Cell Structure and Function

General Structure of Prokaryotic and Eukaryotic Cells

The cell is the fundamental unit of life, and all living organisms are composed of cells. Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Cell Theory: All cells arise from preexisting cells and are the smallest units of life.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Possess a nucleus and membrane-bound organelles. Examples include Plants, Animals, Fungi, and Algae.

• Shared Properties: Both cell types reproduce, contain DNA/RNA, have organized metabolic pathways, respond to stimuli, and possess plasma membranes.

Plasma Membrane Structure and Function

The plasma membrane is a dynamic, fluid-mosaic structure composed of a phospholipid bilayer with embedded proteins, lipids, and carbohydrates.

• Integral Proteins: Embedded within the bilayer, often involved in transport.

• Peripheral Proteins: Located on the membrane's surface, providing structural support or acting as receptors.

• Functions: Structural support, molecule transport, enzyme regulation, hormone reception, and surface antigens.

Glycocalyx

The glycocalyx is an extracellular matrix surrounding animal cells and some bacteria, playing roles in communication, protection, immune modulation, and biofilm formation.

• Biofilm Formation: Glycocalyx enables bacteria to adhere to surfaces, forming biofilms that are significant in healthcare and industry.

Cell Wall Structure

Cell walls provide structural support and protection. Their composition varies among organisms.

• Bacteria: Cell wall composition is used for identification and affects antimicrobial drug selection.

• Gram-Positive Bacteria: Thick peptidoglycan layer, contains teichoic acids, minimal periplasmic space.

• Gram-Negative Bacteria: Thin peptidoglycan layer, outer membrane, large periplasmic space.

• Archaea: Lack peptidoglycan; may have pseudopeptidoglycan.

• Plants: Cellulose, hemicellulose, pectin.

• Algae: Cellulose, glycoproteins, polysaccharide inclusions.

• Fungi: Cellulose, glucosamine, chitin.

Surface Appendages

Cells may possess appendages for movement, attachment, or absorption.

• Prokaryotes: Cilia, pili (fimbriae).

• Eukaryotes: Cilia, flagella, microvilli.

• Flagella: Provide motility; structure differs between prokaryotes and eukaryotes.

• Pili (Fimbriae): Rigid, composed of pilin, used for attachment.

• Microvilli: Increase surface area for absorption in eukaryotes.

Types of Bacterial Flagella

• Monotrichous: Single flagellum at one end.

• Amphitrichous: Single flagellum at each end.

• Lophotrichous: Tufts of flagella at one or both ends.

• Peritrichous: Flagella distributed over the surface.

Biofilms

Biofilms are communities of microbes attached to surfaces and embedded in a polysaccharide matrix. They are important in healthcare, industry, and environmental processes.

• Formation: Initial colonizers adhere and anchor; new cells join, and the biofilm grows.

• Significance: Biofilms resist cleaning and antibiotics, facilitate plasmid exchange, and can be used in bioremediation.

Cell Components

Cells contain various organelles and structures, each with specific functions.

• Cytoplasm: Site of biochemical activities.

• Cytoskeleton: Provides structure; includes actin filaments, intermediate filaments, microtubules.

• Nucleus: Control center, contains DNA (eukaryotes only).

• Endoplasmic Reticulum (ER): rER (protein synthesis), sER (lipid synthesis).

• Golgi Apparatus: Modifies, sorts, and packages proteins; produces lysosomes and vesicles.

• Lysosomes: Digestive enzymes for intracellular digestion.

• Peroxisomes: Remove toxic substances via oxidative enzymes.

• Mitochondria: Site of ATP production; contain their own DNA; divide by binary fission.

• Chloroplasts: Photosynthesis in plants and algae; contain DNA.

• Ribosomes: Protein synthesis; 80S in eukaryotes, 70S in prokaryotes.

• Vacuoles: Storage; contractile vacuoles expel excess water in protozoa.

• Vesicles: Transport, metabolism, enzyme storage.

• Inclusions: Reserve deposits.

• Endospores: Survival structures in some bacteria.

Fluid Compartments and Membrane Transport Mechanisms

Fluid Compartments

Cells exist in distinct fluid compartments, which are critical for maintaining homeostasis.

• Intracellular Fluid (ICF): Largest compartment; high in potassium, phosphate, magnesium.

• Extracellular Fluid (ECF): Surrounds cells; high in sodium, chloride, bicarbonate.

Membrane Transport Mechanisms

Substances move across cell membranes via passive or active transport mechanisms.

• Passive Transport: No energy required; includes diffusion, facilitated diffusion, osmosis, filtration.

• Active Transport: Requires energy (ATP); includes pump transport, endocytosis, exocytosis.

Passive Transport

• Diffusion: Movement from high to low concentration until equilibrium.

• Facilitated Diffusion: Carrier-mediated, specific, competitive, saturable.

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

• Filtration: Movement driven by hydrostatic pressure or gravity.

Active Transport

• Pump Transport: Uses ATP; moves substances against concentration gradient.

• Endocytosis: Bulk transport into cell; includes phagocytosis (cell eating), pinocytosis (cell drinking), receptor-mediated endocytosis.

• Exocytosis: Bulk transport out of cell; vesicles formed by Golgi complex.

Cellular Metabolism

Metabolic Pathways

Cellular metabolism encompasses all chemical reactions in a cell, organized into catabolic and anabolic pathways.

• Catabolism: Breakdown of molecules; releases ATP.

• Anabolism: Synthesis of molecules; requires ATP.

Enzymes

Enzymes are biological catalysts that accelerate chemical reactions without being consumed.

• Structure: Most are proteins with active and allosteric sites.

• Specificity: Lock and key model; shape changes can inhibit activity.

• Classification: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

• Exoenzymes: Secreted outside the cell.

• Endoenzymes: Function within the cell.

• Cofactors/Coenzymes: Nonprotein helpers; apoenzyme (protein), coenzyme (organic), cofactor (metal ion), holoenzyme (enzyme + cofactors).

• Regulation: Influenced by temperature, pH, substrate/enzyme/product concentration, cofactors/coenzymes.

• Inhibition: Competitive (active site), noncompetitive (allosteric site).

Cellular Respiration

Cellular respiration is the process by which cells generate ATP from nutrients. It can be aerobic or anaerobic.

• Aerobic Respiration: Glycolysis, Krebs cycle, electron transport chain (ETC).

• Anaerobic Respiration: Glycolysis, fermentation; final electron acceptor is not oxygen.

Glycolysis

• Location: Cytoplasm.

• Process: Glucose converted to pyruvic acid.

• Equation:

• Acetyl CoA Formation: Pyruvic acid decarboxylated, combines with CoA.

Krebs Cycle

• Location: Plasma membrane (prokaryotes), mitochondrial matrix (eukaryotes).

• Process: Acetyl-CoA combines with oxaloacetic acid to form citric acid; cycle regenerates oxaloacetic acid.

• Products: 1 GTP, 3 NADH, 1 FADH2 per cycle.

Electron Transport Chain (ETC)

• Location: Mitochondrial cristae (eukaryotes).

• Process: Electrons transferred through protein complexes; energy used to produce ATP.

• ATP Yield: 36 from ETC, 4 from glycolysis, minus 2 used; total 38 ATP per glucose.

Phosphogluconate Pathway (Pentose Phosphate Pathway)

• Alternate pathway: Used by some bacteria; produces lactic acid, ethanol, CO2.

Anaerobic Respiration and Fermentation

• Lactic Acid Pathway: NADH transfers H to pyruvate, forming lactic acid.

• Final Electron Acceptors: Carbonates, nitrates, sulfates (not oxygen).

• Fermentation: End products vary (lactic acid, ethanol, acetic acid, etc.).

Photosynthesis

• Process: Light energy converted to chemical energy.

• Equation:

• Light-dependent Reaction: ATP production via photophosphorylation (cyclic and noncyclic).

• Light-independent Reaction: Calvin Benson cycle; CO2 fixed into sugars.

Protein Synthesis

Central Dogma

Genetic information flows from DNA to RNA to protein via transcription and translation.

• Transcription: RNA polymerase synthesizes mRNA from DNA template; occurs in cytoplasm (prokaryotes) or nucleus (eukaryotes).

• Translation: mRNA codons translated into amino acids by tRNA; occurs in ribosomes.

Messenger RNA (mRNA)

• Codons: Three-base sequences coding for specific amino acids.

Transfer RNA (tRNA)

• Structure: Cloverleaf; carries anticodon and corresponding amino acid.

• Function: Binds to mRNA codon during translation.

DNA Replication, Cell Division, and Genetics

DNA Replication

DNA replication ensures genetic continuity before cell division. Each new DNA molecule contains one old and one new strand.

• Bacteria: Circular DNA.

• Eukaryotes: Double helix.

Binary Fission

Bacteria reproduce asexually by binary fission, producing two identical daughter cells.

• Process: Cell elongates, DNA replicates, septum forms, cell divides.

Cell Cycle and Mitosis

Eukaryotic cells undergo a cell cycle with distinct phases, culminating in mitosis and cytokinesis.

• G1 Phase: Metabolic activity.

• S Phase: DNA replication.

• G2 Phase: Growth, preparation for mitosis.

• M Phase: Mitosis and cytokinesis.

Mitosis Stages

• Prophase: Chromosomes visible, spindle forms, nuclear membrane disappears.

• Metaphase: Chromosomes align at cell equator.

• Anaphase: Chromatids separate to opposite poles.

• Telophase: Nuclear membrane reforms, cytokinesis completes division.

Meiosis

Meiosis is essential for sexual reproduction, producing haploid cells through two divisions after one chromosome duplication.

• First Division: Homologous chromosomes pair and segregate.

• Second Division: Chromatids separate, resulting in haploid cells.

Genetic Mutations and Transfer

Mutations are changes in DNA/RNA sequence, which can be spontaneous or induced. Bacteria can acquire new genetic material via several mechanisms.

• Mutation: Random, unpredictable changes passed on by cell division.

• Genetic Transfer: Transformation (uptake of DNA), transduction (virus-mediated), conjugation (direct transfer between cells).

Comparison of Gram-Positive and Gram-Negative Bacterial Cell Walls

Summary Table: Membrane Transport Mechanisms

Example: The difference in cell wall structure between Gram-positive and Gram-negative bacteria is crucial for antibiotic selection and infection control.

Additional info: Academic context was added to clarify mechanisms, provide definitions, and expand on examples for completeness.