Cell Structure, Function, and Membranes: Prokaryotic and Eukaryotic Cells, Energy, and Enzymes

Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Prokaryotic Cell Structure

Prokaryotic cells, including bacteria and archaea, are simpler than eukaryotic cells and lack membrane-bound organelles. They have unique features that allow them to thrive in diverse environments.

• Plasmids: Small, circular DNA molecules that replicate independently of chromosomal DNA. They often carry genes for antibiotic resistance or metabolic functions.

• Cytoplasm: Contains ribosomes, enzymes, and sometimes cytoskeletal elements.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes).

• Cell Membrane: Boundary of the cell, involved in ATP production and transport.

• Cell Wall: Contains peptidoglycan (bacteria) or pseudopeptidoglycan (archaea). Provides structural support and shape.

• Glycocalyx (capsule/slime layer): Polysaccharide layer outside the cell wall, protects against desiccation and immune attack.

• Flagella and Fimbriae: Structures for motility (flagella) and attachment (fimbriae).

Bacterial Cell Wall Types

Type	Structure	Example
Gram-Positive	Thick peptidoglycan layer	Bacillus subtilis
Gram-Negative	Thin peptidoglycan, outer membrane	Escherichia coli

Archaea vs. Bacteria

• Archaea have unique membrane lipids and cell wall components (no peptidoglycan).

• Some archaeal structures are more similar to eukaryotes than bacteria.

Eukaryotic Cell Structure and Function

Eukaryotic cells are more complex, containing membrane-bound organelles that compartmentalize cellular functions.

• Nucleus: Contains genetic material (DNA), surrounded by a double membrane (nuclear envelope).

• Nucleolus: Site of rRNA synthesis and ribosome assembly.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles.

• Mitochondria and Chloroplasts: Organelles responsible for energy transformation (ATP production, photosynthesis).

• Cytoskeleton: Network of protein filaments (actin, intermediate filaments, microtubules) providing structure, shape, and movement.

• Plasma Membrane: Selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

Membrane-Bound Organelles and Their Functions

• Increase the area of membranes for metabolic reactions.

• Specialize in synthesis, modification, and transport of proteins and lipids.

• Compartmentalize cellular processes.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes, binary fission-like division.

Energy and Metabolism

Forms of Energy

• Potential Energy: Stored energy (e.g., chemical bonds).

• Kinetic Energy: Energy of motion.

First Law of Thermodynamics

• Energy cannot be created or destroyed, only transformed from one form to another.

Second Law of Thermodynamics

• Entropy (disorder) of a system tends to increase; energy transformations are not 100% efficient.

Gibbs Free Energy ($G$)

• Portion of a system's energy available to do work.

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

• $\Delta H$ = change in enthalpy (total energy)

• $T$ = absolute temperature (Kelvin)

• $\Delta S$ = change in entropy (disorder)

Exergonic vs. Endergonic Reactions

• Exergonic: $\Delta G < 0$, spontaneous, release energy.

• Endergonic: $\Delta G > 0$, require energy input.

Metabolism

• Catabolism: Breakdown of complex molecules, releases energy (e.g., glycolysis).

• Anabolism: Synthesis of complex molecules, requires energy (e.g., protein synthesis).

ATP: The Energy Currency

• ATP (Adenosine Triphosphate): Stores and transfers energy for cellular processes.

• Energy released by ATP hydrolysis is used to drive endergonic reactions.

Enzymes and Catalysis

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy ($E_a$).

• Bring reactants together in the correct orientation.

• Stabilize the transition state.

• Are not consumed in the reaction.

Mechanisms for Lowering Activation Energy

• Induced fit: Enzyme changes shape to better fit the substrate.

• Microenvironment: Provides optimal conditions for the reaction.

Factors Affecting Enzyme Activity

• Temperature and pH

• Substrate and enzyme concentration

• Presence of cofactors (metal ions, vitamins)

Regulation of Enzyme Activity

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

• Allosteric Regulation: Inhibitor or activator binds to a site other than the active site, changing enzyme activity.

• Feedback Inhibition: End product of a pathway inhibits an enzyme earlier in the pathway.

Membranes and Membrane Transport

Structure of Biological Membranes

• Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails inward.

• Membrane Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Cholesterol: Modulates membrane fluidity in animal cells.

Membrane Fluidity

• Increased by unsaturated fatty acids and higher temperatures.

• Decreased by saturated fatty acids and cholesterol at high temperatures.

Membrane Permeability

• Selective permeability: Allows some molecules to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) diffuse freely.

• Large or charged molecules require transport proteins.

Types of Membrane Transport

Type	Energy Required?	Example
Simple Diffusion	No	O2, CO2
Facilitated Diffusion	No	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Na+/K+ pump

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves from low solute concentration to high solute concentration.

Endomembrane System and Protein Trafficking

• Proteins synthesized in the rough ER are modified and sorted in the Golgi apparatus.

• Vesicles transport proteins to their destinations (e.g., lysosomes, plasma membrane).

• Exocytosis: Secretion of proteins out of the cell.

• Endocytosis: Uptake of materials into the cell via vesicles.

Cytoskeleton

• Actin Filaments: Provide shape, enable movement (muscle contraction, cell crawling).

• Intermediate Filaments: Provide mechanical strength (e.g., keratin).

• Microtubules: Hollow tubes, involved in cell division, organelle movement, and structure (e.g., cilia, flagella).

Motor Proteins

• Kinesin and dynein move along microtubules, transporting vesicles and organelles.

• Myosin interacts with actin for muscle contraction and cell movement.

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell wall	Yes (most)	Some (plants, fungi)
Size	Small (1-10 μm)	Larger (10-100 μm)

Example: Escherichia coli (prokaryote) vs. human liver cell (eukaryote)

Additional info: Some details, such as the specific roles of the endomembrane system and the mechanisms of protein targeting, were expanded for clarity and completeness.