Cell Structure and Function

Methods Used to Study Cells: Types of Microscopy

• Light Microscopy: Uses visible light to observe cells; suitable for living cells and general cell structure.

• Electron Microscopy: Uses electron beams for much higher resolution; includes Transmission Electron Microscopy (TEM) for internal structures and Scanning Electron Microscopy (SEM) for surface details.

• Fluorescence Microscopy: Uses fluorescent dyes to label specific cell components for visualization.

• Confocal Microscopy: Provides optical sectioning for detailed 3D images of cells.

• Example: TEM is used to view the detailed structure of organelles like mitochondria.

Prokaryotes vs. Eukaryotes: Structural Differences

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotes: Have a true nucleus and various membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus). Examples: Plants, Animals, Fungi, Protists.

• Example: Escherichia coli is a prokaryote; Homo sapiens cells are eukaryotic.

Plant Cells vs. Animal Cells: Key Differences and Organelles

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

• Animal Cells: Lack cell walls and chloroplasts; contain centrioles and lysosomes.

• Common Organelles: Nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, ribosomes.

• Functions:

  • Nucleus: Stores genetic material (DNA).

  • Mitochondria: Site of cellular respiration and ATP production.

  • Chloroplasts: Site of photosynthesis (plants only).

  • Vacuole: Storage and structural support (plants).

  • Lysosomes: Digestion of macromolecules (mainly animals).

Free Ribosomes vs. Bound Ribosomes

• Free Ribosomes: Float in the cytosol; synthesize proteins used within the cell.

• Bound Ribosomes: Attached to the rough ER; synthesize proteins for export or for membranes.

• Structure: Both types are made of rRNA and proteins, forming large and small subunits.

Microtubule Arrangements: Centrioles, Cilia, and Flagella

• Cilia and Flagella: Have a "9+2" arrangement—nine pairs of microtubules surrounding two central microtubules.

• Centrioles: Have a "9+0" arrangement—nine triplets of microtubules with no central pair.

• Function: Cilia and flagella are involved in cell movement; centrioles organize microtubules during cell division.

Extracellular Matrix (ECM) and Cell Junctions

• ECM Components: Glycoproteins (e.g., collagen, fibronectin), proteoglycans, and integrins.

• Functions: Provides structural support, cell signaling, and anchorage for cells.

• Cell Junctions:

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Tight Junctions: Seal cells to prevent leakage of extracellular fluid.

  • Gap Junctions: Allow communication and passage of ions/small molecules between cells.

Cell Membrane Structure and Function

Fluid Mosaic Model of the Plasma Membrane

• Description: The membrane is a fluid structure with a "mosaic" of proteins embedded in or attached to a bilayer of phospholipids.

• Fluidity: Lipids and proteins can move laterally within the layer, allowing flexibility and self-healing.

Components of the Cell Membrane and Their Functions

• Phospholipids: Form the bilayer; hydrophilic heads face outward, hydrophobic tails inward.

• Cholesterol: Maintains membrane fluidity and stability, especially at temperature extremes.

• Proteins: Serve as channels, carriers, receptors, enzymes, and anchors.

Saturated vs. Unsaturated Fatty Acids and Membrane Fluidity

• Saturated Fatty Acids: No double bonds; pack tightly, making the membrane less fluid.

• Unsaturated Fatty Acids: Have double bonds; create kinks, increasing membrane fluidity.

Substances Crossing the Plasma Membrane Without Transport Proteins

• Small, nonpolar molecules: e.g., O2, CO2, and some lipids can diffuse freely.

• Water: Can cross slowly by simple diffusion, but more efficiently via aquaporins (facilitated diffusion).

Types of Cellular Transport

• Passive Transport: Movement down a concentration gradient; no energy required.

  • Simple Diffusion: Direct movement through the membrane.

  • Facilitated Diffusion: Uses transport proteins (channels/carriers).

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

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

  • Uniporters: Transport one substance in one direction.

  • Symporters: Transport two substances in the same direction.

  • Antiporters: Transport two substances in opposite directions.

  • Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Bulk Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Osmosis and Tonicity

• Osmosis: Water moves from areas of low solute concentration to high solute concentration.

• Tonicity: Describes the effect of a solution on cell volume.

  • Isotonic: No net water movement; cell volume remains stable.

  • Hypotonic: Water enters the cell; animal cells may burst (lyse).

  • Hypertonic: Water leaves the cell; cell shrinks (crenates).

• Example: An animal cell in a hypotonic solution will swell and may burst.

Metabolism and Enzyme Function

Metabolism, Catabolism, and Anabolism

• Metabolism: All chemical reactions in a cell.

• Catabolism (Exergonic): Breakdown of molecules, releasing energy.

• Anabolism (Endergonic): Synthesis of molecules, requiring energy input.

• Example: Cellular respiration is catabolic; photosynthesis is anabolic.

Laws of Thermodynamics

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

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

Energy and ATP in Metabolism

• Energy: The capacity to do work.

• ATP (Adenosine Triphosphate): Main energy currency of the cell; provides energy for cellular processes.

• If cells cannot make ATP: Cellular processes stop, leading to cell death.

• ATP Hydrolysis Equation:

Spontaneous Processes

• Definition: Occur without energy input; increase entropy.

• Determined by: Change in free energy (); if , the process is spontaneous.

• Equation:

• Where is change in enthalpy, is temperature in Kelvin, is change in entropy.

Potential vs. Kinetic Energy

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

• Kinetic Energy: Energy of motion (e.g., movement of molecules, muscle contraction).

• Example: Glucose has potential energy; moving ions across a membrane uses kinetic energy.

Enzyme Activity and Regulation

• Effect of Temperature and pH: Each enzyme has optimal temperature and pH for activity; deviations reduce activity or denature the enzyme.

• Allosteric Regulation: Enzyme activity is regulated by molecules binding to sites other than the active site, causing conformational changes.

• Inhibition:

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

  • Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape and function.

• Metabolic Pathways: Series of enzyme-catalyzed reactions; substrates and inhibitors regulate pathway flow.

Activation Energy

• Definition: The energy required to start a chemical reaction.

• Effect: Enzymes lower activation energy, increasing reaction rates without being consumed.