Core Concepts in General Biology: Cell Structure, Energy, Metabolism, and Communication

Cell Structure and Membrane Function

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Phospholipids: Amphipathic molecules forming a bilayer, with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and lipids moving laterally.

• Cholesterol: Modulates membrane fluidity and stability.

• Membrane Proteins: Include channel, carrier, and receptor proteins, each with specific functions.

• Selective Permeability: Only certain molecules can cross freely; others require transport proteins.

Osmosis and Diffusion are passive transport processes driven by concentration gradients.

• Tonicity: Refers to the relative concentration of solutes (isotonic, hypertonic, hypotonic) affecting cell volume.

• Channel and Carrier Proteins: Facilitate movement of ions and molecules across the membrane.

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

• Electrochemical Gradient: Combination of concentration and electrical gradients influencing ion movement.

Bulk Transport

Cells use endocytosis and exocytosis for the transport of large molecules.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids.

• Receptor-mediated Endocytosis: Specific uptake via receptor binding.

Cellular Energy and Metabolism

Energy Concepts

Cells require energy to perform work, which is governed by the laws of thermodynamics.

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy, such as in chemical bonds.

• Thermodynamics: First Law (energy conservation), Second Law (entropy increases).

• Free Energy (G): Energy available to do work; exergonic reactions release energy, endergonic require input.

Equation for free energy change:

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

• Exergonic Reaction: Releases energy ($\Delta G < 0$).

• Endergonic Reaction: Requires energy input ($\Delta G > 0$).

Enzymes and Catalysis

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

• Substrate: The reactant an enzyme acts upon.

• Active Site: Region on enzyme where substrate binds.

• Activation Energy: Minimum energy required to start a reaction.

• Competitive Inhibitor: Binds active site, blocking substrate.

• Noncompetitive (Allosteric) Inhibitor: Binds elsewhere, changing enzyme shape.

Cellular Respiration

Overview

Cellular respiration is the process by which cells extract energy from glucose and other molecules.

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

• Pyruvate Oxidation: Converts pyruvate to Acetyl CoA.

• Krebs Cycle (Citric Acid Cycle): Completes oxidation of Acetyl CoA, producing NADH and FADH2.

• Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis to produce ATP.

Key molecules:

• NAD+ and NADH: Electron carriers.

• FAD and FADH2: Electron carriers.

• ATP Synthase: Enzyme that synthesizes ATP using proton gradient.

Summary Table: Major Steps of Cellular Respiration

Step	Main Location	Key Products
Glycolysis	Cytoplasm	Pyruvate, ATP, NADH
Pyruvate Oxidation	Mitochondrial Matrix	Acetyl CoA, NADH, CO2
Krebs Cycle	Mitochondrial Matrix	NADH, FADH2, ATP, CO2
Electron Transport Chain	Inner Mitochondrial Membrane	ATP, H2O

Fermentation

When oxygen is absent, cells use fermentation to regenerate NAD+.

• Lactic Acid Fermentation: Produces lactate (in muscle cells).

• Alcohol Fermentation: Produces ethanol and CO2 (in yeast).

Photosynthesis

Overview

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

• Chloroplasts: Organelles where photosynthesis occurs.

• Thylakoids: Membranous sacs containing chlorophyll.

• Grana: Stacks of thylakoids.

• Stroma: Fluid surrounding thylakoids.

Light Reactions and Calvin Cycle

• Light Reactions: Occur in thylakoid membranes; convert light energy to ATP and NADPH.

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

• Chlorophyll: Pigment that absorbs light energy.

• Photosystems: Complexes that capture light (I and II).

• Rubisco: Enzyme that fixes CO2 in Calvin Cycle.

Summary Table: Photosynthesis Pathways

Pathway	Main Features	Example Organisms
C3 Photosynthesis	CO2 fixed directly by Rubisco	Most plants
C4 Photosynthesis	CO2 fixed in mesophyll, then bundle sheath cells	Corn, sugarcane
CAM Photosynthesis	CO2 fixed at night, stored as malate	Cacti, succulents

Cell Communication and Signal Transduction

Overview

Cells communicate using chemical signals, which are detected and processed by receptors.

• Autocrine Signaling: Cell targets itself.

• Paracrine Signaling: Cell targets nearby cells.

• Endocrine Signaling: Hormones travel through bloodstream to distant cells.

• Synaptic Signaling: Nerve cells communicate via neurotransmitters.

Signal Transduction Pathways

• Receptors: Proteins that bind signaling molecules (ligands).

• GPCRs (G-protein coupled receptors): Transmit signals via GTP/GDP exchange.

• RTKs (Receptor Tyrosine Kinases): Activate signaling cascades via phosphorylation.

• Second Messengers: Small molecules (e.g., cAMP) that amplify signals.

• Protein Kinases: Enzymes that add phosphate groups to proteins.

• Protein Phosphatases: Remove phosphate groups.

• Transcription Factors: Regulate gene expression in response to signals.

• Apoptosis: Programmed cell death, important for development and homeostasis.

Summary Table: Types of Cell Signaling

Type	Distance	Example
Autocrine	Self	Immune cell signaling
Paracrine	Nearby	Growth factors
Endocrine	Distant	Hormones (e.g., insulin)
Synaptic	Very close	Neurotransmitters

Additional info:

• Some terms (e.g., "Energy Coupling", "Scaffolding Proteins") are included for completeness but not expanded due to brevity in original notes.

• All major topics are grouped and expanded for clarity and exam preparation.