Cell Structure and Microscopy

Types of Microscopes

Microscopes are essential tools for studying cells and their structures. The choice of microscope depends on the level of detail required.

• Light Microscope: Uses visible light to view living or preserved cells. Suitable for observing cell shape, size, and some organelles.

• Scanning Electron Microscope (SEM): Provides detailed 3D images of cell surfaces. Used for studying external structures.

• Transmission Electron Microscope (TEM): Offers high-resolution images of internal cell structures by passing electrons through thin sections.

• Example: TEM is used to view mitochondria, while SEM is used to examine cilia on cell surfaces.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structural features.

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

• Eukaryotic Cells: Have a nucleus and various membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Similarities: Both have cytoplasm, cell membrane, and ribosomes.

• Differences: Eukaryotes have organelles; prokaryotes do not due to their simpler structure.

Universal Cell Components

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

• Cell Membrane: Phospholipid bilayer that controls entry and exit of substances.

• Ribosomes: Sites of protein synthesis; found in all cells.

Cell Organelles and Their Functions

Overview of Organelles

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

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

• Nucleolus: Produces ribosomal RNA.

• 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.

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

• Vesicles: Transport materials within cells.

• Vacuoles:

  • Central Vacuole: Large storage organelle in plant cells.

  • Contractile Vacuole: Expels excess water in some protists.

• Mitochondria: Site of cellular respiration; generates ATP.

• Chloroplasts: Site of photosynthesis in plant cells.

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

• Cilia and Flagella: Provide movement; cilia are short and numerous, flagella are long and few.

• Extracellular Matrix: Supports and connects animal cells.

• Cell Junctions:

  • Tight Junctions: Prevent leakage between cells.

  • Anchoring Junctions: Attach cells to each other or to the extracellular matrix.

  • Gap Junctions: Allow communication between cells.

• Cell Wall: Provides support in plant cells.

• Plasmodesmata: Channels between plant cells for communication.

Organelles by Function

Additional info: Some organelles are unique to plants (chloroplasts, central vacuole, cell wall) or animals (lysosomes, extracellular matrix).

Endosymbiont Theory

Origin of Mitochondria and Chloroplasts

The Endosymbiont Theory explains how mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have their own DNA, double membranes, and reproduce independently within cells.

• Example: Mitochondria resemble aerobic bacteria; chloroplasts resemble photosynthetic bacteria.

Plasma Membrane Structure and Function

Fluid Mosaic Model

The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Selective Permeability: Allows certain molecules to pass while blocking others.

Membrane Proteins and Their Functions

• Transport: Move substances across the membrane.

• Enzymes: Catalyze reactions at the membrane surface.

• Attachment: Anchor the membrane to cytoskeleton or extracellular matrix.

• Signaling: Receive and transmit signals.

• Junctions: Connect adjacent cells.

• Glycoproteins: Serve as cell identity markers.

Membrane Transport

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

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

• Facilitated Diffusion: Uses transport proteins for molecules that cannot cross the membrane directly.

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

• Aquaporins: Channel proteins that facilitate water movement.

Tonicity and Effects on Cells

Example: Placing a plant cell in a hypotonic solution causes it to become turgid.

Active Transport

Active transport requires energy (ATP) to move substances against their concentration gradient.

• Example: Sodium-potassium pump in animal cells.

Transport of Large Molecules

• Exocytosis: Exporting materials out of the cell via vesicles.

• Endocytosis: Importing materials into the cell.

  • Phagocytosis: "Cell eating" of large particles.

  • Receptor-mediated Endocytosis: Specific uptake of molecules via receptors.

Energy and Cellular Metabolism

Potential vs. Kinetic Energy

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

• Kinetic Energy: Energy of motion.

Conservation of Matter and Energy, Heat, and Entropy

• Law of Conservation: Matter and energy cannot be created or destroyed, only transformed.

• Heat: Energy lost during transformations increases entropy (disorder).

Endergonic vs. Exergonic Reactions

• Endergonic: Require energy input (e.g., photosynthesis).

• Exergonic: Release energy (e.g., cellular respiration).

ATP: Energy Currency of the Cell

• ATP Structure: Adenosine triphosphate; three phosphate groups.

• Energy Storage: Energy stored in phosphate bonds.

• Energy Release: Hydrolysis of ATP releases energy for cellular work.

Equation:

Enzymes as Biological Catalysts

• Function: Lower activation energy, speeding up reactions.

• Specificity: Each enzyme acts on a specific substrate.

Photosynthesis and Cellular Respiration

Comparison

Additional info: Photosynthesis is endergonic; cellular respiration is exergonic.

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• NADH and FADH2: Electron carriers in cellular respiration.

Stages of Cellular Respiration

Glycolysis

• Location: Cytoplasm

• Reactants: Glucose

• Products: 2 Pyruvate, 2 ATP, 2 NADH

• Substrate-level Phosphorylation: Direct transfer of phosphate to ADP.

• Oxygen Needed? No

Pyruvate Oxidation

• Location: Mitochondrial matrix

• Purpose: Converts pyruvate to acetyl-CoA for entry into the citric acid cycle.

Citric Acid Cycle

• Location: Mitochondrial matrix

• Reactants: Acetyl-CoA

• Products (per turn): 2 CO2, 3 NADH, 1 FADH2, 1 ATP

Oxidative Phosphorylation

• Location: Inner mitochondrial membrane

• Purpose of Proton Gradient: Drives ATP synthesis via chemiosmosis.

• Electron Cascade: Electrons pass through the electron transport chain, releasing energy.

• ATP Generation: ATP synthase uses proton flow to make ATP.

• Final Electron Acceptor: Oxygen

Equation:

Fermentation

Types and Differences from Cellular Respiration

• Fermentation: Occurs without oxygen; produces less ATP.

• Lactic Acid Fermentation: Converts pyruvate to lactic acid; occurs in animal muscles and some bacteria.

• Alcohol Fermentation: Converts pyruvate to ethanol and CO2; occurs in yeast and some plants.

Metabolism of Other Molecules

Use of Carbohydrates, Fats, and Proteins

• Carbohydrates: Broken down to glucose for cellular respiration.

• Fats: Yield more ATP per molecule; enter respiration as fatty acids and glycerol.

• Proteins: Amino acids can be used for energy or biosynthesis after deamination.

Additional info: Cells can use these molecules for energy or as building blocks for biosynthesis.