabolic (synthesis) pathways.

• Catabolism: Breaks down molecules, releasing energy (e.g., cellular respiration).

• Anabolism: Builds complex molecules, consuming energy (e.g., protein synthesis).

• Energy: The capacity to do work; exists as kinetic or potential energy.

Example: Glucose breakdown in glycolysis is catabolic; synthesis of DNA is anabolic.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics, which govern energy transformations.

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

• Free Energy ($G$): The portion of a system's energy available to do work.

Equation:

$\Delta G = \Delta H - T\Delta S$

Where $\Delta G$ is change in free energy, $\Delta H$ is change in enthalpy, $T$ is temperature, and $\Delta S$ is change in entropy.

ATP & Energy Coupling

ATP (adenosine triphosphate) is the cell's main energy currency, coupling exergonic and endergonic reactions.

• Hydrolysis of ATP: Releases energy by breaking a phosphate bond.

• Energy Coupling: Uses exergonic reactions to drive endergonic processes.

Equation:

$ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Enzymes & Regulation

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Substrate: The reactant an enzyme acts upon.

• Active Site: Region on enzyme where substrate binds.

• Competitive Inhibition: Inhibitor binds to active site, blocking substrate.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Allosteric Regulation: Regulation by molecules binding at sites other than the active site.

Example: Feedback inhibition in metabolic pathways.

Chapter 9: Cellular Respiration & Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP.

• Aerobic Respiration: Requires oxygen; produces most ATP.

• Anaerobic Respiration/Fermentation: Occurs without oxygen; produces less ATP.

• Equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Stages of Cellular Respiration

• 1. Glycolysis: Occurs in cytoplasm; breaks glucose into 2 pyruvate, produces 2 ATP and 2 NADH.

• 2. Pyruvate Oxidation: Pyruvate converted to Acetyl-CoA, producing NADH and CO2.

• 3. Citric Acid Cycle (Krebs Cycle): Completes glucose breakdown, produces ATP, NADH, FADH2, and CO2.

• 4. Oxidative Phosphorylation: Electron transport chain and chemiosmosis; produces most ATP.

Fermentation

Fermentation allows ATP production without oxygen.

• Lactic Acid Fermentation: Pyruvate reduced to lactate (e.g., muscle cells).

• Alcohol Fermentation: Pyruvate converted to ethanol and CO2 (e.g., yeast).

Example: Muscle fatigue during intense exercise is due to lactic acid fermentation.

Chapter 10: Photosynthesis

Overview of Photosynthesis

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

• Equation:

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

• Occurs in: Chloroplasts of plant cells.

• Two Stages: Light reactions and Calvin cycle.

Light Reactions

Light reactions convert solar energy to chemical energy (ATP and NADPH).

• Location: Thylakoid membranes.

• Inputs: Water, light.

• Outputs: Oxygen, ATP, NADPH.

Calvin Cycle

The Calvin cycle uses ATP and NADPH to fix carbon dioxide into glucose.

• Location: Stroma of chloroplast.

• Does not require light directly.

• Phases: Carbon fixation, reduction, regeneration of RuBP.

Alternative Forms of Photosynthesis

• C4 Plants: Spatial separation of carbon fixation and Calvin cycle; adapted to hot, dry environments.

• CAM Plants: Temporal separation; fix CO2 at night, Calvin cycle during day.

Example: Corn is a C4 plant; cactus is a CAM plant.

Chapter 11: Cell Communication

Overview of Cell Communication

Cells communicate via chemical signals to coordinate activities. Signal transduction pathways convert signals received at a cell’s surface into cellular responses.

• Local Signaling: Paracrine (nearby cells), synaptic (neurons).

• Long-Distance Signaling: Endocrine (hormones in bloodstream).

Stages of Cell Signaling

• 1. Reception: Signal molecule (ligand) binds to receptor protein.

• 2. Transduction: Signal is relayed and amplified by a cascade of molecular interactions (often involving phosphorylation).

• 3. Response: Cellular activity is altered (e.g., gene expression, enzyme activity).

Types of Receptors

• Membrane Receptors: G protein-coupled receptors (GPCRs), ion channel receptors, receptor tyrosine kinases.

• Intracellular Receptors: Found in cytoplasm or nucleus; bind small or hydrophobic ligands.

Signal Transduction Pathways

• Phosphorylation Cascade: Series of protein kinases activate each other by adding phosphate groups.

• Second Messengers: Small molecules (e.g., cAMP, Ca2+) relay signals inside the cell.

Example: Epinephrine signaling in liver cells triggers glucose release.

Summary Table: Key Processes in Cellular Metabolism

Process	Location	Main Inputs	Main Outputs	ATP Yield
Glycolysis	Cytoplasm	Glucose	Pyruvate, NADH, ATP	2
Pyruvate Oxidation	Mitochondrial Matrix	Pyruvate	Acetyl-CoA, NADH, CO2	0
Krebs Cycle	Mitochondrial Matrix	Acetyl-CoA	NADH, FADH2, CO2, ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	NADH, FADH2, O2	ATP, H2O	~26-28
Fermentation	Cytoplasm	Pyruvate	Lactate or Ethanol, CO2	2
Photosynthesis (Light Rxns)	Thylakoid Membrane	Light, H2O	O2, ATP, NADPH	---
Photosynthesis (Calvin Cycle)	Stroma	CO2, ATP, NADPH	Glucose	---

Additional info: Some details, such as the exact ATP yield and the distinction between C4 and CAM plants, were expanded for clarity and completeness.