The Cell: Structure and Function

Cytology

Cytology is the study of cell structure and function. Cells are the basic units of life, and their organization underpins all biological processes.

• Size and Shape: Cells vary in size and shape, which relates to their function. Most cells are microscopic, ranging from 1 to 100 micrometers.

• Surface Area/Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of material exchange. This ratio is crucial for cell efficiency.

Microscopy

Microscopy allows scientists to observe cells and their components.

• Simple vs. Compound Microscopes: Simple microscopes use one lens; compound microscopes use multiple lenses for higher magnification.

• Magnification: The process of enlarging the appearance of an object.

• Resolution: The ability to distinguish two close points as separate.

• Types of Microscopy:

  • Light Microscopy: Uses visible light to observe living or stained cells.

  • Phase-Contrast Microscopy: Enhances contrast in transparent specimens without staining.

  • Scanning Electron Microscopy (SEM): Provides detailed surface images.

  • Transmission Electron Microscopy (TEM): Reveals internal cell structures at high resolution.

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on internal organization.

• Prokaryotes: Lack a nucleus and membrane-bound organelles (e.g., Bacteria, Archaea).

• Eukaryotes: Have a nucleus and organelles (e.g., animals, plants, fungi, protists).

• Animal vs. Plant Cells: Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells do not.

Cell Organelles

Organelles are specialized structures within eukaryotic cells, each with distinct functions.

• Nucleus: Contains genetic material (DNA) and controls cell activities.

  • Nuclear Envelope: Double membrane surrounding the nucleus.

  • Nuclear Pores: Allow exchange of materials between nucleus and cytoplasm.

  • Nucleolus: Site of ribosome synthesis.

  • Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

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

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

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris.

• Vacuoles and Vesicles: Storage and transport of substances; plant cells have large central vacuoles.

• Endomembrane System: Includes ER, Golgi, lysosomes, vesicles; coordinates synthesis and transport of cellular products.

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

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plant cells.

• Centrosomes: Organize microtubules; contain centrioles in animal cells.

• Cytoskeleton: Network of protein fibers for support and movement.

  • Microtubules: Hollow tubes; shape, transport, cell division.

  • Microfilaments: Actin filaments; cell shape, movement.

  • Intermediate Filaments: Structural support.

• Cilia and Flagella: Motile structures for movement.

• Extracellular Matrix (ECM): Network outside animal cells; provides support and signaling.

• Glycocalyx: Carbohydrate-rich coating on animal cells; protection and cell recognition.

• Cell Junctions:

  • Tight Junctions: Seal cells together.

  • Desmosomes: Anchor cells together.

  • Gap Junctions: Allow communication between animal cells.

  • Plasmodesmata: Channels between plant cells for transport and communication.

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

Membrane Structure and Function

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts a dynamic arrangement of lipids and proteins.

• Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails inward.

• Integral Proteins: Embedded in the membrane; include transmembrane proteins (span the membrane) and embedded proteins (partially inserted).

• Functions of Membrane Proteins: Transport (channels, pores), receptors, markers, enzymes, adhesion.

• Peripheral Proteins: Loosely attached to membrane surface.

• Freeze-Fracture Technique: Used to study membrane structure by splitting the bilayer.

• Membrane Fluidity: Influenced by unsaturated (cis-) fatty acids and cholesterol content.

Membrane Transport

• Passive Transport: Movement of substances without energy input.

  • Diffusion: Movement from high to low concentration.

  • Facilitated Diffusion: Uses transport proteins for polar or large molecules.

  • Osmosis: Diffusion of water across a semipermeable membrane.

  • Tonicity: Relative solute concentration; affects cell volume (isotonic, hypertonic, hypotonic).

• Active Transport: Movement against concentration gradient; requires energy (ATP).

  • Electrochemical Gradients: Combined effect of concentration and electrical charge.

  • Na+-K+-ATPase Pump: Maintains sodium and potassium gradients across the membrane.

  • H+ Pump: Moves protons across membranes, important in plants and mitochondria.

  • Co-transport: Coupled transport of two substances (symport or antiport).

• Bulk Transport:

  • Endocytosis: Uptake of materials via vesicles.

    • Phagocytosis: "Cell eating" of large particles.

    • Pinocytosis: "Cell drinking" of fluids.

    • Receptor-Mediated Endocytosis: Specific uptake using receptors.

  • Exocytosis: Release of substances from the cell via vesicles.

Introduction to Metabolism

Metabolic Pathways

Metabolism encompasses all chemical reactions in a cell, divided into two main types:

• Anabolism: Synthesis of complex molecules from simpler ones; requires energy.

• Catabolism: Breakdown of complex molecules into simpler ones; releases energy.

Energy and Thermodynamics

• Energy: The capacity to do work.

  • Kinetic Energy: Energy of motion.

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

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

Free Energy and Reactions

• Exergonic Reactions: Release free energy; spontaneous.

• Endergonic Reactions: Require input of energy; non-spontaneous.

• Free Energy Change (): Determines whether a reaction is spontaneous.

  • Equation:

• Activation Energy (): Energy required to start a reaction.

• Catalysis: Acceleration of a reaction by lowering .

• Coupling Reactions: Linking exergonic and endergonic reactions, often via ATP hydrolysis.

ATP and Enzymes

• ATP Hydrolysis: Releases energy to drive cellular work.

  • Equation:

• Enzymes: Biological catalysts; speed up reactions without being consumed.

  • Active Site: Region where substrate binds and reaction occurs.

  • Co-factors: Non-protein helpers (metal ions or organic molecules).

  • Effects of pH and Temperature: Each enzyme has optimal conditions; extremes can denature enzymes.

• Regulation of Enzyme Activity:

  • Inhibition: Competitive (binds active site) or noncompetitive (allosteric site).

  • Reversible vs. Irreversible Inhibition: Reversible inhibitors can detach; irreversible cannot.

  • Allostery: Regulation by binding at a site other than the active site.

  • Feedback Regulation: End product inhibits pathway to prevent overproduction.

Cellular Respiration and Fermentation

Redox Reactions

Cellular respiration involves oxidation-reduction (redox) reactions, transferring electrons to release energy.

• NAD+ / NADH: Electron carriers; NAD+ is reduced to NADH during catabolism.

ATP Production Mechanisms

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP to form ATP.

• Oxidative Phosphorylation: ATP synthesis powered by electron transport and chemiosmosis.

Anaerobic Pathways

• Glycolysis: Occurs in cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

  • Investment (Priming): Uses ATP to phosphorylate glucose.

  • Cleavage and Rearrangement: Splits 6-carbon sugar into two 3-carbon molecules.

  • Payoff: Produces ATP, NADH, and pyruvate.

• Fermentation: Occurs when oxygen is absent; regenerates NAD+ by converting pyruvate to ethanol (yeast) or lactate (animals).

Aerobic Respiration

• Pyruvate to Acetyl CoA: Pyruvate is oxidized, producing NADH and CO2.

• Krebs (Citric Acid) Cycle: Completes glucose breakdown; produces NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation:

  • Electron Transport Chain: Electrons from NADH and FADH2 pass through protein complexes, releasing energy.

  • Chemiosmosis: Proton gradient drives ATP synthesis via ATP synthase.

  • Products: ATP, NAD+, FAD.

• Feedback Regulation: Cellular respiration is regulated by the availability of substrates and energy needs of the cell.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Table: Types of Membrane Transport

Example: ATP Yield from Aerobic Respiration

• Glycolysis: 2 ATP (net), 2 NADH

• Pyruvate to Acetyl CoA: 2 NADH

• Krebs Cycle: 2 ATP, 6 NADH, 2 FADH2

• Oxidative Phosphorylation: ~26-28 ATP (from NADH and FADH2)

• Total ATP per glucose: ~30-32 ATP

Additional info: The actual ATP yield can vary depending on the cell type and shuttle systems used for transporting electrons into mitochondria.