Cell Theory and Basic Cell Components

Cell Theory

Cell theory is a fundamental concept in biology stating that all living organisms are composed of cells, which are the basic units of life.

• Proteins: Perform most cellular functions.

• Nucleic acids: Store, transmit, and process genetic information.

• Carbohydrates: Provide chemical energy, carbon, support, and identity.

• Lipids: Selectively permeable membrane components.

Prokaryotic Cell Structures

General Features

Prokaryotic cells, such as bacteria and archaea, have distinctive structures and functions.

• Typically have a single, circular chromosome.

• Contain many protein-synthesizing ribosomes.

• Phospholipid components:

  • Bacterial phospholipids: Fatty acids bound to glycerol.

  • Archaeal phospholipids: Branched isoprenoid chains bound to glycerol.

• Cytoplasm: All contents of the cell inside the membrane, made of cytosol (liquid component).

• Plasmids: Small, supercoiled DNA molecules that can help spread antibiotic resistance.

Chromosome

• Most prokaryotes have a single, circular chromosome located in the nucleoid region.

• DNA is often associated with proteins for structural support.

Ribosomes

• Prokaryotes have ribosomes for protein synthesis.

• Ribosomes are macromolecular structures.

Cell Wall

• Composed of peptidoglycan in bacteria.

• Some bacteria have an outer membrane made of glycolipids.

Other Structures

• Flagella: Long filaments that rotate to propel the cell.

• Fimbriae: Needlelike projections that promote attachment to other cells or surfaces.

Eukaryotic Cell Structures and Functions

Compartmentalization

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions.

• Allows separation of incompatible chemical reactions.

• Increases efficiency of chemical reactions.

Mitochondria

• Site of ATP (energy) production.

• Double-membraned: inner and outer membranes.

• Contains mitochondrial matrix and its own DNA (mtDNA).

• Manufactures some of its own proteins.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes and processes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies, and stores calcium ions.

Golgi Apparatus

• Processes, sorts, and ships proteins in vesicles.

• Composed of stacked, flat membranous sacs called cisternae.

• Has distinct polarity (cis and trans faces).

Nucleus

• Highly organized, double-membrane bound compartment.

• Contains nucleolus (site of rRNA synthesis and ribosome assembly).

• Studded with nuclear pores for transport of molecules.

Animal-Specific Structures

• Centrioles: Two per centrosome; organize microtubules.

• Lysosomes: Contain hydrolytic enzymes for digestion; maintain low pH via proton pumps.

• Extracellular matrix: Provides structural support; composed of secreted proteins and polysaccharides.

Plant-Specific Structures

• Chloroplasts: Sites of photosynthesis; contain thylakoids, grana, and stroma.

• Cell wall: Composed of cellulose; provides rigidity.

• Central vacuole: Stores water, ions, and macromolecules; maintains cell shape.

Cytoskeleton

• Provides shape, stability, and organization.

• Transports materials within the cell.

• Composed of actin filaments, intermediate filaments, and microtubules.

Endomembrane System and Cellular Transport

Endomembrane System

• Manufactures, ships, and recycles cargo within the cell.

• Includes ER, Golgi apparatus, lysosomes, and vesicles.

• Proteins destined for secretion have a molecular zip code (signal sequence).

Vesicle Transport

• Vesicles bud from the ER, move to and through the Golgi, and are targeted to specific destinations.

Phagocytosis and Autophagy

• Phagocytosis: Cell engulfs large particles or other cells.

• Autophagy: Cell recycles its own damaged organelles or macromolecules.

Enzymes and Chemical Reactions

Energy in Cells

• Cells require energy to perform work, respond to the environment, and build molecules.

• Enzymes catalyze reactions, increasing their speed and efficiency.

Metabolic Pathways

• Series of chemical reactions that build up (anabolic) or break down (catabolic) molecules.

• Each step is catalyzed by a specific enzyme.

Types of Energy

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or configuration (e.g., chemical bonds).

Laws of Thermodynamics

• First Law: Energy is conserved; it cannot be created or destroyed, only transformed.

• Second Law: Entropy (disorder) increases in spontaneous processes; energy transformations are inefficient.

Enthalpy and Reaction Types

• Enthalpy (H): Total energy in a molecule, including potential energy in bonds.

• Exothermic reactions: Release heat; products have less potential energy than reactants ( is negative).

• Endothermic reactions: Absorb heat; products have more potential energy than reactants ( is positive).

Gibbs Free Energy (G)

• Determines whether a reaction is spontaneous.

• Negative indicates a spontaneous (exergonic) reaction.

• Positive indicates a nonspontaneous (endergonic) reaction.

ATP and Energy Coupling

• ATP hydrolysis releases free energy ( is highly negative).

• Energy from ATP hydrolysis is used to drive endergonic reactions via phosphorylation.

• Phosphorylation changes the shape and activity of target molecules.

Enzyme Function and Kinetics

• Enzymes lower activation energy, speeding up reactions.

• Each enzyme is specific to its substrate (active site binding).

• Enzyme catalysis involves three steps:

  1. Initiation: Substrates bind to the active site.

  2. Transition state facilitation: Enzyme stabilizes the transition state.

  3. Termination: Products are released.

• Enzymes exhibit saturation kinetics: reaction rate increases with substrate concentration but plateaus at high concentrations.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Key Equations

• Gibbs Free Energy:

• ATP Hydrolysis: