Cell Structure and Function

Prokaryotic Cell Components

Prokaryotic cells, such as bacteria and archaea, have a simpler structure compared to eukaryotic cells. Understanding their components is essential for distinguishing cell types and their functions.

• Plasma Membrane: A phospholipid bilayer that controls the movement of substances in and out of the cell.

• Cell Wall: Provides structural support and protection; composed of peptidoglycan in bacteria.

• Cytoplasm: Gel-like substance where cellular processes occur.

• Ribosomes: Sites of protein synthesis; smaller than eukaryotic ribosomes.

• Nucleoid: Region containing the cell's circular DNA; not membrane-bound.

• Flagella: Tail-like structures used for movement (not present in all prokaryotes).

• Pili/Fimbriae: Hair-like structures for attachment to surfaces or other cells.

Example: Escherichia coli uses flagella for motility and pili for attachment during infection.

Eukaryotic Cell Components

Eukaryotic cells are more complex and contain membrane-bound organelles. Some organelles are unique to plant or animal cells.

• Nucleus: Contains genetic material (DNA); surrounded by a double membrane with nuclear pores.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

• Lysosomes: Contain digestive enzymes; break down waste (mainly in animal cells).

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water and maintains turgor pressure.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) for structure and movement.

• Cell Wall: Found in plants, fungi, and some protists; provides support and protection.

Additional info: Animal cells lack cell walls and chloroplasts, while plant cells have both.

Nuclear Transport

The nucleus is separated from the cytoplasm by the nuclear envelope, which contains nuclear pores for selective transport.

• What is transported: mRNA, tRNA, ribosomal subunits (out); proteins, nucleotides (in).

• How transport occurs: Nuclear pore complexes recognize nuclear localization signals (NLS) on proteins, allowing passage.

• Directionality: Import and export are regulated by transport proteins (importins/exportins) and energy (GTP hydrolysis).

Endomembrane System

The endomembrane system is a group of organelles involved in protein and lipid synthesis, modification, and transport.

• Components: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, plasma membrane.

• Functions: Synthesis, modification, and transport of proteins and lipids; waste processing.

Signal Hypothesis (Protein Targeting to the Endomembrane System)

The signal hypothesis explains how proteins are directed to the ER for secretion or membrane insertion.

1. Protein synthesis begins on free ribosomes.

2. Signal sequence emerges from ribosome.

3. Signal recognition particle (SRP) binds signal sequence.

4. SRP halts translation temporarily.

5. SRP binds to SRP receptor on ER membrane.

6. Ribosome docks onto translocon (protein channel).

7. SRP is released; translation resumes.

8. Protein is threaded into ER lumen or membrane.

9. Signal sequence is cleaved off (if necessary).

10. Protein folds and undergoes modifications in ER.

Endocytosis and Autophagy

Cells internalize materials and recycle components through various processes:

• Receptor-mediated endocytosis: Specific uptake of molecules via receptor binding and vesicle formation.

• Phagocytosis: Engulfment of large particles or cells by membrane extension.

• Autophagy: Degradation of damaged organelles or proteins within lysosomes.

Cell-Cell Interactions and Communication

Extracellular Matrix (ECM) in Animals vs. Plants

The ECM provides structural support and mediates cell signaling.

• Animal ECM: Composed of proteins (collagen, elastin) and polysaccharides; provides support, regulates cell behavior.

• Plant ECM (Cell Wall): Made of cellulose, hemicellulose, and pectin; provides rigidity and protection.

Comparison Table:

Cell-Cell Connections

Cells connect and communicate through specialized structures:

• Tight Junctions: Seal cells together, preventing leakage (animal cells).

• Desmosomes: Anchor cells together, providing mechanical strength (animal cells).

• Gap Junctions: Channels allowing direct transfer of ions and molecules between animal cells.

• Plasmodesmata: Channels through plant cell walls for molecular exchange.

Connections for Molecular Transfer: Gap junctions (animals) and plasmodesmata (plants) facilitate direct communication.

Cell Signaling

Cells communicate over long distances using signaling molecules:

• Reception: Target cell detects signaling molecule via receptor (membrane-bound or intracellular).

• Transduction: Signal is relayed and amplified inside the cell, often via second messengers (e.g., cAMP).

• Response: Cell changes behavior (gene expression, metabolism, etc.).

• G-protein-coupled receptors (GPCRs): Activate G-proteins, which trigger second messenger cascades.

Example: Epinephrine binding to a GPCR triggers cAMP production, leading to glycogen breakdown.

Energy, Enzymes, and Metabolism

Thermodynamics in Biology

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

• Potential Energy: Stored energy due to position or structure.

• Enthalpy (H): Total energy of a system.

• Entropy (S): Measure of disorder or randomness.

• Gibbs Free Energy (G): Energy available to do work.

Gibbs Free Energy Equation:

• Exergonic (spontaneous):

• Endergonic (non-spontaneous):

Redox Reactions and Phosphorylation

Cells couple exergonic and endergonic reactions to drive metabolism.

• Redox Reactions: Transfer of electrons; oxidation (loss), reduction (gain).

• Phosphorylation: Addition of phosphate group, often from ATP, to activate molecules.

Enzyme Function and Regulation

Enzymes lower activation energy and increase reaction rates.

• Hurdles: Activation energy barrier and proper molecular orientation.

• Enzyme Mechanisms: Induced fit, substrate binding, transition state stabilization.

• Substrate Saturation: At high substrate concentrations, enzyme activity plateaus (Vmax).

• Regulation:

  • Covalent: Chemical modification (e.g., phosphorylation).

  • Noncovalent: Reversible binding of regulators.

  • Competitive: Inhibitor binds active site.

  • Allosteric: Inhibitor/activator binds elsewhere, changing enzyme shape.

Example: Bamboo shark enzyme activity varies with temperature and pH (experimental analysis).

Cellular Respiration and Fermentation

Metabolic Pathways

• Catabolic: Breakdown of molecules to release energy (e.g., cellular respiration).

• Anabolic: Synthesis of complex molecules from simpler ones (e.g., protein synthesis).

Cellular Respiration Overview

• Inputs: Glucose, oxygen

• Outputs: CO2, H2O, ATP

Four Steps of Cellular Respiration:

1. Glycolysis: Cytosol

2. Pyruvate Processing: Mitochondrial matrix

3. Citric Acid Cycle (Krebs): Mitochondrial matrix

4. Electron Transport Chain & Oxidative Phosphorylation: Inner mitochondrial membrane

Key Processes in Respiration

• Redox Chemistry: NAD+/NADH, FAD/FADH2 electron carriers

• Phosphorylation: Substrate-level and oxidative phosphorylation

• Feedback Inhibition: ATP and citrate inhibit phosphofructokinase in glycolysis

Oxygen and Fermentation

• Oxygen: Final electron acceptor in the electron transport chain; essential for maximal ATP yield.

• Fermentation: Occurs when oxygen is scarce; regenerates NAD+ but produces less ATP.

Photosynthesis

Chlorophyll and Light Capture

Chlorophyll is the main pigment in plants that absorbs light energy for photosynthesis.

• Absorbs mainly blue and red wavelengths; reflects green.

• Transfers energy to reaction centers in photosystems.

Photosynthesis Reactions

• Light-dependent reactions: Occur in thylakoid membranes; produce ATP, NADPH, and O2.

• Light-independent reactions (Calvin Cycle): Occur in stroma; use ATP and NADPH to fix CO2 into sugars.

Oxygen Production: O2 is produced during the light-dependent reactions when water is split.

Carbon Fixation: CO2 is fixed during the Calvin Cycle by the enzyme Rubisco.

Photosynthesis vs. Cellular Respiration

Rubisco and Photorespiration

• Rubisco: Enzyme that catalyzes CO2 fixation in the Calvin Cycle.

• Photorespiration: Occurs when Rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

• Consequence: Wastes energy and decreases sugar production in plants, especially under hot, dry conditions.