Cell Structure and Function

Microscopy: Tools for Studying Cells

• Light Microscope (LM): Uses visible light to magnify specimens up to about 1000x. Allows observation of living cells.

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens, revealing internal structures in high detail.

• Scanning Electron Microscope (SEM): Scans the surface of a specimen with electrons, producing detailed 3D images of cell surfaces.

• Example: SEM is used to visualize the surface of pollen grains, while TEM can show the internal structure of mitochondria.

Prokaryotes vs. Eukaryotes

• Prokaryotes: Cells without a nucleus or membrane-bound organelles (e.g., Bacteria, Archaea).

• Eukaryotes: Cells with a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

• Shared Features: All cells have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

• Differences: Eukaryotes are generally larger, have complex organelles, and compartmentalization.

Cell Structures and Organelles

• Nucleus: Contains DNA, controls cell activities.

• Mitochondria: Site of cellular respiration; double membrane; produces ATP.

• Chloroplasts: Site of photosynthesis in plants; double membrane; contains chlorophyll.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.

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

• Lysosomes: Contain digestive enzymes; break down waste.

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

• Ribosomes: Synthesize proteins; found free in cytoplasm or on rough ER.

• Vacuoles: Storage organelles; large central vacuole in plant cells for water and solute storage.

• Plasma Membrane: Semi-permeable barrier; controls entry/exit of substances.

Organelles with Double Membranes

• Mitochondria and Chloroplasts both have inner and outer membranes, supporting the endosymbiont theory.

Plant vs. Animal Cell Structures

• Plant Cells: Have cell walls, chloroplasts, and a large central vacuole.

• Animal Cells: Have lysosomes and centrioles; lack cell walls and chloroplasts.

Endomembrane System

• Definition: A network of membranes inside eukaryotic cells that work together in synthesis, transport, and metabolism.

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

• Function: Proteins and lipids are synthesized in the ER, modified in the Golgi, and transported via vesicles.

Endosymbiont Theory

• Explanation: Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, their own DNA, and ribosomes similar to bacteria.

Cytoskeleton

• Microtubules: Hollow tubes; maintain cell shape, aid in cell division, and serve as tracks for organelle movement.

• Microfilaments (Actin Filaments): Thin fibers; support cell shape, involved in muscle contraction and cell movement.

• Intermediate Filaments: Provide mechanical support and maintain cell integrity.

Plasmodesmata

• Definition: Channels between plant cell walls that allow transport and communication.

Cell-to-Cell Recognition

• Explanation: Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.

Membrane Structure and Function

Plasma Membrane Composition

• Phospholipid Bilayer: Main structure; amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Proteins: Integral (span the membrane) and peripheral (attached to surface); serve as channels, carriers, receptors.

• Cholesterol: Stabilizes membrane fluidity.

• Glycoproteins: Proteins with carbohydrate chains; involved in cell recognition.

Membrane Properties

• Semi-permeable: Allows selective passage of substances.

• Amphipathic: Molecules have both hydrophilic and hydrophobic regions.

Transport Across Membranes

• Passive Transport: No energy required; substances move down their concentration gradient.

  • Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Uses channel or carrier proteins (e.g., aquaporins for water).

  • Osmosis: Diffusion of water across a semi-permeable membrane.

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Sodium-Potassium Pump: Pumps 3 Na+ out and 2 K+ in, maintaining electrochemical gradients.

  • Proton Pump: Moves H+ ions across membranes, creating a proton gradient.

• Bulk Transport: Movement of large molecules via vesicles.

  • Endocytosis: Uptake of materials into the cell.

    • Phagocytosis: "Cell eating"; engulfing large particles.

    • Pinocytosis: "Cell drinking"; uptake of fluids.

    • Receptor-Mediated Endocytosis: Specific uptake using receptor proteins.

  • Exocytosis: Release of materials from the cell.

Osmotic Terms

• Turgid: Plant cell in hypotonic solution; firm due to water uptake.

• Flaccid: Plant cell in isotonic solution; limp.

• Plasmolysis: Plant cell in hypertonic solution; membrane pulls away from wall.

• Lysis: Animal cell bursts in hypotonic solution.

Membrane Proteins

• Integral Proteins: Span the membrane; involved in transport and signaling.

• Peripheral Proteins: Loosely attached to membrane surface.

• Channel Proteins: Provide hydrophilic channels for specific molecules.

• Carrier Proteins: Bind and transport substances across the membrane.

• Aquaporins: Channel proteins specifically for water transport.

Cell Signaling

Stages of Cell Signaling

• Reception: Cell detects a signaling molecule (ligand) via receptor proteins.

• Transduction: Signal is relayed and amplified through a series of molecular events.

• Response: The cell carries out a specific activity (e.g., gene expression, enzyme activation).

Metabolism and Enzymes

Metabolic Pathways

• Metabolism: All chemical reactions in a cell.

• Catabolic Pathways: Break down molecules, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules, consuming energy (e.g., protein synthesis).

Energy in Biological Systems

• Chemical Energy: Stored in bonds of molecules (e.g., glucose, ATP).

• Light Energy: Used in photosynthesis to convert CO2 and H2O into glucose.

• Energy Coupling: Using exergonic (energy-releasing) reactions to drive endergonic (energy-consuming) ones.

• Exergonic vs. Endergonic: Exergonic reactions release energy (ΔG < 0), endergonic require energy input (ΔG > 0).

• Spontaneous Reactions: Occur without energy input (ΔG < 0); non-spontaneous require energy (ΔG > 0).

• Available Energy (Gibbs Free Energy):

Thermodynamics in Biology

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

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

Enzymes

• Definition: Biological catalysts that speed up reactions by lowering activation energy.

• Substrate Specificity: Each enzyme acts on a specific substrate.

• Mechanism: Enzyme binds substrate at active site, forms enzyme-substrate complex, releases product.

• Regulation: Enzyme activity can be regulated by inhibitors, activators, or feedback inhibition.

• Denaturation: Loss of enzyme structure (and function) due to heat, pH, or chemicals; renaturation may restore function if conditions normalize.

• Activation Energy: The energy required to start a reaction; enzymes lower this barrier.

• Coenzymes: Organic molecules (often vitamins) that assist enzyme function.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, regulating pathway activity.

ATP and Energy Transfer

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• ATP Hydrolysis:

• Phosphorylated Intermediate: A molecule that receives a phosphate group from ATP, making it more reactive.

Types of Cellular Work

• Chemical Work: Synthesis of macromolecules (e.g., proteins, nucleic acids).

• Transport Work: Pumping substances across membranes (e.g., sodium-potassium pump).

• Mechanical Work: Movement (e.g., muscle contraction, movement of cilia/flagella).

Key Comparison Table: Prokaryotes vs. Eukaryotes

Key Comparison Table: Plant vs. Animal Cells

Summary

• Understanding cell structure, membrane function, and metabolism is fundamental to biology.

• Cells are highly organized, with specialized organelles and complex transport mechanisms.

• Energy flow and enzyme regulation are central to cellular processes.